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Ectomycorrhizae and forest biogeochemistry:
The role of ectomycorrhizal communities
in forests impacted by global change
by
Nicholas Paul Rosenstock
A dissertation submitted in partial satisfaction of the
requirements for the degree of
Doctor of Philosophy
in
Environmental Science, Policy and Management
in the
Graduate Division
of the
University of California, Berkeley
Committee in charge:
Professor Thomas D. Bruns, Chair Professor John W. Taylor Professor Ron Amundson
Fall 2010
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Abstract
Ectomycorrhizae and forest biogeochemistry:
The role of ectomycorrhizal communities
in forests impacted by global change
by
Nicholas Paul Rosenstock
Doctor of Philosophy in Environmental Science, Policy and Management
University of California, Berkeley
Professor Thomas D Bruns, Chair
Ectomycorrhizal communities may play a major role in preventing decreases in forest productivity associated with the depletion of nutrients caused by anthropogenic nitrogen and in facilitating increased productivity in response to elevated atmospheric CO2 concentrations. In my doctoral research I attempted to shed some light on the following question: As nutrient demand by forest trees is altered by human induced global change how will the functioning of ectomycorrhizal communities respond?
In chapter 1, I present the results of a study of the effects of nitrogen fertilization on ectomycorrhizal communities in an Eastern US hardwood forest. I found the ectomycorrhizal communities of the organic and mineral horizon to be quite distinct, and that high, but not moderate levels of nitrogen fertilization altered the ectomycorrhizal community composition and decreased ectomycorrhizal species richness. I also found that ectomycorrhizal colonization intensity increased in the mineral soil, and when considered in conjunction with other studies conducted in the same research forest, this may indicate that the ectomycorrrhizal community is shifting in accordance with the shifting nutrient demands of the forest. In Chapter 2, I attempted to elucidate the role of soil heterogeneity in shaping fungal community composition in the mineral soil. Depth and soil carbon content were consistently correlated with fungal community composition. The parent material from which the overlying soil is derived and soil calcium content may be important in determining fungal community composition but our sampling scheme did not allow us to isolate these chemical factors from the potential influence of geographic location on fungal community. In chapter 3, I present a literature review that endeavored to determine whether there is potential for ectomycorrhizal communities or individuals to alter their mineral weathering capabilities and nutrient provision to host plants in response to altered nutrient demand from their host plants. Knowledge of the mechanisms that control belowground carbon allocation by plants in response to nutrient
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demand is limited, but there is a potential for plants to respond to increased demand of phosphorous and potassium by allocating more carbon to the fungal symbionts most adept at providing these nutrients. Future studies on ectomycorrhizal weathering should explicitly test the role of host nutrient demand in stimulating fungal weathering. In chapter 4, I investigated the role of plants, ectomycorrhizae, and low molecular weight organic acids in stimulating mineral weathering, as well as the potential for elevated carbon dioxide to affect biotic weathering. We found that plants, but not their associated ectomycorrhizae, stimulated weathering. Elevated CO2 did not affect weathering rates. The lack of an effect of ectomycorrhizal colonization may have been due to low levels of mycorrhizal colonization. The biotic weathering observed in this study was driven by the uptake of nutrient cations, and not by substrate acidification or root exudation. My doctoral research suggests that ectomycorrhizae may play an important role in mineral nutrient provision.
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Table of contents Acknowledgments………………………………………………………………..…………...…………………………iii Introduction…………………………………………………………………………..………….…………………………iv Chapter 1: Long-‐term additions of nitrogen have altered..……………………………………….……..1 ectomycorrhizal community composition and abundance in a temperate deciduous forest.
Abstract…………………………………………………………………………….……………………1 Introduction…………………………………………………………………..………………….……2 Methods………………………………………………………………………………..…………..……4 Results…………………………………………………………………………………………….……..7 Discussion…………………………………………………………………………………….………14 Conclusion…………………………………………………………………………………….………19 References cited…………………………………………...……………………………….………20
Chapter 2: Linking small scale soil chemical variability ………………………………………………..27 to fungal niche preference
Abstract……………………………………………………………………….……………..…..….…27 Introduction…………………………………………………………………………………….……28 Methods…………………………………………………………………………………..……….…...29 Results…………………………………………………………………………………….………....…31 Discussion………………………………………………………………….……………………...….38 Conclusion………………………………………………………………………………….…………42 References cited………………………………………………………………………….…...……43
Chapter 3: Can ectomycorrhizal weathering activity ………………………………..…………………..46 respond to host nutrient demands?
Abstract…………………………………………………………………….…………………..…...…46 Introduction…………………………………………………………………………………..…...…47 Relevant aspects of root physiology………………………………..….……………...…...47 Evidence from field and microcosm studies……………………….….…………...…...50 Conclusion………………………………………………………………………………….…………55 References cited…………………………………………….…………………………….……..…56
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Table of contents (continued) Chapter 4: The role of ectomycorrhizae, organic acids,…………………………………………………61 and Pinus sylvestris seedlings in mineral weathering: nutrient uptake increases weathering rates.
Abstract…………………………………………………………………….…………………….……61 Introduction…………………………………………………………………………………….……62 Methods…………………………………………………………………………………..……………64 Results…………………………………………………………………………………….……………68 Discussion………………………………………………………………….…………………………75 Conclusion……………………………………………………………………………………….……82 References cited…………………………………………………………………………...….……83
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Acknowledgements: I cannot overstate the role that my late father Robert Rosenstock (1935-‐2004) has had in forming the man that I am today, and contributing to the modest accomplishments I have achieved. He always encouraged my curiosity and, by example, inspired me to pursue a career path that is inspiring to me and valuable to society. I owe a great deal of gratitude to all of the teachers and professors I have had who have encouraged my inquisitive (albeit, at times, overeager) nature. In particular, Dr. Wood, my high school chemistry teacher, showed me the joy and fulfillment that a curious mind can find in science. My undergraduate forest ecology professor, Tim Fahey’s statement in reference to the evolution of the mycorrhizal symbiosis that “the fungi got smart enough to grow trees”, as well as his biogeochemistry lectures likely laid the foundation for this thesis. At Berkeley, professor Garrison Sposito reaffirmed the belief that I could learn and understand anything I put my mind to, with many of his office hours required, and that one could be successful in science without sacrificing an interest in the humanities. The friends that I made during my doctoral studies, were a great help to my mental health, which otherwise may have suffered (more) from thousands of DNA extractions and millions of pipette depressions. Particular debts of gratitude are due to Adriana, for her friendship and support, Russell, for showing me there are always new friends to be made (and for his home stretch efforts in the submission of this document), and Ashley, for distracting me to no end from my research, and energizing me to complete it. The great northern nation of Sweden, its citizens (particularly Anna, Patrick, Niklas, and Elisabet), and immigrants (Martin, Raphaelle, Dragos, Jonas, Linda, Inga) are all due some credit (and bear some responsibility) for my progress in the field of ecology. I have been lucky in many ways in my life. Perhaps my greatest good fortune thus far was to have landed in the Bruns lab at Berkeley. Tom Bruns has been a good friend and a great advisor and mentor. I don’t due well in overly formal settings, and there is nothing overly (or even moderately) formal about the rapport Tom maintains with his students. His general policy of being always available for questions, his encouragement to pursue my research passions (not his), his sense of humor, his compassion, and his attempts to improve my “very long, convoluted (sort of Germanic)” writing style have all been instrumental in my doctoral studies. Lastly, I think I should thank the Forest Service for not hiring me in the Summer of 2003, and the Bush administration, for its insidious attempts to systematically underfund all benevolent, and potentially regulatory, government institutions. Had I gone down my first-‐choice career path after Cornell, there is no telling where I would be right now, but it wouldn’t be here.
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Introduction Man’s emissions of carbon dioxide and nitrogen are impacting large portions of the Earth’s forests. The implications of these impacts for forest productivity and soil carbon pools remain largely unknown. In this era of greenhouse gas induced climate change understanding the trajectory of global trends in forest productivity and soil carbon storage is more important than ever. Ectomycorrhizal fungi play a major role in many of the processes that govern how forests will respond to global change. My PhD thesis attempted to shed light on one of the most important questions facing forest ecology today: As nutrient demands by forest trees is altered by human induced global change, how will the functioning of ectomycorrhizal communities respond? In the proceeding introductory pages I will give some background information about ectomycorrhizal fungi and the effects of anthropogenic nitrogen deposition and elevated atmospheric CO2 levels on forests and ectomycorrhizal communities. I will then discuss how my PhD research has contributed to a greater understanding of how forests will be affected by the next century of human alteration of the global carbon and nitrogen cycles.
Ectomycorrhizal fungi are important to forest health. The majority of land plants associate with some fungi in mycorrhizal symbiosis (Smith and Read, 2008). A mycorrhiza is a symbiotic union between a fungus and a plant root, and, while the role of this symbiosis is complex and variable, nearly all mycorrhizae share a common trait of being a mutualistic exchange of plant derived carbon for fungus supplied nutrients. Ectomycorrhizae are one of a number of mycorrhizal types. Ectomycorrhizae are structurally distinct in that the fungus forms a sheath around the root tips, and sends hyphae (small hair like threads of fungal tissue) into the root, but does not penetrate the plant cell walls. This type of mycorrhiza, ectomycorrhizae, associates with a relatively small portion of all plant species, perhaps only about 3%, but this 3% represents the majority of trees of the temperate and boreal forests (particularly the plant families Fagaceae and Pinaceae), so, in terms of land area, the majority of the earth’s forests are dependant on ectomycorrhizal fungi (Smith and Read, 2008). Ectomycorrhizae have been shown to improve their host plant’s resistance to drought (Muhsin and Zwiazek, 2002), pathogens (Buscot et al., 1992), and phytotoxic concentrations of heavy metals (Smith and Read, 2008), but it is their ability to provide nutrients to their hosts that is generally considered their most growth promoting effect. This enhanced nutrient uptake is a result of ectomycorrhizal fungi’s ability to greatly increase the amount of soil the plants can be in contact with through their extensive mycelial networks as well as their ability to solubilize and take up nutrients from pools not available to plant roots. The majority of research on growth promotion by ectomycorrhizal fungi has focused on their ability to provide nitrogen to their host plants. Ectomycorrhizae can provide up to 80% of a plant’s nitrogen demand (Hobbie and Colpaert, 2003). However, ectomycorrhizae have also been shown to provide their host plants with significant amounts of the mineral derived nutrients calcium (Jentschke et al., 2001), potassium (van Scholl et al., 2006), magnesium (Jentschke et al., 2000), and phosphorous (Ekblad et al., 1996). Studies have shown that ectomycorrhizal fungi may also play a role in the weathering of soil minerals, enhancing mineral nutrient uptake from these otherwise highly recalcitrant nutrient pools (Landeweert et al., 2001). Ectomycorrhizal communities are species rich, with well over a
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hundred ECM species having been documented in monodominant forests (Parrent and Vilgalys, 2007; Taylor et al, 2010), and dozens or more on individual trees (Bruns, 1995). Our knowledge of the respective ecological niches of ECM fungi is poor, but there is ample evidence that suggests discreet, non-‐overlapping niches of habitat preference (Rosling et al., 2003, Dickie et al., 2002) and nutrient acquisition (van Scholl et al., 2006; Abuzinadah and Read, 1986) exist for some species.
As atmospheric CO2 concentrations rise, forest growth and tree’s nutrient demands may increase. The Earth’s atmospheric concentrations of CO2 are increasing due to fossil fuel combustion, agriculture, and deforestation, and are predicted to continue to rise, even if we arrest the increasing rate of anthropogenic CO2 emissions. A number of studies have shown that plants grow faster and fix more CO2 when CO2 concentrations are increased above ambient levels (Ainsworth and Long, 2005). This increased growth however, is dependant on increased nutrient uptake to support increased standing plant biomass (Pinkard et al., 2010) There is evidence that forests are responding to this increased nutrient demand caused by CO2-‐stimulated-‐growth-‐enhancement by increasing root growth and developing a deeper distribution of roots (Iversen, 2010).
Anthropogenic nitrogen pollution threatens to alter the productivity and carbon storage of temperate and boreal forests. Anthropogenic nitrogen pollution (ANP) from energy generation, transport, and agriculture has more than doubled the inputs of nitrogen to terrestrial ecosystems (Vitousek et al., 1997). Emissions of the other major component of acid rain, sulfur, were successfully reduced in the early 1990’s, and public attention to acid rain has since diminished greatly. ANP however, has either remained steady or increased somewhat in the developed world, and has risen sharply, and is predicted to rise even more sharply in the 21st century in the developing world (Galloway et al., 2004). Nitrogen put into the atmosphere by transport and energy generation returns to earth as HNO3 and can fall as wet or dry deposition many hundreds of miles from pollution sources (Driscoll et al., 2003). Soil nitrogen status is, for temperate and boreal forests, the dominant edaphic factor controlling forest productivity and shaping species composition (Cleland and Harpole, 2010). Anthropogenic nitrogen pollution (ANP) has facilitated invasive species establishment in many forests of the temperate and boreal zone and contributed to widespread species loss (Cleland and Harpole, 2010). There is ample evidence that moderate levels of ANP may significantly increase the net primary productivity of temperate forests (Pregitzer et al., 2008; Janssens et al., 2010). Beyond a certain amount of accumulated ANP forest productivity may drop sharply as a result of soil acidification and excess N inputs leaching out other essential nutrients, which then become limiting to forest productivity. This shift from nitrogen limitation to limitation or co-‐limitation by phosphorous (Gradowski and Thomas, 2006), potassium (Gleeson and Good, 2003), or calcium (Long et al., 2009) due to prolonged ANP has already been observed in a number of forests in Eastern North America. Thus, the continued productivity of forests sustaining heavy nitrogen deposition will become dependant on the uptake of these mineral derived nutrients. Mineral weathering increases the supply of these nutrients and neutralizes the acidifying effects of nitrogen deposition. Increased nitrogen availability decreases belowground carbon allocation (Ekblad et al., 1996; Ericsson, 1995). Decreased belowground carbon allocation equates to decreased inputs of carbon into deep soil; carbon inputs which may lead to longer-‐term
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soil carbon retention than aboveground litter inputs (Gill and Burke, 2002). This decreased belowground carbon allocation also has profound effects on mycorrhizal relations.
Understanding how nitrogen deposition and elevated atmospheric CO2 concentrations will affect forest productivity and soil carbon storage is essential to predicting how future anthropogenic emissions of carbon will affect the Earth’s climate. At present, an amount of carbon equivalent to 600 % of our annual CO2 emissions is taken up by the planet’s terrestrial biota each year, the majority in forests (Lal, 2008). Even small increases in forest productivity could be a major negative feedback to greenhouse gas-‐induced climate change. There is five times as much carbon stored in soils as there is in living plant biomass, a change of just 1 % in soil carbon pools is equal to 3 years of anthropogenic carbon emissions (Lal, 2008). There is a wide variety of effects of anthropogenic emissions of N and C that may affect these huge stocks of soil carbon, with the net effect, increase or decrease, very much unclear.
Ectomycorrhizal communities may play a major role in determining how forest productivity and soil carbon stocks are affected by anthropogenic carbon and nitrogen emissions. Elevated CO2 has been found to alter ectomycorrhizal community composition (Parrent and Vilgalys, 2007) and increase mycorrhizal colonization (Alberton et al., 2005). Anthropogenic nitrogen pollution has been found to alter ectomycorrhizal community composition (Wallenda and Kottke, 1998 and references therein), decrease ECM diversity (Lilleskov et al., 2002) and decrease colonization intensity (ECM biomass per unit root biomass) (Treseder et al., 2004). The potential loss of ECM species from nitrogen deposition reduces forest biodiversity and may represent a reduction in forests’ resiliency to future environmental change. ECM represent a very large sink for fixed carbon; studies have found more than 60% of recent carbon assimilation (Rosling et al 2004) and net primary production (Godbold et al., 2006) may be allocated to ectomycorrhizal symbionts, though most estimates are closer to 15% (Hobbie, 2006). Ectomycorrhizal biomass may be much more recalcitrant than fine root biomass (Langley et al., 2006). Reductions in C allocation to ECM may significantly reduce soil C storage and serve as a positive feedback to global change. The reductions in carbon allocation to ectomycorrhizae observed under nitrogen limitation may continue as more N deposition occurs or may level off as other nutrients become limiting to forest growth. The increase in belowground carbon allocation observed with elevated CO2 may continue as global CO2 levels increase, or may level off or reverse if plants become sufficiently nutrient limited that carbon fixation rates are reduced. If ectomycorrhizal community shifts observed under elevated CO2 and nitrogen inputs represent a shift towards ectomycorrhizal species that are better able to provide the nutrients most limiting to plant growth, then forests may likely adapt to their shifting nutrient demands and continue to increase in productivity in response to increasing CO2 levels and nitrogen deposition. If, on the other hand, these shifts in community composition, and reductions in mycorrhizal colonization reflect temperate and boreal forests’ adaptation to limitation by nitrogen and only nitrogen, then increasing amounts of forest may experience reduced productivity in response to continued nitrogen deposition and the fertilization effect from CO2 enrichment will likely decrease as forests become more severely nutrient limited. My dissertation attempts to shed light on which
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of these two scenarios is likely to unfold over the coming decades of continued anthropogenic global change.
In chapter one, I investigated the effects of nitrogen addition on ectomycorrhizal community composition and colonization in a deciduous forest. Very high levels of nitrogen fertilization significantly changed ectomycorrhizal community composition, decreased ectomycorrhizal diversity, and shifted the ectomycorrhizal distribution more towards the mineral soil. These results suggest that the ectomycorrhizal community may be shifting to meet the changing nutrient demands of the forest and outline a potential mechanism for increased soil carbon storage under anthropogenic nitrogen deposition. Based on the high fungal diversity found in the mineral soil, and the fact that the ectomycorrhizal abundance in the mineral soil increased in response to nitrogen deposition I decided to investigate how the heterogeneous distribution of nutrients in mineral soil determines fungal species distribution. In chapter two, I tried to assess which soil properties determine fungal species distribution. Our sampling design prevented us from assessing the role of some of the chemical properties examined, but carbon content and depth emerged as the most influential soil properties determining fungal community composition. Calcium content also appeared to be important in determining fungal community composition.
While pure culture studies have shown that ectomycorrhizal fungi vary in their ability to stimulate mineral weathering and take up mineral nutrients, the observed species shifts in ectomycorrhizal communities can only reflect shifting nutrient demands from host plants if plants can allocate carbon to the mycorrhizal fungi that are providing the most mineral nutrients. In Chapter 3, I present a literature review on the current state of knowledge on how plant nutrient demand drives fungal weathering. Within the plant physiology literature there is evidence that plants may be able to respond to phosphorous and potassium limitation with increased carbon allocation to mycorrhizal fungi providing those nutrients. Magnesium limitation reduces belowground carbon allocation, and the effects of calcium limitation on carbon allocation are unclear. In studies of ectomycorrhizal weathering there is a distinct lack of explicit testing of the role of host nutrient status in driving fungal weathering. I conclude by making a number of recommendations for how future studies can address this important question. For the fourth chapter of this dissertation I investigated how elevated CO2 affects plant growth, biotic weathering, and organic acid exudation, as well as the roles of plants, their ectomycorrhizal symbionts, and organic acids in stimulating mineral weathering. Elevated CO2 increased plant growth but did not increase mineral weathering. Pine seedlings but not ectomycorrhizae significantly increased mineral weathering, though there was some indication that one of the two ectomycorrhizal species examined, Piloderma fallax, may have stimulated mineral weathering. These results do not support the hypothesis that increased nutrient demand by plants, caused by increased CO2 availability, will stimulate weathering, though our system’s departures from forest soil conditions hamper our ability to directly relate our results to processes in forested ecosystems.
In my doctoral research I set out to determine whether the ecological, chemical, and physiological nature of the ectomycorrhizal symbiosis will allow for ectomycorrhizal communities to shift their functioning in accordance with the changing nutrient demands
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of forests experiencing global change. My research indicates that ectomycorrhizal communities may shift to accommodate the shifting nutrient demands of forest undergoing sustained heavy nitrogen deposition, but failed to find an effect of elevated CO2 on mycorrhizal fungi or biotic weathering. I also identified a number of ways in which future studies could address these questions in a more targeted, verifiable manner. References Cited Abuzinadah RA, Finlay RD, Read D. 1986. The role of proteins in the nitrogen nutrition of ectomycorrhizal plants. II Utilization of proteins by mycorrhizal plants of Pinus contorta. New Phytologist 103: 495-‐506. Ainsworth EA, Long SP. 2005. What have we learned from 15 years of free-‐air CO2 enrichment (FACE)? A meta-‐analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165: 351–372. Alberton O, Kuyper TW, Gorissen A. 2005. Taking mycocentrism seriously: mycorrhizal fungal and plant responses to elevated CO2. New Phytologist 167:859-‐868. Bruns TD. 1995. Thoughts on the processes that maintain local species-‐diversity of ectomycorrhizal fungi. Plant and Soil 170: 63-‐73. Buscot F, Weber G, Oberwinkler F. 1992. Interactions between Cylindricarpon-destructans and ectomycorrhizas of Picea abies with Laccaris laccata and Paxillus involutus. Trees-‐Structure and Function 6:83-‐90. Cleland EE, Harpole WS. 2010. Nitrogen enrichment and plant communities. Annals of the New York Academy of Sciences 1195:46 -‐61. Dickie IA, Xu B, Koide RT. 2002. Vertical niche differentiation of ectomycorrhizal hyphae in soil as shown by T-‐RFLP analysis. New Phytologist 156: 527–535. Driscoll CT, Whitall D, Aber J, Boyer E, Castro M, Cronan C, Goodale C, Groffman P, Hopkinson C, Lambert K, Lawrence G, Ollinger S. 2003. Nitrogen pollution in the northeastern United States: sources, effects, and management options. Bioscience 53: 357– 374.
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Ekblad A, Wallander H, Carlsson R, Huss-‐Danell K. 1995. Fungal biomass in roots and extraradical mycelium in relation to macronutrients and plant biomass of ectomycorrhizal Pinus sylvestris and Alnus incana. New Phytologist 131:443-‐451. Ericsson T. 1995. Growth and shoot: root ratio of seedlings in relation to nutrient availability. Plant and Soil 168-‐169:205-‐214. Galloway JN, Dentener FJ, Capone DG, Boyer EW, Howarth RW, Seitzinger SP, Asner GP, Cleveland CC, Green PA, Holland EA, Karl DM, Michaels AF, Porter JH, Townsend AR, Vorosmarty CJ. 2004. Nitrogen cycles: past, present, and future. Biogeochemistry 70:153-‐226. Gill RA, Burke IC. 2002. Influence of soil depth on the decomposition of Bouteloua gracilis roots in the shortgrass steppe. Plant and Soil 241: 233–242. Gleeson SK, Good RE. 2003. Root allocation and multiple nutrient limitation in the New Jersey Pinelands. Ecology Letters 6: 220–227. Godbold DL, Hoosebeek MR, Lukac M, Cotrufo MF, Janssens IA, Ceulemans R, Polle A, Velthorst EJ, Scarascia-‐Mugnozza G, De Angelis P, Miglietta F, Peressotti A. 2006. Mycorrhizal hyphal turnover as a dominant process for carbon input into soil organic matter. Plant and Soil 281:15-‐24. Gradowski T, Thomas SC. 2006. Phosphorus limitation of sugar maple growth in central Ontario. Forest Ecology and Management 226:104–109. Hobbie EA. 2006. Carbon allocation to ectomycorrhizal fungi correlates with belowground allocation in culture studies. Ecology 87:563-‐569. Hobbie EA, Colpaert JV. 2003. Nitrogen availability and colonization by mycorrhizal fungi correlate with nitrogen isotope patterns in plants. New Phytologist 157:115-‐126. Iversen CM. 2010. Digging deeper: fine-‐root responses to rising atmospheric CO2 concentration in forested ecosystems. New Phytologist 186: 346–357. Janssens IA, Dieleman W, Luyssaert S, Subke JA, Reichstein M, Ceulemans R, Ciais P, Dolman AJ, Grace J, Matteucci G, Papale D, Piao SL, Schulze ED, Tang J, Law BE. 2010. Reduction of forest soil respiration in response to nitrogen deposition. Nature Geoscience 3:315-‐322. Jentschke G, Brandes B, Kuhn AJ, Schröder WH, Becker JS, Godbold DL. 2000. The mycorrhizal fungus Paxillus involutus transports magnesium to Norway spruce seedlings. Evidence from stable isotope labeling. Plant and Soil 220:243–246. Jentschke G, Godbold DL, Brandes B. 2001. Nitrogen limitation in mycorrhizal Norway
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spruce (Picea abies) seedlings induced mycelial foraging for ammonium: implications for Ca and Mg uptake. Plant and Soil 234:109–117. Lal R. 2008. Sequestration of atmospheric CO2 in global carbon pools. Energy and Environmental Science 1: 86–100. Landeweert R, Hoffland E, Finlay RD, Kuyper TW, van Breemen N, 2001. Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals. Trends in Ecology and Evolution 16: 248-‐254. Langley JA, Chapman SK, Hungate BA. 2006. Ectomycorrhizal colonization slows root decomposition: the post-‐mortem fungal legacy. Ecology Letters 9: 955–959. Lilleskov EA, Fahey TJ, Horton TR, Lovett GM. 2002. Below-‐ground ectomycorrhizal fungal community change over a nitrogen deposition gradient in Alaska. Ecology 83:104-‐115. Long RP, Horsley SB, Hallett RA. 2009. Sugar Maple growth in relation to nutrition and stress in the northeastern United States. Ecological Applications 19:1454-‐1466. Muhsin TM, Zwiazek JJ. 2002. Ectomycorrhizas increase apoplastic water transport and root hydraulic conductivity in Ulmus americana seedlings. New Phytologist 153:153-‐158. Parrent JL, Vilgalys R. 2007. Biomass and compositional responses of ectomycorrhizal fungal hyphae to elevated CO2 and nitrogen fertilization. New Phytologist 176: 164–174. Pinkard EA, Beadle CL, Mendham DS, Carter J, Glen M. 2010. Determining photosynthetic responses of forest species to elevated [CO2]: Alternatives to FACE. Forest Ecology and Management 260:1251–1261. Pregitzer K S, Burton AJ, Zak DR, Talhelm AF. 2008. Simulated chronic nitrogen deposition increases carbon storage in northern temperate forests. Global Change Biology 14:142–153. Rosling A, Lindahl BD, Taylor AFS, Finlay RD. 2004. Mycelial growth and substrate acidification of ectomycorrhizal fungi in response to different minerals. FEMS Microbiology Ecology 47:31-‐37. Rosling A, Landeweert R, Lindahl BD, Larsson KH, Kuyper TW, Taylor AFS, Finlay RD. 2003. Vertical distribution of ectomycorrhizal fungal taxa in a podzol soil profile New Phytologist 159: 775–783. Smith SE, Read DJ. 2008. Mycorrhizal Symbiosis: 3rd Edition. London: Academic Press. 787 p.
