Influences of global change on carbon sequestration by agricultural and forest soils

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  • 161

    Influences of global change oncarbon sequestration by agriculturaland forest soils

    Ole Hendrickson

    Abstract: Global change including warmer temperatures, higher CO2 concentrations,increased nitrogen deposition, increased frequency of extreme weather events, and landuse change affects soil carbon inputs (plant litter), and carbon outputs (decomposition).Warmer temperatures tend to increase both plant litter inputs and decomposition rates,making the net effect on soil carbon sequestration uncertain. Rising atmospheric carbondioxide levels may be partly offset by rising soil carbon levels, but this is the subject ofconsiderable interest, controversy, and uncertainty. Current land use changes have a netnegative impact on soil carbon. Desertification and erosion associated with overgrazing andexcess fuelwood harvesting, conversion of natural ecosystems into cropland and pasture land,and agricultural intensification are causing losses of soil carbon. Losses increase in proportionto the severity and duration of damage to root systems. Strategic landscape-level deploymentof plants through agroforestry systems and riparian plantings may represent an efficient wayto rebuild total ecosystem carbon, while also stabilizing soils and hydrologic regimes, andenhancing biodiversity. Many options exist for increasing carbon sequestration on croplandswhile maintaining or increasing production. These include no-till farming, additions ofnitrogen fertilizers and manure, and irrigation and paddy culture. Article 3.4 of the KyotoProtocol has stimulated intense interest in accounting for land use change impacts on soilcarbon stocks. Most Annex I parties are attempting to estimate the potential for increasedagricultural soil carbon sequestration to partly offset their growing fossil fuel greenhouse gasemissions. However, this will require demonstrating and verifying carbon stock changes, andraises an issue of how stringent a definition of verification will be adopted by parties. Soilcarbon levels and carbon sequestration potential vary widely across landscapes. Wetlandscontain extremely important reservoirs of soil carbon in the form of peat. Clay and siltsoils have higher carbon stocks than sandy soils, and show a greater and more prolongedresponse to carbon sequestration measures such as afforestation. Increased knowledge ofsoil organisms and their activities can improve our understanding of how soil carbon willrespond to global change. New techniques using soil organic matter fractionation and stableC isotopes are also making major contributions to our understanding of this topic.

    Key words: climate change, carbon dioxide (CO2), nitrogen, soil respiration, land use change,plant roots, afforestation, no-till.

    Received 12 September 2003. Accepted 5 January 2004. Published on the NRC Research Press Web site athttp://er.nrc.ca/ on 16 March 2004.

    O. Hendrickson. Environment Canada, Gatineau, QC K1A 0H3, Canada (e-mail: ole.hendrickson@ec.gc.ca).

    Environ. Rev. 11: 161192 (2003) doi: 10.1139/A04-001 2003 NRC Canada

  • 162 Environ. Rev. Vol. 11, 2003

    Rsum : Le changement global incluant des tempratures plus leves, de plus fortesteneurs en CO2, des dpositions accrues dazote, une augmentation des frquences desvnements mtorologiques extrmes, et lutilisation des terres affectent limmobilisationdu carbone dans les sols (litires vgtales) et la libration du carbone (dcomposition). Lestempratures plus leves ont tendance augmenter la fois les taux dimmobilisation dansles litires et ceux de la dcomposition, rendant incertain leffet sur la squestration nettede carbone dans les sols. Laugmentation des teneurs en dioxyde de carbone pourrait enpartie contrebalancer laugmentation du carbone dans les sols, mais ceci est sujet beaucoupdintrt, de controverse et dincertitude. Les changements actuels de lutilisation des terresont un impact ngatif sur le carbone du sol. La dsertification et lrosion associes ausurpturage et lexcs de rcolte du bois de feu, la conversion des cosystmes naturelsen cultures et en pturages, et lintensification de lagriculture entranent des pertes nettesen carbone du sol. Les pertes augmentent en proportion de la svrit et de la dure desdommages occasionns aux systmes racinaires. Un dploiement stratgique de plantes auniveau du paysage, dans les systmes agro-forestiers et les plantations riveraines, peuventconstituer une manire efficace de reconstruire le carbone cosystmique total, tout enstabilisant les sols et les rgimes hydrologiques, et en augmentant la biodiversit. Plusieursoptions se prsentent pour augmenter la squestration du carbone dans les terres en culture,tout en maintenant ou en augmentant la production. Celles-ci comprennent la culture sanslabour, laddition de fertilisants azots et de fumiers, ainsi que lirrigation et la culture enrizires. Larticle 3.4 du protocole de Kyoto a provoqu un intrt marqu pour tenir comptedes impacts du changement de lutilisation des terres sur les rserves en carbone du sol.La plupart des participants inscrits lAnnexe 1 tentent dvaluer le potentiel daugmenterla squestration de carbone dans les sols agricoles afin de contrebalancer leurs missionscroissantes en gaz effet serre. Cependant, ceci va demander des dmonstrations et desvrifications sur le changement des rserves en carbone, et soulve la question, savoircomment les participants adopteront une dfinition contraignante de la vrification. Lesteneurs en carbone du sol et le potentiel de squestration varient largement travers lespaysages. Les terres humides contiennent des rservoirs de carbone du sol extrmementimportants sous forme de tourbe. Les sols argileux et limoneux ont des rserves en carboneplus leves que les sols sablonneux, et montrent une raction plus importante et plusprolonge suite aux mesures de squestration du carbone, telles que lafforestation. Uneaugmentation de la connaissance des organismes du sol et de leurs activits peut amliorernotre comprhension sur la faon aves laquelle le carbone du sol ragira au changementglobal. De nouvelles techniques utilisant le fractionnement de la matire organique dusol et les isotopes stables du C apportent galement une contribution majeure notrecomprhension sur ce sujet.

