Soil Carbon || Challenges for Soil Organic Carbon Research

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<ul><li><p>3A.E. Hartemink and K. McSweeney (eds.), Soil Carbon. Progress in Soil Science,DOI 10.1007/978-3-319-04084-4_1, Springer International Publishing Switzerland 2014</p><p> Abstract The soil body is the largest terrestrial pool of organic carbon globally. Lately, research related to soil organic carbon has been a main focus worldwide, motivated by the potential the soil inhabits to become a manageable sink for atmo-spheric carbon dioxide and thus to mitigate climate change and the known benefi ts of increased soil organic carbon for the functioning of soils. Here, challenges are highlighted for soil organic carbon research that we are currently facing. Knowledge on soil organic carbon dynamics in the soil system is briefl y reviewed, followed by an elaboration on how soil organic carbon dynamics and soil organic carbon stocks have been modelled in space and time and where modelling needs to go. </p><p> Chapter 1 Challenges for Soil Organic Carbon Research </p><p> Alex B. McBratney , Uta Stockmann , Denis A. Angers , Budiman Minasny , and Damien J. Field </p><p> A. B. McBratney (*) Faculty of Agriculture , Food &amp; Natural Resources, The University of Sydney , Australia e-mail: </p><p> U. Stockmann Soil Security Laboratory, Department of Environmental Sciences , Faculty of Agriculture and Environment, The University of Sydney , Sydney , NSW 2015 , Australia e-mail: </p><p> D. A. Angers Agriculture and Agri-Food Canada , 2560 Hochelaga Boulevard , Quebec City , QC , G1V 2J3 , Canada B. Minasny Soil Security Laboratory , Faculty of Agriculture and Environment, The University of Sydney , Sydney , NSW 2006 , Australia e-mail: </p><p> D. J. Field Department of Environmental Sciences, Faculty of Agriculture and Environment , The University of Sydney , Biomedical Building C81, Suite 401, 1 Central Avenue, Australian Technology Park , Eveleigh , NSW 2015 , Australia e-mail: </p></li><li><p>4 Keywords Soil organic carbon Soil carbon sequestration Soil function SOC saturation Soil organic carbon modelling Soil organic carbon mapping </p><p> Introduction </p><p> There is currently a wide and deep interest in the science of soil carbon, which motivated the IUSS Global Soil C conference and the consequent book. This is largely driven by the potential of soil carbon to mitigating climate change, i.e. soil as a potential sink for greenhouse gases (GHG); but also by the effect of enhanced or decreased carbon on soil function, i.e. SOC as one of the main indicators for the soils condition or quality. </p><p> This chapter briefl y documents the growth in interest, reviews the consequences of increased or decreased soil carbon concentrations and stocks, and then focuses on key scientifi c challenges around soil carbon that must be taken up to help meet societal demands for climate and food security into the foreseeable future. </p><p> Motivation Behind the Interest in SOC Globally </p><p> Over the years, the interest in soil organic carbon (SOC) has risen signifi cantly in the science community. The number of publications on SOC over time, for example, has increased steadily since the early 2000s (refer to Fig. 1.1 which shows the number of publications on SOC per year from 1960 to 2012 listed in the scientifi c search engines </p><p> Fig. 1.1 Number of papers on soil carbon each year over time listed in the scientifi c search engines Scopus and Web of Science (from 1960 to 2012). The lower amount of publications each year found in the Web of Science search engine is due to the selected consideration of journals which is based on the journals impact factors. In addition, conference papers and book chapters are not included here </p><p>A.B. McBratney et al.</p></li><li><p>5Scopus and Web of Science.). In broad terms, most of these publications tackle research related to the forms of SOC and their organisation in the soil architecture, the role of SOC in nutrient cycling, soil fertility and yield production as well as SOC dynamics in relation to climate change, management practices and modelling.</p><p> The steady increase in publications on SOC, however, is more or less due to two aspects. </p><p> Soil as a Potential Sink for Greenhouse Gases (GHG) </p><p> First , the increase stems from the attention that SOC has received globally due to its potential to play a role in mitigating GHG through Soil Carbon Sequestration (SCS). SCS can be defi ned as the net removal of atmospheric carbon dioxide (CO 2 ), i.e. the transfer into long-lived pools such as soil organic matter (SOM), so that the build-up of CO 2 concentration in the atmosphere is reduced or slowed. As such, SCS can be a recognised offset in a market-based carbon pollution reduction scheme (Lal 2004 ). This is mainly owing to the size of the soil C pool (estimated at 2,344 Gt of OC) compared to the terrestrial biotic C pool (estimated at 560 Gt of OC) (Jobbgy and Jackson 2000 ). In addition, this is due to anthropogenic infl uences mainly management induced changes in C on agricultural land that have resulted in an estimated loss of SOC of up to 78 Gt per annum (Lal 2004 ). The soil body may therefore have signifi cant C storage potential. This possibility is demonstrated in Fig. 1.2 which shows the decrease in C storage with time, induced by the establishment </p><p> Fig. 1.2 An example of changes in C storage with time since land clearance, induced by management practices (Cotton production, Lower Namoi Valley, Australia) (Based on Minasny et al. 2006 , used with permission ) </p><p>1 Challenges for Soil Organic Carbon Research</p></li><li><p>6of irrigated cotton production in the Lower Namoi Valley, Australia, and subsequently highlights the maximum C storage potential of the local soil to 30 cm (~1.51.0 dag kg 1 which represents the original average value of SOC under native vegetation). Studies such as this consider trends in C storage with time, starting with a reference value, employing long-term fi eld experiments as well as soil legacy data. These are necessary to help with the assessment of maximum C storage potential for a range of soil types and agricultural regions. On the other hand, rates of SCS have been measured globally for principal biomes by adoption of best management practices and have been synthesized by Stockmann et al. ( 2013 , Table 8, p. 93). Based on this selection of actual measured SCS rates, we can assume a rounded average of half a tonne of carbon per hectare per year can be sequestered globally under best manage-ment cropping practice.</p><p> Soil Function </p><p> Secondly , the interest in SOC stems from the known benefi ts of SOC for soil fertility, plant growth and production (Janzen 2006 ). SOC also plays an important role in soil and water conservation. SOC is associated with aggregate stability which in turn deter-mines erodibility. Increased SOC has also been linked to increased available water holding capacity (AWC) which may provide greater landscape resilience under climate change. Moreover, SOC is known to increase the soils nutrient cycling capability. </p><p> The notion that higher SOC contents result in an improvement of the soils condition has been discussed widely within the soil quality concept (Andrews et al. 2004 ) and has been addressed more recently as part of the Soil Security framework ( Bouma and McBratney 2013 ). In both concepts, soil C is considered one of the key indicators of soil function (e.g. in regards to agricultural production or mitigation of climate change). </p><p> Scientifi c Challenge 1: Critical Levels of SOC and the Concept of SOC Saturation </p><p> Together with the benefi ts of SOC for a soil to function, the existence of a critical texture-dependent concentration of SOC as well as the possibility of SOC saturation in agricultural soil, has been widely discussed (Hassink 1997 ; Six et al. 2002 ). A critical concentration of SOC is referring to a SOC level below which a soils function is reduced signifi cantly, whereas the concept of SOC saturation relates to the limiting capacity of a soil to accumulate carbon in dependence on the amount of fi ne particles in the soil (e.g. clay and the fi ne silt content). </p><p> Critical Levels of SOC for Soil Function </p><p> Zvomuya et al. ( 2008 ), for example, identifi ed total SOC content as one of the key soil quality indicators associated with differing yield of spring wheat in southern </p><p>A.B. McBratney et al.</p></li><li><p>7Canada. A critical concentration of 20 g SOC kg 1 (2 % SOC) was found fi tting a broken-stick regression model above which no change in yield response was expected and below which the productivity of the soil was reduced notably (Fig. 1.3 ). Similarly, Yan et al. ( 2000 ) identifi ed a critical level of 1.76 % SOC above which soil microbial diversity remained constant and did not increase further. Stockmann et al. ( 2013 ) proposed a notional critical as well as saturation curve of SOC accumu-lation as a function of soil texture as well as climate, soil material and topographic position (Fig. 1.4 ) which is intended to be used as a conceptual guide for land managers to identify critical concentrations of SOC.