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Taylor DL, Herriott IC, Stone KE, McFarland JW, Booth MG, Leigh MB. 2010. Structure and resilience of fungal communities in Alaskan boreal forest soils. Canadian Journal of Forest Research 40: 1288–1301. Treseder KK. 2004. A meta-‐analysis of mycorrhizal responses to nitrogen, phosphorus, and atmospheric CO2 in field studies. New Phytologist 164:347-‐355. van Scholl L, Smits MM, Hoffland E. 2006. Ectomycorrhizal weathering of the soil minerals muscovite and hornblende. New Phytologist 171:805-‐814. Vitousek PM, Aber JD, Howarth RW, Likens GE, Matson PA, Schindler DW, Schlesinger WH, Tilman DG. 1997. Human alterations of the global nitrogen cycle: Sources and Consequences. Ecological Applications 7:737–750. Wallenda T, Kottke I. 1998. Nitrogen depositon and ectomycorrhizas. New Phytologist 139:169-‐18
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Chapter 1
Long-‐term additions of nitrogen have altered ectomycorrhizal community composition and abundance in a temperate deciduous forest. Abstract
Studies have found that prolonged additions of nitrogen to temperate forests have profound effects on the ectomycorrhizal community composition. Most studies note decreased diversity and altered community composition, and some have found decreased ectomycorrhizal abundance. Few of these studies have been performed in deciduous forests and only one has looked at the effects of nitrogen addition on the ectomycorrhizal community in different soil layers. We sampled the ectomycorrhizal community in a mixed species oak-‐dominated deciduous forest at the Harvard Forest Long-‐Term Ecological Research Chronic Nitrogen Enrichment site in the Northeast US. These plots have undergone 18 years of NH4NO3 additions at high (150 kg N/ha/ yr) and low (50 kg N/ha/ yr) levels. We used soil cores to sample the ECM community in the organic and mineral horizons in the control, low N, and high N plots. Individual root tips were sequenced with Sanger sequencing. First and second order roots were collected for ectomycorrhizal abundance measurements.
A total of 1806 ectomycorrhizal root-‐tips were sequenced to yield 495 ectomycorrhizal sequences representing 65 ectomycorrhizal fungal species (based on 97% sequence similarity in the ITS region). Ectomycorrhizal community composition was significantly different and diversity significantly lower in the high N treatment. Neither diversity nor community composition were significantly different between the low N and control treatments. The ectomycorrhizal communities were significantly different in the organic and mineral soil and diversity was higher in the mineral soil. Ectomycorrhizal abundance was not significantly different between any horizons or N addition levels except for the high N mineral horizon, which had significantly higher ectomycorrhizal abundance. We could not detect an interactive effect of nitrogen and horizon on community composition. While certain species exhibited clear nitrophillic (Lactarius quietus, Russula atropurpurea, Amanita sp. 1, Lactarius sp. 1) or nitrophobic (Cenococcum geophilum, Tomentella sp. 1, Russula sp. 1, Tricholoma aff. sejunctum) reactions to nitrogen addition, these affinities do not seem to be generalizable to genus or family. Distinct horizon preferences were found for species, genera, and families. We suggest that our results indicate a possible adaptation to increased nutrient demand for phosphorous and/or base cations.
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Introduction Anthropogenic nitrogen pollution is a global problem that threatens the ecological
integrity of many terrestrial and aquatic ecosystems. Anthropogenic nitrogen pollution (ANP) from energy generation, transport, and agriculture has more than doubled the inputs of nitrogen to terrestrial ecosystems (Vitousek et al., 1997). Emissions of the other major component of acid rain, sulfur, were successfully reduced in the early 1990’s and public attention to acid rain has since diminished greatly. Anthropogenic nitrogen pollution however, has either remained steady or increased somewhat in the developed world, and has risen sharply, and is predicted to rise even more sharply in the 21st century in the developing world (Galloway et al., 2004). Nitrogen pollution from agriculture either leaches into streams, lakes, and groundwater as nitrate or volatilizes off of fields and manure deposits to return to the earth in the form of ammonium. Nitrogen put into the atmosphere by transport and energy generation returns to earth as HNO3 and can fall as wet or dry deposition many hundreds of miles from pollution sources. As a result of both these processes, but primarily due to the more far reaching HNO3 (Driscoll et al., 2003), large forested regions are receiving inputs of HNO3 that threaten to dramatically alter their ecology and species composition (Gilliam_2006), and, when prolonged severe deposition occurs, reduce their productivity (Fenn et al, 2006). Ectomycorrhizal fungi are essential to forest health and are particularly important to the nitrogen nutrition of temperate and boreal forests. They form intimate associations with tree roots, providing the roots with nutrients and receiving fixed carbon from their plant hosts. Ectomycorrhizae form symbiosis with less than 3% of the world’s plant species but with many of the dominant trees of temperate and boreal forests, particularly the plant families Pinaceae and Fagaceae, (Smith and Read, 2008). Ectomycorrhizal communities are species rich, with well over a hundred ECM species having been documented in monodominant forests (Parrent and Vilgalys, 2007; Taylor et al, 2010), and dozens or more on individual trees (Bruns, 1995). Ecomycorrhizae have been shown to transfer significant amounts of P (Ekblad et al., 1996), Mg (Jentschke et al., 2000), Ca (Jentschke et al., 2001), and K (van Scholl et al., 2006) to their plant hosts, but their provision of nitrogen is generally considered their primary contribution to plant health. Studies have shown that ECM may provide up to 80% of a host plant’s total N uptake (Hobbie and Colpaert, 2003). The extraradical mycelia of ECM greatly increase the volume of soil that roots can exploit (Smith and Read, 2008). Through the use of a diverse suite of enzymes ECM may be able to solubilize and take up nitrogen from organic N pools that roots cannot utilize (Abuzinadah and Read, 1986).
Anthropogenic nitrogen pollution threatens to alter the productivity and carbon storage of temperate and boreal forests. Soil nitrogen status is, for temperate and boreal forests, the dominant edaphic factor controlling forest productivity and shaping species composition (Cleland and Harpole, 2010). Anthropogenic nitrogen pollution (ANP) has facilitated invasive species establishment in many forests of the temperate and boreal zone, and contributed to widespread species loss (Gilliam_2006; Cleland and Harpole, 2010). There is ample evidence that moderate levels of ANP may significantly increase the net primary productivity of temperate forests (Pregitzer et al., 2008; Janssens et al., 2010). Beyond a certain amount of accumulated ANP forest productivity may drop sharply. This is the core of the “nitrogen saturation hypothesis” developed by Aber et al.
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(1998), and it is used by many as a theoretical framework to understand the ecological processes at work over the course of prolonged periods of ANP (Fenn et al., 1998; Fenn et al., 2006; Vadeboncoeur, 2010). According to the Nitrogen Saturation Hypothesis, the tipping point and proceeding drop in forest productivity is a result of soil acidification, and excess N inputs leaching out other essential nutrients, which then become limiting. This shift from nitrogen limitation to limitation or co-‐limitation by phosphorous (Gradowski and Thomas, 2006), potassium (Gleeson and Good, 2003), or calcium (Long et al., 2009; Baribault et al., 2010) due to prolonged ANP has already been observed in a number of forests in Eastern North America. Increased nitrogen availability decreases belowground carbon allocation (Ekblad et al., 1996; Ericsson, 1995; Magill, 2004). Decreased belowground carbon allocation involves decreased inputs of carbon into deep soil; carbon inputs which may lead to longer-‐term soil carbon retention than aboveground litter (Gill and Burke, 2002). This decreased belowground carbon allocation also has profound effects on mycorrhizal relations. Anthropogenic nitrogen pollution has been shown in fertilization and deposition gradient studies to have large impacts on ectomycorrhizal communities. N fertilization and deposition is commonly associated with a shift in ECM species composition with some species increasing in abundance while others disappear under high N fertilization or deposition (Wallenda and Kottke, 1998 and references therein; Avis et al., 2008; Lucas and Caspar, 2008). Many studies have also observed a decrease in both ECM diversity (Lilleskov et al., 2002; Avis et al., 2008; Lucas and Caspar, 2008) and colonization intensity (ECM biomass per unit root biomass) (Treseder et al., 2004; Wölllecke et al., 1999) with nitrogen additions.
The effects of ANP on ectomycorrhizal communities may have serious negative implications for ecosystem integrity. Our knowledge of the respective ecological niches of ECM fungi is poor, but there is ample evidence that suggests discreet, non-‐overlapping niches of habitat preference (Rosling et al., 2003, Dickie et al., 2002) and nutrient acquisition (van Scholl et al., 2006; Abuzinadah and Read, 1996) exist for some species. The potential loss of ECM species from nitrogen deposition reduces forest biodiversity and may represent a reduction in forests’ resiliency to future environmental change. ECM represent a very large sink for fixed carbon; studies have found more than 60% of recent carbon assimilation (Rosling et al 2004) and net primary production (Godbold et al., 2006) may be allocated to ectomycorrhizal symbionts, though most estimates are closer to 15% (Hobbie, 2006). Ectomycorrhizal biomass may be much more recalcitrant than fine root biomass (Langley et al., 2006). Reductions in C allocation to ECM may significantly reduce soil C storage and serve as a positive feedback to global change. The great majority of studies on nitrogen fertilization and ECM have been conducted in conifer stands and have focused on the organic horizon, yet there is evidence that forests dominated by broad-‐leafed angiosperms may react differently than coniferous gymnosperms to ANP (Aber et al., 1998; Fenn et al., 1998). Very few studies examining the effects of ANP on ECM communities or even on ECM communities for any purpose have looked at the ECM community in the mineral soil. Over half of all ECM biomass may be in the mineral soil (Rosling et al., 2003; Scattolin et al, 2008) and ECM
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community composition varies significantly with soil horizon (Dicke et al., 2002; Rosling et al., 2003).
To examine the effect of N enrichment in broad-‐leaved forests and in mineral and organic horizons we investigated the ECM community in a mixed broad-‐leaved forest at the Harvard Forest Chronic N Enrichment research site. This National Science Foundation-‐sponsored Long Term Ecological Research facility is the longest running nitrogen fertilization study in the US. Methods Study Site The study was conducted in the hardwood stand at the Harvard Forest Chronic Nitrogen Enrichment site. This is a National Science Foundation sponsored Long-‐Term-‐Ecological -‐Research study that commenced in 1988 to investigate the effects of nitrogen addition on forests. The Harvard Forest is located in central Massachusetts (42o30’N, 72o10’W). Mean monthly temperatures range from 19oC in July to -‐12oC in January. Average annual precipitation is 112 cm and is distributed evenly throughout the year. Atmospheric nitrogen deposition is approximately 8 kg/ha/yr. The hardwood stand is dominated by red (Quercus rubra) and black oak (Quercus velutina) (approximately 50%) with lesser amounts of black birch (Betula lenta), red maple (Acer rubra), american beech (Fagus gransifolia) and white birch (Betula alba), and is approximately 60-‐100 years old. The soils are stony to sandy loams classified as Typic Dystrochrepts. More detail about the soils, climate, and stand history can be obtained from Magill et al. (2004).
In 1988, Three 30m X 30m plots were established, which were each further divided into 36 5m X 5m subplots. Since 1988, the low N and high N plots have received 50 kg N/ha/yr and 150 kg N/ha/yr, respectively, divided evenly amongst 6 applications at 4 week intervals by backpack sprayer from May through September. The interior 16 subplots are used for analysis, and the perimeter 20 subplots are not. More detail about the nitrogen addition treatment can be obtained from Magill et al. (2004). Sample collection and processing Ectomycorrhizal sampling was performed in July, 2005. We randomly selected 4 5m X 5m subplots from the interior 16 subplots. We used 30cm (depth) X 2.5cm (interior diameter) PVC pipe to collect soil cores. We sampled 4 subplots from each of the control, low N addition, and high N addition hardwood plots (3 plots, 12 subplots total). Six cores were taken from each subplot. Each core was processed on the same day it was collected from the forest. Because sampling was conducted over the course of a month during which time rooting dynamics may change, sample collection was divided evenly between different subplots and nitrogen treatments over time.
Each core was divided into organic and mineral soil. There was typically a distinct border between the organic horizon and mineral horizon, and to prevent any cross-‐contamination 0.5-‐2 cm of soil at the interface between the cores was not kept. While the depth of the organic horizon varied somewhat, the organic horizon was generally around 5 cm thick, and the mineral horizon we sampled 25cm. After each core was divided, each portion of the core was washed over a 2mm sieve with distilled water. Fine roots were
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then collected and put into a petri dish filled with water for ectomycorrhizal root tip sampling and quantification.
Roots were examined under a dissecting microscope and roots that were not turgid or that appeared senescent were removed, as were any roots that were higher than second branching order. Any roots that appeared to be red maple were also removed. Red maple associates with arbuscular mycorrhizal fungi, not the ectomycorrhizal fungi that this study focused on. Fortunately, red maple roots are quite morphologically distinct as they have a much lighter color and a unique beaded morphology that makes them easy to distinguish form the ectomycorrhizal roots. The live, ectomycorrhizal fine roots from each core were placed in a water-‐filled petri dish for community characterization and quantification. A subset of all samples (3 cores /subplot) was examined for percentage root length colonized before they were assessed for community composition. Petri dishes (10cm diameter) were placed over a piece of transparency paper with a black grid. The dish was illuminated from underneath and the roots were examined under a dissection microscope at low magnification. Each line of the grid was followed visually and every intersection with a root was recorded as either mycorrhizal if the grid intersected the root at an ectomycorrhizal root tip or non-‐mycorrhizal if the grid intersected the root at a point on the root that was not an ectomycorrhiza. If the grid crossed a coralloid cluster of mycorrhizae each discrete intersection of the line with a seprate branchlet of the cluster was counted. All gridlines, vertical and horizontal were counted for each dish (20 lines in all) and then the grid was rotated, the roots were mixed and the count was done again. For each sample 5 counts were done, and the percentage root length colonized (% RLC) for each sample was calculated as the average amount of total mycorrhizal intersections/divided by the average amount of total root intersections.
Either following quantification, or immediately after washing and sorting, 14 root tips were randomly selected from each horizon from each core (+1 sample control with no root tip) and placed in 300uL 2X CTAB buffer (100mM Tris-‐HCl(pH 8.0), 1.4 M NaCl, 20mM EDTA, 2% CTAB, 0.2% β-‐mercapto-‐ethanol) and immediately frozen for later extraction and sequencing. In all, 2,160 ectomycorrhizal samples were collected [3 Plots X 4 Subplots X 6 cores X 2 horizons X 15 tips (14 + 1 control) = 2160 tips collected]. Molecular analysis DNA from root tips was extracted by chloroform/isopropanol/ethanol precipitation as per Gardes and Bruns (1993). The ITS region was selected for sequencing, and PCR amplification was done with the primer pair ITS 1F (Gardes and Bruns 1993) and ITS 4 (White et al., 1990). The ITS region is the most commonly sequenced region for fungal identification and has a large database of vouchered sequences. Its high variability makes it a suitable region for species identification (Horton and Bruns, 2001). PCR products were treated with ExoSAP IT (USB Corp, Cleveland, OH, USA) to remove primers and inhibitory salts. PCR amlicons were sequence directly without cloning using ABI Big Dye version 3.1 (Applied Biosystems, Foster City, CA, USA) and pre sequencing cleanup was performed with the ABI recommended ethanol/EDTA precipitation. Single pass sequencing was conducted on an ABI 3100 16 capilary Sanger-‐sequencing machine. Sequences were analyzed using Sequencher 4.2 (Gene Codes Corp., Ann Arbor, MI, USA). Sequences were edited to remove priming sites and poor quality portions of the
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sequences at the 3’ and 5’ ends. Only sequences with at least 200 clear distinct base pairs were used. Many were disqualified due to apparent contamination or co-‐occurrence of other fungal PCR-‐product. Acceptable sequences were identified by comparison with the sequence database at the National Center for Bioscience Informatics (NCBI) using the basic local alignment search tool (BLAST) to identify rough phylogenetic identity. Sequences were then grouped according to these approximate phylogenetic groupings (typically family) and clustered using Sequencher 4.2 with a minimum overlap of 50% and minimum sequence identity of 97%. High quality sequences (long sequences, no unclear peaks) were then selected from clusters for BLASTing against the NCBI database again. When matches at 97% or higher were found, the best match to a vouchered sporocarp sequence was used as the taxa name. In many cases no match at 97% or higher for a vouchered sporocarp was found and we felt that we could only reliably identify the genus of these taxa. These taxa are called “genus” sp. “#” (ex. Russula sp. 2). Statistical analysis Diversity was assessed using the chao1 estimate, species accumulation curves generated by analytical rarefaction, the Shannon index, and Simpson’s index. Rarefaction analysis was performed using Analytic Rarefaction (Hunt Mountain Software, 2009. Analytic Rarefaction, version 2.0). The Shannon diversity index was calculated as
-‐ ∑ pi ln ( pi ) .
The Simpson diversity index was calculated as 1 / (∑ pi2)
Species evenness was calculated as
H / log(S) . Here pi is the proportion of total number of species made up of the ith species, H is the shannon index, and S is the total number of observed species. The similarities between communities were assessed with ordination methods using the statistical software PC-‐ORD (MjM Software Design, version 5.0, Glene-‐ den Beach, OR, USA). For ordination, each community consisted of all successfully identified ectomycorrhizal sequences from the 6 cores taken for a specific horizon in a specific subplot (24 communities total). The communities were compared across nitrogen treatments (n = 8/N treatment) or between soil horizons (n= 12 / horizon). Mantel tests were independently conducted to assess whether the communities in different horizons or nitrogen treatments were significantly different. Differences were visualized using non-‐parametric-‐multi-‐dimensional scaling (NMS). NMS is a suitable method for comparing complex microbial communities because it does not assume a normalized distribution of species or equivalent variance between communities (Clarke, 1993). For NMS ordination an initial run was performed using the “medium thoroughness” default settings to identify the optimum dimensionality of the ordination. After that, the ordination was performed again (Sorenson’s/Bray-‐Curtis distance measure, 400 maximum iterations, 0.00001 instability criterion, recommended dimensionality, 100 real runs and 200 randomized trials). To account for unequal amounts of root tips between
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communities, the abundance of each taxa in each sample was divided by the total number of root tips in that sample such that the total abundance of all taxa within each sample equaled 1. Only the 45 most abundant taxa (3 or more root tips found) were used for community analysis. The effect of horizon and nitrogen treatment on the abundance of the 25 most common taxa was assessed using a two way ANOVA followed by post-‐hoc comparisons using the student’s t-‐test (for horizon) or Tukey’s HSD test (nitrogen treatment). The effect of horizon and nitrogen treatment on groups of taxa (genera, families, orders) was assessed for monophyletic groups that had at least 3 species, each of which occurring on at least 3 (out of 12) subplots. For such groups of species the standardized species’ abundances (as detailed for ordination methods) were used with the additional standardization step of equalizing the total abundance of each species, so that the total abundance of each species was set to equal 1; this was done so that one very abundant species did not bias the whole genus or family. All ANOVA were done with JMP v5.0.1 (SAS Institute Inc., Cary, NC, USA).
Results Out of 1800 ectomycorrhizal root tips extracted, we obtained 495 (243 organic, 252 mineral horizon) ectomycorrhizal sequences of sufficient quality to use in our analysis. A small number (36) of readable sequences were members of the Heliotiales or other known saprobe groups and were not used in analysis. The majority of DNA extracts that failed to yield useable sequences did so due to multiple fungi being present in the DNA extract. The number of useable sequences amounted to 13-‐30 sequences per subplotXhorizon (n = 24), 31-‐55 per subplot (n = 12), and 59-‐102 per plotXhorizon (n = 6). Examining the overall species accumulation curve (figure 1) it appears that we sampled adequately to capture the overall diversity; the chao1 estimator of total diversity is 67.25 +/-‐ 1.75 (S.D.). When the community from each plotXhorizon combination is examined separately, the species accumulation curves do not plateau, and the chao1 estimators of diversity vary between 26.3 and 58.2 (figure 2).
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Figure 1: Rank abundace plot (bars, axis left and below) and species accumulation curve (line, axis above and right) for total ectomycorrhizal community across all three nitrogen treatments and both horizons. 495 sequences grouped into 65 species. Error bars on species accumulation curve are variance.
Figure 2: Species accumulation curves for each horizon of each nitrogen treatment. Vertical bars are the variance associated with each estimate. Numbers to the right of each line are the chao1 estimates of total diversity.
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Response to Nitrogen Horizon Preference Taxa name # of tips # of subplots (out of 12)
P value P value
1. Cenococcum geophilum 62 12 -‐ high 0.02 No pref 2. Lactarius quietus var. incanus 53 11 + high 0.011 org 0.082 3. Russula atropurpurea 44 11 + high 0.011 No pref 4. Tomentella sp. 1 27 9 -‐ high 0.02 No pref 5. Lactarius camphoratus 21 8 No effect min 0.0374 6. Lactarius imperceptus 19 7 + low 0.013 org 0.013 7. Amanita sp.1 18 4 + high 0.009 min 0.088 8. Russula sp. 1 16 6 -‐ high 0.05 No pref 9. Russula sp. 10 15 8 No effect No pref 10. Russula sp. 7 12 6 No effect org 0.024 11. Russula sp. 2 11 7 No effect org 0.083 12. Tricholoma aff. sejunctum 10 6 -‐ high 0.033 min 0.07 13. Inocybe sp. 1 9 3 + low 0.049 No pref 14. Lactarius deceptivus 8 5 No effect org 0.021 15. Thelephoraceae sp. 1 7 4 No effect No pref 16. Lactarius sp. 1 7 4 + high 0.037 org 0.059 17. Lactarius theiogalus 7 4 + low 0.04 org 0.037 18. Cortinarius flexipes 6 2 + control 0.02 No pref 19. Ascomycetous ecto 6 4 No effect min 0.079 20. Basidiomycetous ecto 6 6 + low 0.087 min 0.0042 21. Clavulina castaneipes 6 2 No effect No pref 22. Clavulinaceae sp. 1 6 4 No effect min 0.065 23. Piloderma sp. 1 6 4 + control 0.077 No pref 24. Amanita flavoconia 5 4 No effect No pref 25. Byssocorticium atrovirens 5 4 No effect min 0.048 Table 1: List of 25 most common species observed across all three nitrogen treatments and both horizons. Number of tips = number of root tips observed for that species. Number of subplots = number of subplots in which at least one tip was collected and successfully sequenced for that species. Responses to nitrogen: + control = significantly reduced abundance in both low N and high N plots, -‐ high = significantly lower abundance in high N plots only, + high = significantly greater abundance in high N plots , = significantly greater abundance in low N plots.
