Forest soils and carbon sequestration

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Forest soils and carbon sequestrationR. Lal *Carbon Management and Sequestration Center, OARDC/FAES, The Ohio State University, Columbus, OH 43210, USAAbstractSoils in equilibrium with a natural forest ecosystem have high carbon (C) density. The ratio of soil:vegetation C densityincreases with latitude. Land use change, particularly conversion to agricultural ecosystems, depletes the soil C stock. Thus,degraded agricultural soils have lower soil organic carbon (SOC) stock than their potential capacity. Consequently, afforestationof agricultural soils and management of forest plantations can enhance SOC stock through C sequestration. The rate of SOCsequestration, and the magnitude and quality of soil C stock depend on the complex interaction between climate, soils, treespecies and management, and chemical composition of the litter as determined by the dominant tree species. Increasingproduction of forest biomass per se may not necessarily increase the SOC stocks. Fire, natural or managed, is an importantperturbation that can affect soil C stock for a long period after the event. The soil C stock can be greatly enhanced by a careful sitepreparation, adequate soil drainage, growing species with a high NPP, applying N and micronutrients (Fe) as fertilizers orbiosolids, and conserving soil and water resources. Climate change may also stimulate forest growth by enhancing availability ofmineral N and through the CO2 fertilization effect, which may partly compensate release of soil C in response to warming. Thereare significant advances in measurement of soil C stock and fluxes, and scaling of C stock from pedon/plot scale to regional andnational scales. Soil C sequestration in boreal and temperate forests may be an important strategy to ameliorate changes inatmospheric chemistry.# 2005 Elsevier B.V. All rights reserved.Keywords: Soil organic carbon; Sequestration; Carbon cycle; Climate change; Forest Ecology and Management 220 (2005) 2422581. IntroductionCarbon (C) storage in forest ecosystems involvesnumerous components including biomass C and soil C(Fig. 1). The total ecosystem C stock is large and indynamic equilibrium with its environment. Because ofthe large areas involved at regional/global scale, forestsoils play an important role in the global C cycle* Tel.: +1 614 292 9069; fax: +1 614 292 7432.E-mail address:$ see front matter # 2005 Elsevier B.V. All rights reserveddoi:10.1016/j.foreco.2005.08.015(Detwiler and Hall, 1988; Bouwman and Leemans,1995; Richter et al., 1995; Sedjo, 1992; Jabaggy andJackson, 2000). Land use change causes perturbationof the ecosystem and can influence the C stocks andfluxes. In particular, conversion of forest to agricul-tural ecosystems affects several soil properties butespecially soil organic carbon (SOC) concentrationand stock. The conversion to an agricultural land useinvariably results in the depletion of SOC stock by2050% (Schlesinger, 1985; Post and Mann, 1990;Davidson and Ackerman, 1993). The depletion of.R. Lal / Forest Ecology and Management 220 (2005) 242258 243Fig. 1. Components of the terrestrial carbon stock.Table 1Carbon stock in selected biomes of the world (recalculated fromAdams et al., 1990; Eswaran et al., 2000; Dixon et al., 1994; Malhiet al., 1999)Biome Area(Mha)C density (Mg/ha) C stock (Pg)Vegetation Soil Vegetation SoilTundra 927 9 105 8 97Boreal/Taiga 1372 64 343 88 471Temperate 1038 57 96 59 100Tropical 1755 121 123 212 216Wetlands 280 20 723 6 202Total 5672 Mean 54 Mean 189 373 1086SOC stock is attributed to numerous factors including:decrease in the amount of biomass (above- and below-ground) returned to the soil, change in soil moistureand temperature regimes which accentuate the rate ofdecomposition of organic matter, high decomposa-bility of crop residues due to differences in C:N ratioand lignin content, tillage-induced perturbations,decrease in soil aggregation and reduction in physicalprotection of the soil organic matter, and increase insoil erosion. Thus, agricultural soils and especiallyeroded agricultural soils usually contain lower SOCstock than their potential capacity. Afforestation ofagricultural land can reverse some of the degradationprocesses and cause enhancement or sequestration ofSOC stock (Ross et al., 2002; Silver et al., 2000).Interest in the ability of forest soils to sequesteratmospheric CO2 derived from fossil fuel combustionhas increased because of the threat of projectedclimate change. Thus, understanding the mechanismsand factors of SOC dynamics in forest soils isimportant to identifying and enhancing natural sinksfor C sequestration to mitigate the climate change.This manuscript synthesizes available informationon forest soils as a sink for atmospheric CO2, andidentifies the management options that may enhancethe capacity for C capture/storage in forest soils. Themanuscript focuses on SOC stock in forest soils andfactors affecting its dynamics, assesses the role offorest management on SOC sequestration, outlinessoil sampling and analytical procedures and modelingoptions, describes the likely effects of projectedclimate change on SOC stock in forest soils, andoutlines challenges of achieving the potential. Ratherthan being a comprehensive literature review, thefocus is to highlight the principle issues andopportunities and build upon previous reviews onthe topic (e.g. Bouwman, 1990; McFee and Kelly,1995; Lal, 2001; Kimble et al., 2003).R. Lal / Forest Ecology and Management 220 (2005) 242258244Table 2World forest area and area change (adapted from FAO, 2003)Region Land area (Mha) Forest cover (Mha) Mangrove forest (Mha)Area in 2000 Plantations Change Area in 2000 Annual change (%/year)Africa 2978.4 649.9 8.0 0.526 3.35 0.3Asia 3084.7 547.8 115.9 0.036 5.83 1.2Europe 2260.0 1039.3 32.0 +0.88 North and Central America 2137.0 549.3 17.5 0.57 1.97 1.4Oceania 849.1 197.6 2.8 0.37 1.53 1.0South America 1754.7 885.6 