</p><p> Fig. 1.3 Critical concentration of SOC in relation to biomass yield response an example from Lethbridge, Alberta, Canada (Zvomuya et al. 2008 , used with permission) </p><p> Fig. 1.4 Notional saturation, achievable and critical curve of SOC accumulation as a function of soil texture. Here, k is a site- or region-specifi c proportionality constant that depends on a variety of interacting factors (e.g. climate, soil material and topographic position) (Stockmann et al. 2013 , used with permission) </p><p>1 Challenges for Soil Organic Carbon Research</p></li><li><p>8 SOC Saturation Concept </p><p> Angers et al. ( 2011 ) estimated the soil carbon saturation defi cit for French agricultural soils which was assumed to be largely dependent on the amount and type of the soils fi ne fractions (clay plus fi ne silt content). Here, a saturation defi cit represents C that may be stored in the soil in a stable form and represents the difference between the theoretical SOC saturation value (Hassink 1997 ) and the present SOC content. This study found that most of the agricultural soils in France are below their SOC satura-tion potential. We used this data set to fi t an upper boundary level to the distribution (Shatar and McBratney 2004 ), which represents the maximum SOC storage potential in dependence on the soils clay and fi ne silt content (Fig. 1.5 ).</p><p> Scientifi c Challenge 2: Infl uence of Management Practices on SOC Storage </p><p> Over decades, research has clearly shown that agricultural management practices signifi cantly infl uence the soil C content and associated soil properties. A study comparing bio-dynamic to conventionally managed dairy pastures, for example, demonstrated the vital relationship of SOC and soil structure and its dependence on management strategies (Lytton-Hitchins et al. 1994 ). Results showed that 18 years of bio-dynamic farming resulted in more favourable physical and chemical soil conditions with greater macro-porosity, smaller bulk density values and a larger soil organic matter content (Fig. 1.6 ). However, we must note here that even so organic farming does result in an increase of SOC at a local scale; it does not necessarily </p><p> Fig. 1.5 Fit of the upper boundary line (95 % confi dence level) for the data set of French agricultural soils (Based on Angers et al. 2011 , used with permission from Unit Infosol INRA, Orlans) </p><p>A.B. McBratney et al.</p></li><li><p>9translate to SCS, i.e. a net removal of CO 2 from the atmosphere (Leifeld et al. 2013 ). Often it is simply a transfer of one terrestrial C pool to another and has no impact on climate change per se .</p><p> In general, research suggests that the adoption of best management practices will increase the amount of SOC accumulated in agricultural soil (Lal 2008 ). However, the infl uence of management practices on crop residue and SOC decom-position as well as the distribution with depth is still poorly understood and under discussion. Meta-data analysis, for example, has shown that the SOC content in the surface layers increased under no-till (NT) when compared to full-inversion tillage </p><p> Fig. 1.6 An example of management effects on SOC content and soil structure. Conventionally managed versus bio-dynamically managed dairy pastures (Redrawn, based on Lytton-Hitchins et al. 1994 ) </p><p>1 Challenges for Soil Organic Carbon Research</p></li><li><p>10</p><p>(FIT) and that this tendency increases with time (Angers and Eriksen-Hamel 2008 ; Luo et al. 2010 ; Virto et al. 2012 ). Limiting effects of NT on SOC stocks have been found when the whole soil profi le is considered (Angers and Eriksen-Hamel 2008 ; Luo et al. 2010 ). Cases have also been published where NT does not increase SOC contents (e.g. Loke et al. 2012 ) or where NT resulted in SOC increase at great depth when NT has been used with legume cover crops (e.g. Boddey et al. 2010 ). </p><p> Scientifi c Challenge 3: The Nature of Soil Carbon </p><p> Soil organic C has been described as a continuum of materials in varying states of decomposition with differing residence times (Table 1.1 : Forms of (soil) organic C presented in the scientifi c literature) that are located within the soils architecture (Jastrow and Miller 1998 ). Within the soil matrix, SOC is stabilized through physico-chemical processes, i.e. SOC is protected within aggregates which translates to its spatial inaccessibility for the soil microbes and limited oxygen availability. In addition, SOC is stabilized within the soils architecture through interactions with mineral surfaces as well as metal ions (Six et al. 2004 ). These stabilization processes can provide a priori limits to the soils capacity to store or sequester C which has been discussed previously.</p><p> Traditionally, SOC dynamics have been explained by the Humus Concept where composition is assumed to be at least partly controlled by the molecular structure of C inputs. Chemical extractions (alkaline/acid) result in unextractable residues with complex polymeric macrostructures that are believed to be inherently chemically stable (humins and humic acids). However, chemical recalcitrance of SOC as </p><p> Table 1.1 Forms of (soil) organic carbon found in the scientifi c literature (Adapted from Baldock 2007 ) Form Composition Pool category Surface plant residue Plant material residing on the surface </p><p>of the soil, including leaf litter and crop/pasture material </p><p> Fast (or labile) pool Decomposition occurs at a </p><p>timescale of days to years Buried plant residue Plant ma...</p></li></ul>


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