Ectomycorrhizal Community Composition The 495 ectomycorrhizal sequences were grouped into 65 OTU’s, which we will henceforth refer to as species. They varied in abundance from 62 tips (Cenococcum geophilum) to a number of species for which only one tip was found. The rank abundance curve for the ECM community has the shape of a typical soil microbial community (figure 1) with a few abundant species and many rare species. The 5 most abundant species account for 42% of all tips. The most abundant genera were Lactarius, Russula, Cenococcum, Thelephora/tomentella, and Amanita, which comprised 23.2%, 22.6%, 12.5%, 8.2%, and 6.9% of the total number of tips sampled, respectively. A complete list of species, and their affinity for nitrogen treatment and horizon (top 25 species) is detailed in tables 1 and 2. Four nitrophilic (significantly increased abundance in N-‐fertilized plots) and 5 nitrophobic (significantly decreased abundance in N-‐fertilized plots) species could be identified amongst the most abundant species, as well as three species that had significantly higher abundance in the low N treatment (table 1). Four of the most abundant species exhibit a clear preference for the organic horizon, and 3 for the mineral horizon. There does not appear to be any interaction between horizon preference and reaction to nitrogen fertilization but the number of suitable candidate species was too low to allow significance testing. When we scale up and look at species groups at either
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the genus, family, or order level we see that the Agaricales (mineral), Russulaeae (organic), and Calvulinaceae (mineral) exhibit strong preference for horizon, while only the Clavulinaceae has a consistent reaction to nitrogen fertilization (nitrophobic) (table 3).
Taxa name # of tips # of subplots (out of 12)
26. Inocybe sp. 2 5 3 27. Coltricia cinnamomea 5 2 28. Tomentella sp. 2 4 4 29. Scleroderma citrinum 4 3 30. Sebacinaceae sp. 1 4 4 31. Clavulinaceae sp. 3 4 2 32. Inocybe sp. 4 4 2 33. Tricholoma saponaceum 3 3 34. Uncultured ectomycorrhiza (Piloderma) 3 2 35. Cortinarius sp. 2 3 2 36. Laccaria ochropurpurea 3 1 37. Sebacinaceae sp. 2 3 2 38. Russula sp. 6 3 2 39. Russula sp. 8 3 1 40. Amanita cf. subphalloides 3 2 41. Amanita sp. 5 3 2 42. Tomentella sublilacina 3 2 43. Sebacina epigaea 3 2 44. Tricholoma sp. 1 3 2 45. Scleroderma citrinum 3 1 46. Sebacinaceae sp. 3 2 1 47. Inocybe sp. 3 2 2 48. Inocybe sp. 5 2 2 49. Sebacinaceae sp. 4 2 1 50. Clavulinaceae sp. 2 2 1 51. Laccaria sp. 1 2 2 52. Russula sp. 3 2 2 53. Russula sp. 4 2 2 54. Russula sp. 5 2 1 55. Russula sp. 9 2 2 56. Entoloma sinuatum 2 2 57. Amanita sp. 4 2 2 58. Cortinarius sp. 1 2 2 59. Amanita sp. 2 2 2 60. Tricholoma sp. 2 1 1 61. Melanogaster sp. 1 1 1 62. Cortinarius illitus 1 1 63. Tricholoma sp. 3 1 1 64. Amanita sp. 3 1 1 65. Hydnum repandum 1 1 Table 2: List of 40 least common species observed across all three nitrogen treatments and both horizons. Number of tips = number of root tips observed for that species. Number of subplots = number of subplots in which at least one tip was collected and successfully sequenced for that species
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Response to Nitrogen
Horizon Preference
Taxa name # of tips
# of OTU’s
P value P value Order: Agaricales 76 13 No effect min 0.0002 Family: Tricholomataceae 19 4 No effect min 0.006 Genus: Inocybe 18 3 No effect min 0.018 Genus: Amanita 29 4 No effect No pref Family: Russulaceae 213 11 No effect org 0.0001 Genus: Russula 98 5 No effect org 0.0011 Genus: Lactarius 115 6 No effect org 0.0001 Family: Thelephoraceae 41 4 No effect No pref Family: Atheliaceae 14 3 + control 0.09 No pref Family: Clavulinaceae 16 3 -‐ high 0.027 min 0.013 Table 3: Effects of nitrogen treatment and preference for horizon of higher phylogenetic groups. Number of tips = number of root tips observed for that group. Number of OTU’s = number of taxa included in that group. Responses to nitrogen: + control = significantly lower abundance in low N and high N plots, -‐ high = significantly lower abundance in high N plots. Note Agaricales includes the Tricholomataceae, Incocybe, and Amanita; Russulaceae includes the genera Russula and Lactarius.
The three dimensional non-‐parametric multi-‐dimensional scaling (MDS) plot (figure 3) explained 83% of the variation between the ectomycorrhizal communities (axis 1, 30.1%; Axis 2, 37.7%; axis 3, 15.1%). The final stress in the 3 dimensional solution was 12.36 (p < 0.02). The MDS plot illustrates that the communities of the mineral and organic horizons are distinct (Axis 2), and that the high N treatment ECM community is distinct from the communities in the control and low N plots (Axis 1). Mantel tests for both nitrogen treatment and horizon indicate that both factors are important in shaping community composition (p=0.001, r=0.348).
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Figure 3: NMDS plot of ectomycorrhizal community composition as affected by nitrogen treatment and horizon. Each point represents all tips collected from one horizon in one subplot in a particular nitrogen treatment (n=24). Hollow symbols are organic horizon, filled symbols are mineral. Squares are high N, triangles low N, and circles control. Ordination represents axis 1 and 2 of a three dimensional ordination with a final stress of 12.67 (p < 0.02). Ectomycorrhizal Diversity Richness, as measured by observed # of species, chao1 estimator, and diversity as measured by shannon’s or simpson’s index, were significantly lower in the high N plot than in either the low N plot or the control plot (table 3). Diversity was not appreciably different between the control and the low N plot (table 3). Thirty-‐four species were found in the organic soil, and 57 were found in the mineral soil. By all diversity metrics employed (table 3) the mineral soil was more species rich than the organic horizon within each nitrogen treatment (figure 2). The overall chao1 estimated for the organic and mineral soil were 58 +/-‐11.8 (S.D.) and 70 +/-‐2.0 (S.D.) respectively, and the Shannon diversity index: 2.87 and 3.55, respectively.
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Plot/Horizon Control Organic
Control Mineral
Low N Organic
Low N Mineral
High N Organic
High N Mineral
# of species 22 34 25 30 14 20 Shannon index 2.57 3.27 2.76 3.02 1.91 2.45
Simpson index (1/D) 9.5 21.4 11.4 14.7 4.1 7.7 Evenness 0.83 0.93 0.86 0.89 0.72 0.82
Total species/ treatment (% shared min/org)
46 (24% shared)
43 (28% shared)
27 (26% shared)
Chao1 estimator (+/-‐ S.D.) 78.8 (+/-‐ 9.5) 84.3 (+/-‐ 21.3) 39.5 (+/-‐ 3.2)
Shannon index 3.32 3.29 2.56 Simpson index 1/D 18.9 17.8 8.0
Evenness 0.87 0.87 0.78 Table 3: Effects of nitrogen treatment and horizon on various diversity metrics. Ectomycorrhizal Colonization Intensity The percentage of root length colonized (%RLC) was quite consistent between treatments (23.4-‐26.6%), except for the high N mineral horizon, which had significantly higher %RLC (33.6%, p < 0.001)(figure 4)
Figure 4: Ectomycorrhizal colonization intensity measured as percent root length colonized. Error bars are standard error. Bars with a different letter next to them are significantly different (p < 0.001).
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Discussion Many studies have demonstrated that high levels of nitrogen addition significantly impact ectomycorrhizal communities, though very few of them have looked at deciduous stands, and none of those have looked at the mineral and organic horizons separately. In this study, we found that high levels of nitrogen fertilization (150 kg/ha/yr) have significantly altered the ectomycorrhizal community in the Harvard Forest NSF LTER Chronic N Enrichment study. There is no evidence that the community composition, diversity, or colonization intensity have been appreciably affected by “low” levels of nitrogen fertilization (50 kg/ha/yr). We had a very low sequencing success rate (<30%) and this appeared to be due to contamination of many samples by saprotrophic other fungal infection. Our sampling was done in mid summer, and this may be a time of high root turnover. The levels of nitrogen fertilization applied in this study termed “high” and “low” would be considered “extremely high” and “very high” in natural settings. The highest rates of nitrogen deposition in the eastern US are generally below 20 kg N/ha/yr (Driscoll et al., 2003), and many forests considered to be exhibiting nitrogen saturation associated decline are below 15 kg N/ha/yr (Fenn et al., 2006). The high and low levels of fertilization employed here are 20 and 6 fold higher than the atmospheric deposition levels in this part of the northeast (Magill et al., 2004). Because there was no evidence of an interactive effect of nitrogen treatment and horizon with regards to community composition, we will first discuss our findings in the context of nitrogen addition experiments and then discuss the implications of our findings on horizon preference. There is some evidence that deciduous forests may respond differently to nitrogen deposition than coniferous forests (Aber et al., 1998; Fenn et al., 1998), so, when possible, we will focus on studies done in deciduous forests. We will also not discuss studies that focus exclusively on ectomycorrhizal sporocarp (mushrooms) inventories. Sporocarp inventories formed the majority of early studies on the effects of acid deposition on ectomycorrhizal communities, and could be considered the canary in the coal mine that has spurred 2 decades of research following the seminal work by Arnolds (1991). However, there is ample evidence that sporocarp abundance may not be reflective of mycorrhizal abundance (Gardes and Bruns, 1996; Koren and Nylund, 1997; Wallenda and Kottke, 1998). We will address the results of our colonization intensity measurements in the context of rooting distribution, ecosystem biogeochemistry and ecosystem responses to nitrogen deposition. Effects of Nitrogen on Ectomycorrhizal Community Composition Our findings of a significant shift in ectomycorrhizal species composition are generally in agreement with other investigations on the effects of N deposition on ectomycorrhizal commmmunities. Avis et al. (2003, 2008) and Lucas and Casper (2008) found that nitrogen fertilization significantly impacted ECM community composition in oak forests of the eastern US, as have numerous studies in coniferous forests (see review by Wallenda and Kottke, 1998, and regional study by Cox et al., 2010). Our results stand apart from those of Avis et al. (2008) and Lucas and Casper (2008) in that they observed significant effects on N fertilization on ECM community composition at N fertilization levels between 20 and 35 kg/ha/yr and in as little as two years, while we found no significant effects of 18 years of 50 kg/ha/yr of N fertilization on ectomycorrhizal community composition.
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Avis et al. (2003) also failed to find a significant community change after 18 years of fertilization at 54 kg/ha/yr. The majority of N addition studies on coniferous plots have found a significant impact of 30-‐50 kg N/ha/yr on ECM community composition (Lilleskov et al. 2002; Cox et al., 2010, Wallenda and Kottke, 1998), though Ishida and Nordin (2010) failed to find an effect of 12 years of 50 kg N/ha/yr on ECM community composition in a spruce forest. Wallenda and Kottke (1998) identified a general threshold for 20-‐30 kg N/ha/yr before marked changes in the ECM community composition are likely to be observed, although they caution that forest-‐specific factors may change this threshold a great deal in either direction, and at the time of their review, there were no studies on the effects of N addition on ECM communities in deciduous forests.
Our findings of a marked reduction in ECM diversity with high N fertilization are also in line with the findings of Avis et al (2008), and Lucas and Casper (2008), which found significantly reduced ECM diversity in their high N treatments, though again, they noted significant decreases in ECM diversity at levels comparable to our “low” N treatment, while we did not. Avis et al. (2003) failed to find a significant effect of N addition (at levels comparable to our High N treatment) on ECM diversity. Looking at studies in coniferous forests, a reduction in ECM diversity with N addition seems to be the general finding (Lilleskov et al. 2002; Cox et al., 2010; Carfae et al., 2006, Wallenda and Kottke, 1998), but studies have also found no reduction in ECM diversity in forests fertilized with moderate (Ishida and Nordin, 2010; Jonsson et al., 2000) or very high levels of N addition (Kåren and Nylund, 1997).
We could clearly identify certain species to be nitrophilic or nitrophobic but we could not make such characterizations for any higher-‐level phylogenetic groups other than the Clavulinaceae. We found that the Clavulinaceae were generally nitrophobic and this is in agreement with the findings of Avis et al. (2008). Among our 5 most abundant Lactarius species, one (L. quietus, the second most abundant species overall) was nitrophilic, two were more common in the low nitrogen treatment, and nitrogen had no effect on the abundance of the other two. Lactarius quietus was also quite abundant in Avis et al.’s (2008) study on oak, but they found no consistent reaction to nitrogen fertilization. In their study across a depositional gradient in an Alaskan spruce forest Lilleskov et al. (2002) found Lactarius theiogalus to be dramatically nitrophilic, shifting from 7% to 69% of all root tips from the low to high end of their nitrogen gradient; we found L. theiogalus to be mildy nitrophilic, occurring in greatest abundance in the low N treatment. Cox et al.’s (2010) study across european Picea abies forests identified the genera Lactarius and Thelepohra/Tomentella to be nitrophilic. We found our only common Tomentella species to be nitrophobic. Our most abundant species was Cenococcum geophilum, and it was termed nitrophobic due to its sharply reduced abundance in the high N treatment. Avis et al. (2008) found Cenococcum abundance across N treatments variable, with one TRFLP type being nitrophilic and another nitrophobic. Lucas and Casper (2008) found Cenoccocum geophilum abundance increased on oak roots in response to nitrogen fertilization (fertilization levels below our low N treatment levels). Avolio et al. (2009) looked at ECM community composition on pine and oak seedlings in response to nitrate fertilization and found that Cenococcum abundance increased markedly in response to N fertilization on oaks, and decreased markedly in response to N fertilization on pines. Cenococcum geophilum is widely thought to be a
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cryptic species complex (Douhan et al., 2007), and we examined the possibility that we had sub-‐taxa within C. geophilum that might have a distinct affinity for nitrogen or horizon. Clustering our C. geophilum sequences at different % similarities did yield different clusters, but did not divide this large group according to any horizon or nitrogen affinity. This inability to define Cenococcum’s niche is not uncommon; in a 2007 commentary, Dickie (2007) identified Cenococcum as “one notable exception to the rule of niche differentiation”.
Role of horizon is shaping ectomycorrhizal community composition The ECM communities in the organic and mineral soil were quite distinct, both across all treatments and within a given nitrogen treatment. Our findings of different ECM communities in the mineral and organic horizon is common to other studies which have examined the ECM communities in organic and mineral soil separately (Rosling et al., 2003; Dickie et al., 2002; Scattolin et al, 2008, Tedersoo et al., 2003), though to date few studies (<10) have done so. We found evidence of increased diversity in the mineral soil. Rosling et al. (2003) found higher diversity in the mineral soil, while Dickie et al. (2002) found lower diversity in the mineral soil, and Scattolin et al. (2008) found moderately (though not significantly) increased diversity of ECM species in the mineral soil. All three of these studies were done in coniferous forests. Certain species in our study exhibited clear preferences for one horizon or another, and these trends were also applicable to higher phylogenetic classifications. In particular the Russulaceae exhibited a preference for the organic horizon while members of the genus Inocybe, the order Agaricales, and the families Tricholomataceae and Clavulinaceae were significantly more abundant in the mineral soil. In contrast to our findings, Baier et al.(2006) and Scattolin et al.(2008) found that Lactarius and Russula were generally more abundant in the mineral soil, though they found individual species within both genera that were more abundant in the organic soil; both studies were done in high elevation coniferous forests. Tedersoo et al. (2003) found, as we did, that the Agaricales exhibited strong preference for mineral soil, though they also found the Clavulinaceae to be significantly more abundant in the organic horizon, in contrast to our findings. In general, our data suggests greater specialization in the mineral horizon than in the organic horizon. Of the 65 species we found, 31 were found only in the mineral soil, while only eight were found exclusively in the organic horizon. Rosling et al. (2003) also found a higher proportion of species occurring exclusively in mineral soil. Mycorrhizal colonization intensity and ecosystem responses to nitrogen deposition
Across all treatments, the ectomycorrhizal colonization intensity (%RLC) was very similar between the mineral and organic horizon, and this stands in some contrast to the published literature. Very few studies have exhaustively sampled the ectomycorrhizal community in the mineral soil (our sampling only considered the top 25 cm in the mineral soil), but those that have, have found as many, if not more, ECM (total biomass) in the mineral soil as in the organic soil (Rosling et al., 2003; Scattolin et al., 2008). More studies have sampled the upper layers of the mineral soil, and ECM colonization intensity, measured as either percent root length colonized (%RLC) or percentage of fine root tips that are mycorrhizal, is generally lower in the mineral soil (Rosling et al, 2003, Baier et al,
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2006, Jonsson et al., 2000). However, Scattolin et al. (2008) found that the A and B horizons in a montane spruce forest in Italy had significantly higher mycorrhizal colonization intensity than the organic soil, so our finding of equal (control and low N) or slightly elevated (high N) colonization intensity in the mineral soil is not unprecedented. Magill et al. (2004) assessed the fine root biomass in the same plots we did two years before we sampled these stands. They found that between 55% and 62% of all fine roots were found in the mineral soil, though they only sampled the top 20cm of mineral soil. Thus, our findings of equivalent or higher colonization intensity in the mineral soil can be interpreted as equivalent or higher mycorrhizal biomass in the mineral soil.
Nitrogen addition had no effect on the colonization intensity in the low N treatment or in the organic horizon of the high N treatment, and increased the percentage root length colonized (%RLC) in the mineral soil in the high N treatment. According to Magill et al. (2004), the fine root biomass in the organic horizon in these same stands is 25% and 27% lower in the low and high N treatments, respectively. Fine root biomass in the top 20 cm of the mineral horizon has not been significantly affected by nitrogen treatment, thus the percentage of fine roots found in the mineral horizon increased marginally from 55% to 62% with nitrogen addition (Magill et al., 2004). We can thus infer that there is a significantly higher fraction of total ectomycorrhizae in the mineral soil (69% of total in high N vs. 51.4% of total in control) in the high N treatment. There is considerable variation in the literature in reported responses of ECM colonization and fine root biomass to nitrogen addition. The consensus seems to be that nitrogen addition decreases both, but the large number of reports finding the opposite suggests that characteristics of the individual forest being examined may be important to consider. In a meta-‐analysis of 6 studies conducted in 14 forest stands, Treseder et al. (2004) found a moderate (though significant) decrease in %RLC of 5.8% with nitrogen fertilization, but also noted that the responses to N were very heterogeneous. In a meta-‐analysis of boreal forests’ responses to nitrogen addition, Cudlin et al. (2007) found a not significant decrease of 10% in percent mycorrhizal colonization. Carfrae at al. (2006) and Koren and Nylund (1997) found significantly increases in %RLC with nitrogen fertilization. Wöllecke et al. (1999) noted a sharp decrease in % RLC with nitrogen fertilization, but this was much more pronounced for the organic horizon than the mineral horizon, also indicating increased fraction of total mycorrhizal activity in the mineral soil under nitrogen addition. To the authors’ knowledge no published studies have examined the effects of nitrogen on ECM colonization intensity in deciduous forests. The literature on the effects of nitrogen on rooting activity is no more consistent. While, many micro-‐ and mesocosm studies have demonstrated a decreased ratio of root biomass to shoot biomass with increased N availability (Ekblad et al., 1996; Ericsson, 1995; Marschner, 1995 and references therein), and the mechanisms for this are well understood (Walch-‐Liu et al., 2005), the effect is generally driven by increases in aboveground biomass and the effects of nitrogen fertilization on belowground biomass in forests vary considerably. Cudlin et al. (2007) found an increase in root biomass of 10% in their meta-‐analysis of 22 forest studies, while Ostonen et al. (2007) found a 20% decrease in specific root length in a meta-‐analysis of 54 studies, both of these studies reported such high variation in responses to N that neither trend was significant.
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The altered mycorrhizal colonization we observed in the high N plot may be an adaptation to limitation or co-‐limitation by phosphorous or base-‐cations. Many studies have shown that anthropogenic nitrogen deposition is altering the nutrient status of eastern forests from nitrogen limitation to co-‐limition or limitation by phosphorous or base cations (Aber et al., 2003; Fenn et al., 2006; Long et al., 2008). Minocha et al. (2000) measured the foliar and soil contents of base cations and phosphorous in the same plots we did 5 years before we sampled them, and found significantly reduced amounts of Ca, Mg, K, and P in the organic horizon in the high N plots at Harvard Forest, while the mineral soil amounts were not affected. Currie et al. (1999) studied the same plots and demonstrated that loss of cations was due to downward leaching of these important nutrients due to nitrogen addition. Minocha et al. (2000) also showed significantly reduced foliar concentrations of calcium and foliar Mg:N ratio. In light of the findings of Minocha et al. (2000) and Magill et al. (2004) we suggest that the increased ectomycorrhizal colonization we observed in the mineral soil of the high N plot in conjunction with a deeper rooting distribution may be a response to greater demand for mineral derived nutrients. The two species that we found to increase in the mineral soil of the high N sites were Amanita sp. 1, which was significantly more abundant, and Russula atropurpurea, which showed a substantial but non-‐significant increase. One way to examine whether shifting nutrient demand on the part of the host trees is driving the observed changes in species composition and distribution would be to examine whether these species have enhanced ability to take up and translocate phosphorous or base cations. A split chamber mesocosm similar to that employed by Jentschke et al. (2000) would be ideal to address such questions. The effects of nitrogen addition on root and mycorrhizal distribution must be considered in models of how soil carbon stocks will respond to nitrogen deposition. There is evidence that soil carbon storage increases in response to elevated nitrogen deposition (Pregitzer et al., 2008; meta-‐analysis by Janssens et al., 2010), and some studies ascribe this to reduced decomposition, not increased litter inputs (Waldrop et al., 2004, Zak et al., 2008). In a recent meta-‐analysis Nave et al. (2009) found that nitrogen addition had no effect on carbon storage in organic horizons but significantly increased carbon stocks in the mineral soil by 12%. Few studies have exhaustively sampled how rooting distribution responds to nitrogen addition. There is, however, increasing evidence that CO2 enrichment causes deeper rooting distribution among forest trees in response to greater nutrient demand (Norby et al., 2004; review by Iversen, 2010). Studies investigating the role of depth on root turnover and decomposition are limited but there is evidence that root decomposition is significantly slower at greater depth (Gill and Burke, 2002). Ectomycorrhizal roots decompose significantly more slowly and contribute more carbon to recalcitrant carbon pools with much slower turnover rates than nonmycorrhizal fine roots (Langley et al., 2006; Fann and Guo, 2010). The results of our experiment and Magill et al. (2004) have demonstrated increased mycorrhization and proportionally more rooting in the mineral soil in response to N addition. This study is the first to address how ectomycorrhizal abundance and distribution in deciduous forests is affected by nitrogen deposition, and one of the first in any forest to combine information on how colonization intensity responds to nitrogen addition with concurrent examination of root biomass responses. If our finding that the amount of ectomycorrhizal biomass in the mineral soil
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increases in response to nitrogen deposition is representative of other forests, then this should be considered in models that predict how soil carbon stores may respond to anthropogenic nitrogen deposition. Conclusions Eighteen years of heavy nitrogen fertilization have significantly impacted the ectomycorrhizal community at Harvard Forest. The community composition has shifted, diversity has decreased and the total mycorrhizal activity has shifted significantly towards the mineral soil. The low nitrogen treatment, though six times higher than ambient deposition, does not appear to have changed the ectomycorrhizal community appreciably. Our study only sampled roots to a depth of 25 cm, while many studies fail to sample the mineral soil at all. Future studies of ectomycorrhizal communities or ecosystem responses to nitrogen addition should take care to address rooting and mycorrhizal processes in the mineral soil.
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Chapter 2 Linking small scale soil chemical variability to fungal niche preference Abstract Soil heterogeneity is often proposed to be an important contributing factor to fungal diversity. Significant evidence exists to suggest that, at least on the scale of horizon-to-horizon variation, consistent shifts in fungal (including just ectomycorrhizal) community can be found to correlate with soil chemical variables. However, significant diversity is found within a given soil horizon, suggesting that if soil chemical factors and niche heterogeneity are driving fungal diversity then a smaller scale of sampling is necessary. We looked at a large number (150) of small (<5g) soil samples taken from a bishop pine forest at Point Reyes National Seashore in northern California. Six 1 X 1 m pits were dug in soils formed from two parent materials (three granitic, three sandstone). Three 70 cm vertical transects were drawn across each soil profile and eight samples were taken per vertical transect. Plant community and stand history were both constant between pits and sites. For each sample %C, %N, pH, and BaCl2-exchangeable Na, Ca, K, and Mg were measured. The fungal community within each sample was characterized using terminal restriction fragment length polymorphism. When all samples from both parent materials are examined together Ca, Mg, pH, and Na stand out as the dominant factors potentially shaping fungal community. However, the variance in fungal community and these soil chemical variables co-correlate with parent material so we cannot isolate the effects of soil chemistry from other site-specific factors. When pits are examined individually depth, %C, and, to a lesser extent, Ca stand out as soil factors strongly correlated with fungal community composition. Other associations between soil chemical factors and fungal community were found (K, pH, %N, Mg), but our sampling design did not allow us to separate the potential influence of geographic location from the influence of these soil parameters. To our knowledge this is the first study which links soil chemical variability to fungal community composition on a scale relevant to individual hyphae.