10.5 3.71 1.97 1.0World 13063.9 3869.5 186.7 9.39 14.65 1.02. World forestsForests ecosystems covering about 4.1 billionhectares globally (Dixon and Wisniewski, 1995) area major reserve of terrestrial C stock. There are threeprinciple forest biomes: boreal, temperate and tropical(Table 1). The boreal or taiga forest occupies acircumpolar belt. Temperate forests cover mid-latitudes between 25 and 508 north and south of theEquator, and comprise both evergreen and deciduousspecies. Tropical forests occur about 258 north andsouth of the Equator, and comprise both evergreen anddeciduous species. Predominant types of tropicalforests include lowland rainforest, montane forestsand mangrove forests.The geographical distribution of the worlds forestsindicates large areas in South America, Asia, Europe,and North and Central America and Africa (Table 2).Worldwide, forest cover is decreasing at the net rate ofabout 9.4 Mha/year mostly due to deforestation of thetropical rainforest (TRF) in Brazil, Sumatra and Westand Central Africa. The deforestation and conversionof TRF into agricultural ecosystems results inTable 3Estimates of terrestrial carbon stock in worlds forest zones (Pre-ntice, 2001)Biome Area(Mha)Terrestrial carbonstock (Pg)Carbondensity(Mg C/ha)Plants Soil Total Plants SoilTropical forests 1.76 340 213 553 157 122Temperate forests 1.04 139 153 292 96 122Boreal forests 1.37 57 338 395 53 296Total 4.17 536 704 1240 emission of 1.61.7 Pg C/year into the atmosphere(IPCC, 2000).3. Carbon stock in forest ecosystemsForest vegetation and soils contain about 1240 Pg ofC (Dixon et al., 1994), and the C stock varies widelyamong latitudes. Of the total terrestrial C stock in forestbiomes, 37% is in low latitude forests, 14% in mid-latitudes and 49% in high latitudes. The above-groundplant C density increases with decreasing latitude fromtundra to tropical rainforest (Fisher, 1995).Typical plantCdensityrangesfrom40to60 Mg C/hainborealforests,60 to 130 Mg C/ha in temperate forests and 120 to194 Mg C/ha in tropical forests (Table 3), with the Cdensity of an undisturbed TRF as high as 250 Mg C/ha.Nonetheless, asmuchas two-thirdsof the terrestrialC inforest ecosystems is contained in soils (Dixon et al.,1994).ThesoilCstockmaycompriseasmuchas85%ofthe terrestrial C stock in the boreal forest, 60% intemperate forests and 50% in TRF (Dixon et al., 1994).The ratioof soil:plantC stockmay range from3 to17 forhigh latitude, 1.2 to 3 for mid-latitude and 0.9 to 1.2 forlowlatitude(Table4).Alargepartofthetotalsoilorganiccarbon stock occurs in soils of tundra, pre-tundra andtaiga regions. Ping et al. (1997) reported that density ofSOC in selected pedons of Alaska ranged from 162 to1292 Mg C/ha. The SOC stock was 692 Mg C/ha in anarctic coastal marsh pedon, 314599 Mg C/ha in anarctic tundra pedon, and nearly 1300 Mg C/ha in anorganic soil. Boreal forests also have been documen-ted to contain large quantities of soil C (Bonan andVan Cleve, 1992; Rapalee et al., 1998). In the borealforest ecosystem of West Alberta, Canada, BanfieldR. Lal / Forest Ecology and Management 220 (2005) 242258 245Table 4Relative abundance of vegetation and soil carbon density in forests of different ecosystems (adapted from Dixon et al., 1994)Latitude Region (country) Soil C:vegetation C Carbon flux (Pg C/year)High Russia 3.38 +0.30 to +0.50Canada 17.29 +0.08Alaska 5.44 +0.48 0.1Mean 8.70 7.50Mid USA 1.74 +0.10 to +0.25Europe 2.81 +0.09 to +0.12China 1.19 0.02Australia 1.84 TracesMean 1.90 0.67 +0.26 0.09Low Asia 1.05 0.50 to 0.90Africa 1.21 0.25 to 0.45Americas 0.92 0.50 to 0.70Mean 1.06 0.15 1.65 0.40Net effect 0.9 0.40et al. (2002) reported that the regional average above-ground C stock ranged from 43 to 50 Mg C/hacompared with SOC stock of 83156 Mg C/ha.Kolchugina and Vinson (1993) estimated that soilsof the forest biome of the former Soviet Union (FSU)contained 319 Pg C. They calculated that C stock inthe phytomass of the FSU forest biome was 128 Pg,and together the terrestrial C stock (phytomass plussoil) accounted for 16% of the terrestrial C stock of theworld. Stolbovoi and Stocks (2002) estimated thatsoils of Russia contain 373 Pg of SOC and 75 Pg ofsoil inorganic carbon (SIC) in the 20 cm layer. Inaddition to differences in the total amount, soils of thenorthern latitudes (arctic and boreal biomes) may alsocontain a large proportion of labile C (1040%) thatTable 5Impact of projected global warming on soil organic carbon stock based oBiome LocationBoreal forests CanadaWorld forest GlobalTundra and boreal forests GlobalMid-latitude forest Massachusetts, USAArctic and boreal Fairbanks, AlaskaMid-latitude Norfolk, VirginiaWorld forests GlobalWorld forests GlobalWorld forests Globalcould be easily decomposed (Neff and Hooper, 2002).In contrast to high latitudes, soils of TRF arecharacterized by a low SOC density. For example,tropical soils of Costa Rica contain only 50140 Mg C/ha (Powers and Schlesinger, 2002)(Table 5).The SOC concentration in forest soils mayrange from 0% in very young soils to as much as50% (w/w) in some organic or wetland soils (Trettinand Jurgensen, 2003), with most soils containingbetween 0.3 and 11.5% in the surface 20 cm of mine-ral soil (Perry, 1994). Similar to SOCdensity, SOC con-centration in mineral soils is lower in tropical forestsand higher in montane and boreal forests (Jones,1989). Whereas deforestation of TRF emits 1.61.7 Pgn modeling soil C dynamicsChange in SOC stock ReferenceDecrease Bonan and Van Cleve (1992)Decrease Ceulemans et al. (1999)Decrease Wang and Polglase (1995)Decrease Melillo et al. (2002)Decrease Neff and Hooper (2002)Increase Dilustro et al. (2002)Decrease Nisbet (2002)Decrease Morgan et al. (2001)Uncertain Zak et al. (2000)R. Lal / Forest Ecology and Management 220 (2005) 242258246C/year into the atmosphere (about 20% of the anth-ropogenic emissions) (Watson et al., 1995; IPCC,2000), temperate and boreal forests are net sink ofatmospheric CO2 (Tans et al., 1990). Dixon et al. (1994)estimated