carbon sequestration in soils: some cautions amidst optimism

Agriculture, Ecosystems and Environment 82 (2000) 121–127

Carbon sequestration in soils: some cautions amidst optimism

William H. Schlesinger∗Duke University, Durham, NC 27708-0340, USA

Abstract

A sink for atmospheric carbon (i.e., CO2) in soils may derive from the application of conservation tillage and the regrowth ofnative vegetation on abandoned agricultural land. Accumulations of soil organic matter on these lands could offset emissionsof CO2 from fossil fuel combustion, in the context of the Kyoto protocol. The rate of accumulation of soil organic matter isoften higher on fertilized fields, but this carries a carbon “cost” that is seldom assessed in the form of CO2 emissions duringthe production and application of inorganic fertilizer. Irrigation of semiarid lands may also produce a sink for carbon in plantbiomass, but its contribution to a sink for carbon in soils must be discounted by CO2 that is emitted when energy is used topump irrigation water and when CaCO3 precipitates in the soil profile. No net sink for carbon is likely to accompany the useof manure on agricultural lands. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Carbon sequestration; Fertilizer; Irrigation; Kyoto protocol; Manure; Soil carbon; Soil carbonate; Soil organic matter

1. Introduction

When fully implemented, the Kyoto protocol willrequire most nations of the world to reduce their netemissions of greenhouse gases by agreed, specifiedamounts by the year 2012. Despite the large poolof carbon (C) in soils, and large changes in soil or-ganic matter (SOM) that often accompany human ac-tivities, changes in soil C are not explicitly includedin the current version of the protocol (cf., Article3.4). Agronomists have long recognized the benefits ofmaintaining and increasing SOM, which adds to soilfertility, water retention, and crop production. Increas-ingly, many soil scientists now suggest that the seques-tration of atmospheric carbon dioxide, derived fromfossil fuel combustion, should be added to this listof benefits, contributing to the Kyoto protocol (Bruceet al., 1999). Presently, the IPCC Panel on Land Use,Land Cover and Forestry is devising guidelines by

∗ Tel.: +1-919-660-7406; fax:+1-919-660-7425.E-mail address:[email protected] (W.H. Schlesinger).

which changes in soil C might be included in nationalcarbon accounts.

In the US, lands set aside under the conserva-tion reserve program (CRP) have been small sinksfor atmospheric CO2, accumulating C at rates upto 110 g/m2/yr, or 17× 1012 g C/yr, during the pastdecade (Gebhart et al., 1994). Conservation tillage,including no-till, is also an effective process to se-quester C in some agricultural soils (Rasmussen andCollins, 1991; Reeves, 1997; Lal, 1997; Paustianet al., 1998), although its success varies with soil tex-ture (Needelman et al., 1999), and increases in SOMin the surface layers are sometimes matched by lossesat depth (Angers et al., 1997; McCarty et al., 1998;Campbell et al., 1999; Six et al., 1999). By reducingthe frequency of cultivation, conservation tillage alsoreduces the emissions of CO2 from fossil fuel use inthe agricultural sector (Frye, 1984). Kern and Johnson(1993) calculate that the conversion of large areas ofcropland to conservation tillage during the next 30years could sequester all of the CO2 emitted fromagricultural activities and up to 1% of the total annual

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122 W.H. Schlesinger / Agriculture, Ecosystems and Environment 82 (2000) 121–127

fossil fuel emissions (at today’s levels) in the US.Similarly, improved management and alternative landuse for agricultural soils in Europe could potentiallyprovide a net sink for about 0.8% of the world’s cur-rent annual CO2 release from fossil fuel combustion(Smith et al., 1997).

Beyond conservation tillage, many of the other tech-niques recommended to increase C sequestration insoils contain hidden carbon “costs” in terms of greateremissions of CO2 and other “greenhouse” gases to theatmosphere. The objective of this paper is to examineseveral agricultural practices frequently recommendedto increase C sequestration in soils and estimate howmuch we should “discount” their net contribution tosoil C storage as a result of considering the ancillaryC emissions associated with each practice.