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Introduction Fungal communities in forests are remarkably diverse. Recent advances in molecular techniques have allowed the identification of hundreds of fungal taxa in small volumes of forest soil (Lim et al., 2010; Buee et al., 2009). This tremendous diversity has been observed for both the saprotrophic and ectomycorrhizal components of the fungal community. Fungal community composition and diversity is altered by elevated CO2 (Parrent and Vilgalys, 2007) and anthropogenic nitrogen deposition (Wallenda and Kottke, 1998 and references therein). Lilleskov et al. (2002a) and Parrent and Vilgalys (2007) suggested that these shifts in community composition might be a result of changing nutrient dynamics within the soil. An understanding of what soil factors control fungal community composition is necessary to understand the implications of global change-‐induced shifts in fungal communities.
We do not have a clear understanding of what factors control fungal community composition and maintain fungal diversity. Studies have demonstrated that different soil horizons in forest soils have distinct fungal communities (Rosling et al., 2003; Dickie et al., 2002; Lindahl et al., 2007), but this scale of examination is too coarse to explain the observed diversity in fungal communities and may not capture variations in soil chemistry relevant to individual genets. Many studies have found discreet niches in resource use among both ectomycorrhizal (Lilleskov et al., 2002b; Abuzinadah and Read, 1986) and fungal saprotrophic (McQuire et al., 2010; Gleeson et al., 2005) communities. The majority of these studies have been done with either labeled substrate addition or in vitro pure culture studies. In their reviews on the factors shaping fungal diversity in forest soil, Taylor (2002) and Bruns (1995) both identified small-‐scale (finer scale than horizon) variation in soil chemistry as a potentially important factor shaping fungal community composition and called for studies to address the effects of soil heterogeneity on fungal species distributions.
No studies have investigated the role of small-‐scale variation in soil chemistry in structuring fungal communities in forest soils. A few studies have examined the role of soil chemistry in structuring fungal communities in grassland soils (Moore et al., 2010, Allison et al., 2007; Fierer et al., 2003). They found depth and carbon content to be the soil parameters most strongly associated with fungal community composition These studies, however, have all used large composite soil samples (>0.5 l), and have thus likely obscured the role of soil heterogeneity in structuring fungal communities, focusing instead on broad, depth-‐related trends.
We took a large number of small mineral soil samples across a range of depths in a pine forest to elucidate the role of soil chemistry in determining fungal community composition. We used a PCR-‐based community fingerprinting method in conjunction with a novel sequential chemical analysis procedure to maximize the amount of soil chemical information from each sample while minimizing the size of the sample required. In doing so, we hoped to capture the heterogeneous distribution of elements in the soil at a scale relevant to fungal hyphae.
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Methods Study site Point Reyes National Seashore is located in west Marin County, California (38 °04N by 122 °50W), approximately 60 km north of San Francisco. It has a mixture of vegetation types but the areas we sampled were dominated Bishop Pine (Pinus muricata) forest. It has a Mediterranean climate characterized by a warm dry summer and a cool wet winter. Approximately 80% of annual rainfall (80-‐160cm per year) falls between November and March. Average air temperatures range between 11oC and 14oC, and average summer temperatures are around 18oC. In October 1995, a major stand-‐replacing wildfire occurred that burned approximately 5000 ha of the park.
Our two study sites were located in even-‐aged 13 year-‐old Bishop pine forests derived from the 1995 fire. One site (38.085N, 122.865W) was on soils derived from Silurean granite. The granitic soil is an inceptisol in the soil series Sheridan variant. It is classified as a coarse-‐loamy, mixed, isomesic ustic dystrocrept. This site is located at approximately 350 m above sea level and is 3.5 km east of the Pacific Ocean. The other site (38.043N, 122.873W) was on soils derived from greywacke sandstone. The sandstone soil is an alfisol in the soil series Tomales. It is classified as a fine, mixed, mesic ultic paleustalf. This site is located at approximately 150 m a.s.l. and is 1.5 km east of the Pacific ocean. The two study sites are approximately 2 km from each other. Sampling We conducted our sampling in mid-‐November of 2007, approximately one month after the start of the Fall rains. On each study site (granitic and sandstone) we dug three pits within 50 meters of each other. Each pit was approximately 1 m wide and 1 m deep. On one wall of each pit we cut a straight vertical profile, and used a brush to remove any cross contamination from different depths that may have occurred while digging. On each profile we made three vertical transects. Each transect was a line drawn from the soil surface straight down to a depth of 75 cm. The three vertical transects within each pit were 10 cm apart. We sampled from 5 cm beneath the organic mineral horizon interface to a depth of 70 cm. We sampled each vertical transect at depths of 5cm, 10cm, 15cm, 20cm, 30cm, 40cm, 50cm, and 60cm, and for each pit we took one sample at 70cm. Sampling was performed with a small circular scoop (a mellon baller). We removed one scoop (~1-‐2 cm3) from the exposed soil face for each sample and placed it in a small sealable plastic bag. In between samples, the scoop was cleaned with ethanol. Samples were immediately put in a cooler with ice and stored at 4oC until processed. Molecular Analysis Within 2 days of sampling, samples were homogenized and a 0.5 g subsample was taken for DNA extraction. DNA was extracted from soil using the PowerSoil DNA Isolation Kit (Mo Bio Laboratories Inc., Carlsbad, CA) as per manufacturer’s instructions. DNA was then diluted 1:25 for PCR amplification with fluorescent primers. PCR was conducted in 50 µl reaction volumes with the primers ITS1-‐F (Gardes & Bruns, 1993) and ITS4 (White et al., 1990) using the cycling parameters detailed in Gardes and Bruns (1993) with the following modifications: two microliters molecular-‐grade bovine serum albumin (20mg/ml) was added to each reaction to improve PCR efficiency, and primers were
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labeled with WellRED fluorescent dyes; ITS-‐1F primer was labeled with D4-‐PA and ITS4 with D3-‐PA (Proligo, Boulder, CO, USA). Three 5 µl aliquots from each PCR product were separately digested with the restriction enzymes AluI (Amersham Biosciences, Freiburg, Germany), CfoI (Promega, Madison, Wisconsin, USA), and TaqI (Fermentas, Glen Burnie, Maryland, USA), according to the manufacturer’s instructions. The terminal restriction fragment length polymorphism (here after TRFLP) patterns were analyzed with a Beckman Coulter CEQ 8000 Genetic Analysis System, using the CEQ DNA Size Standard Kit-‐600. TRFLP results were screened for quality and only samples with clear distinct peaks (>5000 intensity units; max threshold 150,000) for each fluorescent primer were included. Excessive “pullup” (a high intensity peak causes the occurrence of a spurious high intensity peak of the other fluorescent dye 1-‐3 bp longer, and slightly lower in amplitude) was grounds for excluding samples, as was excessive stuttering (a series of peaks, each 1-‐2 bp longer than the last with decreasing amplitude). Only samples with clear, high quality TRFLP profiles for each of the three restriction enzymes were included in the final analysis. Soil Chemical Analysis After subsampling for DNA extraction the remaining soil (~4g) was divided into 2 portions. One portion was dried at 60oC for 3 days in an oven, and the other was used for pH and exchangeable cation contents. Before drying, the soil was weighed moist, and then weighed again after drying to calculate the moisture content. After weighing the portion intended for pH and exchangeable cation analysis, it was immersed in a solution of 0.1M BaCl2, [1:5 soil mass (g): solution volume (ml)] and pH was measured, after which another 10ml 0.1M BaCl2 was added for cation exchange measurement. BaCl2/soil slurries were shaken overnight, centrifuged at 5000g at 4oC for 15 minutes, and the supernatant solution was carefully removed to avoid collecting any soil particles with the solution. The supernatant soil solution was then measured for elemental content on a Perkin-‐Elmer atomic optical emission inductively coupled plasma emission spectrometer (AOE-‐ICP). A set of four standards was established based on preliminary analysis of elemental concentrations. In addition to hourly rerunning of standards, duplicates and an internal scandium standard were run to ensure an accuracy of elemental contents to +/-‐ 1%. The calibration blank was an aliquot of the same 0.1M BaCl2 solution used for extraction. We used BaCl2 as our cation extractant, as opposed to the more commonly used ammonium acetate, in order to allow pH measurement from the same sample, and reduce the required sample size. Dried soil was pulverized in a ball mill and analyzed for carbon and nitrogen content on a combustion elemental analyzer. Exchangeable cation contents are reported as µmol per gram soil, and carbon and nitrogen contents as %C and %N by weight. Statistical Analysis Correlations between soil chemical data were assessed by one-‐way ANOVA using JMP software version 5.01a (SAS Institute, Inc., Cary, NC, USA). Differences between fungal communities and associations between fungal community composition and soil chemistry were assessed with the statistical software PC-‐ORD (MjM Software Design, version 5.0, Gleneden Beach, OR, USA). Communities were examined at different scales; individual pit,
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all 3 pits on a parent material, and all 6 pits together. All TRFLP peak data was treated as presence/absence for each enzyme/primer/fragment length combination. At each community scale examined, the TRFLP profiles were reorganized, and peaks that occurred in less than 10% of all samples or more than 75% were not considered. Community differences were visualized using nonparametric multidimensional scaling (NMS). NMS is a suitable method for comparing complex microbial community data because it does not assume a normalized distribution of species or equivalent variance between communities (Clarke, 1993). For NMS ordination an initial run was performed using the “medium thoroughness” default settings to identify the optimum dimensionality of the ordination. After that, the ordination was performed again (Sorenson’s/Bray-‐Curtis distance measure, 400 maximum iterations, 0.00001 instability criterion, recommended dimensionality, 100 real runs and 200 randomized trials). For NMS ordination vectors are displayed in ordination plots to indicate the strength of association between soil chemical data and community composition, however these vectors only indicate relative explanatory power of the different soil chemical variables and do not test for statistically significant associations. Mantel tests were independently conducted to assess the strength of association between the different soil chemical variables and fungal community composition (Mantel, 1967). Mantel tests were performed using the default parameters associated with the “randomization” method with 5000 iterations, and the Mantel test statistic “r” is reported as a measure of effect size (McCune and Grace, 2002). We examined the relationship between fungal community and each individual soil parameter (parent material, pit, pH, %C, %N, Ca, Mg, K, Na) All soil parameters were combined into a distance matrix using NMS ordination which was then also examined for its relationship to fungal community using the Mantel test. This combined relationship (not including pit or parent material) will be referred to as “aggregate soil chemistry”. Results Across all 6 pits on both parent materials we obtained 115 TRFLP profiles that met our quality criteria out of 149 soil samples (55 sandstone, 60 granite; 16-‐23 samples/pit). A total of 34 were excluded because of excessive “pull-‐up”, excessive stuttering of peaks, or failing to obtain peaks above a minimum intensity threshold for each primer/enzyme pair. Our final data set had 186 peak sizes in total for the 6 restriction enzyme/primer pairs.
We did not measure the total contents of calcium, magnesium, sodium, or potassium, so, for brevity, all data for BaCl2-‐exchangeable Ca, BaCl2-‐exchangeable Mg, BaCl2-‐exchangeable Na, and BaCl2-‐exchangeable K will be referred to as Ca, Mg, Na, and K, respectively. Data for the seven chemical parameters measured (pH, %C, %N, Ca, Mg, Na, and K) are presented in figure 1 (contents and co-‐correlation of all seven parameters across all six pits and both parent materials), figure 3 (carbon, magnesium, and calcium contents as well as co-‐correlation of all seven parameters from the three pits overlying granitic parent material), and figure 5 (carbon, magnesium, and sodium contents as well as co-‐correlation of all seven parameters from the three pits overlying granitic parent material ). In general, and particularly for Ca, Mg, Na, and pH, there was more variation in chemical parameters between soils on different parent materials than within the all soil samples on one parent material (figure 1). For each parent materials there was more
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variation with depth than between pits, but Na (sandstone), Ca (granite), Mg (granite) and carbon (granite) also varied considerably between pits. Many chemical parameters were strongly co-‐correlated (figure 1). We only sampled the mineral soil, and there was very limited visual indication of distinct horizons within the mineral soil. According to national soil surveys, the pits on granitic parent material should have been entirely in the A horizon, and the sandstone pits would have included the upper 10 cm of B horizon (USDA NRCS, http://casoilresource.lawr.ucdavis.edu).
Figure 1: Soil chemistry data for all 6 pits on both parent materials. A:Mg, B:Ca, C:pH, D:Na, E: percent carbon (by weight), F: percent nitrogen (by weight), G:K, H: correlation coefficient for soil chemical variables (n=150). For figures A-‐G each point is the average across all samples from one depth from one parent material (n=9) and horizontal error bars are = two standard deviations. For figure H r2 correlations significant at p<0.05 are marked with an *, p < 0.01 are in bold, and p <0.001 are in bold and larger font.
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Non-‐parametric multidimensional scaling was performed for each pit, each parent
material, and for all samples combined (figures 2,4,6). The final statistics for each ordination are presented in table 1. In most cases a three dimensional solution was optimal, and for all NMS plots the dominant three axis explained 54 – 85% of the total variation. Final stress ranged from 9.4 to 22.3 and was proportional to the number of samples examined. Fungal Community Across Sandstone and Granitic Soils Examining the NMDS plot (figure 2), a very clear division of the fungal community according to parent material is apparent. All 7 chemical factors examined (pH, %C, %N, Ca, Mg, Na, and K) are significantly associated with community composition according to the Mantel tests (Mantel’s “r” > 0.068, p < 0.02). Soil chemistry is also strongly associated with fungal community when the soil all 7 soil chemical parameters are aggregated into one “total soil chemistry” parameter (Mantel’s “r” = 0.311, p < 0.0002). However, all 7 soil chemical factors co-‐vary quite strongly with parent material (figure 1). The four chemical factors that have the strongest association with fungal community composition are Mg, Na, Ca, and pH (figure 2; Mantel’s “r” > 0.2, p < 0.0002), and these are the four chemical factors that most distinguish the soils on the two parent materials (figure 1). Additionally, these 4 chemical factors are all very strongly co-‐correlated (r2 >0.3, p < 0.001). Overall, parent material is such a dominant factor shaping the community, and chemistry varies so much with parent material, that we can’t determine which chemical factors are most influential upon fungal community composition when we look across both parent materials.
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Figure 2: NMS plot of all samples (n=115) and results of Mantel tests for each soil chemical parameter as well as parent material. Vectors in ordination represent soil chemical variables that had a strong correlation with the primary axis (r2 >0.2), and vector length is proportional the strength of correlation. (see table 1 for final NMS statistics)
Fungal Community on Granite On granite, the three pits have distinct communities, as revealed by the NMDS plot and Mantel test (figure 4). Pit 2 separates more strongly from pits 1 and 3, primarily along axis 2 of the NMDS plot (35.1 % of total variation), and %C is strongly associated with this axis. Pit 3 separates from pits 1 and 2 along axis 1, and Mg and Ca are strongly associated with this axis. Across the three granite pits nearly all chemical variables are significantly associated with fungal community composition. The factors most strongly associated with fungal community composition are pit, Ca, aggregated soil chemistry, Mg, depth, % C, and pH (figure 4). Pit is clearly the dominant factors shaping the fungal community composition across the three soil pits on granitic parent material. The two chemical factors most strongly associated with fungal community composition across the three pits, Ca and Mg, separate very strongly by pit. Pit 3, which separated from the other pits along axis 1, has markedly lower calcium and magnesium contents (figure 3). The only pit within which Ca or Mg were significantly associated with community composition was Ca in pit 3. pH is significantly associated with community composition across the three pits, but it is strongly correlated with both carbon (r2 =0.242, p < 0.001) and depth (r2 =0.322, p < 0.001) (figure 3). The only pit within which the pH is significantly associated with the fungal community is pit 3, and this pit is also the one in which pH is most strongly associated with depth (r2,: pH with depth, pit 3: 0.64; pit 1: 0.255; pit 2: 0.264). The significant association of nitrogen with fungal community in pit 1 can also be explained by more consistency with depth in plot 1: (r2: N with depth: overall: 0.245; pit 1: 0.635 ; pit 2: 0.224 ; pit 3: 0.192). Across all three plots carbon is strongly associated with axis 2 (35.1
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% of total variation), and this axis also separates the fungal community on plot 2 from plots 1 and 3. While % C changes more with depth and parent material than it does with pit (figures 1,3,5), pit 2 does have a measurably higher carbon content throughout its depth than pits 1 and 3 (figure 3). Carbon content and depth are strongly associated with fungal community composition across all three plots and within each plot (figure 4). Within each plot %C and depth are associated with the same NMS axis (figure 4). Across all 3 granite pits, %C and depth are very strongly correlated with one another (r2 = 0.625, p < 0.001), and we cannot resolve which is more strongly associated with fungal community composition.
ordination
number of
samples
dimensionality of final solution
% variation explained by 3
dimensions final
stress final
instability
number of
iterations all samples 119 3 53.80% 22.26 0.0008 143
all sandstone 55 3 69% 17.4 0.00001 149 sand pit 1 20 3 79.10% 11.77 0.00001 104 sand pit 2 18 3 70.90% 13.54 0.00001 135 sand pit 3 17 3 76.70% 12.62 0.00001 107 all granite 60 3 64.80% 18.8 0.00009 103
granite pit 1 16 4 81.90% 9.42 0.00009 99 granite pit 2 21 4 71.60% 14.19 0.00009 195 granite pit 3 23 4 79% 0.00001 156 Table 1: Ordination results and statistics
Figure 3: Magnesium (top left), calcium (top right), and percent carbon content (bottom left) for all 3 pits on granitic parent material. Correlation coefficient (bottom right) for soil chemical variables (n=60); correlation significant at p<0.05 are marked with an *, p < 0.01 are in bold, and p <0.001 are in bold and larger font.
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Figure 4: NMS plots of all granite samples (top left, n=60) and the samples within pit 1 (top right, n=16), pit 2 (bottom left, n=21), and pit 3 (bottom right, n=23) on granite derived soils and results of Mantel tests for each soil chemical parameter. Vectors in ordinations represent soil chemical variables that had a strong correlation with the primary axis (r2 >0.2), and vector length is proportional the strength of correlation (within each ordination, not between ordinations)(see table 1 for final NMS statistics). Fungal Community on Sandstone On sandstone, the three pits have distinct communities, as revealed by the NMDS plot and Mantel test (figure 6). Pit 6, in particular, separates strongly from pits 4 and 5, primarily along axis 1 of the NMDS plot (34.1 % of total variation), and the soil chemical variables Na (very strongly) and Ca (less strongly) are associated with this axis (figure 6). Across the three sandstone pits the factors most strongly associated with fungal community composition are pit, Na, aggregated soil chemistry, % C, and calcium, with a weak, marginally significant association with depth (figure 6). Pit is clearly the dominant factor shaping the fungal community. Sodium is very strongly associated with fungal community when all three pits are examined together, but this seems to be entirely explained by pit 6 (figure 6), and when we examine the soil chemistry data (figure 5) for Na on sandstone we see that pit 6 had markedly higher Na throughout, so we cannot separate the effect of
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Na from the effect of pit location on fungal community. When we look at each pit individually, depth is significantly associated with fungal community composition on two of the three pits (pit 4:“r” =0.169, p < 0.034; pit 6:“r” = 0.165, p < 0.045), while %C (pit 4:“r” =0.233, p < 0.037; pit 6:“r” = 0.155, p < 0.1) and Ca (pit 4:“r” =0.165, p < 0.1; pit 6:“r” = 0.279, p < 0.007) are both significantly associated with fungal community composition on one of the three pits, and marginally associated with fungal community composition on another (figure 6). Across all 3 sandstone pits, %C, Ca, and depth are very strongly co-‐correlated (r2 >0.39, p < 0.001), and, we cannot resolve which of the three soil factors is more strongly associated with fungal community composition.
Figure 5: Calcium (top left), percent carbon (top right), and sodium content (bottom left) for all 3 pits on sandstone parent material. Correlation coefficient (bottom right) for soil chemical variables (n=55); correlation significant at p<0.05 are marked with an *, p < 0.01 are in bold, and p <0.001 are in bold and larger font.
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Figure 6: NMS plots of all sandstone samples (top left, n=55) and the samples within pit 1 (top right, n=20), pit 2 (bottom left, n=18), and pit 3 (bottom right, n=17) on sandstone derived soils and results of Mantel tests for each soil chemical parameter. Vectors in ordinations represent soil chemical variables that had a strong correlation with the primary axis (r2 >0.2), and vector length is proportional the strength of correlation (within each ordination, not between ordinations)(see table 1 for final NMS statistics).
Discussion What Part of the Fungal Community are we Sampling? This study was originally intended to investigate the effects of soil chemistry on ectomycorrhizal fungal community composition. This is why we did not sample the organic horizon or the uppermost mineral soil, where we assumed saprotrophic fungi would dominate the fungal community. While the relative ratio of ectomycorrhizal fungi to saprotrophic fungi is much higher in the mineral soil (Lindahl et al., 2007), there are likely to be many saprotrophic fungi in the mineral soil, and we had planned to eliminate these from our dataset by using a database of known ectomycorrhizal fungi. We attempted to use an extensive collection of ectomycorrhizal sporocarps in conjunction with cloning and sequencing to build a reference database of known TRFLP types, similar to the method used by Dickie et al. (2002). We had many problems assembling this database so we had to restrict our analysis to a “fingerprint” of the total fungal community. We did, however, assemble 6 clone libraries from a total of 36 of our samples
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(evenly distributed across parent material, pit and depth). Of 118 sequenced clones, 56% were attributed to being of ectomycorrhizal origin, belonging to 22 operational taxonomic units (OTU’s), and 44% were either known or suspected saprotrophic fungi, belonging to 24 OTU’s. Clones were picked after examination by gel electrophoresis in order to maximize taxonomic coverage of our sequencing so clone abundance cannot be taken as an accurate indication of relative distribution. However, since there were more OTU’s of putatively saprotrophic fungi, and more clones of ectomycorrhizal origin, it seems reasonable to conclude that at least half of our TRFLP peaks profiles are of ectomycorrhizal fungi. However, given the significant amount of saprotrophic diversity observed, we do not know whether the observed shifts in fungal community composition are due to shifts in saprotrophic community or ectomycorrhizal community, and this has significant implications for the interpretation of our results. Role of Parent Material in Fungal Community Composition Parent material was the dominant factor shaping fungal community across the 6 soil pits. Soils formed from different parent materials can be expected to vary in many respects as a result of the mineral from which they formed (Shaw, 1930; Jenny, 1994) Across both parent materials all soil chemical characteristics we examined varied significantly with parent material, and the soil chemical factors that varied most with parent material were the most strongly associated with fungal community composition. Given that carbon was strongly associated with fungal community composition at the pit scale but very weakly across both parent materials, it is clear that location or parent material exerts a strong influence on fungal community composition, but we cannot assess the relative importance of any particular chemical factor in determining fungal community composition at this broad scale. When examining the effect of parent material, we cannot separate the effects of chemistry from the role of geographic location in shaping fungal community. Rainfall and sea salt inputs may contribute to the different communities observed on the different parent materials. The elevation of the sandstone plots is approximately 150 meters lower than the granite plots, and the sandstone plots are located approximately 2km closer to the sea (~1.5 km vs. ~3.5 km). As a result of these geographic factors, the annual rainfall is approximately 10% higher, and one can assume that any ocean derived salt inputs from precipitation significantly higher at the sandstone plots. Sea salt inputs have been shown to significantly lower soil pH (Lydersen and Henriksen, 1995) and this may explain the lower pH of the soils formed on sandstone. The higher precipitation inputs may be expected to have significant effects on the fungal community of these seasonally dry soils, though at the time of our sampling we were 1 month into the wet season, and we did not detect any significant difference in soil moisture between the different soils (data not shown).