that temperate forests of high and mid-latitudes absorb 0.7 0.2 Pg C/year (Table 4).4. Factors affecting the forest soils carbonconcentration and stockA variety of factors will affect the amount andconcentration of SOC in forest soils. For example,climate has a pronounced effect on SOC concentration.Important climatic factors include precipitation, poten-tial evapotranspiration (PET) and the ratio betweenPETand annual precipitation also known as the PET ratio.For any given rate of annual precipitation, SOC storageincreaseswith a decreasing PETratio (Post et al., 1982).In addition, there are numerous other soil and landscapefactors that also affect SOCstockwithin forests (Wilcoxet al., 2002). Prichard et al. (2000) observed a strongeffect of slope and aspect on the SOC stock of a sub-alpine forest in the Olympic Mountains of Washingtonstate. The SOC concentration was relatively higher onthe northeastern slopes, ranging from 43 to 143 g/kg,than in southwestern slopes ranging from 27 to 162 g/kg. The SOC stock, especially in soils of high latitude isalso influenced by permafrost dynamics and drainage(Hobbie et al., 2000). Landscape position can impactSOC stock because of its influence on soil water regime(Gulledge and Schimel, 2000). The SOC stock alsodepends on cation exchange properties (Chandler,1939), soil texture and aggregation. Borchers and Perry(1992) observed that in comparison to silt loam andsandy loam soils, coarser soils had lower total SOCconcentration. In West Alberta, Canada, Banfield et al.(2002) also observed an exponential relationshipbetween soil texture and biomass C, and the latter isalso related to SOC stock. For volcanic soils in CostaRica, Powers and Schlesinger (2002) observed thatSOC concentration was positively correlated to theamount of non-crystalline clays (e.g. allophane,imogolite and ferrihydrite) in the high elevation soils,and also positively correlated to aluminum in organo-metal complexes in the low elevation sites.The forest soil C stock is affected by both naturaland anthropogenic factors (Larionova et al., 2002). Anatural disturbance can be a destructive event withdrastic perturbation of an ecosystem, such as wind,fire, drought, insects and diseases. Severe naturaldisturbance is followed by changes in soil moistureand temperature regimes, and succession of forestspecies with differences in quantity and quality ofbiomass returned to the soil. The impact of naturaldisturbances on SOC stock has been described byOverby et al. (2003). Fire and other naturaldisturbances may also change the canopy cover, andthereby affect soil erosion (Elliot, 2003), which alsoaffects SOC stock of the surface layer.Anthropogenic factors, which may affect SOC inforests, include forest management activities, defor-estation, afforestation of agricultural soils and sub-sequent management of forest plantations. Althoughforestland management is generally less intensive thancropland management, there are several managementoptions that may enhance or increase SOC stock inforests. Management systems that maintain a con-tinuous canopy cover and mimic regular natural forestdisturbance are likely to achieve the best combinationof high wood yield and C storage (Thornley andCannell, 2000). Management activities that mayimpact the SOC stock include harvesting and sitepreparation, soil drainage and planting of adaptedspecies with high NPP and more below-groundbiomass production, fertilization and liming (Hoover,2003). Because management strategies may differ forboreal forests (Hom, 2003), high elevation forests(Bockheim, 2003) and arid and semi-arid forestecosystems (Neary et al., 2003), the intensity of effectsmay also vary among forest types. Finally, manage-ment activities can influence the labile fraction of theSOC stock (Ellert and Gregorich, 1995), and affectsoil quality and productivity (Chandler, 1939;Henderson, 1995).4.1. Forest harvesting and soil carbon stockThe most common forest management activitiesare harvesting and site preparation. Because the forestfloor comprises the most dynamic part of SOC stock,estimating the effects of these activities on SOCdynamics are critical to predicting the local effects onecosystem sustainability and global C exchange withthe atmosphere (Yanai et al., 2003). The so-calledCovington Curve, which described SOC dynamicsR. Lal / Forest Ecology and Management 220 (2005) 242258 247Fig. 2. A schematic showing the effects of timber harvest and logging on soil organic carbon stock (a) with soil disturbance and mixing of litterlayer and mineral soil and (b) without soil disturbance and adoption of improved management practices.following forest harvesting (Covington, 1981), statesthat SOC stock declines sharply following harvest,with as much as 50% of SOC lost within first 20 yearsor more (Fig. 2a, Bouwman, 1990; Johnson, 1992;Davidson and Ackerman, 1993). The loss of SOCstock was attributed to decreased litter input, shifts inabundance of woody and herbaceous vegetation,changes in depth distribution of plant roots, alteredsoil water and temperature regimes which acceleratedecomposition, and a decrease in NPP (Covington,1981; Johnson et al., 1995; Jackson et al., 2000).Knoepp and Swank (1997) studied the SOC dynamicsin five watersheds in the southern Appalachian regionand compared their results to the Covington model.They reported that the SOC and N concentrationsgenerally declined during the first year following thewhole tree harvest, but SOC remained stable 14 yearsafter cutting. In California, Black and Harden (1995)also observed that timber harvest resulted in an initialloss of SOC (15%) within 17 years due to oxidationand erosion. For 17 years of forest re-growth, therewas a continued loss of SOC (another 15%) despite theslight accumulation of new litter and roots. After 80years of re-growth, rates of C accumulation exceededrates of loss. Over the 80-year period, the SOC stockdid not recover to the pre-harvest level. In Oregon,Law et al. (2001) observed