2. Nitrogen fertilizer

Applications of nitrogen (N) fertilizer are oftenrecommended to increase SOM, particularly on landsthat have already experienced significant losses ofSOM as a result of cultivation. Rasmussen and Ro-hde (1988) show a direct linear relationship betweenlong-term N additions and the accumulation of soil

Table 1Gross and net soil carbon sequestration under different crop rotations and low fertilizer applications (from Varvel, 1994)

Continuousa Rotationsb

C SB SG C–SB SG–SB C–OCL–SG–SB C–SB–SG–OC

Mean nitrogen application (kg N/ha/yr) 90 34 90 62 62 62 62

Cumulative N-application(8 years) (mol N/m2)

5.14 1.94 5.14 3.54 3.54 3.54 3.54

Cumulative CO2-“cost” ofN-fertilizerc (g C/m2)

86.4 32.6 86.4 59.5 59.5 59.5 59.5

Change in soil carbon content (g C/m2)0–15 cm +18.6 −20.3 +131.6 +18.6 +22.8 +64.4 +101.215–30 cmd 0 −69.3 −147.0 −29.4 −8.4 −69.3 −54.6

Total +18.6 −119.6 −15.4 −10.8 +14.4 −4.9

CO2 released from fertilizer as aproportion of sequestration (%)

465% nme nm nm 413% nm 128%

a C: corn; SB: soybean; SG: sorghum.bCombinations of these indicate 2- and 4-year rotations involving these crops and clover (OCL).c Calculated using a factor of 1.4 mol C released as CO2 per mole of N fixed in fertilizer.d Assumes bulk density of 1.4 Mg m−3 in 15–30 cm layer.enm: not meaningful due to net loss of soil organic carbon.

organic C in some semiarid soils of Oregon. At 100%efficiency, the stoichiometry of the Haber–Boschprocess for the industrial production of ammonia in-dicates an emission of 0.375 mol of C per mole of Nproduced:

3CH4 + 6H2O → 3CO2 + 12H2 (1)

4N2 + 12H2 → 8NH3 (2)

Accounting for inefficiencies in this industrial process,the IPCC (1996) recommends a factor of 0.58 mol ofC released as CO2 per mole of N fixed in fertilizerproduction, and a factor of 1.4 best represents a fullaccounting of the emissions of CO2 associated with themanufacture, transport and application of N fertilizer(Cole et al., 1993; Izaurralde et al., 1998).

If the factor of 1.4 is applied to the data reportedby Varvel (1994) for the continuous and rotationalcropping of corn (Zea mays), soybean (Glycine max),and sorghum (Sorghum bicolor) in Nebraska, thereported increases in soil organic C in the 0–30 cmlayer are not sufficient to balance the CO2 emissionsassociated with the use of N fertilizer. At low fertil-izer applications, as much as 465% of the apparentC sink in soils was released as CO2 during fertilizerproduction (Table 1). CO2 emissions from fertilizer

W.H. Schlesinger / Agriculture, Ecosystems and Environment 82 (2000) 121–127 123

Table 2Gross and net soil carbon sequestration under different crop rotations and high fertilizer applicatons (from Varvel, 1994) (abbreviation asin Table 1)

Continuous Rotations

C SB SG C–SB SG–SB C–OCL–SG–SB C–SB–SG–OC

Mean nitrogen application(kg N/ha/yr)

180 68 180 124 124 124 124

Cumulative N-application(8 years) (mol N/m2)

10.3 3.9 10.3 7.1 7.1 7.1 7.1

Cumulative CO2-“cost” ofN-fertilizera (g C m2)

173 66 173 119 119 119 119

Change in soil carbon content (g C m−2)0–15 cm +135.8 −36.3 +153.8 −6.9 +89.2 +142.0 +126.115–30 cmb −21.1 −84.0 −157.5 −94.5 +29.4 −54.6 −12.6

Total +114.8 −120.3 −3.7 −101.4 +118.6 +87.4 +113.5

CO2 released from fertilizer as a proportionof sequestration (%)

151% nmc nm nm 100% 136% 105%

a Calculated using a factor of 1.4 mol C released as CO2 per mole of N fixed in fertilizer.b Assumes bulk density of 1.4 Mg/m3 in 15–30 cm layer.cnm: not meaningful due to net loss of soil organic carbon.