Role of Soil Chemistry in Fungal Community Composition On each parent material, carbon content and depth stand out as the dominant variables that correlate with fungal community composition. Magnesium and calcium are strongly associated with fungal community composition, but Ca and Mg also correlate strongly with pit. Calcium is significantly associated with fungal community composition in one pit
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on each parent material including pit 3 on the granite derived soil. This pit also has the lowest calcium content of all pits. Overall though, the lack of a consistent effect of calcium, nor any significant effect of magnesium on fungal community at the pit scale, in conjunction with their tendency to vary considerably between pits suggests that Mg and Ca contents may be proxies for pit location and not significant factors in shaping fungal community composition. Sodium is strongly associated with fungal community composition on soils formed from sandstone, but Na is not a macronutrient for plants, and it is more abundant in soils formed from sandstone parent material. Given that it forms less than 5% of extractable cations (Mg and Ca form >80 %) it seems unlikely that Na is driving fungal community composition. Pit 6 on sandstone, had a significantly different fungal community than pits 4 and 5 and markedly higher sodium content. While it did not appear to have a more ocean-‐facing aspect than the other pits (<50 meters apart, all in closed canopy forest with similar slope and aspect), it is possible that the higher exchangeable Na from this pit is a marker of significantly higher sea salt effects. However, we would expect a lower pH and lower contents of non-‐marine base cations (K, Ca, Mg) to be associated with markedly higher sea salt inputs, and we did not observe this.
The strong association between fungal community and carbon content suggests that the differences we observed in the fungal community may be primarily driven by shifts in the saprotrophic fungal community. While carbon correlates with nitrogen, pH and base cations across all soil samples, it consistently stands out as the dominant factor shaping community composition. There is evidence that ectomycorrhizal fungi can obtain energy from soil carbon (Cullings et al., 2008, Courty et al., 2007), but saprotrophic activity by ectomycorrhizal fungi appears to only be significant when carbon allocation by the host plant is cut-‐off (drought, girdling, defoliation) (Baldrian et al., 2009), and we sampled the fungal community during the period of maximum carbon allocation to roots. Given that these forests are considered to be nitrogen-‐limited, and there was considerable range of nitrogen content observed, we would expect nitrogen to be an important determinant of fungal community composition if the observed changes in the fungal community were driven by shifts in the distribution and abundance of ectomycorrhizal fungi. Nitrogen was only associated with fungal community composition in two pits, and only marginally significantly (p < 0.06, p <0.1). The carbon to nitrogen ratio (C:N) was 9.5 across all soils (did not vary markedly with depth, or parent material, data not shown); this is well below the fungal biomass C:N ratio (Wallander et al., 2003), indicating that the saprotrophic fungal community is limited by carbon availability. The lack of a strong effect of nitrogen or potassium (the most-‐commonly growth limiting cation in temperate forests) on fungal community composition and the low soil C:N suggest that the relationship between soil carbon and fungal community is driven by shifts in the saprotrophic fungal community.
Our finding that carbon content was very strongly associated with fungal community composition is not in agreement with other studies in forests, though no studies have examined the effects of mineral soil carbon distribution on fungal community composition in forests. Lauber et al. (2008) and Kasel et al. (2008) both compared fungal communities across multiple forest types and failed to find an effect of soil carbon content on fungal community. However, both studies took large (>1 liter) soil samples that included the litter layer, organic horizon and only the top few cm (<8) of
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mineral soil, and any carbon effects were thus dominated by the thickness of the organic horizon and litter layer. These are, to the author’s knowledge, the only studies that have investigated the role of soil carbon in determining fungal community composition in forest soils. A number of studies have investigated the effect of soil carbon content in the mineral soil on microbial community composition in grasslands, and carbon content is consistently found to be the chemical factor most strongly correlated with fungal community composition (Moore et al., 2010, Allison et al., 2007; Fierer et al., 2003).
To the author’s knowledge, the only studies investigating depth effects on fungal community composition in forests have done so by comparing the ectomycorrhizal community on root tips between soil horizons. These studies consistently find a strong effect of horizon on ectomycorrhizal fungal community composition (Rosling et al., 2003; Dickie et al., 2002; Scattolin et al., 2008). A number of studies have found a strong association between total fungal community composition and depth in grassland soils. Robinson et al. (2009) used nearly identical TRFLP fingerprinting methods to ours to determine the effect of depth on the fungal community composition in a grassland soil approximately 5 km from our study site. They sampled 2 depths (1.5 cm, 14.5cm) and found that across a wide range of spatial scales the TRFLP profiles of the fungal community were more similar between different cores taken from the same depth (25 cm to 96m apart) than between the two depths of an individual soil core (13cm apart). Moore et al. (2010), Allison et al. (2007), and Fierer et al. (2003) found that, though very strongly associated with carbon, depth was an even stronger determinant of fungal community than carbon alone. Allison et al. (2007) and Moore et al. (2010) both suggest that carbon is the most important depth-‐related factor in determining fungal community composition, but also indicate that calcium availability may be an important component of the association of microbial community composition with depth. Rooting distribution, and the associated exudation of labile carbon may also exert a controlling effect on fungal community composition in forest soils (Brant et al., 2006), and this may be another depth-‐related factor controlling fungal community composition. Methodological Considerations There are limitations associated with using terminal restriction fragment length polymorphism as a community “fingerprinting” method (see review by Avis et al., 2006). Differences in peak profiles can be caused by incomplete restriction digest. Multiple OTU’s may yield the same peak profile, and individual taxa may yield multiple restriction profiles. Compiling a database of peak profiles from known fungal species will reduce the deleterious effects of these potential sources of error in TRFLP methods. However, TRFLP fingerprinting methods are reproducible (Thies et al., 2007; Edel Hermann et al., 2004), and widely used. Our methods were internally consistent, and our results were sufficiently strong and our sampling sufficiently intensive that we do not believe that these possible sources of error caused spurious correlations. One limitation of any community profiling method that we have already addressed is that it cannot be used to isolate polyphyletic groups, such as ectomycorrhizal fungi. A limitation of any molecular method involving DNA extraction from soil is that we cannot separate the potential contributions of fungal spores from that of fungal hyphae (our target) to our peak profiles, but we find that only very high levels of spores can be detected (Bruns, pers. observ.).
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Our sampling strategy was flawed in its spatial design due to incorrect assumptions about the soil chemical variability we would encounter. We were unable to determine the relative importance of geographic location or certain soil chemical factors strongly associated with fungal community in shaping fungal community composition. We anticipated more variation in soil chemical variables within pits and less between pits or parent materials. We designed our sampling strategy to maximize soil chemical variability, but we did so underestimating how much soil chemistry would vary with distance. We assumed that autocorrelation of fungal community with distance would drop so rapidly that pit separated by more than 5 meters should be independent. (Lilleskov et al., 2004), but given that we cannot separate the effects of distance from the effects of parent material, magnesium, nitrogen, sodium, pH, or calcium on fungal community composition, this may not be true at the scale we have sampled. Calcium, though, does appear to be more strongly associated with fungal community than location alone could explain. Phosphorous is an important nutrient, and we neglected to include it in our soil chemical analysis because we did not think we would have enough material left over from our very small soil samples after analyzing the seven parameters we analyzed to perform phosphorous analysis. In actuality, we had extra material and likely could have both collected smaller samples (potentially increasing soil chemical variability) and performed phosphorous analysis. Conclusion In the mineral soil of Pinus muricata forests at Point Reyes National Seashore soil carbon and depth have a very strong influence on fungal community composition. Parent material and calcium content also appear to exert considerable influence on fungal community composition but our sampling scheme did not allow us to eliminate location or distance as a possible cause for their apparent influence on fungal community composition. This study has demonstrated the potential of a novel approach to elucidating the role of very small-‐scale variation in soil chemistry in shaping microbial communities. In future studies, sampling schemes should be designed with consideration for the spatial scale of soil chemical variation inherent to the study area.
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References Cited Abuzinadah RA, Read DJ. 1986. The role of proteins in the nitrogen nutrition of ectomycorrhizal plants. I. Utilization of peptides and proteins by ectomycorrhizal fungi. New Phytologist 103:481-‐493. Albrecht R, Perissol C, Ruaudel F, Le Petit J, Terrom G. 2010. Functional changes in culturable microbial communities during a co-‐composting process: Carbon source utilization and co-‐metabolism. Waste Management 30:764-‐770. Allison VJ, Yermakov Z, Miller RM, Jastrow JD, Matamala R. 2007. Using landscape and depth gradients to decouple the impact of correlated environmental variables on soil microbial community composition. Soil Biology & Biochemistry 39: 505–516. Baldrian P. 2009. Ectomycorrhizal fungi and their enzymes in soils: is there enough evidence for their role as facultative soil saprotrophs? Oecologia 161:657–660. Brant JB, Myrold DD, Sulzman EW. 2006. Root controls on soil microbial community structure in forest soils. Oecogia:148: 650-‐659 . Buee M, Reich M, Murat C, Morin E, Nilsson RH, Uroz S, Martin F. 2009. 454 Pyrosequencing analyses of forest soils reveal an unexpectedly high fungal diversity New Phytologist 184: 449-‐456. Clarke KR. 1993. Non-‐parametric multivariate analyses of changes in community structure. Australian Journal of Ecology 18:117–143. Cullings K, Ishkhanova G, Henson J. 2008. Defoliation effects on enzyme activities of the ectomycorrhizal fungus Suillus granulatus in a Pinus contorta (lodgepole pine) stand in Yellowstone National Park. Oecologia 158:77–83 Courty PE, Breda N, Garbaye J. 2007. Relation between oak tree phenology and the secretion of organic matter degrading enzymes by Lactarius quietus ectomycorrhizas before and during bud break. Soil Biology and Biogeochemistry 7: 1655-‐1633. Dickie IA, Xu B, Koide RT. 2002. Vertical niche differentiation of ectomycorrhizal hyphae in soil as shown by T-‐RFLP analysis. New Phytologist 156: 527–535. Dickie IA, Richardson SJ, Wiser SK. 2009. Ectomycorrhizal fungal communities and soil chemistry in harvested and unharvested temperate Nothofagus rainforests. Canadian Journal of Forest Research 39:1069-‐1079. Edel-‐Hermann V, Dreumont C, Perez-‐Piqueres A, Steinberg C. 2004. Terminal restriction fragment length polymorphism analysis of ribosomal RNA genes to assess changes in fungal community structure in soils. FEMS Microbiology Ecology 47:397-‐404.
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Fierer N, Schimel JP, Holden PA. 2003. Variations in microbial community composition through two soil depth profiles Soil Biology & Biochemistry 35: 167–176. Gardes M, Bruns TD. 1993. ITS primers with enhanced specificity for basidiomycetes -‐ application to the identification of mycorrhizae and rusts. Molecular Ecology 2:113-‐118. Gleeson DB. Clipson N, Melville K, Gadd GM, McDermott FP. 2005. Characterization of fungal community structure on a weathered pegmatitic granite. Microbial ecology 50:360-‐368. Jenny H, Amundson. 1994. Factors of soil formation: A System of Quantitative Pedology. Dover Publications, New York. Foreword by Amundson R, reprint of original work: Factors of soil formation : a system of quantitative pedology by Jenny H. 1941.New York: McGraw-‐Hill. Kasel S, Bennett LT, Tibbits J. 2008. Land use influences soil fungal community composition across central Victoria, south-‐eastern Australia. Soil Biology & Biochemistry 40:1724–1732. Lilleskov EA, Fahey TJ, Horton TR, Lovett GM. 2002a. Below-‐ground ectomycorrhizal fungal community change over a nitrogen deposition gradient in Alaska. Ecology 83:104-‐115. Lilleskov EA, Hobbie EA, Fahey TJ. 2002b. Ectomycorrhizal fungal taxa differing in response to nitrogen deposition also differ in pure culture organic nitrogen use and natural abundance of nitrogen isotopes. New Phytologist 154: 219–231. Lim YW, Kim BK, Kim C, Jung HS, Kim BS, Lee JH, Chun J. 2010. Assessment of soil fungal communities using pyrosequencing. The Journal of Microbiology 48:284-‐289. Lindahl BD, Ihrmark K, Boberg J, Trumbore SE, Högberg P, Stenlid J, Finlay RD. 2007. Spatial separation of litter decomposition and mycorrhizal nitrogen uptake in a boreal forest. New Phytologist 173: 611–620. Lydersen E, Henriksen A. 1995. Seasalt effects on the acid neutralizing capacity of streamwaters in Southern Norway. Nordic Hydrology 26:369-‐388. Mantel, N. 1967. The detection of disease clustering and generalized regression approach. Cancer Research 27, 209-‐220. McCune, B., Grace, J.B., 2002. Analysis of Ecological Communities. MjM Software Design, Gleneden Beach, Oregon, USA. McGuire KL, Bent E, Borneman J, Majumder A, Allison SD, Treseder KK. 2010. Functional diversity in resource use by fungi. Ecology 91: 2324-‐2332.
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Moore J, Macalady JL, Schulz MS, White AF, Brantley SL. 2010. Shifting microbial community structure across a marine terrace grassland chronosequence, Santa Cruz, California. Soil Biology & Biochemistry 42:21–31. Parrent JL, Vilgalys R. 2007. Biomass and compositional responses of ectomycorrhizal fungal hyphae to elevated CO2 and nitrogen fertilization. New Phytologist 176: 164–174. Robinsona CH, Szaro TM, Izzo AD, Anderson IC, Parkin PI, Bruns TD. 2009. Spatial distribution of fungal communities in a coastal grassland soil. Soil Biology & Biochemistry 41: 414–416. Rosling A, Landeweert R, Lindahl BD, Larsson KH, Kuyper TW, Taylor AFS, Finlay RD. 2003. Vertical distribution of ectomycorrhizal fungal taxa in a podzol soil profile New Phytologist 159: 775–783. Scattolin L, Montecchio L, Mosca E, Agerer R. 2008. Vertical distribution of the ectomycorrhizal community in the top soil of Norway spruce stands. European Journal of Forest Research 127:347–357. Shaw CF. 1930. Potent factors in soil formation Source: Ecology 11: 239 -‐245 Taylor, AFS. 2002. Fungal diversity in ectomycorrhizal communities: sampling effort and species detection. Plant and Soil 244: 19–28. Thies JE. 2007.Soil Microbial Community Analysis using Terminal Restriction Fragment Length Polymorphisms. Soil Science Society of America Journal 71: 579-‐591. Wallander H, Nilsson LO, Hagerberg D, Rosengren U. 2003. Direct estimates of C:N ratios of ectomycorrhizal mycelia collected from Norway spruce forest soils. Soil Biology & Biochemistry 35:997–999. Wallenda T, Kottke I. 1998. Nitrogen depositon and ectomycorrhizas. New Phytologist 139:169-‐187. White TJ, Bruns TD, Lee SB, Taylor JW. 1990. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. In: Innis MA, Gelfand DH, Sninsky JJ, White TJ. PCR Protocols-‐ a Guide to Methods and Applications. New York, Academic Press. p315-‐322.
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Chapter 3 Can ectomycorrhizal weathering activity respond to host nutrient demands? Abstract Ectomycorrhizal fungi may make a significant contribution to mineral weathering in temperate and boreal forests. It is important to know how this weathering activity will be affected by changing nutrient demands of forests affected by global change and nitrogen deposition. This review looks at how belowground carbon allocation in plants is affected by nutrient demand and at a number of experiments that have examined ectomycorrhizal weathering activity. Plant physiology literature indicates that plants can respond to phosphorus limitation by allocating more carbon belowground and increasing root branching in areas of high P availability. Increasing expression and upregulation of phosphorus and potassium uptake transporters has been observed under P-‐ and K-‐ limitation, respectively. There is evidence for a negative feedback between magnesium-‐ and potassium-‐ deficiency and belowground carbon allocation. There are very few ectomycorrhizal weathering experiments that explicitly test how weathering activity responds to nutrient demand. Field studies suggest that hyphal colonization of readily available P sources does increase with increased P demand of the host. In microcosm studies there is indirect evidence that weathering activity may increase in response to P, K, or Mg demand. Recommendations are made for how future ectomycorrhizal research can better address this question. More research on how plants sense and respond to nutrient limitation, as well as genomic data from gymnosperms would also aid our understanding of this important aspect of forest ecology.
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Introduction Ectomycorrhizae are considered to be weathering agents in temperate and boreal ecosysytems (see reviews by van Scholl et al., 2008; Hoffland et al., 2004; Landeweert et al., 2001; van Breeman et al., 2000 and other articles in this issue). There is considerable debate about the significance of this biotic component of weathering. Understanding and quantifying the ectomycorrhizal contribution to mineral weathering may be particularly important to forest managers attempting to minimize time between stand harvest without depleting mineral nutrient levels in the soil and to policy makers determining critical loads of atmospheric pollutants.
Ectomycorrhizal fungal communities are dynamic, composed of many species, with somewhat discreet ecological niches. Nitrogen deposition (Avis et al., 2008; Lilleskov et al., 2002) and increasing CO2 levels (Parrent and Vilgalys, 2007) have been observed to cause a shift in the community composition of ectomycorrhizal fungi (ECM) in a given forest. While it is unclear what causes these changes, it has been suggested that trees may be selecting for ECM species that best satisfy the host plant’s altered nutrient demand (Lilleskov et al., 2002; Treseder, 2004). It is important to know how ectomycorrrhizal weathering rates will respond to changing nutrient status of forests to determine what measures must be taken to guarantee forest health as humans continue to alter the global carbon and nitrogen cycles. This review examines what is known about how weathering activity of ectomycorrhizal fungi (both communities and individual species) responds to nutrient demand. First, attention is given to our understanding of how host plant nutrient demand is sensed by the plant and communicated to the roots, then research on ectomycorrhizal weathering is discussed with recommendations for further studies. For the purposes of this review we will focus on nutrient demand of P, K, Mg, and Ca, although S and sometimes Fe are also important nutrients to consider in regards to weathering activity. Ectomycorrhizal weathering activity will be defined as any activity on the part of the fungus that increases the rates of element release from mineral surfaces, such as proton exudation, organic acid exudation, or element removal from the mineral surface. Relevant aspects of root physiology When considering how ectomycorrhizal weathering rates may respond to host plant nutrient demand two important questions stand out: ♦ Is nutrient demand of the aboveground portions of the host plant communicated to
the roots? And if so, how? ♦ How can ectomycorrhizae respond to host nutrient demand?
The first question addresses belowground carbon allocation and root growth of the plant, and the focus is almost entirely on the plant, whereas the second question incorporates plant physiology, fungal physiology and community ecology and the primary focus is the root apoplast-‐fungal interface. Plant responses to nutrient demand is a broad subject; what follows is a concise review of aspects of plant nutrient sensing and root physiology relevant to the central question of whether ectomycorrhizal weathering can respond to plant-‐host demand for P, K, Mg, and Ca. There are three (overlapping) mechanisms by which a plant can increase nutrient uptake by its roots.
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♦ Increased C allocation belowground ♦ Increased C allocation to-‐ and/or root branching in-‐ a particular part of the root
system ♦ Increased expression or upregulation of nutrient transporters.
The first is sometimes called “systemic” control and the second and third “localized”, implying that belowground allocation is controlled by overall plant nutrient status while localized root branching/proliferation and transporter expression and regulation are controlled by the roots, but there is ample evidence that the “localized” characters may also be systemically controlled (Ford and Lorenzo, 2001).
Nearly all of the research done to date on plant nutrient status sensing and regulation of nutrient acquisition has been done on model organisms or crop plants, all of which are angiosperms (as opposed to gymnosperms which comprise the majority of temperate and boreal forest cover), and nearly all of which form arbuscular mycorrhizae (as opposed to ectomycorrhizae which are the primary focus of this article). However, in the limited number of cases where ECM forming host plants are examined their reactions to nutrient limitation seem qualitatively similar.
Phosphorus deficiency inhibits the growth of new plant tissues, particularly aboveground shoots and leaves (Radin and Eidenbock, 1984), leading to a buildup of sugars and starches in leaves which in turn causes increased phloem loading of sugars, and C transport to the roots (Cakmak et al., 1994a; Cakmak et al., 1994b; Qiu and Israel, 1992). Increased P demand has been found in a number of studies (including the EM associate Betula pendula) to cause increased belowground carbon allocation and increased root: shoot ratios (Andrews et al., 1999; Cakmak et al., 1994a; Ericsson et al., 1995; Hermans et al., 2006). Increased root growth and branching is generally found in patches of high P availability when plants are P-‐limited (Robinson, 1994; Drew et al., 1975; Lopez-‐Bucio et al., 2003). Numerous studies have found increased expression and up regulation of high-‐affinity P transporters in the roots of phosphorous deficient plants covering a wide range of species (see refs. in Wang et al., 2006).
Potassium deficiency causes a buildup of carbohydrates in the leaf, but also inhibits phloem loading of carbohydrates (Hermans et al., 2006). Andrews et al. (1999) found either no effect (Pisum sativum, Phaseolus vulgaris) or a marginal increase in root:shoot with increasing K limitation (Triticum aestivum). In general, however, K deficiency has been found to decrease belowground carbon allocation and root:shoot (including on ECM associate Betula pendula) (Cakmak et al., 1994; Ericsson and Kahr, 1993; Ericsson, 1995). Numerous studies find plants (including ECM gymnosperm associate Picea stitchensis) unable to respond to local concentration gradients of potassium by enhanced growth or branching in areas of elevated K availability (Gile and Carrero, 1917; Drew, 1975; Philipson and Coutts, 1977; Ashley et al., 2006). A number of studies have found high affinity root K transporters are up-‐regulated in response to K deprivation (see refs in review by Wang et al., 2006).
Mg deficiency causes a sharp inhibition of sugar loading into the phloem, and this is generally found to result in a sharp decrease in belowground carbon allocation in a wide range of plants (including both gymnosperm and angiosperm ECM associates), even at only moderate levels of Mg deficiency (Ericsson, 1995: Ericsson and Kahr, 1995; Cakmak and Kirby, 2008; Andrews, 1999). Ericsson and Kahr (1995) also observed that
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ectomycorrhizal development was strongly suppressed in Mg-‐limited spruce seedlings. Indirect evidence suggests that plant root systems are not able to alter root growth patterns to take advantage of localized patches of high Mg availability. There is very little data available on root Mg transporters (Wang et al. 2006).
Ca deficiency generally inhibits the formation of new tissues. This may cause an increase in leaf soluble carbohydrate concentrations and result in increased root:shoot and belowground C allocation. A slight increase in root: shoot with decreasing plant Ca concentrations has been observed in a number of studies (Lopez-‐Lefebre et al., 2001; Ingestad and Lund, 1986), but this increase is always marginal and the increased root:shoot likely reflects growth inhibition of aboveground apical meristems and not an increase in belowground C allocation. The majority of calcium is thought to enter the plant with bulk flow of evapotransporation via the apoplastic pathway in parts of the root that lack a casparian band, but it is also known that there are symplastic pathways. Little is known about the relative importance of the apoplastic and symplastic pathways in Ca uptake (White and Broadley, 2003). The author could not find any data on whether roots are able to increase branching in patches of localized Ca abundance. Overall, our understanding of how plants sense and respond to nutrient status is
severely limited. The development of-‐ and subsequent research upon-‐ nitrate reductase mutants of tobacco plants greatly advanced out understanding of nitrogen responsiveness in plants, but no such milestones have been achieved for P, K, Mg, or Ca. As a result we have very little idea about the mechanisms of plant nutrient status sensing for these important nutrients. More research is needed in this fundamental area of plant physiology for mycorrhizal ecologists to gain an understanding of how mycorrhizae may respond to shifting host nutrient demand in ecosystems undergoing continued acid deposition and/or global change. Nonetheless, the research performed to date does provide the field of mycorrhizal ecology with some useful information in predicting how ECM communities may respond to host nutrient demand: ♦ Belowground allocation increases in response to increased host P demand, is not
significantly affected by plant Ca status, and is reduced significantly by K and Mg deficiency.
♦ Plants can respond to localized patches of P by increased root branching. ♦ Plants can either increase expression of or upregulate existing root nutrient
transporters to increase uptake of K and P in response to deficiency. Increased carbon allocation belowground necessarily correlates with increased carbon allocation to ectomycorrhizae (Hobbie, 2006) and thus, it follows logically, to increased weathering activity by ectomycorrhizal fungi. Roots that can preferentially grow and/or branch in localized areas of high nutrient availability will favor the ectomycorrhizal species which are providing the largest amounts of this nutrient, because by ramifying in those areas they allow those fungal species the opportunity to colonize a larger root area. It is unclear if increased expression and up-‐regulation of high affinity transporters is possible under the ectomycorrhizal mantle. If so, one would assume this greater sink strength on the part of plant root would increase nutrient loss from the fungus, and thus increase nutrient transport in ECM rhizomorphs. However, it remains poorly understood
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how fungi exude non-‐N nutrients into the fungal-‐plant interface. A better understanding of this mechanism is necessary to understand how increased expression of nutrient transporters in plant roots will affect ectomycorrhizal activity.