that SOC stock wasconsistently lower at all soil depths compared to pre-disturbance conditions.Other studies have, however, shown that theobserved post-harvesting decline in SOC is generallydue to mixing and movement of the organic materialor litter layer into the mineral soil (Yanai et al., 2003).Harvesting operations often cause drastic soil dis-turbance (Nyland, 2001) mixing the forest floor intothe mineral soils. The exposure of the soil alsoexacerbates losses due to soil erosion (Elliot, 2003),and leaching of dissolved organic carbon (Kalbitzet al., 2000). Numerous studies have shown thatdecomposition rates of surface litter generallydecrease after clear cutting because of the reductionin biotic activity and decrease in soil moisture content.Consequently, some studies have documented anincrease in forest floor carbon several years afterharvest (Mattson and Swank, 1989; Johnson and Todd,1998; Johnson et al., 1985; Mroz et al., 1985). If forestharvesting is done with sufficient care, and does notresult in disruption of natural processes, there may bea little or no effect on SOC stock (Fig. 2b). Further,any decline in biomass input may be compensated bythe large amount of harvest residues left behind (Post,2003; Yanai et al., 2003).A goal of site preparation, following the use ofheavy machinery for harvesting and other vehiculartraffic, is to alleviate soil compaction. Therefore, sub-soiling can be useful to improve seedling growth andvigor. Improving soil moisture storage in the root zoneis another goal of site preparation. Carmean (1970)observed low tree growth in soils of low availablewater capacity. Improving sub-soil drainage can alsoenhance tree growth. Kelting (1999) observed thatproductivity of loblolly pine was strongly influencedR. Lal / Forest Ecology and Management 220 (2005) 242258248by the water table depth. Schoenholtz et al. (1991)observed a strong relationship between soil physicalproperties (e.g. bulk density, hydraulic conductivity,total and macroporosity) on growth of nuttalli oak(Quercus nuttalli). Zou (2001) observed a strongrelationship between root growth of radiata pine(Pinus radiata) and soil physical quality parameters.Because of the strong impact of soil physical qualityon biomass productivity (Wagnet and Hustson, 1997),site preparation to enhance soil quality is crucial toincreasing terrestrial C pool in forest plantations.4.2. Fire and soil carbon dynamicsFire is anothermajor disturbance that can impact soilC stock in a forest ecosystem, and may have aparticularly long-term impact on C stock in soils of theboreal regions. The impact of fire on SOC stockdepends on fire temperature and duration, SOC stockand its distribution in the soil profile, and change in thedecomposition rate of SOC following the fire event(Page-Dumroese et al., 2003). Changes in C stock andflux may be due to alterations in soil temperature andwater regimes, and in thickness of the active thaw layer.Forest fires in tundra regionsmay transform a landscapethat was a net C sink into a net C source. ONeill et al.(2002) monitored CO2 flux following fire in blackspruce, white spruce and aspen stands of interiorAlaska. They observed that these soils becamesignificantly warmer following fire and C exchangebecame more sensitive to fluctuations in surface waterconditions. The mean seasonal temperature increasedby 58 8C in the upper 1 m of the soil profile, whichresulted in a 200% increase in the depth of active thawlayer and a corresponding reduction in themean surfacesoil water potential. These environmental changes mayhave enhanced decomposition of C previously immo-bilized by permafrost. In the boreal forests of Quebec,Canada, Smith et al. (2000) reported that soil N contentsof the surface organic layer of recently burned siteswere significantly lower than those under an older burnsite. In Maine, USA, Parker et al. (2001) used threepaired watersheds to study the effects of N depositionand fire, and observed that 50 years after wild fire, theburned watershed with hardwood regeneration hadsignificantly lower forest floor C and N concentrationsthan the referencewatershed dominated by a softwood.In this study, any perturbations (e.g. fire, N deposition)decreased forest floor C. Fernandez et al. (1999) studiedthe impact of fire on SOC stock for 2 years after theevent in two Galician forests in Spain. Fire resulted in asubstantial decrease in the SOC concentration, andSOC mineralization decreased immediately after thefire. However, the percentage of total C mineralizedincreased. In contrast, Johnson and Curtis (2001)reported that fire resulted in no significant overalleffects on either C or N stocks, but there was asignificant effect of time since fire on these stocks.Thus, the effects offire onSOCconcentration or stock isnot always negative.4.3. FertilizationFertilization is commonly used in intensivelymanaged forest stands. Many forest ecosystems arenitrogen (N) limited, and an increase in N supply canenhance NPP and SOC stock. In the forests of thenortheastern US, Magill and Aber (2000) observed thatforests are N-limited even though soil microbialpopulations may be limited by the amount of Crequired as an energy source. In Maine, USA, Parkeret al. (2001) observed that 8 years of applying(NH4)2SO4 as N deposition also significantly reducedforest floor C:N ratios from 30.6 to 23.4. Forest floor Cand N stocks were lower in the treated watershed(38 Mg C/ha, 1612 kg N/ha) compared with thereference watershed. Resh et al. (2002) observedgreater soil C sequestration (0.11 0.07 kg/m2 of totalSOC) under N-fixing trees compared with Eucalyptustrees in Hawaii and Puerto Rico.In addition to N, biomass production in forestecosystems may also be limited by deficiency of somemicronutrients. Benemann (1992) suggested thatfertilizing forests with Fe and other trace elementsmight enhance primary productivity. In southeasternUSA, Shan et al. (2001) assessed the effects offertilization and understory management