also exceeded apparent soil C sequestration at all highfertilizer applications (Table 2). A slightly more fa-vorable balance is calculated from the data of Ismailet al. (1994), in a 20-year study of corn receiving Nfertilizer in Kentucky (Table 3). The annual rates ofC sequestration in these soils (62.5 and 87 g C/m2/yr,respectively) are significantly higher than the rate offormation of humic compounds seen in most naturalsoils (2.5 g C/m2/yr; Schlesinger, 1990). However,during the continuous cultivation of corn, CO2 emis-sions from the use of N fertilizer discount 27–65%of the C sequestration in SOM, and 81% of the in-cremental soil C storage between the 168 kg N/ha/yrtreatment and the 336 kg N/ha/yr treatment. Under

Table 3Gross and net soil carbon sequestration under corn in different management regimes and fertilizer applications (from Varvel, 1994)

Continuous cultivation No-till

N-application (kg N/ha/yr) 0 84 168 336 0 84 168 336

Cumulative N-application in 20 years (mol/m2) 0 12 24 48 0 12 24 48

Cumulative CO2-“cost” of N-fertilizera (g C/m2) 0 201.6 4.03 806.4 0 201.6 403.2 806.4

Change in soil carbon content (0–30 cm) g C/m2 vs. Control +745 +755 +1250 +655 +950 +1005 +1740

CO2 released from fertilizer as a proportion of sequestration (%) 27.1 53.4 64.5 0 21.2 40.1 46.3

a Calculated using a factor of 1.4 mol C released as CO2 per mole of N fixed in fertilizer production, following IPCC guidelines.

no-till practices, the application of N fertilizer at168 kg N/ha/yr produced no net gain in C sequestra-tion over implementation of no-till without any Nfertilizer.

Similarly, data presented by Paustian et al. (1992)show that 41% of the C sequestered in small agri-cultural plots in Sweden must be discounted by theCO2 emissions associated with fertilizer use during a30-year period. During 32 years of corn production inOntario, 62% of the apparent increase in soil organicC was lost via the hidden CO2 costs of N fertilizer(Gregorich et al., 1996). The data of Drinkwater et al.(1998) show that 71% of the C sequestration in fieldswith conventional tillage and N fertilizer is offset by


CO2 emissions associated with fertilizer production.Potter et al. (1997) report no effect of fertilizer onthe sequestration of organic C in semiarid soils of thesouthern Great Plains, and for a dryland cropping sys-tem in Colorado, the fertilizer discount was 65–88% ofapparent C sequestration (Halvorson et al., 1999). Aneven less favorable situation was reported by Jenkin-son (1990) from the continuous wheat (Triticum aes-tivum) plots at Broadbalk, UK. The production of theN fertilizer applied during a 140-year period released905 g C/m2 to the atmosphere, with no apparentincrease in soil organic C.

Given the increasing recognition of environmen-tal problems associated with the release of excessivequantities of N to the environment (Vitousek et al.,1997), it would seem difficult to argue for a greateruse of inorganic N fertilizer as a means of increas-ing the sink for C in soils. Indeed, nitrogenous ferti-lizers are also recognized as a major source of N2O— a potent greenhouse gas (Eichner, 1990). Emis-sions of N2O from the intensification of agricultureappear to account from most of the increase in thisgas in the atmosphere during the past century (Kroezeet al., 1999). Alternatively, the large C “cost” of N fer-tilizer is avoided by agronomic systems that includeleguminous crops that fix N (Van Kessel et al., 1994;Drinkwater et al., 1998).

For a full accounting of potential C credits associ-ated with the management of agriculture soils, ana-logous costs should be estimated for the productionand distribution of phosphorus fertilizer, pesticidesand herbicides. Kern and Johnson (1993) estimatethat the manufacture and application of herbicidesemits the equivalent of 2 g C/m2/yr in no-till systemsof the Great Plains. Notwithstanding, in calculatingC “credits” to the Kyoto protocol, few workers haveprovided appropriately discounted estimates of thenet C sequestration that can be attributed to the appli-cation of fertilizers and the implementation of no-tillpractices on agricultural soils (e.g., Lal et al., 1999).

