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N.H. Batjes (Y)International Soil Reference and Information Centre (ISRIC),P.O. Box 353, NL-6700 AJ Wageningen, The Netherlandse-mail: batjes6isric.nl, Fax: c31-317-471700

Biol Fertil Soils (1998) 27:230235 Q Springer-Verlag 1998

ORIGINAL PAPER

N.H. Batjes

Mitigation of atmospheric CO2 concentrationsby increased carbon sequestration in the soil

Received: 1 December 1997

Abstract The International Panel on Climate Change

distinguished three main options for the mitigation ofatmospheric CO2 concentrations by the agriculturalsector: (1) reduction of agriculture-related emissions,(2) creation and strengthening of C sinks in the soil,and (3) production of biofuels to replace fossil fuels.Options for sustained sequestration of C in the soilthrough adapted management of land resources are re-viewed in the context of the ongoing discussion on theneed to reduce greenhouse gas concentrations in the at-mosphere. Enhanced sequestration of atmospheric CO2in the soil, ultimately as stable humus, may well prove amore lasting solution than (temporarily) sequestering

CO2 in the standing biomass through reforestation andafforestation. Such actions will also help to reverseprocesses of land degradation, thus contributing to sus-tained food productivity and security for the people inthe regions concerned.

Key words Carbon dioxide 7 Carbon sequestration 7Climate change 7 Greenhouse gases 7 Mitigationoptions

Introduction

The expanding use of fossil fuels and large-scalechanges in land use have led to increased concentra-tions of radiatively active trace gases in the atmosphere,which affect the global climate (Watson et al. 1996).Current international climate negotiations are aimed atdeciding upon targets for the reduction of greenhousegas emissions after the year 2000. The proposal of theEuropean Union, for the third conference of the parties

to the UN Climate Convention in Kyoto (December

1997), is that the industrialized countries should reducetheir emissions of greenhouse gases to 15% below 1990levels by the year 2010. Consequently, many researchinstitutes and industries have been developing technol-ogies to this avail. How effective can such technologicalmeasures be in periods of economic growth, associatedwith higher levels of fuel use by industry and the trans-port sector? And can these measures, alone, adequatelyreduce current atmospheric CO2 levels as envisaged bythe policy makers?

The average, global anthropogenic release of C tothe atmosphere, in the 1980s, was 7.1B1.1 Pg C year1

(1 Pgp1 Gtp103 Tgp1015 g), 1.6B1.0 Pg C year1 ofwhich were from changes in tropical land use and5.5B0.5 Pg C year1 from fossil fuel combustion andcement production. During the period under review,the atmosphere gained 3.2B0.2 Pg C year1 and theoceans absorbed 2.0B0.8 Pg C year1 (Schimel 1995).Current budgets for human-induced perturbations ofCO2 levels point to a low, inferred, terrestrial uptake ofabout 12 Pg C year1, part of which is due to the CO2fertilization effect (1.0B0.5 Pg C year1), forest re-growth in the northern hemisphere (0.5B0.5 Pg Cyear1), and increased N deposition (0.6B0.3 Gt C

year1

) (Schimel 1995).Direct effects of elevated atmospheric CO2 concen-

trations on soil C cycling are unlikely in view of highambient CO2 concentrations in the soil. Evidence ex-ists, however, that plant growth and soil C sequestra-tion are increasing due to the so-called physiologicalCO2 fertilization effect, associated with increased atmo-spheric CO2 levels (Allen et al. 1996; Bazzaz et al.1996), improved water-use efficiency (Van de Geijnand Goudriaan 1996), more favourable temperaturesfor plants and increased anthropogenic N emissions(Hudson et al. 1994; Mellilo 1996). In this context, the

possibility of enhanced and sustained C sequestrationin standing vegetation and the soil should be consid-ered as a possible means of mitigating increases in theconcentrations of atmospheric CO2.


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Fig. 1 Factors and main proc-esses of soil formation (from:Arnold et al. 1990)

Soil organic matter

Most C in the soil is associated with organic matter.Levels of CO3-C can be significant in calcareous soils ofsemi-arid and arid regions, while charcoal may be animportant constituent in ecosystems subject to frequentfires (Sanford et al. 1985; Skjemstad et al. 1990). Themass of soil organic C in the upper 1 m of soil is about12001600 Pg C (see Batjes and Sombroek 1997), andabout 23762456 Pg C in the upper 2 m of soil (Batjes1996). On average, the soil contains about 2.5 timesmore organic C than the vegetation (;650 Pg C) andabout twice as much C as is present in the atmosphere(;750 Pg C).