on soil Cstorage. They observed that soil C storage was lower inplots where the understory was eliminated with orwithout fertilization, probably because of the reducedlitter fall input and reducedfine root growth.Thegrowthof fine roots plays an important role in soil C dynamics,and it is influenced by soil fertility and other properties.Hubbert et al. (2001) observed that SOC concentrationwas negligible in rocky and shallow soils under Jeffreypine plantation, butwas higherwithin the joint fracturesR. Lal / Forest Ecology and Management 220 (2005) 242258 249(3.7%) of the bedrock where roots were concentratedthan within the A horizon (2.7%). Furthermore, C:Nratios weremuch lower in the soil A horizon (19.6) thanin the bedrock fractures (62.0).Application of biosolids (e.g. sludge and compost)to forest soils may offer another management oppo-rtunity for soil C sequestration. Harrison et al. (1995)assessed the soil C profile in a biosolids-amended sitecompared with a control treatment at the University ofWashington Pack Forest. Application of biosolidsincreased SOC concentration to 45 cm depth and alsodecreased soil bulk density to 17 cm depth.4.4. Afforestation and plantationsAfforestation/reforestation and the managerschoice of species are also important to enhancing soilC stock. Reforestation of abandoned/marginal agri-cultural lands can increase SOC stock (Akala and Lal,2001). In northern Belgium, Schauvlieghe and Lust(1999) assessed C budgets under different land usesystems. The total C stock was 128 Mg/ha underpasture, 173 Mg/ha under 29-year-old forest and232 Mg/ha under 69-year-old stand. The total C stockwas 117 Mg/ha under 27-year-old pin oak stand and227 Mg/ha under 69-year-old oak beech stand. It wasobserved that the older the stand, the larger theimportance of soil C became, especially the stable soilC. In east-central Minnesota, Johnston et al. (1996)reported an average increase in SOC stock at the rate of0.8 Mg C(ha year) in the mineral soil over a 40-yearperiod due to afforestation of degraded agriculturalsoils.Afforestation of reclaimedmine soil, an importanteconomic activity affecting deforestation, has a strongimpact on SOC sequestration (Akala and Lal, 2000).Establishing bioenergy plantation crops is anotheroption for enhancing SOC stock and offsetting fossilfuel combustion. Tolbert et al. (2000) observed that inAlabama, conversion from traditional corn (Zea mays)production to biomass crops increased SOC stock. Allwoody crops (e.g. sweet gum, sycamore and cottonwood) sequestered considerable organic matter below-ground primarily as stumps and large roots (79%) andto a lesser extent as fine roots (21%). There was ameasurable increase in SOC under sweet gum with acover crop (tall fescue) established between rows.Urban forests exhibit a unique ecosystem structureand function in comparison with suburban and ruralforest stands. Transforming landscapes from non-urbanto urban landuse also has the potential to greatlymodifythe terrestrial C stock and fluxes. McPherson et al.(1997) studied the impact of urban forests on air qualityin Chicago. They observed that each year Chicagoforests sequester 0.32 TgC, and contain stock of 5.6 TgC or equivalent of 1418 mg C/ha (Nowak, 1994).Large (>77 cm dbh) and medium (3146 cm dbh)sized trees store an average of 3186 and 399 kg of Cand annually sequester 93 and 19 kg of C, respectively.In comparison, C storage by shrubs is 4% of the amountstored by trees. Pouyat et al. (2002) observed that urbanforest stands had significantly higher organic Cdensities than the suburban and rural stands.The highest SOC densities were observed in loamyfill (28.5 kg/m2) with the lowest in clean fill and olddredge materials (1.4 and 6.9 kg/m2, respectively)with low (15.5 1.2 kg/m2) densities in residentialareas.While the effects of urbanization on the global Cpool and fluxes may be relatively less than those ofother ecosystems, the changes associated withurbanization are likely to persist for longer timesthan those due to other land use conversions. Indirecteffects of urbanization, due to change in air qualityand microbial communities with the attendant effectson litter decomposition, are complex and variable.McDonnell et al. (1997) studied red oak stands locatedon similar soils along an urbanrural gradient fromNew York City to Litchfield County, Connecticut.They observed that rural litter decomposed faster thanthe urban litter in all sites, suggesting that rural litter isof higher quality. Thus, the flow of C between plantsand microbial communities may be quite different inurban versus rural forest. Groffman et al. (1995)reported that urban forests had lower labile C andhigher total passive C than rural forests. Groffmanet al. (1995) concluded that urban forests along thetransect have the potential for sequestering and storingmore carbon than the rural forests.5. Climate change and carbon dynamics inforest soilsAtmospheric CO2 levels are increasing at the rateof 0.4% per year and are predicted to double during the21st century. The implications of this increase arecomplex and not very well understood (Dixon andR. Lal / Forest Ecology and Management 220 (2005) 242258250Wisniewski, 1995). Climate change may affectecosystem productivity, allocation of above-groundversus below-ground biomass and microbial popula-tion (Joyce and Birdsey, 2000). For example, it hasbeen predicted that the standing biomass (above-ground C stock) in minimally disturbed northernforests would increase and the SOC stock woulddecrease (Morgan et al., 2001).However, the response of forested ecosystems tothe projected increase in temperature is complex.Both climate and disturbance interact to influencelatitudinal patterns of vegetation and soil C storage(McGuire et al., 2002). Further, it is the processes atthe landscape scale that control regional C fluxes. Itis, therefore, difficult to extrapolate site-levelmeasurement to regional scales (Hobbie et al.,2000). Yet, there exists a general consensus