3. Greening the desert

Increasing the production of plants on marginal,semiarid lands is another method frequently profferedto increase the storage of C in soils. In most cases, in-creasing plant production on these lands will require

irrigation, yet irrigation waters are potentially asso-ciated with large CO2 emissions to the atmosphere.From data presented by Maddigan et al. (1982) onthe electricity used to pump irrigation waters, one cancalculate that 22.5 g C/m2/yr are released during theirrigation of agricultural lands in a 22-state area ofthe US. Similarly, Morris (1998) has estimated thatthe energy used to pump irrigation water amountsto 83 g C/m2/yr for irrigated corn. These emissionsare likely to exceed any net C sequestration on irri-gated agricultural lands. For instance, Lueking andSchepers (1985) report increases in soil organic C of11 g/m2/yr during 15 years of irrigation of croplandsin Nebraska’s sandhill region.

Groundwaters are often extracted from subsur-face environments wherepCO2 is as high as 0.01 vs.pCO2 = 0.00036 in the Earth’s atmosphere (Woodand Petraitis, 1984). When allowed to equilibrate atthe surface, these waters will degas CO2 to the atmo-sphere. The chemistry of these groundwaters oftenevolves in equilibrium with calcite in closed-systemconditions; thus, the groundwaters, and many surfacewaters, of arid regions often contain high concentra-tions of dissolved calcium (Ca). On the High Plains ofTexas, Wood and Petraitis (1984) report Ca concen-trations in groundwater ranging from 38 to 43 mg/l.If such waters are applied to arid lands, dissolved Caprecipitates in the soil, forming CaCO3 and releasingCO2 to the atmosphere, viz.

Ca2+ + 2HCO−3 → CaCO↓ +H2O + CO2 ↑ (3)

Precipitation of calcite is also favored when largeamounts of gypsum (CaSO4·H2O), providing a readysource of Ca, are used to remediate dryland soils(e.g., Amundson and Lund, 1987).

In experiments with several artificial solutions thatwere taken to be representative of the irrigation watersused in arid regions, Bower et al. (1965) and Miyamotoet al. (1975) calculated the degassing of CO2 and pre-cipitation of calcite that would occur if these waterswere transferred from closed- to open-system condi-tions. Annual application of 1 m of irrigation water at40 mg Ca/l would liberate 12 g C/m2/yr as CO2 duringthe precipitation of carbonate. Taking the water-useefficiency of aridland plants as 1428 g H2O lostper gram of biomass produced (Fischer and Turner,1978), and assuming 50% C content in plant biomass,this irrigation would be expected to increase plant


production by about 350 g C/m2/yr. If 1% of addedplant production (i.e., 3.5 g C/m2/yr) contributes tolong-term C sequestration in the soil (Schlesinger,1990), the extraction and application of Ca-rich irri-gation water actually transfers CO2 from soils to theatmosphere! In such a system, net C sequestrationwould only be realized through a greater productionand storage of C in a crop of woody biomass.

Many workers have suggested that the higherwater-use efficiency of plants grown in high CO2will be particularly effective in increasing plant pro-duction and soil C storage in aridland soils as CO2rises in Earth’s atmosphere. Wood et al. (1994) reportsmall increases in soil organic C as a result of thegrowth of cotton (Gossypium hirsutum) at high CO2,using free-air CO2 enrichment (FACE) technology onarid agricultural lands in Arizona. On their wet-plottreatments, application of∼1.0 m of irrigation water(Mauney et al., 1994), likely containing as much as40 mg/l of Ca, would release 12 g C/m2/yr to the at-mosphere by the precipitation of CaCO3-equivalentto about 14% of the sequestration of organic C in the0–20 m layer of these soils during a 2-year period.

For a C credit in terms of the Kyoto protocol, theCO2 emissions associated with the supply of water andfrom the equilibration of those waters in the surfaceenvironment must be subtracted from the apparent netsequestration of C in dryland soils.

