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-groun

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