thatCO2 enrichment is likely to enhance C storage for50100 years (Norby et al., 1995; Nisbet, 2002).However, other C fluxes may also increase. Wangand Polglase (1995) predicted positive C gainsthrough increased NPP particularly in high latitudes,and predicted that under projected CO2 andtemperature increases, the tundra and boreal forestswill emit increasingly more C to the atmospherewhile the humid tropical forest will continue to storeC. King et al. (2001) used the free-air CO2-enrichment (FACE) experiment to elevate concen-trations of CO2 to 535 ppm. Analyses indicated thatwhile native soil organic matter still dominated thesystem, soil respiration increased (39%) due to anincrease in fine root biomass of aspen (96%).Andrews et al. (2000) also observed that respirationrates increased exponentially with an increase intemperature, which was accompanied by a change inthe microbial community structure. In contrast,Schortemeyer et al. (2000) observed no effect ofelevated CO2 on bacterial numbers in the rhizo-sphere. Zak et al. (2000) summarized 47 publishedreports on soil C and N dynamics and concluded thatthere are insufficient data to predict how microbialactivity and rates of soil C and N cycling maychange with enrichment of atmospheric concentra-tion of CO2. Important knowledge gaps include ourunderstanding of the fine-root biology/longevity andthe response of microorganisms.There is also a lack of understanding about thedirection in which nutrient availability may beaffected (Ceulemans et al., 1999), and about theinteraction between various ecophysiological pro-cesses, such as C and N cycles and below-groundresponse. Most studies have shown a shift in whole-tree C allocation pattern towards below-ground partswith increased atmospheric CO2 concentration. Ceule-mans et al. (1999) concluded that at the ecosystemlevel, a large amount of C being allocated below-ground could lead to: (i) more root growth andturnover, (ii) enhanced activity of root-associatedmicroorganisms, (iii) larger microbial biomass stocksand enhanced microbial activity and (iv) increasedlosses of soil C through respiration.Matamala and Schlesinger (2000) investigated theeffects of elevated CO2 concentrations (ambient +200 ppm) on the root production and soil C dynamicsin a loblolly pine (Pinus taeda) FACE experiment nearDurham, NC, USA. Live fine root mass increased by86% in elevated CO2 treatment with an increase of37 g/m2 above the control plots after 2 years of CO2fumigation. Dilustro et al. (2002) studied the effects ofelevated CO2 (700 ppm) in an oak-palmetto scrubecosystem in Florida, USA. Fine root productionincreased with elevated CO2, suggesting that there wasa potential of increased rate of C input into the soil.Climate change itself may also have a fundamentaleffect on soil properties and processes, which mayinfluence SOC stock in forest soils. There is someconcern that an increase in global temperature mayresult in a long-term loss of the SOC stock (Jenkinsonet al., 1991). A significant warming at high latitudesmay make tundra and boreal forests a net C source(Wang and Polglase, 1995; Ping et al., 1997).Increases in soil temperature may influence decom-position dynamics in arctic and boreal ecosystemsaffecting the flux of dissolved organic carbon (DOC)and nitrogen (DON) to aquatic systems. Neff andHooper (2002) measured the release of CO2, DOC andDON from tundra soils from Alaska at 10 and 30 8C.These soils released 80370 mg CO2-C/g soil/C and546 mg DOC/g soil/C at high temperatures. Thesedata show that soils of the arctic region contain a largeproportion of labile carbon (941%) that may be easilydecomposed with an increase in temperature.Melillo et al. (2002) reported the results from adecade long warming experiment conducted in a mid-latitude hardwood forest. They observed that soilwarming accelerated the soil organic matter decayR. Lal / Forest Ecology and Management 220 (2005) 242258 251and CO2 fluxes to the atmosphere. However, thisresponse was small and short-lived because of thelimited size of the labile soil C stock, which may notbe the case in soils of tundra and taiga regions.Warming also increased the availability of mineral Nto plants, which in a N-limited forest may stimulate Cstorage in biomass to compensate for the C loss fromsoils.6. Challenges in assessing carbon dynamics inforest soilsA principle difference in forest versus agriculturalecosystems and one of the main challenges to under-standing C dynamics in forest soils lies in the longerduration of the rotation or growth cycle in forests (Hartand Sollins, 1998). Interest in assessing the dynamics ofSOC stock in forest ecosystems with regards to climatechange dates back only to mid 1990s as a potentialstrategy to off-set fossil fuel emissions. Therefore, mostexperiments on which current conclusions on SOCstock, rate of sequestration and turnover time are basedwere not specifically designed to assess soil C dynamicsover long time periods. The chronosequence approachhasbeenwidely used in forest soil studies, inwhich sitesof different ages are assumed to represent points in time.This space-for-time-substitution approach, whileuseful (Crews et al., 1995; Lighter, 1998; Yanai et al.,2003), can produce erroneous results if soil character-istics differ among sites. Selecting age chronose-quences on sites with similar parent material, soil type,landscape position and prior land use history canminimize sources of error. Other challenges common toforest soils are discussed below.6.1. Relative complexity of forest soils versusagricultural soilsThe data on SOC available in the literature aremostly confined to the top (015 cm) soil layer andoften without any information on the litter layer or thedetritus material and soil bulk density. Forest soilsmay be many meters in depth with great variability intexture and other soil properties among horizons. Landuse conversion from/into a forest ecosystem can affectSOC stock to 1 m or often to 2 m