There is a great variation in the amount and verticaldistribution of organic matter in boreal, temperate,tropical and subtropical soils. Soil organic matterstored in the topsoil contributes most actively to nu-trient cycling in the soil-water-plant system and to ga-

seous exchanges with the atmosphere, but the subsoilcan also be important (Nepstad et al. 1991; Davidson etal. 1993).

Major environmental factors which control the be-haviour of organic matter in soil are moisture status,soil temperature, O2 supply (drainage), soil acidity, soilnutrient supply, clay content and mineralogy. The turn-over time of organic matter increases with depth in thesoil, ranging from several years for litter to 1540 yearsin the upper 10 cm and over 100 years below a depth ofabout 25 cm (Harrison et al. 1990; Lobo et al. 1990).

The favourable effects of soil organic matter on the

physical, chemical and thermal properties of the soil

and on biological activity, and thus in sustaining soilproductivity and biodiversity, are well known. Theseaspects may be seen as an added benefit over direct Cmitigation techniques that would only lead to the stor-age of CO2 in the deeper subsoil (e.g. old gasfields,mines and aquifers).

Possible effects of human-induced changes on Csequestration

The soil-forming factors, notably climate as well localbiological activity in which humans are often a predom-inating factor, control the amount of soil organic matterthat corresponds with equilibrium conditions in a cer-tain natural ecosystem or agro-ecosystem (Fig. 1). Aftereach disturbance, a period of constant management isrequired in order to reach a new steady-state. This maytake 1050 years or more for soil organic C, and be-tween 1520 years for N, depending on climate. Upon

the introduction of adapted management practices onpresent and newly cultivated soils or sites of reforesta-tion, whereby litter inputs exceed decomposition, theorganic C content in the soil gradually increases to-wards a new steady-state. This new equilibrium may belower, similar or higher than the original one.

Notable examples of enhanced C sequestrationthrough adapted management include: agricultural soilsto which large amounts of farm refuse, rich in P, havebeen added over prolonged periods; agricultural soilssupplied with large amounts of farmyard manure an-nually for over 100 years; and soils under maize which

have been supplied annually with NPK fertilizers for 30


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years (Gregorich et al. 1996). Some of these increasesare of a long-lasting nature, as is the case for the TerraPreta-do-Indio found throughout the Amazon Basinand Plaggen soils in north-western Europe (Sombroek1995). In other cases, the increase in C levels were notstable after the beneficial land management practicesstopped because, apparently, most of the soil C was stillpresent in a labile form.

The potential effects of a changing climate and high-er atmospheric CO2 levels on soil C sequestration arehighly interactive and complex. Both net primary pro-ductivity (NPP) and organic matter decomposition, be-ing (micro)biologically mediated, are likely to be en-hanced by increasing temperature, provided water andnutrients are not limiting. According to Kirschbaum(1995), organic matter decomposition is likely to bestimulated more than NPP, which would lead to the re-lease of more CO2 from the soil to the atmosphere inthe event of global warming (Schimel et al. 1990;

Kirschbaum 1995). Neither of these studies, however,consider the interactive effects of temperature andmoisture limitation on plant responses and decomposi-tion rates, as extensively reviewed by Bazzaz et al.(1996). Studies by Klein Goldewijk et al. (1994) whichused models, proposed that the effects of increasedtemperature and water availability on soil respiration implying CO2 release would be smaller than those as-sociated with the CO2 fertilization effect. Post et al.(1996) reported an increase in soil organic C content intheir transient and equilibrium climate change scenar-ios, in which NPP was varied as a function of climate

and atmospheric CO2 concentration, as opposed to adecrease in scenarios which considered solely climatechange or climate change and concomitant changes inNPP.

Basically, as shown in Fig. 2, it is still uncertain inwhich direction soil C pools will actually evolve (Krn-er 1996). Under elevated CO2 levels, most plants arefound to produce tissues that contain more C and lessN. The assumption then is that this CO2-induced in-crease in the C/N ratio, and possibly increased lignincontent, will lead to reduced rates of decomposition(Ball 1997), and thereby facilitate C sequestration in

the soil. Krner (1996), however, illustrated there is an-other pathway by which CO2 fertilization may influencethe soil environment. That is, through priming-effectsof increased rates of turnover of fine roots and higherexudation of low molecular weight organic compoundsto the rhizosphere (Paterson et al. 1997); quantitatively,root exudation may be much less important in foreststhan in grasslands. It appears that only a small portionof the exuded, labile C can be stabilized in soil organicmatter through interactions with clay minerals, contraryto other more recalcitrant plant constituents such as lig-nin and cellulose (Hungate et al. 1997). The soil miner-

al status is a strong controlling factor with respect topassive C pools. According to Torn et al. (1997), theeffect of mineralogy on soil C storage is of the samemagnitude as that attributed to climate or vegetation.