It is generally assumed that there is a tit-‐for-‐tat relationship between the ectomycorrhizal fungus and the plant host. The former providing nutrients in exchange for carbon from the latter. Much research has gone into developing a mechanistic understanding of how this arrangement might be structured at the fungus-‐root interface, nearly all of it focusing on nitrogen as the fungal exchange currency. Nehls et al. (2007) suggest that carbon provision by the fungus may be dependant on plant phosphorylation of hexose uptake transporters. This mechanism would allow the host plant to very quickly control carbon loss to the fungus as a function of nutrient provision from the ectomycorrhizal fungus. If these phsophorylation sites are controlled by the supply of nutrients other than N, such as P, K, Mg, or Ca, then this could be a key point of communication between host nutrient demand and fungal nutrient acquisition. Evidence from field and microcosm studies Field studies A variety of microcosm and field experiments have been conducted to examine how host plant nutrient status affects the weathering activity of ectomycorrhizal fungi. There are tradeoffs associated with each scale of experimentation. Field studies take into account the full range of biotic and abiotic interactions which govern nutrient demand and mycorrhizal activity. In field studies, plants generally grow under realistic nutrient regimes, and in the presence of dynamic, complex microbial communities. However, it may be very difficult to isolate nutrient deficiency as a single treatment in field studies. Fertilization treatments are one method for examining the effects of nutrient demand on weathering activity; using existing productivity gradients is another. Fertilization may, however, cause other, undesirable effects such as increased pH, and it may take many years for the system to fully react to the altered nutrient status. Natural productivity gradients may vary with regards to a number of factors other than the desired nutrient availability. Both of these methods have been employed in a limited number of field studies to look at how nutrient availability and host plant nutrient status affect ectomycorrhizal weathering activity in situ.
While it is not possible to quantify ectomycorrhizal weathering activity in the field on an ecosystem scale, a number of qualitative measures can be employed to compare weathering activity either between different nutrient regimes or between different minerals. One such measure involves the use of in-‐growth bags. Small mesh bags are filled with either quartz sand alone or sand amended with a particular mineral of interest. The mesh is of a size that allows hyphae but not roots to penetrate the bags. Studies by Wallander et al. (2001), Parrent and Vilgalys (2007), and Kjoller (2006) have indicated that the great majority of hyphae found in these bags are ectomycorrhizal fungi. By quantifying the fungal biomass and mineral dissolution in bags one can make inferences about the relative ectomycorrhizal weathering activity in a given soil. One important weakness of this method is that it discriminates against the broad range of ectomycorrhizal fungi that do not produce abundant extraradical hyphae.
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Hagerberg et al. (2003) examined ectomycorrhizal hyphal colonization of mesh bags filled with sand, or sand amended with either a mineral P source (apatite) or a mineral K source (biotite) in a variety of Picea abies forests in southern Sweden encompassing a range of P and K availabilities. Apatite amended bags always had greater mycorrhizal colonization than pure sand bags, but in forests considered to be P-‐limited (spruce needle P content below sufficiency levels) there was significantly greater difference in colonization between apatite-‐amended and pure sand bags. No such response was found for forest K status and biotite colonization, although colonization of biotite amended bags was overall significantly higher than purely sand filled bags. Wallander et al. (2008) revisited this study, also using southern Swedish P. abies stands of varying P status, but they focused on P and added a fertilization treatment. Again, they found a significantly greater difference in colonization between apatite-‐amended and pure sand bags when P levels were growth limiting, but when P levels were increased by fertilization there was no increase in mycelial colonization of apatite amended bags as compared to pure quartz sand filled bags. Taken together these studies suggest that ectomycorrhizal colonization of readily weatherable minerals, though not necessarily actual weathering, can respond to forest phosphorous status.
In another mesh bag study, Wallander et al. (2003) looked at mycelial colonization of quartz filled mesh bags, some of which were amended with wood ash (a rich source of relatively mobile Ca, K, Fe, Mg) or apatite. The study employed particle induced X-‐ray emission spectra to assess the elemental content of specific rhizomorphs which were also molecularly identified to fungal species. Wood ash-‐amended bags had significantly more mycelia than either sand filled or apatite-‐amended bags. Paxillus involutus, by far the most utilized ECM in weathering experiments and the one generally shown to have the highest oxalic acid production in microcosm studies (Wallander and Wickman, 1999; van Scholl, 2005; van Scholl, 2006) was not found on any root tips near the mesh bags, but it was by a large margin the most common ITS phylotype found in the mesh bags. Suillus granulatus hyphae consistently contained more K than other species and generally had Ca-‐oxalate encrustations on its rizomorphs, while P. involutus consistently had higher Ca content than any other species. Some trends were also found for Fe and P contents and other ECM species. These results suggest there may be some consistent trends that can be used to place certain ectomycorrhizal species in distinct biogeochemical niches. However, elevated levels of mineral elements do not necessarily indicate increased weathering activity of minerals containing those elements. On the contrary, elevated levels of a given element could indicate that this fungus passes less of this elelement on to its host plant.
Another potentially useful method for examining weathering activity of ectomycorrhizal fungi is to look at the density of hyphal tunneling in mineral grains. Jongmans et al. (1997) first proposed that regular channels often found in minerals might be fungal in origin. While no “smoking gun” has to date been devised to demonstrate that these tunnels are irrefutably caused by ectomycorrhizal fungi, a number of studies offer circumstantial evidence (Jongmans et al, 1997; Hofland et al, 2004; Hoffland et al, 2002). Looking at a topographic fertility gradient in a northern Swedish boreal forest, Hoffland et al. (2003) found that the density of tunnels of putative ectomycorrhizal origin had a strong positive correlation with ectomycorrhizal tip density and a strong negative one with soil fertility. Assuming that tunneling can be taken as an indicator for overall
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weathering activity (despite the relatively small contribution tunneling makes to overall mineral weathering (Smits et al., 2005)), it remains unclear whether the greater weathering activity in the lower fertility sites is due to lower pH, greater nutrient demand on the part of the host, or greater ectomycorrhizal colonization. Hoffland et al. (2002) assessed tunneling activity across a northern Sweden podzol sequence and found that the occurrence of tunnels in feldspar grains coincided with the disapearence of easily weatherable cation sources such as biotite. Taken together, these tunnel studies imply a correlative, but not a causative, link between weathering activity by ectomycorrhizal fungi and host nutrient demand.
Wilson at al. (2008) used magnetic separation to segregate readily weatherable cation sources such as biotite and orthopyroxene from more cation poor K feldpars. They then used a variety of molecular and microscopic methods to asses the density of microbial colonization and weathering state of these minerals. They found significantly more mycelial (primarily ectomycorrhizal) colonization of readily weatherable cation sources such as biotite and orthopyroxene than on more cation-‐poor K feldpars, but noticed only slightly increased weathering of the biotite compared to the feldspar minerals.
In the aforementioned field studies, there is evidence that ectomycorrhizal fungi may increase mineral foraging and colonization in response to increased demand for phosphorus. There is also evidence that weathered tunnels (a likely indicator of fungal weathering activity) coincide with increased demand for mineral elements other than phosphorus and that ectomycorrhizal hyphae can preferentially colonize mineral fragments which are good sources of mineral nutrients other than phosphorus. However, there is no direct evidence in field studies that foraging for and weathering of K, Mg, or Ca sources by ECM can respond to demand for these nutrients. There are many reports in the literature of forest ecosystems dominated by ectomycorrhizal hosts which, possibly due to anthropogenic acid deposition, are now limited by base cation availability and not nitrogen or phosphorus. The mesh bag approach employed by Wallander and others in Swedish forests may be a good method for examining how the mycorrhizal role in nutrient acquisition has changed with the changing nutrient status of these forests. Especially good sites to use this approach would be the sharp N depositional gradients near industrial or agricultural sites. Pot and microcosm studies Microcosm studies allow weathering to be quantified and can focus on the weathering of a single mineral or any desired mineral mix. Microcosms can be used to examine the weathering potential of individual ectomycorrhizal species and can be employed to isolate the weathering activity of the ectomycorrhizal fungus from that of the plant root. Microcosm studies also allow the researcher to isolate the effects of the availability of just one nutrient on weathering activity. In relatively sterile microcosm experiment it is also much easier to assay for readily decomposable weathering agents, particularly low-‐molecular-‐weight organic acids (LMWOA), and examine how LMWOA production affects weathering rates. In soils, measured bulk-‐solution LMWOA concentrations are generally too low to significantly enhance mineral weathering due to their rapid degradation by soil microbiota. However in semi-‐sterile microcosms and at the fungus-‐mineral interface in
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natural soils, LMWOA concentrations may be high enough to greatly enhance weathering rates via proton-‐promoted and ligand-‐promoted dissolution. (van Hees et al. 2005; Banfield et al., 1999)
Van Scholl et al. (2006) looked at how organic acid production was influenced by nutrient deficiency of Mg, N, P, and K. Decreasing P or N increased organic acid production, while reducing Mg or K either had no effect or slightly decreased overall LMWOA, although reducing Mg did increase oxalate production in some treatments. There were also significant differences between individual fungal species organic acid exudation profiles and how they reacted to different nutrient deficiencies.
Paris et al. (1995, 1996a, 1996b) conducted a series of studies examining how weathering activity of ectomycorrhizal fungi in azenic culture is affected by nutrient availability. They found that Ca (Paris et al., 1995), K, and Mg (Paris et al., 1996a) had no effect of weathering activity when one element was deficient, however when Mg and K were simultaneously deficient both phlogopite weathering (Paris et al., 1996a) and oxalic acid production (Paris et al., 1996b) increased.
In order to test whether weathering activity can respond to nutrient demand there must be a nutrient-‐sufficient treatment and a nutrient-‐deficient treatment, both with added minerals. The great majority of microcosm studies investigating ectomycorrhizal weathering fail to have both a nutrient-‐deficient and a nutrient-‐sufficient treatment. Only the work by van Scholl et al. (2005) and Paris et al. (1995, 1996a, 1996b), explicitly tested whether weathering activity can respond to nutrient demand. From these studies it does appear that there is potential for the ectomycorrhizal fungus alone, or the ectomycorrhizal seedling to respond to deficiencies in P, Mg, or K by enhancing weathering activity, however the study by van scholl et al. (2005) had no added minerals and thus doesn’t actually measure weathering, and the studies by Paris et al. (1995, 1996a) are azenic pure culture studies. More studies are clearly needed to address this specific question.
If the ectomycorrhizal fungus is also below its optimal level for a particular nutrient, then increases in weathering or nutrient uptake observed by ectomycorrhizae in a –nutrient treatment are not necessarily a reaction to host plant nutrient demand. Increased weathering may be a reaction to ectomycorrhizal nutrient demand only. Having separate mycorrhizal and rooting compartments would help to resolve this question as would an additional treatment in which the growth medium is kept very nutrient poor but the plant is foliarly fertilized. The two-‐compartment system used in Jentschke et al. (2001) would be a very effective way to segregate the ecomycorrhizal nutrient demand from plant nutrient supply.
While they do not explicitly test whether weathering activity can respond to changing nutrient status, a number of other studies can offer insight into the study of ectomycorrhizal weathering and some discussion of them is warranted in this review. Ectomycorhizae have been found to increase weathering in a number of microcosm studies (Ochs et al., 1993; van Hees et al., 2004; van Scholl et al., 2006; Wallander et al., 2000), while others have not found any increase in weathering with ectomycorrhizal colonization (Balogh-‐Brunstad et al., 2008; Wallander et al., 1999). Many of these studies find increased weathering with one ectomycorrhizal species but not another or with one nutrient treatment or mineral type but not another. Generally, studies that deny P or K
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and add a weatherable P or K source such as apatite or biotite do find increased weathering with ectomycorrhizal colonization. The same cannot be said for Mg; no study has yet looked at how weathering by ectomycorrhizal plants is affected by Ca status. The proposed weathering mechanism varies from one study to another and even from fungus to fungus. Wallander (2000) found that all 3 ECM strains tested increased weathering rates above the non-‐mycorrhizal (NM) control, but the mechanism of increased weathering was different for each strain: decreasing solution pH (proton-‐promoted dissolution), oxalic acid prodution (ligand-‐promoted dissolution) and greater P uptake (removing transport limitation to weathering). The most commonly proposed mechanism for ectomycorrhizal enhancement of mineral weathering is greater nutrient uptake and transport away from the mineral surface (van Scholl et al., 2006; Wallander. 2000; van Hees et al., 2004).
Organic acid production by seedlings is generally found to be altered, though not necessarily increased, by ectomycorrhizal colonization. Organic acid exudation does not respond in a consistent way to nutrient demand or to the presence of certain minerals, nor is it generalizable across different ectomycorrhizal species. When one LMWOA is linked to increased weathering rates it is most commonly oxalic acid. Oxalic acid is produced in particularly large quantities by P. involutus, which also happens to be the most commonly used ectomycorrhizal species in weathering experiments. Ochs et al. (1993) found that there were strong weathering agents in the root exudates of H. crustiliniforme, present in very low concentrations (µM) which were likely not LMWOA’s.
The work of Calvaruso et al. (2006) and Uroz et al. (2007) give convincing evidence for a key role that bacteria may play in ectomycorrhizal weathering. They found that bacteria isolated from the symbiotic mantle of ectomycorrhizosphere of oak mycorrhizas have significantly higher weathering capacity than phylogenetically closely related bacteria isolated from the adjacent bulk soil (Uroz et al., 2007). Calvaruso et al. (2007) demonstrated that one of these bacteria has the potential to enhance nonmycorrhizal seedling growth by alleviating Mg and K limitation by stimulating biotite weathering. These results strongly suggest that further research into the field of mycorrhizal helper bacteria and ectomycorrhizal weathering is warranted. It also suggests that some of the highly reductionist experiments with either no bacteria or a much simplified bacterial community may fail to account for a key mechanism of ectomycorrhizal weathering.
Often the rooting area in pot or microcosm studies is quite small such that the roots are far more densely packed than they would be in a natural setting. As a result, the ectomycorrhizosphere is no larger than the rhizosphere in nonmycorrhizal treatments. This eliminates one of the major proposed advantages of mycorrhizal colonization, and possibly a key mechanism by which ectomycorrhizae may confer a greater weathering ability on root systems: greater mineral surface contact and uptake of weathering products directly from mineral surfaces. The majority of microcosm experiments employ an artificial rooting medium (such as semi-‐sterile peat, or sterile quartz sand) and/or an inorganic nutrient solution, both of which may be a poor recreation of the nutrient environment of field settings. Nutrient starvation may be achieved when minor nutrient limitation, more representative of field conditions, is desired. Most microcosm studies also have either no or a highly simplified bacterial community, which may significantly
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alter weathering dynamics from natural settings. Another key drawback of microcosm studies is that the carbon and nutrient exchange dynamics of isolated seedlings in a laboratory may bear little resemblance to that of seedlings or mature trees in the field. In field settings hyphal networks may allow seedlings to avoid some of the initial carbon investment involved in establishing mycorrhizal colonization. Mature ectomycorrhizal trees are generally considered to be dependent on ectomycorrhizal communities for survival, while seedlings in the lab often experience growth reductions in response to mycorrhizal colonization and uncolonized seedlings can be far larger and more vigorous. The following recommendations will improve our ability to relate the results of weathering experiments in pots or microcosms to field processes: ♦ Experiments must have both a +nutrient and –nutrient experiment. ♦ If weathering activity is to be attributed to feedback from plant demand, some
segregation of plant and fungal nutrient demand is necessary. ♦ Pots should be big enough so that root systems do not completely fill potting
medium. ♦ Weathering agents other than LMWOA should be assayed for. ♦ A variety of mycorrhizal species should be used. ♦ When possible a diverse community of soil bacteria should be used. ♦ Pilot studies should be done to ensure that nutrients are limiting but not deficient. ♦ When possible, larger, older seedlings (1-‐2 yr. old instead of 2-‐6 mo.) should be used.
Conclusion There is limited evidence in the literature that ectomycorrhizal fungi can respond to increased nutrient demand with an increase in weathering activity, particularly with regards to phosphorous and potassium. Most studies however fail to properly isolate nutrient demand as a single factor, and no studies isolate fungal nutrient demand from plant nutrient demand. More research is needed to address the important question of how ectomycorrhizal communities will buffer changes in soil nutrient regimes caused by human activities. In the field, mesh bag studies in a variety of affected forests and nutrient gradients will improve our understanding of how weathering may respond to nutrient demand. In microcosm studies, experiments should be designed to better capture the range of ecological interactions present in natural systems, and more controls are needed to attribute altered weathering activity to host plant nutrient demand.
An impressive amount of research has been devoted to how plant root physiology is affected by nutrient demand, but we still lack a clear mechanistic understanding for how plants sense and respond to nutrient demand. What little we do know is based on angiosperm herbs, not the gymnosperm trees for which we may wish to interpret the results. Gymnosperm genomes are enormous and thus none of them have been sequenced to date, as more plant genomes are sequenced we can expect our understanding of plant nutrient sensing to increase.
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Philipson JJ, Coutts MP, 1977. The influence of mineral nutrition on the root development of trees. II. The effect of specific nutrient elements on the growth of individual roots of Sitka spruce. Journal of Experimental Botany 28: 864-‐871. Qiu J, Isreal DW, 1992. Diurnal starch accumulation and utilization in phosphorus deficient soybean plants. Plant Physiology 47: 404-‐408. Radin JW, Eidenbock MP, 1984. Hydraulic conductance as a factor limiting leaf expansion of phosphorous-‐deficient cotton plants. Plant Physiology 88: 725-‐730. Robinson D, 1994. The responses of plants to non-‐uniform supplies of nutrients. New Phytologist 127: 635-‐674. Rosling A, Lindahl B, Finlay R, 2004. Carbon allocation to ectomycorrhizal roots and mycelium colonizing different mineral substrates. New Phytologist 162: 795-‐802. Smits MM, Hoffland E, Jongmans AG, van Breemen N, 2005. Contribution of mineral tunneling to total feldspar weathering. Geoderma 125: 59-‐69. Treseder K.K. 2004. A meta-‐analysis of mycorrhizal responses to nitrogen, phosphorus, and atmospheric CO2 in field studies. New Phytologist 164: 347-‐355. Uroz S, Calvaruso C, Turpaul MP, Pierrat JC, Mustin C, Frey-‐Klett P, 2007. Effect of the mycorrhizosphere on the genotypic and metabolic diversity of the bacterial communities involved in mineral weathering in a forest soil. Applied and Environmental Microbiology 73: 3019-‐3027. van Breemen N, Finlay R, Lundstrom U, Jongmans AG, Giesler R, Olsson M, 2000. Mycorrhizal weathering: A true case of mineral plant nutrition? Biogeochemistry 49: 53-‐67. van Hees PAW, Jones DL, Jentschke G, Godbold DL, 2004. Mobilization of aluminum, iron and silicon by Picea abies and ectomycorrhizas in a forest soil. European Journal of Soil Science 55: 101-‐111. Van Hees PAW, Jones DL, Jentschke G, Godbold DL, 2005. Organic acid concentrations in soil solution: effects of young coniferous trees and ectomycorrhizal fungi. Soil Biology and Biogeochemistry 37: 771-‐776. Van Scholl L, Smits MM, and Hoffland E, 2006. Ectomycorrhizal weathering of the soil minerals muscovite and hornblende. New Phytologist 171: 805-‐814. Van Scholl L, Hoffland E, van Breemen N, 2006. Organic anion exudation by ectomycorrhizal fungi and Pinus sylvestris in response to nutrient deficiency. New Phytologist 170: 153-‐163.
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Van Scholl L, Kuyper TW, Smits MM, Landeweert R, Hoffland E, van Breemen N, 2008. Rock-‐eating mycorrhizas: their role in plant nutrition and biogeochemical cycles. Plant and Soil 303: 35-‐47. Wallander H, Thelin G, 2008. The stimulating effect of apatite on ectomycorrhizal growth diminishes after PK fertilization. Soil Biology and Biochemistry 40: 2517-‐2522. Wallander H, Mahmood S, Hagerberg D, Johansson L, Pallon J, 2003. Elemental composition of ectomycorrhizal mycelia identified by PCR-‐RFLP analysis and grown in contact with apatite or wood ash in forest soil. FEMS Microbial Ecology 44: 57-‐65. Wallander H, 2000. Uptake of P from apatite by Pinus sylvestris seedlings colonized by different ectomycorrhizal fungi. Plant and Soil 218: 249-‐256. Wallander H, Johansson E, Pallon J, 2002. PIXE analysis to estimate the elemental composition of ectomycorrhizal rhizomorphs grown in contact with different minerals in Forest soil. FEMS Microbiology Ecology 39: 147-‐156. Wallander H, Wickman T, 1999. Biotite and microcline as potassium sources in ectomycorrhizal and non-‐mycorrhizal Pinus sylvestris seedlings. Mycorrhiza 9:25-‐32. Wallander H, Nilsson LO, Hagerberg D, Baath E, 2001. Estimation of the biomass and seasonal growth of external mycelium of ectomycorrhizal fungi in the field. New Phytologist 151: 753-‐760. Wang H, Inukai Y, Yamauchi A, 2006. Root development and nutrient uptake. Critical Reviews in Plant Sciences. 25: 279-‐301. White PJ, Broadley MR, 2003. Calcium in plants. Annals of Botany 92: 487-‐511. Wilson MJ, Certini G, Campbell CD, Anderson IA, Hiller S, 2008. Does the preferential microbial colonization of ferromagnesian minerals affect mineral weathering in soil. Naturwissenschaften 95: 851-‐858.
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Chapter 4 The role of ectomycorrhizae, organic acids, and Pinus sylvestris seedlings in mineral weathering: nutrient uptake increases weathering rates. ABSTRACT Calculating the weathering rates in forest soils is critically important to forest managers, air quality policy, and models of forest productivity. Understanding biotic weathering, the relative contributions from bacteria, plants and fungi, and the influence of elevated CO2 on biotic weathering is essential to constraining weathering estimates. We employed a column mesocosm system to look at the effects of elevated CO2 and Pinus sylvestris seedlings, with or without the ectomycorrhizal fungi Piloderma fallax and Suillus variegatus on rhizosphere soil solution concentrations of low molecular weight organic acids (LMWOA) and weathering of primary minerals. Seedlings significantly increased mineral weathering. Elevated CO2 increased plant growth but had no effect on weathering. Ectomycorrrhizae demonstrated some ability to increase weathering, particularly P. fallax, but did not significantly increase overall weathering rates. LMWOA production was strongly correlated with seedling biomass but not with weathering rates. Our results indicate that nutrient uptake is the primary mechanism of biotic enhancement to weathering. Biotic uptake of nutrients and water reduces transport limitation to weathering, and it is this process, not proton-‐ or ligand-‐promotion, that is the primary mechanism by which biota enhanced mineral weathering. While this system departs from conditions in forest soils in a number of ways, these results are in line with weathering studies done at the ecosystem, mesocosm, and microcosm scale.
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INTRODUCTION Calculating the weathering rates in forest soils is critically important to forest managers, air quality policy, and models of forest productivity. Any removal of timber from a forest represents a removal of mineral nutrients; understanding how quickly those nutrients are replenished by atmospheric deposition or mineral weathering is a key component of a sustainable harvesting cycle (Duschesne and Houle, 2006; Sverdrup et al., 2006; Akselsson et al., 2007). Mineral weathering rates determine a soil’s buffering capacity and are the single most important properties determining an ecosystem’s ability to buffer the effects of acidifying pollutants (McDonnell et al., 2010; Hodson and Langan, 1999; Whitlfield et al., 2006). Mineral weathering rates in soils are also the single most poorly constrained component of models designed to calculate acceptable airborne pollutant loads of nitrogen and sulfur deposition from power generation, transport, and agriculture (Shindo et al., 1995; McDonnell et al., 2010; Li and McNulty, 2007). Accurate estimates of net primary productivity of forests over the course of the next century are critically important to global carbon models. Forest productivity is predicted to increase due to elevated CO2 (Lindner et al., 2010; Ainsworth and Long, 2005). The extent of this negative feedback to elevated CO2 levels is largely dependant on forest trees’ ability to meet their increased carbon availability and water use efficiency with increased nutrient uptake (Pinkard et al., 2010; Norby et al., 1999). As the effects of anthropogenic nitrogen deposition continue to accumulate, large areas of forest are limited by base cation availability (Naples and Fisk, 2010; Baribault et al., 2010), which is a function of mineral dissolution. In coniferous trees, elevated CO2 has been shown to increase the ratio of root to shoot biomass (root:shoot) (Alberton et al., 2007; Janssens et al., 2005) and allocation to mycorrhizal symbionts (Fransson et al., 2007; Compant et al., 2010). To understand how forest productivity and forest carbon stocks will be affected by global change we must first understand whether increased carbon allocation to nutrient uptake organs actually results in increased nutrient uptake and whether this increased carbon allocation is a result of increased nutrient demand.