depth. Measurementof soil bulk density is a major challenge for gravellyand rocky soils but also in situations where large treeroots form a large fraction of the soil volume.6.2. ReplicationSimilar to other soil properties (e.g. texture,infiltration rate and bulk density), SOC concentrationand stock are highly variable over space (horizontallyand vertically) and time (Giese et al., 2000; Wiggintonet al., 2000). Thus, adequate replication of treatmentsand soil samples is essential to account for soilvariability. Assessment of differences among land useand soil management treatments can only be made onsufficiently replicated large plots. The paired-plottechnique, although useful for comparative analyses,is not always adequate to assess management-induceddifferences in SOC dynamics.6.3. Antecedent soil carbon stock and baselinedataBecause of the long duration, either the antecedentSOC stock is not assessed or the archive samples arenot available to compare temporal changes in SOCstock for different growth stages of the forest stand.Without the data on the antecedent SOC stock, it isdifficult to conclude whether the present differencesare the result of differing initial condition or due tomanagement action (Hoover, 2003).6.4. Sampling protocolThere is a strong need to develop and follow astandardized sampling protocol for assessment ofSOC stock and fluxes (Lal et al., 2001). In addition toassessment of the litter layer or the detritus material,total depth and depth increment for obtaining soilsamples are also important. Measurement of (theproportion by weight and volume) gravel or skeletalfraction is essential for gravelly and rocky soils. It isimportant to consider whether samples are obtained byspecific depth or genetic horizon.6.5. Scale of measurementAssessment of the potential of soil C sequestration inforest ecosystems requires a thorough understanding ofthe biogeochemical mechanisms responsible for CR. Lal / Forest Ecology and Management 220 (2005) 242258252stock and fluxes at the molecular, aggregate, pedon,soilscape, landscape, regional and global scales. It isnecessary to understand the structure and dynamics ofthe below-ground component of terrestrial C stocks(Metting et al., 2001). In this regard, the choice ofmethods is critical so that results are reliable, com-parable andextrapolatable.Malhi et al. (1999)proposedan approach based on combination of micrometeor-ological, ecophysiological and forestry data.Eddy co-variance measurement is a widely usedtechnique for assessing C dynamics. Law et al. (2001)used this technique in Oregon, USA and showed thatsoil disturbance decreases soil C stock. Curtis et al.(2002) used micrometeorological methods and con-cluded little if any changes in soil C stock. Billingset al. (1998) suggested that Ficks lawmight be used topredict surface CO2 flux from the information on CO2concentration in soil and the canopy.There are also techniques of developing theterrestrial C budget by measuring above- and below-ground components. An important consideration inusing this approach is the large-scale spatial variabilityand poor statistical sensitivity in both above- andbelow-ground components (Homann et al., 2001;Yanaiet al., 2000). Inherent spatial variability of soil C limitsthe precision of measurement of changes in soil C.Conant et al. (2003) proposed a protocol for a samplingdesign to address the variability. They observed thatdifferences in the order of 2.0 Mg C/ha could bedetected by collection and analyses of at least five coresfrom a cropland and two cores per plot from a forestedtreatment. Johnson and Curtis (2001) used metaanalyses to assess the impact of forest harvesting onsoil C and N stocks, and Yanai et al. (2000) used achronosequence approach to study soil C dynamics. Aswith soil sampling, there is also a strong need fordeveloping/standardizing a protocol for soil C analysesover long time periods. Palmer et al. (2002) proposed aForest Health Monitoring (FHM) protocol using a plotstudy. They observed that for total soil C, 40% of thevariancewas betweenplots, 40%between sub-plots and20% within sub-plots.6.6. Analytical procedures for soil carbonconcentrationThe analytical procedures used in the laboratory areanother important step in credible assessment of soil Cstock. A wide range of analytical methods exists(McColl and Gressel, 1995), based on physical,chemical and biological procedures (Schoth et al.,2003). New procedures are being developed to assesssoil C rapidly, economically and precisely. Wang andHsieh (2002) proposed the accelerator mass spectro-metry (AMS) in radiocarbon measurements to studyrates of C cycling and turnover times. They proposedthis technique for comparing soil C dynamics in diverseecosystems such as in arid soils of the Mojave Desert,loess soils of Iowa, forest soils of Sierra Nevada, etc.Hoover et al. (2002) proposed pyrolysis molecularbeam mass spectrometry (Py-MBMS) for the mea-surement and characterization of C in forest soils. Thistechnique is especially useful to distinguish recent frommore stable humic substances and to relate thisinformation to the ecology and history of the site.Cremers et al. (2001) proposed a method using laser-induced breakdown spectroscopy (LIBS). Theydetermined appropriate spectral signatures, calibratedthe technique with the conventional dry combustionmethod, and found it to be rapid, accurate and precise.The LIBS technique has excellent detection limits(300 mg/kg), precision (45%) and accuracy (314%),and is readily adaptable to a field-portable instrument.6.7. Modeling and aggregating the dataThe laboratory measurements must then be scaledup to obtain regional and global stocks and fluxes.Assessing the soil C stock at regional and nationalscales is of interest in the context of internationalpolicy agendas on greenhouse gas emissions mitiga-tion. There are several techniques available foraggregating the data. Bhatti et al. (2002) used threedifferent approaches to estimate soil C stocks in threeprairie