4. The myth of manure

Since biblical times, farmers have returned animalwastes to farmland as a means of increasing cropyields (Luke13:8). Applications of manure are oftenassumed to increase C sequestration in soils (Smithet al., 1997), but manure is not likely to yield anetsink for C in soils, as would be required by the Kyotoprotocol. Buyanovsky and Wagner (1998) show in-creasing SOM as a function of increasing C input fromresidues and manure in the Sanborn plots in Missouri.Manure was applied at a rate of 1340 g/m2/yr to fieldsof corn and wheat. In the same fields, the highest levelsof plant production were found in corn, ranging up to1100 g/m2/yr. If this crop were all used for silage andthe digestion efficiency of livestock is 60% (NationalResearch Council, 1996), then the production of ma-nure would be 440 g/m2. The entire aboveground plant

production on 3.0 ha of land would be required to sup-ply the manure to each hectare of manured land. Sim-ilarly, during the 140-year Broadbalk experiment, theannual applications of manure delivered 300 g C/m2/yrto fields in continuous production of wheat (Jenkin-son, 1990). Assuming the 60% efficiency of digestion,this amount of manure would derive from 750 g C infresh plant tissue fed to livestock. This is equivalentto 6.25× the net primary production found in controlplots. Thus, greater levels of SOM in manured fieldscan be expected to be associated with lower inputs ofplant residues on a proportionally larger area of off-sitelands. SOM will decline on those lands, because thereturn of crop residues to the soil is important to themaintenance of SOM in agricultural systems (Havlinet al., 1990; Robinson et al., 1996; Smil, 1999).

Liang and MacKenzie (1992) report on increases insoil organic C during a 6-year experiment with corngrowing with additions of manure (160 g C/m2/yr)and crop residues (355 g C/m2/yr) in eastern Canada.Again, assuming a 60% digestion efficiency, 400 g Cin plant materials were required to produce the160 g C/m2/yr that was added to the experimentalplots as manure. Thus, the total input of organicresidues was equivalent to a hypothetical annual plantproduction of 755 g C/m2 on these fields. Approxi-mately 23.4% (120 g C/m2/yr) of the organic inputswere retained in the soil, or 37 g C/m2/yr derived frommanure and 83 g C/m2/yr from crop residues. Alterna-tively, if 755 g C were allowed to decompose in situ,177 g C would accumulate in SOM. Manuring has anumber of practical applications in agronomy, but netC sequestration does not appear to be one of them.

5. Conclusions

A small sink for C in soils may derive from the ap-plication of conservation tillage and the regrowth ofnative vegetation on abandoned agricultural land. Anyaccumulation of SOM on these lands would contributeto a net sink for CO2 that could offset emissions ofCO2 from fossil fuel combustion and contribute to theKyoto protocol. The rate of accumulation of SOM isoften higher on fertilized fields, but this carries a car-bon “cost” that is seldom assessed in the form of CO2emissions during the production and application ofinorganic fertilizer. Irrigation of semiarid lands may


produce a sink for C in plant biomass, but its contri-bution to a sink for C in soils must be discounted byCO2 that is emitted when energy is used to provideirrigation water and when CaCO3 precipitates in thesoil profile. No net sink for C is likely to accompanythe application of manure on agricultural lands, al-though its use as a soil supplement is often preferableto other means of disposal. Many of these activitiesare important agricultural practices in their own right(Tiessen et al., 1994), but we should not be overzealousin promoting their potential benefits to a sink for C insoils. Intensifying agricultural activity carries substan-tial environmental costs (Matson et al., 1997), whichare better weighed against the demand for food by theEarth’s rapidly increasing human population.

Acknowledgements

The genesis of this paper occurred while its authorenjoyed the hospitality and facilities of the Califor-nia Institute of Technology, where he was a visitingprofessor of biogeochemistry in 1998. It has benefitedfrom discussions and data provided from Kris Havstadand Adele Morris, and critical comments by LaurieDrinkwater, Rattan Lal, Bill Parton, Pedro Sanchez,Pete Smith, Pieter Tans and Daniel Yaalon.

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carbon sequestration in soils: some cautions amidst optimism

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