    Mots cls : changement climatique, dioxyde de carbone (CO2), azote, respiration du sol,changement de lutilisation des terres, racines des plantes, afforestation, culture sans labour.

    [Traduit par la Rdaction]

    Introduction

    The ability of agricultural and forest soils to sequester carbon depends on the balance betweencarbon inputs and carbon outputs. Inputs to soil are mainly in the form of above- and below-groundplant litter derived from photosynthesis. Carbon outputs arise mainly from the respiratory activity ofsoil decomposer organisms.

    Both photosynthesis and respiration are stimulated by higher temperatures, making the net carbonbalance in ecosystems affected by global warming uncertain. Predictions of future carbon stocks arealso rendered imprecise by uncertainty about the magnitude and rate of impending changes in theenvironment. Various aspects of global change higher CO2, warmer temperatures, modified nutrient

    2003 NRC Canada

  • Hendrickson 163

    Table 1. Synergies in meeting multiple political, environmental, economic and social objectives: a broadercontext for carbon sequestration projects.

    Political Environmental Economic Social

    U.N. FrameworkConvention onClimate Change;Kyoto Protocol

    Limiting increase ofgreenhouse gases to alevel compatible withlife

    Least cost approachesto meeting nationaltargets

    Public engagementin implementingmitigation andadaptation measures

    Ramsar Conventionon Wetlands

    Integrated waterresources management;sustaining habitat foraquatic life

    Free ecosystemservices: waterpurification, floodcontrol, etc.

    Reduction of adversehealth impactsassociated with lack ofaccess to clean water

    U.N. Conventionto CombatDesertification

    Restoring productivity ofdegraded lands

    Decreasing reliance onglobal aid and relief

    Sustainable livelihoodapproaches; reductionof refugee influxes

    Convention onBiologicalDiversity

    Linking conservationareas across landscapes

    Sustainable use inresource sectors:fisheries, agriculture,forestry, tourism, etc.

    Ecosystem approach:human communitiesembedded withinecosystems

    cycles, and altered disturbance regimes can each have negative or positive effects on soil carbonpools.

    Climate variation is a dominant influence on biodiversity in agricultural and forest ecosystems.Different genotypes and species show different adaptive responses to climate-mediated phenomenasuch as extreme high and low temperatures, droughts, and episodic plant litter inputs arising from icestorms, fires, or windstorms. Our understanding of how individual genotypes and species respond toclimate variation is limited and imperfect. However, it is also key to predicting how different ecosystemswill respond to global change and to designing mitigation measures appropriate to a particular site.

    Human activities are the ultimate drivers of global change (Millennium Ecosystem Assessment2003). Carbon sequestration projects are implemented in a particular political, environmental, economic,and social context. Climate change is only one of several global change issues addressed by multilateralenvironmental agreements (Table 1). Others include lack of clean water, desertification, and biodiversityloss. Actions to increase soil carbon sequestration can simultaneously address several of these issues. Agrowing view is that projects that provide synergies can maximize return on investment and deservepriority attention.