Fig. 2 Possible fate of C in CO2 fertilized ecosystems (from:Krner 1996)

The complexity of terrestrial ecosystem responses toincreased atmospheric CO2 concentrations, andchanges in climate, illustrates the need for models. Pos-sible impacts of increased atmospheric deposition of Non soil physical, chemical and biotic processes, and ulti-mately C sequestration should be considered also inthese models. This applies also for impacts caused bychanging land management practices and possible shiftsin species diversity and functions (Brinkman and Som-broek 1996; Betts et al. 1997; Jefferies and Maron 1997;Norby 1997).

Management options for enhanced soil C sequestration

Differences between soil types, their suitability for dif-ferent uses and the factors of soil formation must beconsidered when identifying management options forenhanced C sequestration. Organic C contents in artifi-cially drained peat soils are unlikely to increase, unlessthey revert to wetlands. As a result, possibilities for in-creased C sequestration are largely limited to the so-called upland soils.

The role of agriculture in the sequestering of organic

C by soils remains ambiguous. The overall picture iscomplicated by technological, social, economic and cul-tural factors (Fig. 3). These must be addressed specifi-cally in the management of fragile and ecologically sen-


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Fig. 3 Possible impacts of changes in socio-economic and politi-cal factors on land-use and soil C status

sitive ecosystems. Watson et al. (1996) estimated that0.40.8 Pg C year1 could be sequestered in agriculturalsoils globally by implementation of appropriate man-agement practices. This corresponds with about 10% ofthe global anthropogenic production of CO2 for theyear 1990 [;6 Pg C year1].

Management practices should be aimed at optimiz-ing CO2-utilization in photosynthesis to increase cropproductivity and yields, and at increasing especially thepassive or mineral-stabilized fraction of the soil organic

matter pool. Available options include: high residueproduction; mulching; reduction of bare fallow; tillagepractices that limit depth and intensity of disturbances;and fertilization, notably with (rock)phosphates andfarmyard manure. Manipulation of the quality andquantity of organic inputs, for example by cultivar se-lection and introduction of leguminous crops, and ofthe soil fauna are options worthy of further research(Fernandes et al. 1997; Torbert et al. 1997).

The degree to which various management tech-niques will be effective depends on several environ-mental controls, some of which may be altered by cli-

mate change. Management implications of elevatedCO2 concentrations and increased temperatures thusneed to be considered. Increased atmospheric CO2 con-centrations may stimulate crop growth until the supply

of nutrients or water becomes limiting. This means thepotential for additional fertilizer application must beevaluated with respect to the type of soil and crop (e.g.C3, C4 or CAM crops). In addition, land-users will onlyadopt adapted management techniques if they improveprofitability within several years (Izac 1997).

Energy crops or biofuels have a considerable po-tential for mitigation of atmospheric CO2 concentra-tions by counteracting the use of fossil fuels. Biofuelproduction on 1015% of the land currently in agricul-tural use or in agricultural set-asides could substitutefor 0.31.3 Pg C year1 of fossil fuel, while recovery andconversion of crop residues could substitute for an ad-ditional 0.10.2 Pg C year1 of fossil fuel (Cole 1996).These figures do not include the indirect mitigation ofatmospheric CO2 by biofuel production through in-creased C storage in the standing (woody) vegetation,and possibly by increased C sequestration in the soil.Only part of the residues of biofuel crops can be re-

moved without adversely affecting soil productivity.Management of forests and reducing tropical and

subtropical deforestation are viable options for mitiga-tion of atmospheric CO2 (see review by Brown 1996).Such actions would also reduce the extent of anthro-pogenic land degradation, thus contributing to sus-tained food productivity and security for the people inthe regions concerned.

Evidence of enhanced soil C sequestration

Historical decreases in soil C content upon cultivation,as reported for the US dust bowl and the virgin landscheme in the former USSR, were often associatedwith low production levels, inadequate fertilizer appli-cation, removal of crop residues and intensive tillage(e.g. Papendick 1994). Historical examples of a sus-tained doubling of the organic matter component ofsoils under century-long human occupation also exist,albeit for smaller areas. Encouraging reports have be-come available also on enhanced C sequestration underimproved grasslands and in agro-ecosystems both forthe (sub)tropics and temperate regions, as well as on

the positive effects of agro-forestry (Batjes and Som-broek 1997; Fernandes et al. 1997; Paustian et al. 1997;Torbert et al. 1997).