Most forest trees of the temperate and boreal biomes are dependant on ectomycorrhizae for their survival (Smith and Read, 2008). Ectomycorrhizal fungi (EMF) are symbionts that form an intimate association with the fine roots of trees and some woody shrubs. Increased nutrient uptake is generally considered to be the most beneficial effect of EMF on forest trees (Smith and Read, 2008), though EMF have also been shown to increase water uptake (Muhsin and Zwiazek, 2002), provide resistance to aluminum and other toxic metals (Smith and Read, 2008), and increase pathogen resistance (Buscot et al., 1992). EMF take up nitrogen from the soil and provide their host plant significant amounts of it; up to 80 % of total plant N uptake is from EMF (Hobbie and Colpaert 2003). They also take up P, Ca, Mg, and K from the soil and provide their host plant significant amounts of these nutrients (Hatch, 1937, Smith and Read, 2008, Jentschke et al. 2001). In return for this key role in plant nutrition the host plant transfers significant amounts of fixed carbon to their EMF. A recent review by Hobbie (2006) suggests an average of approximately 15% of total fixed carbon is allocated to ectomycorrhizae, but studies have found more than 60% of recent carbon assimilation (Rosling et al 2004) and net primary production (Godbold et al., 2006) may be allocated to EMF.
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In addition to their instrumental role in tree nutrition EMF are believed to be instrumental in forest biogeochemistry. They have a strong effect on processes that govern soil carbon residence time (Gadgil and Gadgil, 1971; Entry et al., 1991; Talbot et al., 2008) and have been the subject of much interest over the last 15 years of biogeochemistry research due to their ability to stimulate mineral weathering. A Web of Science citation search finds that since 1995, 74 articles (including 7 reviews) have been generated on the topic of ectomycorrhizal weathering. Many have found that EMF do stimulate weathering, and the proposed mechanisms include acidification (Balogh-‐Brunstad et al., 2008; Rosling et al., 2004), nutrient uptake (Wallander, 2000, van Hees et al., 2004), production of siderophores (Ochs et al., 1993; Watteau and Berthelin, 1994), and production of low molecular weigh organic acids (Paris et al., 1996, van Scholl et al. 2006a).
Numerous studies point to a significant biotic contribution to mineral weathering in forest soils from prokaryotic (Uroz et al., 2009), fungal saprotrophic (Rosling et al, 2007), mycorrhizal (Wallander, 2000), and plant (Drever, 1994) components of the biota. The relative importance of these different groups in mineral weathering as well as the effects of elevated CO2 on biotic weathering remain poorly understood.
Low molecular weight organic acids (LMWOA) are actively exuded by biota in response to nutrient demand (van Scholl et al., 2006b; Paris et al., 1996) and have been proposed to be key drivers of biotic weathering (van Hees at al., 2002; Stillings et al., 1996). Work by Fransson & Johansson (2009), and Johansson et al. (2010) suggests that LMWOA production by both plants and EMF may increase in response to elevated CO2, either as a result of source sink relationships within the plant caused by increased carbon fixation or as an active response to increased nutrient demand. Assessing the relative contributions of these microbial and plant components of forest biota to LMWOA production, the effects of CO2 on LMWOA production, and the effects of these LMWOA production levels on mineral weathering rates are necessary elements for a better understanding of the importance of biotic weathering.
Previous work by van Hees et al. (2006a, 2006b) demonstrated a slight increase in weathering rates and organic acid production in mesocosms with pine seedlings compared to those without seedlings but limited effects of ectomycorrhizae on weathering rates or organic acid production. In that study pine growth responded poorly to EMF due to low nutrient levels, EMF did not persist in one of the treatments, and overall weathering was very low because a highly weathered substrate was used which was likely well coated with resistant secondary mineral coatings.
Here we focus on the role of LMWOA’s in biological weathering by using a column microcosm system to elucidate the role of Scots pine seedlings, their associated EMF and the effects of elevated CO2 on weathering rates. In contrast to earlier work that used a similar system (van Hees et al., 2006a, 2006b) we have increased nutrient levels, used a rooting substrate derived from primary minerals, used different fungal symbionts, and incorporated an elevated CO2 treatment.
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METHODS Plant, mycorrhizal, and bacterial preculture Seeds of Pinus sylvestris were surface sterilized for 30 min in 33 % H2O2, rinsed in sterilized water and sown in sterile, autoclaved water agar (1.6 %) for 6-‐8 weeks for germination (300 µmol m-‐2 s-‐1 PPFD; 16H light at 18°C, 8 hours dark at 15°C).
Ectomycorrhizal and non-‐ectomycorrhizal seedlings were prepared in petri dishes of peat:vermiculite:Modified Melin-‐Norkrans media (1:4:2 v:v:v) as detailed in Fransson and Johansson (2009). The ectomycorrhizal fungal species Suillus variegatus (Sw.:Fr.) O. Kuntze (isolate code UP597, GenBank accession no. EF493256) and Piloderma fallax (Liberta) Stalpers (UP113, DQ179125), growing on half strength Modified Melin-‐Norkrans media (Marx, 1969) were used for EMF inoculum. After 12 weeks seedlings were removed from petri dishes and planted in 10 cm X 10 cm X 10 cm pots filled with a 1:10 sterilized, autoclaved peat:quartz sand mixture. These pots were grown under the same environmental conditions as detailed for seed germination for 6 months and watered 3 times weekly with ~ 20 ml of nutrient solution. The composition of this nutrient solution was 600 µmol NH4NO3, 140 µmol K2HP04, 150 µmol Ca(NO3) 2, 80 µmol K2SO4, 15 µmol H3BO3, 0.3 µmol Na2MoO4, 0.3 µmol ZnSO4, 0.3 µmol CuSO4, 50 µmol Mg(NO3)2.
In order to incorporate bacterial–ectomycorrhizal interactions which may be important to mineral weathering (Uroz et al., 2007; 2009) we inoculated each column with a fungus free bacterial suspension. This was obtained in April 2008 from E horizon soil and humus collected from a local Pinus sylvestris boreal forest (59.785N, 17.683E, Lunsen forest, Uppsala, Sweden). The collected soil was subjected to sequential centrifugation in winogradsky salt solution (Faegri et al. 1977), followed by nicodenz extraction at ultra high speed (Courtois et al. 2001). The resulting bacterial suspension was tested for the presence of culturable fungi by plating on potato dextrose agar, which yielded no observable fungal colonies. The bacterial suspension was used for inoculation within 2 days of extraction and was stored at 4-‐8°C in the meanwhile.
Soil column system and growth conditions A sand culture system nearly identical to that of van Hees et al. (2006a) was employed with opaque plexiglas tubes (diameter= 4 cm, height = 30 cm) serving as vertical growth columns. Each column was filled with 405 grams (dry weight) of a mineral mix (which mimicked the e-‐horizon of a local boreal forest soil) comprised of 50% quartz sand, 28% oligioclase, 18% microcline, 1.8% hornblende, 0.9% vermiculite, and 0.9% biotite. The quartz sand was acid washed (10% HCl w/v) overnight and washed with DI water until the solution pH was >6 before being mixed with the other ground minerals. The complete mix was approximately 40% silt size class (>71uM, <100um) and 60% sand size class (>100um, <500uM). The tubes were drained by applying suction at the base of the columns with a ceramic lysimeter cup (655X01, Soil Moisture Corp., Santa Barbara, CA) and column leachate was collected in opaque 250-‐ml glass bottles. When tested before planting, this drainage system maintained the mineral mix at a moisture content of 5±2%. Rhizon SMS-‐MOM suction lysimeter samplers (0.3cm diameter, 3 cm length; Rhizosphere Research Products, Wageningen) were inserted horizontally 10 cm below the soil surface
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for the purpose of extracting rhizosphere soil solution to measure LMWOA production. The columns were maintained at 20 ± 0.5°C during the “daytime” and 14-‐16°C
during the “nighttime” by the use of plexiglass chambers placed over the column system with peltier coolers used to regulate temperature. CO2 levels were maintained at 330-‐380 ppm (ambient) and 700-‐750 ppm (elevated) under each chamber. Light was supplied by a high-‐pressure sodium lamp with an intensity of 300 PPFD at the seedling tops (16h photoperiod). The columns were watered 3 times a week with 24-‐36 ml (volumes increased over the course of the experiment) nutrient solution (72-‐108 ml column-‐1 week-‐1). The watering solution contained 33 µmol (NH4) 2HPO4, 407 µmol NH4NO3, 27.5 µmol K2HP04, 55 µmol Ca(NO3) 2, 27.5 µmol K2SO4, 5.5 µmol H3BO3, 1 µmol FeCl3, 0.1 µmol Na2MoO4, 0.1 µmol ZnSO4, 0.1 µmol CuSO4, 55 µmol Mg(NO3)2. The pH was adjusted to 5.0. The molar ratio of N/K/P was 100:10:6, which is comparable to the optimal nutrient use efficiency values for conifers of 100:15:6, as determined by Ingestad (1979), except it is slightly depauperate in K. After addition of the mineral mixture to each column the columns were allowed to equilibrate with the nutrient solution for three weeks. In April 2008, one seedling was planted in each column (except for 8 non-‐planted controls). At this time 2 seedlings of each colonization treatment were dried for future analysis. Black plastic beads (diameter = 0.8 cm) were placed on top of the soil after planting to prevent the growth of algae. One week after planting, each column was inoculated with 5 ml of the fungus-‐free bacterial inoculum described above. Experimental Overview Treatments were factorial with +/-‐seedlings, +/-‐ EMF (2 species), and +/-‐CO2 (there was no treatment with ectomycorrhizae without seedlings). N=4 for the nonmycorrhizal and nonplanted treatments, and 5 for the EMF treatments; in total there were 36 columns, 28 of which were planted. The experiment was run for 9 months. Organic acid and phosphate concentrations were measured in rhizosphere soil solution, and elemental concentrations and pH were measured in column leachate collected during the experiment. Upon harvest, the cation exchange capacity (CEC) of the mineral mix pre and post 9 months of growth, the weight and elemental contents of seedlings, and the chitin contents of the roots, and mineral mix were assessed. This information was used to construct whole column nutrient budgets. Sampling during experiment Rhizosphere soil solution was extracted for low molecular weight organic acid (LMWOA) analysis by applying suction for up to 3h with a 50-‐ml plastic syringe to lysimeter samplers. LMWOA samples were collected five times from each column at 5, 6, 7, 8, and 9 months post planting, with sample volumes ranging from 2 to 12 ml. Sampling was performed 24–36 h after watering, and immediately frozen at -‐20oC for later analysis. Column leachate was sampled every 3-‐4 weeks. Total volume in each bottle was measured and 2 duplicate 15 ml aliquots from each bottle were sampled and frozen for future elemental analysis. Leachate was sampled a total of 11 times for each column.
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Harvest and sample analysis Upon harvest seedlings were removed from the mineral mix, and all possible adhering mineral particles were shaken/brushed off the roots under dry conditions. The last few remaining adhering mineral particles were washed off the roots with distilled water. Each seedling was separated into roots, stem, and needles and dried at 60oC for 24 -‐72 hours until no more mass loss was noted, and we then measured dry weights (DW). The plant DW is the sum of the three compartments and the root:shoot was calculated as root DW / (stem DW + needles DW). Seedlings sampled before planting into columns were given the same treatment.
The mineral mix from the columns was separated into three fractions: bottom, top, and rhizosphere. The bottom fraction consisted of the portion of the column below the extent of lowest roots in that column (the roots extended between 30% and 70% down the 30 cm height of the columns, so the relative ratio of top:bottom is variable), the rhizosphere fraction consisted of all of the soil which remained adhering to the roots when the roots were gently removed from the columns, and the top fraction was the remainder. Each fraction was weighed moist and a subsample of approximately 50 grams (moist weight) was collected and dried at 60°C for 24-‐72 h until no further mass loss was noted. Additional smaller subsamples were freeze-‐dried for chitin analysis. Low molecular weight organic acids (LMWOAs) were determined by capillary electrophoresis by the method of Dahlen et al. (2000). Briefly, LMWOA’s were analyzed on an Agilent 3DCE capillary electrophoresis system (Agilent technologies, Santa Clara). The concentrations of 12 different LMWOA’s were analyzed: acetate, butyrate, citrate, formate, fumarate, lactate, malate, malonate, oxalate, proprionate, succinate, and shikimate, as well as phosphate. To determine oxalate and citrate, EDTA (final concentration 250 mM at pH 9) was added in a separate run to eliminate interference from Al and Fe ions. LMWOA data is presented in µmol/L solution collected from rhizosphere lysimeters and as µmol/L/gram plant DW.
Acid digestion of plant material was undertaken following the procedure of Zarcinas et al. (1987) as follows: 0.1 gram of each seedling component (needles, stems, and roots, all pre-‐ground on a wiley mill) was separately digested at room temperature overnight in 2 ml concentrated HNO3 (10N), heated up to and refluxed at 130oC with a funnel lid for 5-‐7 hours and subsequently diluted with 12-‐15 ml deionized water.
Exchangeable ions were measured for each of the three post-‐harvest mineral fractions for each sample as well as for 9 replicates of the pre-‐experimental mineral mix. Extractions were performed in a 1:10 (m:V) mineral mix:1M NH4AOc suspension by shaking for 5 hours at 100 rpm at room temperature. The supernatant was separated by centrifugation (3000g) and filtered through a pre-‐washed 0.45 µm Na-‐Acetate filter syringe. Before cation exchange, the pre-‐experimental mineral mix was equilibrated with the experimental nutrient solution 3 separate times for 12 hours each (solution was exchanged between each rinse) to mimic the period that the columns were allowed to equilibrate for three weeks before planting.
Plant digests, CEC extracts, and column leachate were all analyzed for elemental contents of Al, Ca, Fe, K, Mg, Mn, Na, P, S, and Si on a Perkin-‐Elmer atomic optical emission
67
inductively coupled plasma emission spectrometer (AOE-‐ICP). A set of four standards was established based on preliminary analysis for each sample type. In addition to hourly rerunning of standards, duplicates and an internal scandium standard were run to ensure an accuracy of elemental contents to +/-‐ 1%. Elemental loss (µmol) though column leachate was calculated from the leachate concentration and the total volume of leachate. Plant roots and growth substrate were assayed for chitin content post harvest to assess fungal biomass. Chitin was extracted and analyzed by HPLC at the Department of Forest Ecology & Management, SLU (Sweden), according to the method in Ekblad and Näsholm (1996). Chitin concentration of each column fraction (top, rhizosphere, bottom, and roots) was multiplied by the mass of that fraction and these sums were added to obtain total chitin content per column. Significant quantities of chitin were not found in any of the bottom fraction samples. To relate fungal biomass to plant biomass the total chitin content (root + rhizosphere + mineral mix) was divided by the total plant biomass in each column. Construction of weathering budgets Elemental concentrations obtained from ICP analysis were used to back-‐calculate: 1. Change (final – initial) in NH4AOc-‐exchangeable elements (ΔCEC); 2. Final elemental content in plants – pre-‐experimental plant contents (seedling uptake, Su); 3. elemental drainage (total leachate, Lt). All values were computed in micromoles and total weathering (Wt) was represented by the following equation. Wt = ΔCEC + Su + Lt -‐ nutrients added Statistical analysis Except where explicitly stated all data are presented as the mean per column. LMWOA and chitin were also presented as mean per column per unit seedling mass. In this experiment we investigate 2 different independent variables: CO2 (ambient and elevated) and seedling treatment (non-‐planted, non-‐mycorrhizal, P. fallax, S. variegatus). If significant interaction effects were found they were indicated. Otherwise, when looking at the effects of seedling treatment (non-‐planted, non-‐mycorrhizal, P. fallax, S. variegatus) the two CO2 treatments were combined (ambient and elevated). When looking at the effects of CO2 the different seedling treatments (non-‐mycorrhizal, P. fallax, S. variegatus) were combined. When looking at the effect of CO2 on any growth or weathering parameters, only planted columns were considered. Statistical analysis was performed using JMP software version 5.01a (SAS Institute, Inc., Cary, NC, USA). Two-‐way ANOVA was used to determine treatment and interaction effects. Significant differences between CO2 treatments were assessed with a student T test, and significant differences between ectomycorrhizal treatments with Tukey’s HSD test, using a one-‐way ANOVA.
68
RESULTS Seedling Growth Elevated CO2 significantly increased the biomass of seedlings (p < 0.04), but had no significant effect on root:shoot. Mycorrhizal treatment had no significant effect on growth or root:shoot (table 1). There was no interactive effect of mycorrhizal treatment and CO2 on either biomass parameter.
Ambient CO2
Elevated CO2
Non-‐ mycorrhizal
Suillus variegatus
Piloderma fallax
Total Biomass (g) 3.84 a 4.49 b 4.69 a 4.05 a 3.88 a Root:Shoot 1.98 a 1.81 a 2.16 a 1.86 a 1.71 a
Table 1: Growth (mean seedling mass) and root:shoot (mean seedling root mass/mean seedling shoot mass) by treatment. Values paired with different letters are significantly different (p < 0.05) either between ambient and elevated CO2 (left half of table) or between non-‐mycorrhizal columns, and columns with seedlings colonized by either S. variegatus , or P. fallax (right half of table).
Mycorrhizal Colonization Upon harvest we observed highly variable rates of mycorrhizal colonization. We classified the seedlings as abundantly colonized (>50% of root tips colonized), moderately colonized (50% > colonization >5%), or sparsely colonized (<5% colonized). Of the 20 seedlings in the mycorrhizal treatments 6, 4, and 10 seedlings were abundantly, moderately, and sparsely colonized, respectively, at the time of harvest. While the root systems extended 12-‐21 cm down the 30 cm columns, no mycorrhizae were found deeper than 6 cm. There were no extraradical hyphae or ectomycorrhizae resembling Piloderma fallax or Suillus variegatus observed in the non-‐mycorrhizal treatment, but there were turgid, smooth, black root tips observed in the non-‐mycorrhizal treatments that may have been Thelephoroid mycorrhizae. Chitin analysis of roots and growing medium showed almost no chitin in our unplanted controls, and significant chitin in both mycorrhizal treatments (Figure 1) (p < 0.002). There were also significant amounts of chitin (~45% of what was found in the mycorrhizal treatments) in the non-‐mycorrhizal planted treatments. In both mycorrhizal treatments most chitin was found in the mineral mix, whereas in the non-‐mycorrhizal treament most chitin was found in the rhizosphere and roots. Combining the two mycorrhizal treatments we found moderately more chitin in the elevated CO2 treatment, 86 (± 17.8 S.E.) mg chitin/column vs. 50 (± 9.1 S.E.) mg chitin / column (p < 0.09). When the chitin content was expressed as total chitin content per gram seedling biomass (figure 2), we see that the mycorrhizal treatments did have significantly more chitin than the non-‐mycorrhizal treatment, and that elevated CO2 was associated with a higher chitin content in the Piloderma treatment.
69
Figure 1: Chitin contents (mean/column) by seedling treatment. Columns that do not share letters were significantly different (p < 0.05).
Figure 2: Chitin content (total/column) per unit seedling mass (mg/g) by seedling treatment and by CO2
treatment. Error bars are equivalent to two standard errors in length. Columns that do not share letters are significantly different (p < 0.05)
Low Molecular Weight Organic Acids Formic, lactic, and acetic acids made up the majority of measured low molecular weight organic acids (LMWOA), comprising 82%, 12%, and 4% of total LMWOA’s, respectively. Much smaller amounts of malonate, oxalate, fumarate, and succinate were occasionally detected, but their occurrence in measurable quantities was not associated with any treatment. Nonplanted columns had significantly lower LMWOA levels than planted columns (p < 0.001) while P. fallax columns had significantly higher LMWOA concentrations than nonplanted, but significantly lower levels than either the
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nonmycorrhizal or S. variegtus columns (p < 0.05). These differences were driven by differences in the production of formate and lactate (Figure 3A). Columns exposed to elevated CO2 produced significantly more total LMWOA’s (p < 0.01), and this difference was driven primarily by significantly greater formic acid production (Figure 3B). When amounts of LMWOA are calculated per gram seedling DW there are no significant differences between mycorrhizal or CO2 treatments (data not shown).
Figure 3: Effect of seedling treatment (A) and CO2 treatment (B: planted columns only) on concentrations of low molecular weight organic acids (LMWOA) in the column leachate (µmol/L). Bars that do not share letters are significantly different (p < 0.05). Capital letters atop each bar refer to total LMWOA.
pH of flow through Solution pH was measured for the leachate from 7 sampling dates. Leachate pH was consistently alkaline (pH 7.0-‐9.4). The nutrient solution used to water the columns was pH 5. There was no significant difference between elevated and ambient CO2 treatments nor between the non-‐mycorrhizal treatments and the mycorrhizal treatments in leachate pH, but the leachate of the non-‐planted controls was significantly lower (p < 0.001) in pH than the planted treatments on 5 of the 7 sampling dates (Figure 4) and moderately but not significantly lower on the other two sampling dates. The pH of the column leachate
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from the planted columns did not change appreciably over time, while the leachate from the unplanted columns decreased slightly over time (~0.4 pH units).
Figure 4: Difference in pH of leachate between planted and non-‐planted columns [planted (n=8) – nonplanted (n=28)] on 7 different sampling dates. Columns topped with an * represent significant differences (p <0.001).
Leachate elemental losses The losses of K, Ca, Mg, and Si were lower under elevated CO2, and higher for the non-‐planted treatment than for the planted treatments, except for Si, the leaching of which was lower for the non-‐planted control columns (table 2). The losses of K, Ca, Mg, and Si were all relatively steady over time (figure 5). Element Ambient
CO2 Elevated CO2
Not planted
Non-‐ mycorrhizal
Suillus variegatus
Piloderma fallax
Ca 467 b 370 a 588 b 431 a 417 a 410 a K 220 b 163 a 416 b 183 a 181 a 209 a
Mg 43.0 b 31.7 a 61.8 b 36.5 a 36.1 a 39.4 a Si 166 b 134 a 103 a 154 b 151 b 145 b
Table 2: Mean total elemental losses in column leachate (total µmol/ column). Values paired with different letters are significantly different (p < 0.05) either between planted columns with ambient and elevated CO2 (left half of table) or between non-‐planted columns, non-‐mycorrhizal columns, and columns with seedlings colonized by either S. variegatus, or P. fallax (right half of table).
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Figure 5: Cumulative elemental losses (mean µmol lost per column), averaged across all columns, in leachate over time.
Cation Exchange Capacity Overall, CO2 had no effect on ΔCEC. For Ca and Si, the presence of seedlings, but not of mycorrhizae, significantly increased ΔCEC, while mycorrhizae, but not seedlings alone, increased the ΔCEC for Mg and K (table 3). For some elements and treatments the CEC was considerably higher before the experiment than after, and thus, the ΔCEC (difference between before treatment and after treatment) is negative for these treatments (table 3). We see, in particular, a sharp decrease in CEC for calcium. The changes in CEC for Mg and K are also negative for some treatments. Element Ambient
CO2 Elevated CO2
Non planted
Non-‐ mycorrhizal
Suillus variegatus
Piloderma fallax
Ca -‐541 a -‐536 a -‐646 a -‐514 b -‐504 b -‐507 b K 10 b 17 b -‐44 a -‐4 a 50. b 38 b
Mg -‐24 a -‐22 a -‐59 a -‐36 a 2 b 10. b Si 118 a 180 a 38 a 141 b 120. b 222. b
Table 3: Change in cation exchange capacity ΔCEC (CECfinal – CECinitial: mean µmol/ column). Values paired with different letters are significantly different (p < 0.05) either between planted columns with ambient and elevated CO2 (left half of table) or between non-‐planted columns, non-‐mycorrhizal columns, and columns with seedlings colonized by either Suillus, or Piloderma (right half of table).
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days since planting
K
Si
Mg
Ca
73
Seedling elemental contents The needle concentrations of Ca, K, Mg, Fe, and P were all at or above deficiency thresholds for P. sylvestris (Breakke, 1994; Bargagli, 1998) (table 4), though P is near the lower limit of optimal growth. CO2 treatment had no effect on needle nutrient concentrations and mycorrhizal treatment only affected needle [Ca] (table 4). Neither CO2 level nor mycorrhizal treatment had a significant effect on total seedling (root + shoot + needle) contents of Ca, K, or Mg (table 5) despite significant differences in seedling biomass between CO2 treatments. The differences in seedling Ca and K contents between treatments correlated tightly with seedling DW, while the opposite trend was found with Mg. Ectomycorrhizal colonization, particularly with Piloderma, increased seedling Mg concentration. Element
(deficiency) Ambient CO2
Elevated CO2
Non-‐ mycorrhizal
Suillus variegatus
Piloderma fallax
Ca (0.4-‐2.0) 10.85 a 9.83 a 10.30 ab 12.02 a 8.69 b K (3.5-‐4.0) 4.97 a 4.97 a 4.82 a 5.02 a 5.05 a Mg (0.4-‐0.7) 1.68 a 1.77 a 1.68 a 1.74 a 1.75 a P (1.1-‐1.3) 1.39 a 1.28 a 1.25 a 1.35 a 1.40 a Fe (0.10) 0.12 a 0.11 a 0.10 a 0.12 a 0.12 a
Table 4: Elemental concentrations (mg/g dry mass) in P. sylvestris needles of major mineral derived nutrients. Values in parenthesis are the deficiency thresholds: ranges strong deficiency -‐ lower level of optimum growth (Breakke, 1994); single value for Fe represents lower level of optimum growth (Bargagli, 1998). Values paired with different letters are significantly different (p < 0.05) either between ambient and elevated CO2 (left half of table) or between non-‐mycorrhizal seedlings, and seedlings colonized by either S. variegatus, or P. fallax (right half of table).