    The nature of soil carbon

    Carbon stocks in above-ground portions of ecosystems are easier to measure than below-groundcarbon stocks. While above-ground carbon is concentrated in living plants, below-ground carbon isdispersed in a complex mixture of plant parts, soil animals, fungi, and bacteria, mostly in variousstages of decay. The bulk of soil carbon can be considered nonliving, but it is completely permeatedand continuously reprocessed (and excreted as wastes) by living organisms. Owing to this continuousreprocessing activity, soil carbon is composed of highly complex biochemical forms with widely varyingresidence times. In addition to the activities of organisms, erosion (wind and water), and leaching oforganic compounds contribute to soil carbon displacement and (or) dispersal.

    Soil organic matter can be grouped by its residence, or turnover time, into three general classes labile (active), stable (slow release), and inert (recalcitrant) (Schlesinger 1986; Smith et al. 1997a; Batjes1999). Labile or active soil organic matter largely consists of recently deposited plant litter (includingroots), living and dead soil microorganisms and soil animals, and their immediate breakdown products.

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  • 164 Environ. Rev. Vol. 11, 2003

    It has a cycling or residence time of 110 years. Roots are a particularly important carbon sourcein many soils (e.g., Balesdent and Balabane 1996). Stable soil organic matter consists of polymericsubstances formed after considerable processing of the active pool. These polymers are extremelydiverse in composition (depending on litter sources and soil-forming conditions) and have a residencetime of between 10 and 50 years or more (Baldock and Nelson 1999). Inert or recalcitrant SOM,which is also diverse in composition, may be stable for up to 500 years.

    The extent to which humic substances are stabilized against microbial mineralization largely dependson the type of bonding with clay minerals (Tate and Theng 1980; Six et al. 2002). Coarse-textured soilshave higher proportions of total soil carbon bound in microbial biomass, making them susceptible tocarbon loss with environmental change (Santruckova et al. 2003). Agricultural practices that involvehigher soil disturbance result in decreased fungal activity, lower ratios of fungi to bacteria, and lessstored soil C compared with less intrusive practices (Bailey et al. 2002). Physical fractionation of soilsand soil organic matter by size and density can provide useful information about stability and persistenceof carbon in soil aggregates (Christensen 1992; Balesdent et al. 1998), particularly when combined withstable carbon isotope analysis (Balesdent et al. 1987; del Galdo et al. 2003).

    Well-developed forest soils tend to show a vertical sequence of layers (FAO/ISRIC 1990) anda progressive decline in carbon concentration with depth (Nakane 1976). At the top is an organicsurface layer or forest floor (O horizon) with fresh, undecomposed plant debris (Oi horizon); semi-decomposed, fragmented organic matter (Oe horizon); and amorphous, well-humified organic matterlargely resulting from the activity of the soil microfauna (Oa horizon). Below this surface layer is amineral surface horizon (A); a leached subsurface mineral horizon (E); a subsurface mineral horizon thatoften shows accumulation of leached minerals and (or) organic matter (B horizon); a mineral horizonpenetrable by roots (C); and underlying bedrock (R). The E, B, C, and R horizons may be lacking, orthe B horizon may be modified by groundwater or stagnant water. Some forests have large amounts ofrelatively stable organic matter arising from inputs of coarse woody debris (Harmon et al. 1986), whicharrives at the soil surface highly depleted in nutrients owing to internal retranslocation processes duringwood formation.

    Agricultural soils associated with rangelands and grasslands often have similar vertical sequences.However, if they are being cultivated (or have been in the past) they may lack the O horizon and theA horizon may be mixed with parts of the E and even the B horizon, resulting in a plow layer (Aphorizon). The B and (or) C horizons may have been broken up by deep cultivation. Soils may havebeen so degraded by past human actions that they can no longer be cultivated. Such soils may still beclassified as agricultural soils and used, for example, for grazing or noncropping production.

    Soil carbon is spatially variable at both fine scales (millimetres to metres) and coarse scales (kilo-metres). Below-ground carbon stocks vary greatly across the landscape (Sombroek et al. 1999) and arehighly influenced by topography and climate. Surface soil has higher total amounts of organic carbon,which is dominantly in forms with short residence times. There are also significant amounts of long-lived organic carbon compounds in deeper soil horizons. Deeper soil carbon may include charcoal fromfires and consolidated carbon in the form of iron-humus pans or concretions. Many soils, particularlyin arid and semi-arid regions, also contain large amounts of inorganic carbon in the form of calciumcarbonate (Schlesinger 1982).

    Of particular importance are wetlands, which generally have low above-ground plant carbon stocksbut significant accumulations of long-lived soil organic carbon as peat. Some forests and some agricul-tural areas (e.g., rice paddies) are themselves wetlands. Wetlands...

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