provinces of western Canada (Alberta, Sas-katchewan and Manitoba): (i) analysis of pedon data,(ii) expert estimation based on Canadian Soil OrganicCarbon Database (CSOCD) and (iii) model simula-tions with the Carbon Budget Model of the CanadianForest Sector (CBM-CFS). Using USDA-NRCSSTATSGO database, Heath et al. (2002) presentedmethodology for estimating C stock in forest soils ofthe USA using FORCARB model. Below-groundconsequences of land use and vegetation change arealso important to modeling SOC dynamics (Jacksonet al., 2000).R. Lal / Forest Ecology and Management 220 (2005) 242258 2536.8. Developing a relationship between soilquality and carbon stockThe soil C stock is also related to soil quality,biomass productivity and soil health (Henderson,1995), although this relationship is not well-devel-oped. Page-Dumroese et al. (2000) proposed soilquality standards and guidelines for forest sustain-ability in northwestern North America, in which theycomputed changes in soil C, N, erosion and CEC usingthreshold variables. Conkling et al. (2002) proposed aNational Forest Health Monitoring program forinvestigating C budgets. They reported that FHMdata are adequate for detecting a 20% change over 10years (2% change per year) in percentage of total Cand C content (mg C/ha).7. An evaluation of the magnitude of potentialof carbon sequestration by forest soilsThe terrestrial C stock in forest biomes is animportant natural sink for sequestering atmosphericCO2, and its role in reducing the rate of enrichment ofatmospheric CO2 cannot be over-emphasized. Schle-singer (1990) estimated that soil C accumulation of0.4 Pg C/year occurs mainly in forest soils. Houghtonet al. (1998) attributed the so-called missing sink of1.8 Pg C/year to the forest biome. Pacala et al. (2001)argues that the missing sink may be containedsomewhere in the terrestrial ecosystem of the northernhemisphere, much of which is forested. A report byDOE (1999) estimated the potential of C sequestrationof world forests at 13 Pg C/year out of the totalpotential of 5.710.1 Pg C/year in all biomes of theworld. Similar estimates were made by Watson et al.(1995) and Trexler (1998).IPCC (2000) estimated the potential of C seques-tration in world forest ecosystems. The potential of Csequestration in Annex I (developed or industrialeconomies) countries (Oberthur and Ott, 2001) wasestimated at 101 Tg C/year by 2010 and 503 Tg C/year by 2040. Similarly, the potential of C sequestra-tion in forest biome of Annex II (developing orindustrializing economies) countries was estimated at69 Tg C/year for 2010 and 200 Tg C/year for 2040. Inaddition, the potential of agroforestry system wasestimated at 12 and 17 Tg C/year for 2010 and 2040 inAnnex I countries, and 14 and 28 Tg C/year for 2010and 2040 in Annex II countries, respectively. There-fore, the total global potential of C sequestration inforest biomes was estimated at 196 Tg C/year in 2010and 748 Tg C/year by 2040 (IPCC, 2000).There may be major technological changesoccurring in agricultural production, which may allowincreases in forest land. Adoption of improvedagricultural practices may result in similar productionlevels on 3050% of the area currently underagricultural land use. The land thus spared may beconverted to nature conservancy or afforestation. Suchland use changes are likely to occur in Europe(Boumer et al., 1998; Liski et al., 2002; Shvidenkoet al., 2002) and in North America. Such conversionsto perennial land use have a potential of terrestrial(biomass and soil) C sequestration.Delcourt and Harris (1980) reported that terrestrialC stock of the temperate forests of the southeasternUSA has been gradually increasing since the 1950s.Kauppi et al. (1992) arrived at similar conclusions forEurope. Yet, such slight or slow increases may notkeep pace with the increasing anthropogenic CO2emissions (Sedjo and Solomon, 1989). Birdsey andHeath (2001) estimated that USA forest ecosystemsare presently removing 0.20.3 Pg C/year from theatmosphere, as terrestrial C in biota and soil. Heathet al. (2003a,b) estimated the potential of Csequestration in forest soils of the US through forestmanagement, land use change and agroforestry. Forlow, medium and high scenarios, respectively, thepotential of C sequestration in forest soils of the USAwas 25, 56 and 103 Tg C/year for forest management,8, 26 and 51 Tg C/year for land use change and 17, 22and 28 Tg C/year for agroforestry. Thus, the totalpotential of C sequestration in forest soils of the USAwas estimated at 49, 106 and 186 Tg C/year for low,medium and high scenarios, respectively. Based on aproductive forestland base of 204 Mha, then thepotential of C sequestration in forest soils of the USAis 108 Tg C/year.8. ConclusionsCarbon sequestration in forest soils has a potentialto decrease the rate of enrichment of atmosphericconcentration of CO2. Increase in C stock of forestR. Lal / Forest Ecology and Management 220 (2005) 242258254soils can be achieved through forest managementincluding site preparation, fire management, affor-estation, species management/selection, use of ferti-lizers and soil amendments. 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Plant Soil 236, 105115.Forest soils and carbon sequestrationIntroductionWorld forestsCarbon stock in forest ecosystemsFactors affecting the forest soils carbon concentration and stockForest harvesting and soil carbon stockFire and soil carbon dynamicsFertilizationAfforestation and plantationsClimate change and carbon dynamics in forest soilsChallenges in assessing carbon dynamics in forest soilsRelative complexity of forest soils versus agricultural soilsReplicationAntecedent soil carbon stock and baseline dataSampling protocolScale of measurementAnalytical procedures for soil carbon concentrationModeling and aggregating the dataDeveloping a relationship between soil quality and carbon stockAn evaluation of the magnitude of potential of carbon sequestration by forest soilsConclusionsReferences