Element
Ambient CO2
Elevated CO2
Non-‐ mycorrhizal
Suillus variegatus
Piloderma fallax
Ca (µmol/seedling) 651 a 757 a 776 a 740. a 611 a K (µmol/seedling) 381 a 432 a 445 a 388 a 395 a Si (µmol/seedling) 13.7 a 17.5 b 16 a 15 a 16 a Mg (µmol/seedling) 429 a 537 a 442 a 454 a 545 a Mg (umol /g dry mass of seedling)
112 a 120. a 95 a 110. ab 139 b
Table 5: Elemental contents (µmol) in P. sylvestris seedlings of Ca, K, Si, and Mg. Values paired with different letters are significantly different (p < 0.05) either between ambient and elevated CO2 (left half of table) or between non-‐mycorrhizal seedlings and seedlings colonized by either S. variegatus, or P. fallax (right half of table).
74
Weathering budgets Whole column weathering budgets for the 4 elements: Si, Ca, K, and Mg show no effect of CO2 on total weathering losses (table 6). The presence of seedlings, mycorrhizal or not, did significantly enhance weathering losses compared to non-‐planted controls (table 6). Mycorrhizal inoculation had no significant effect on the total weathering losses of any of the elements examined. More Si (23%) and Mg (27%) were weathered in columns planted with P. fallax inoculated seedlings, but these differences were not significant (table 6). The major pool of weathering losses for Mg, K, and Ca in the seedling treatments was uptake into the growing seedlings; losses of magnesium were particularly dominated by seedling uptake (figure 6B-‐D). Silica losses were fairly evenly distributed between ΔCEC and column leachate, while seedling uptake was negligible (figure 6A). As stated previously, ΔCEC was negative for calcium and slightly negative for some K and Mg treatments. When the nutrients added during watering were subtracted from the final budgets the net weathering losses of Ca, K, and Mg in the non-‐planted treatment are negative or only slightly positive (table 6), suggesting a “missing sink” for weathering products. Element Ambient
CO2 Elevated CO2
Non planted
Non-‐ mycorrhizal
Suillus variegatus
Piloderma fallax
Ca 416 a 438 a -‐246 a 505 b 465 b 326 b K 270 a 273 a 15 a 266 b 262 b 285 b
Mg 280 a 377 a -‐175 a 264 b 313 b 396 b Si 328 a 342 a 142 a 311 b 286 b 383 b
Table 6: Total weathering (µmol) losses for Ca, K, Mg, and Si (average/ column). Total weathering = Leachate + ΔCEC + Seedling uptake – irrigation additions. Values paired with different letters are significantly different (p < 0.05) either between planted columns with ambient and elevated CO2 (left half of table) or between non-‐planted columns, non-‐mycorrhizal columns, and columns with seedlings colonized by either S. variegatus, or P. fallax (right half of table).
75
Figure 6: Total elemental (µmol) losses for Si (A), Ca (B), Mg (C), and K (D) (average/column). NP= nonplanted columns. NM= nonmycorrhizal columns. P. fal. = columns with seedlings colonized by P. fallax. S. var. = columns with seedlings colonized by S. variegatus. Negative values for ΔCEC indicate that CEC decreased over the course of the experiment.
DISCUSSION Seedling and Ectomycorrhizal Growth Elevated CO2 increased the biomass of the Pinus sylvestris seedlings. Other studies on coniferous seedlings have generally found a growth stimulation with elevated CO2 (see reviews by Ceulemans and Mousseau, 1994; Norby et al., 2005; Hyvönen et al., 2006), and this trend includes several studies on mycorrhizal Pinus sylvestris seedlings (Gorissen and Kuyper, 2000; Alberton et al. 2007; Temperton et al., 2003), although some other studies have failed to find an effect of elevated CO2 on growth (Gorissen and Kuyper, 2000; Fransson and Johansson, 2009; Fransson et al., 2007). We found a slight (though not significant) reduction in root:shoot with elevated CO2. Most studies on ectomycorrhizal P. sylvestris seedlings which have noted a growth stimulation from elevated CO2 also found an increase in root:shoot (Gorissen and Kuyper, 2000; Janssens et al., 2005; Alberton et al.
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76
2007). A decrease in root:shoot does not necessarily indicate reduced belowground C allocation as carbon could be allocated to EM fungi rather than root biomass. If we use Ekblad and Nasholm’s (1996) estimate that 9.5% of fungal biomass is chitin, then our observed 21 mg average difference in chitin between elevated and ambient CO2 treatments would correspond to 220 mg more fungal biomass supported by seedlings in the elevated CO2 treatment. This difference would bring the root: shoot ratios of the two CO2 treatments to near parity. Additionally, a given amount of ectomycorrhizal biomass typically has a significantly higher respiration rate than the same mass of fine roots and thus represents more belowground carbon allocation (Heinonsalo et al., 2010; Rygiewicz and Andersen, 1994; Fitter, 1991) This same explanation for potentially higher belowground carbon allocation despite lower root:shoot applies to our findings of slightly lower root: shoot in the two mycorrhizal treatments vs. the nonmycorrhizal treatment.
Low molecular weight organic acid (LMWOA) production was strongly associated with seedling biomass across both CO2 and mycorrhizal treatments. While EMF are often mentioned in the literature to produce significant amounts of LMWOA’s, our findings seem to fall in line with the majority of studies examining the EMF role in LMWOA production which fail to find an increase in LMWOA production when comparing EMF and non-‐EMF seedlings (van Scholl et al., 2006; van Hees et al. 2005). However, many of these studies do find that EMF significantly alter the composition of LMWOA’s produced, particularly increasing oxalic acid concentrations (van Scholl, 2006; van Hees et al. 2006, Ahonen-‐Jonnarth, 2000), which we did not. Similar to Fransson and Johansson (2009), we did not find that elevated CO2 increased LMWOA production beyond its effect on seedling biomass.
Elevated CO2 was associated with significantly increased chitin content in seedlings colonized by Piloderma fallax, but not with seedlings colonized by Suillus variegatus; this increase was due to elevated chitin levels found in the growth matrix, not on the seedling roots. Increased mycorrhizal growth (increased colonization, fungal biomass, and hyphal growth proportionately greater than increases in root growth) is commonly found in elevated CO2 treatments (see reviews by Alberton et al., 2005 and Compant et al., 2010). Many studies show that this response is highly fungal-‐species-‐specific (Fransson et al, 2005; Fransson et al, 2007; Gorissen and Kuyper, 2000; Parrent and Vilgalys, 2007). It is interesting to note that Fransson and Johansson (2009), in which they assessed the effects of elevated CO2 on mycorrhizal growth of 5 ectomycorrhizal species, also found this strain of Piloderma fallax responded far more to elevated CO2 than any other fungal species examined.
Our finding of no increase (and a moderate though not significant reduction) in seedling biomass in the mycorrhizal seedlings is not uncommon. Despite the fact that Pinus sylvestris is considered obligately mycorrhizal, many studies have also found a growth depression of P. sylvestris with mycorrhizal colonization when mycorrhizal and non-‐mycorrhizal seedlings are compared (Stenström et al. 1990, Alberton et al. 2007, refs contained in review by Jones and Smith, 2004). Given the high needle nutrient concentrations in our non-‐mycorrhizal seedlings, and the very dense rooting we observed, it seems likely that high nutrient availability, and a very restricted rooting zone were the primary reasons we observed no growth stimulation by mycorrhizae
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The generally low levels of mycorrhizal colonization (only 6 out of 20 seedlings were intensely colonized) we observed were likely a result of nutrient levels being too high (we saw luxury consumption of every nutrient measured), or insufficient drainage (mycorrhizae were relegated to only the upper most portion of the roots, and upon harvest most of the growth medium appeared saturated).
The significant amounts of chitin observed in the non-‐mycorrhizal treatments may indicate either the presence of thelephoroid mycorrhizal contamination or saprotrophic fungi growing in the mineral mix. Our visual observations of shiny, turgid, smooth, black roots, and the fact that the majority of the chitin found in the non-‐mycorrhizal treatments was found in the roots and not in the mineral mix suggests some thelephoroid mycorrhizal contamination. The larger size (table 1) of the non-‐mycorrhizal seedlings and their lower chitin levels (figure 2) indicates that thelephoroid infection was minor.
Nutrient levels were sufficient for healthy balanced growth. Leachate concentrations of nutrient cations were steady (figure 5) suggesting that we increased the nutrient amounts sufficiently to keep up with the increasing nutrient demand of the growing seedlings. The fact that none of the needle nutrient concentrations were below sufficiency threshold (table 4) further suggests that none of the seedlings had severe nutrient deficiencies that could have compromised carbon allocation physiology.
Biotic Weathering Activity The overall negative weathering observed for some mineral nutrients suggests a missing sink somewhere in our weathering budget. The two most likely possibilities are the formation of secondary precipitates that were not extracted upon harvest or a large pulse of weathering in the initial 3 weeks of equilibration (in which flowthrough was not collected), before seedlings were planted. We used a chemical speciation and equilibria model Visual MINTEQ (Gustafsson, 2007) to determine if secondary precipitates may have formed. Given the makeup of LMWOA’s observed, the pH’s measured and the elemental concentrations in the drainage as input parameters the only compound likely to have precipitated would have been Ca-‐oxalate, but only in very small quantities <3 uM. This leaves the initial “equilibration flush” as the likely missing sink in our weathering budget. All the columns were treated equally before planting so this missing sink should not affect the merit or interpretation of our results.
Overall, and in every individual elemental flux (seedling uptake, CEC, leachate), seedlings had a significant effect on weathering. For the nutrient cations K, Mg, and Ca, extra weathering products were taken up by the seedlings, while for Si, which was not taken up in appreciable quantities (table 5, figure 6), extra weathering products were found on exchange sites in the mineral matrix. Mycorrhizal colonization did not significantly increase weathering rates or nutrient uptake, but seedlings colonized by Piloderma fallax did exhibit a trend toward increased weathering. More Si and Mg were mobilized in the P. fallax treatment (23% and 27% greater mobilization, respectively, than non-‐mycorrhizal treatments) despite the fact that P. fallax colonized seedlings were on average smaller, though these differences were not statistically significant. P. fallax colonized seedlings also had significantly higher Mg concentrations. Elevated CO2 had no effect on the weathering losses of Ca, K, or Si, but increased Mg losses (though not significantly). While seedlings grown in elevated CO2 did have higher plant and fungal
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biomass, and higher total seedling elemental contents, these increases in nutrient uptake were balanced by reduced leaching losses and not by enhanced mineral dissolution.
Soil biota are capable of stimulating weathering of alumina silicate minerals by four distinct mechanisms. Proton promotion: positively charged hydronium ions exuded by biota bind with partially charged negative surface sites on minerals, displacing cations from the mineral surface and destabilizing Si or Al on mineral surfaces, facilitating their release into solution. Plant growth is generally seen as a net acidifying phenomenon as a plant’s greater uptake of positively charged nutrient cations than negatively charged nutrient anions leads to a plant’s net exudation of protons. Ligand-promotion: an anion, either inorganic or organic, binds to mineral surface cations, again destabilizing the bond energy at the mineral surface stimulating release of surface cations and framework Si or Al. Removal of transport limitation: the removal of weathering products from the surface boundary layer via nutrient uptake or enhanced solution flow eliminates or reduces the constant readsorption of these mineral weathering products that occurs in concert with dissolution, enhancing net dissolution. Physical stimulation of weathering is also possible as hyphae and roots can fracture minerals, increasing the total exposed surface area. There is considerable debate as to which of these four mechanisms dominates the biotic influence on weathering (see reviews by Drever, 1994; Hinsinger et al., 2006; Lucas, 2001; Harley and Gilkes, 2000 and articles by Hinsinger et al., 2001; Bonneville et al., 2009).
Proton-‐promotion was not the primary mechanism of the enhanced weathering observed in columns planted with seedlings. The high pH of the column leachate (7.0-‐9.4) may not reflect the pH of the rooting zone because we measured leachate pH after 30 cm of vertical percolation through ground primary minerals, and ectomycorrhizae and most roots were restricted to the uppermost portions of the columns. However, the pH of the column leachate from planted treatments was significantly HIGHER than the pH of unplanted columns (figure 4), which is the opposite of what we would expect to find if biotic acidification was an important mechanism by which seedlings enhanced weathering. Thus, it seems acceptable to rule out proton exudation as the dominant mechanism through which the seedlings have enhanced weathering here.
Ligand-‐promoted dissolution may be an important mechanism of biotic weathering in our system, but this is not likely due to low molecular weight organic acids (LMWOA’s). The stimulatory effect of LMWOA’s on mineral dissolution is considered to significantly increase at near-‐neutral and slightly alkaline pH’s, as the relative rate of proton-‐promoted dissolution drops sharply (Welch and Ullman, 1993; van Hees et al., 2002; Stillings et al., 1996), but it is di-‐and tri-‐carboxylic acids such as citric and oxalic acids (measureable quantities of which were not consistently found in any columns) that have been shown to strongly increase weathering rates not the mono-‐carboxylic acids formic, lactic, and acetic acids (which comprised the majority of LMWOA’s in this experiment) (Neaman et al., 2006, Drever and Stillings, 1997). Additionally, the concentrations of the most weathering-‐enhancing organic acids required to significantly increase weathering are on the order of millimolar, not micromolar concentrations, as was the case in this experiment (Drever and Stillings, 1997; Drever, 1994, Pokrovsky et al., 2009). So, in this system, LMWOA’s were likely not a contributor to enhanced weathering in the planted columns. This does not mean that we can rule out ligand-‐
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promotion as a potential mechanism of biotic weathering. LMWOA’s represent one of a few possible weathering-‐promoting-‐ligands of biotic origin, and comprise generally well less than 10 % of dissolved organic carbon (see review by Strobel, 2001 and refs therein). Humic and fulvic acids comprise the majority of DOC, but these larger organic acids either have no effect on mineral dissolution or inhibit dissolution rather than promote it, particularly at near neutral pH’s (Pokrovsky et al., 2009; Ochs et al., 1993; Drever and Stillings, 1997). Conversely, organic compounds that are generally found in much lower concentrations than LMWOA’s may be key ligand-‐promoted weathering agents in the soil or in our planted columns. Siderophores can significantly increase weathering rates, with a potential multiplicative effect on organic acid promotion (Lierman et al., 2000; Reichard et al., 2007; Watteau and Berthelin, 1994) and have been found to be actively exuded by EMF (van Hees et al., 2006) and bacteria (Liermann et al., 2000). Rhizosphere (Lemanceau e t al., 2009; Robin et al., 2007) and mycorrhizosphere (Uroz et al. 2007) bacteria have been shown to stimulate mineral weathering more than bacteria from the bulk soil, and siderophore production may be a cause (Uroz et al. 2007; Uroz et al. 2009). Ochs et al. (1993) found that mycorrhizal exudates other than LMWOA’s significantly increased mineral weathering rates, and suggested that these unidentified agents, present in micromolar concentrations, were possibly siderophores. However, Ochs et al. (1993, 1996) found that axenic, non-‐mycorrhizal seedling exudates did not have a stimulatory effect on weathering. It is possible that rhizosphere bacteria, or ectomycorrhizae secreted sufficient siderophores to enhance mineral weathering. This could be why we see evidence of enhanced weathering activity by P. fallax colonized seedlings. However, needle Fe concentrations were at or near the lower range of optimum growth and did not vary significantly between treatments. Additionally, siderophores enhance weathering by binding with and destabilizing mineral-‐surface iron and aluminum atoms, and we did not find detectable levels of iron or aluminum in column leachate (< 0.1µM [Al] and [Fe] vs. > 50µM [Si], data not shown). Thus, siderophores were likely not a significant contributor to biotic weathering enhancement in our system.
Inorganic anions can strongly inhibit dissolution by forming complexes on mineral surfaces. While no measureable levels of phosphate were noted in column leachate, significant levels of phosphorous were noted in lysimeter samples collected for LMWOA analysis from the unplanted columns. Phosphate concentrations in nonplanted columns ranged from 6 to 30 µM and went up steadily over time while there were no measureable concentrations in the samples from planted controls, likely due to seedling uptake. Biber et al. (1994) found that maintaining a solution [phosphate] of 100 µM inhibited iron oxide dissolution by 90% by forming protective bi-‐dentate surface complexes on the mineral surface. The lack of phosphorous in the column leachate or CEC extracts from non-‐planted columns despite our having added it in the nutrient solution at a concentration of 60 µM suggests that strong phosphate-‐mineral complexes may have formed. Seedling uptake of nearly all available phosphate would have prevented the formation of these protective surface complexes, which could have enhanced weathering rates. However, a total of 200µM of phosphate was added to the system, and using the conservative estimates of surface area and mineral surface cation density, this amount of phosphate could not have protected more than of 1% of all mineral surfaces.
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Seedling uptake of nutrient cations and thus inhibition of cation readsorption to mineral surfaces stands out as the most likely mechanism of biotic weathering activity through which P. sylvestris caused elevated weathering. In order for the removal of transport limitation to weathering to be an important mechanism by which seedlings have enhanced mineral weathering, seedlings must significantly reduce the concentrations of weathering products around mineral surfaces and/or significantly alter solution flow rates. The average transpiration rate of seedlings was 23% (+/-‐ 2% S.E.). This is the amount of solution that flowed into the columns which was transpired by the seedlings and was calculated as follows:
(Leachatenp + MCnp) – (leachatep + MCp) total solution volume added to columns (3.4L)
Leachtenp = total solution volume (mean ml/column) collected from column flowthrough in nonplanted treatments Leachtep = total column flowthrough in planted treatments MCnp = total solution volume remaining in nonplanted columns (dry mineral mass X % moisture determined by drying) (mean ml/column) upon harvest MCp = total solution volume remaining in planted columns upon harvest The % uptake of each of the 4 weathering products studied was calculated as follows:
% Uptake = seedling uptake/ (fertilizer input + weathering – ΔCEC)
We subtract ΔCEC because we are specifically looking at uptake from solution. We find that at the root surface seedlings take up significantly less Si (9%+/-‐2% S.E.) and significantly more K (68% +/-‐3% S.E.), Ca (54% +/-‐3% S.E.), and particularly Mg (91% +/-‐3 % S.E.) than can be explained by solution concentrations and transpiration rate. By diverting 23% of column moisture and taking up 54-‐91% of the major nutrient cations, seedlings exerted considerable influence on both column hydrology and elemental concentration gradients through nutrient uptake.
Despite considerable debate as to the importance of transport-‐limitation, our findings are in line with a growing acceptance of the importance of biotic nutrient uptake to mineral weathering rates. Studies using batch reactions have consistently shown that surface detachment of Si and Al complexes, due to either proton-‐ or ligand-‐destabilization, so called “surface reaction control”, is the rate limiting step for mineral dissolution (Barman et al., 1992; Seyama et al., 2002). Reviews by Lasaga et al. (1994), Drever (1994), and Velbel et al. (1993) have also suggested that surface reaction control is the limiting factor for terrestrial mineral weathering rates. However, experiments using column reactors (not unlike ours) that are not saturated and well-‐mixed, as batch-‐reactors are, have shown that reduced transport of weathering products away from mineral surfaces reduces weathering rates by several orders of magnitude, and that weathering rates are directly proportional to flow rates (Evans and Banwart, 2006; van Grinsven and van Riemsdijk, 1991). These abiotic experiments and the results of our mesocosm experiment provide experimental evidence to support a recent comprehensive field study by White et
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al. (2009) as well as two more recent reviews (Maher, 2010; Harley and Gilkes, 2000) that have identified transport limitation as the predominant process governing weathering rates of minerals on the earth’s surface. By taking up nutrients and stimulating solution flow plant growth exerts a major influence on solution composition around mineral surfaces, stimulating weathering. While the actual weathering mechanism is likely ligand-‐ or proton-‐promotion, the weathering rate is controlled by nutrient uptake by plants. In a review of ectomycorrhizal weathering studies, Rosenstock (2010) also identified increased nutrient uptake as the most common mechanism by which ectomycorrhizae stimulate weathering. Despite what seems like growing consensus that the manner in which plants influence weathering rates is predominantly via nutrient and solution uptake, there is a rather distinct lack of mention of transport limitation to weathering in the literature on ectomycorrhizal weathering.
Relevance to Natural Systems Our system departs from natural forest soil conditions in a number of important ways, but our main findings are representative of forest ecosystem processes. The great majority of temperate and boreal forests, and other forests where ectomycorrhizae dominate are acidic soils, generally far below pH 6. Some generalizations have been made suggesting that across very broad pH gradients ectomycorrhizae dominate at increasingly acidic pH’s (Smith and Read, 2008). In these pH ranges proton-‐promotion and ligand-‐promotion are likely to be important components of the biotic influence on weathering. Furthermore, over longer time scales the total exposed surface area of primary minerals is likely to be greatly diminished. In forests that are close to steady state nutrient uptake via plant growth, may be offset by similar nutrient inputs from decomposing plant derived organic matter. Ectomycorrhizae enhance plant growth in natural settings, and the explorative growth patterns and small size of ectomycorrhizal hyphae enable ectomycorrhizal roots to come into contact with and draw from pockets of soil solution in contact with a much larger volume of soil than non-‐mycorrhizal roots. Over long time scales it is likely that the biota are the main physical force for breaking up bedrock into more weatherable smaller rocks (with the obvious exception of glaciation) (Gabet et al., 2003). Our system stands in some contrast to these conditions. We have very high pH due to the abundance of fresh mineral surfaces. Total organic content and organic acid concentrations are likely to be much lower than what would be found in natural soils. Additionally, the high rates of cation weathering we see are also reflective of our use of unweathered freshly ground primary minerals. Numerous weathering experiments have reported an initial early phase of non-‐stoichiometric weathering in which base cations are rapidly dissolved from mineral surfaces followed by the a more stable phase of stoichiometric weathering (Seyama et al., 2002; Malmström and Banwart, 1997). Stoichiometric weathering of our mineral mix would yield a ratio of base cations:Si of around 1:2 -‐ 1:3, depending on the relative contributions of different minerals, instead our base cation:Si was closer to 4:1. Lastly, EMF did not enhance growth in our system and the restricted container volume we employed, and the additional restriction of ectomycorrhizae to only the uppermost portion of these containers greatly decreased the ability of ectomycorrhizae to increase the relative rooting volume of their hosts. Despite our system’s inability to full represent natural soil conditions, our main findings that
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plants primarily stimulate weathering through their influence on soil hydrology and nutrient uptake, are applicable to plants in steady state ecosystems. It has been shown in whole ecosystem weathering models, that failing to account for nutrient uptake by the biota in forested ecosystems, even those near steady state in relation to growth and nutrient cycling, can reduce calculated weathering rates by as much as 80% (Benedetti et al., 1994; Moulton and Berner, 2000). While we found significant stimulation of P. sylvestris growth and ectomycorrhizal colonization with elevated CO2, the growth conditions of our plants are important to consider. In forests and natural soils plants are likely to be more limited by light or nutrients than in our system, and CO2 may have less of a growth-‐stimulatory effect (Körner, 2003).
Conclusions In our experiment seedlings significantly increased weathering rates, while elevated CO2 and ectomycorrhizal colonization had limited effects; Piloderma fallax showed some potential to enhance weathering rates, particularly with respect to Mg. The lack of a mycorrhizal effect may have been due to low mycorrhizal colonization or a lack of mycorrhizal growth promotion. Enhancement of weathering by seedlings was most likely caused by nutrient uptake through its influence on transport-‐limitation. Future studies investigating the weathering effects of ectomycorrhizae should be conducted in sufficiently large containers to allow mycorrhizae and roots to grow in an explorative unrestricted growth pattern. Caution is advised when relating results from ectomycorrhizal weathering experiments in containers with low organic content, primary minerals, and restricted volume to phenomenon occurring in natural forests. Studies on biotic weathering must separate biotic influences on weathering mechanisms (proton promotion, ligand-‐promotion) from biotic influences on limiting factors to weathering (ligand-‐inhibition, transport limitation).
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