The root systems of plants in tropical pastures canbe used effectively to sequester and redistribute Cdeeper in the soil profile (Nepstad et al. 1991), where ittends to be better protected and less susceptible to de-composition. C sequestration in many grasslands insemi-arid areas can be increased by reduction of bio-mass burning, by raising the nutrient status of the soiland by introducing improved grasses and legumes incombination with controlled stocking rates (Fisher et

al. 1994).Lal et al. (1995) estimated that with improved landuse, cultivated and (resilient) degraded soils can se-quester 0.11.0 Pg C yr1, depending on management.


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Assuming a recovery of 5066% of C losses since 1860,Cole (1996) estimated that (improved) cultivated soilsglobally could sequester 2030 Pg C over the next 50years. This figure could be in the order of 2344 Pg C ifpermanent set-asides and restoration of degraded landsare also included (Cole 1996). Sampson et al. (1993)found that, globally, agro-ecosystems could act as a Csink, absorbing up to 7 Pg C during the next 50 years byuse of appropriate soil management practices. Thiswould involve increases in production and major im-provements in management on much of the worlds cul-tivated areas, notably in the economically less-develop-ed regions.

In the European Union there is some potential toincrease soil C stocks over the next decades throughchanges in agricultural practices, notably in the contextof the set-aside policies associated with current agricul-tural overproduction. Regression studies by Smith et al.(1997b), based on data for 14 long-term experiments,

showed the increase in soil C over the next 100 years byaddition of animal manure, sewage sludge or strawwould be less than 15 Tg C year1. These increaseswould be obtained in combination with annual crop-ping, implying a concomitant removal of C-containingagricultural produce. Greater potential seems to existthrough afforestation of surplus arable land (;50 Tg Cyear1), and conversion of current arable land to ley/arable systems (;40 Tg C year1) in which organic resi-dues are re-worked into the soil (Smith et al. 1997b).

The scenarios of Smith et al. (1997b) did not consid-er the potential for increased C sequestration in the soil

of new set-aside lands, under grass or legumes togetherwith application of lime and (rock)phosphates. Suchscenarios are worthy of further research as up to2030% of the European Unions cropland may consistof set-aside land by the year 2010. However, this optionmay no longer be available if policies change, for exam-ple due to changing demands for food and fibre cropsor newly perceived environmental priorities.

Adverse environmental side-effects

Some measures that enhance C sequestration in thebiomass and soil, such as the application of N-fertiliz-ers, increase emissions of greenhouse gases such asN2O. In wetland rice soils, incorporation of fresh or-ganic materials may lead to formation and emission ofCH4, another radiatively active gas. Possible (adverse)environmental side-effects of widespread additions ofmanure and sludges to soil, through increased heavymetal and organic pollutant concentrations, should betaken into consideration also.

Land use and management practices which could re-duce emissions of greenhouse gases such as CH4 and

N2O have been reviewed elsewhere (Watson et al.1996; Neue 1997; Smith et al. 1997a). They includemaintaining existing forest cover, slowing deforesta-tion, regenerating natural forests, establishing tree

plantations, promoting agro-forestry, altering manage-ment of agricultural soils and rangelands, improving ef-ficiency of fertilizer use, restoring degraded agriculturallands and rangelands, recovering CH4 from storedmanure, and improving the quality of ruminantsdiets.

Conclusions

Several management practices are available for increas-ing the C content of the soil. These options deservemore attention in programmes aimed at reducing na-tional and global CO2 budgets, similarly to re- or affor-estation and biofuel programmes.

There is a need for better integration of long-termmonitoring, experimental and modelling programmesat different scales, both in space and time, coupled with

development of spatially explicit databases of climate,vegetation, topography, soils and land-use (Oldemanand Van Engelen 1993; Cole 1996; Ingram and Gregory1996; Cramer and Fischer 1997). Attention should bepaid also to comparative assessments of the cost-effec-tiveness i.e. social and economic dimensions of theavailable technical options, both in terms of enhancedC sequestration in the soil and increased sustainability,for example in terms of improved water management,soil fertility and productivity.

Acknowledgements The author thanks Wim Sombroek for valu-able comments on an earlier version of the manuscript.

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