Carbon sequestration in soils: some cautions amidst optimism
William H. Schlesinger
∗ Duke University, Durham, NC 27708-0340, USAAbstract
A sink for atmospheric carbon (i.e., CO2) in soils may derive from the application of conservation tillage and the regrowth of
native vegetation on abandoned agricultural land. Accumulations of soil organic matter on these lands could offset emissions of CO2from fossil fuel combustion, in the context of the Kyoto protocol. The rate of accumulation of soil organic matter is
often higher on fertilized fields, but this carries a carbon “cost” that is seldom assessed in the form of CO2emissions during
the production and application of inorganic fertilizer. Irrigation of semiarid lands may also produce a sink for carbon in plant biomass, but its contribution to a sink for carbon in soils must be discounted by CO2that is emitted when energy is used to
pump irrigation water and when CaCO3precipitates in the soil profile. No net sink for carbon is likely to accompany the use
of 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 will require most nations of the world to reduce their net emissions of greenhouse gases by agreed, specified amounts by the year 2012. Despite the large pool of 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 included in the current version of the protocol (cf., Article 3.4). Agronomists have long recognized the benefits of maintaining and increasing SOM, which adds to soil fertility, water retention, and crop production. Increas-ingly, many soil scientists now suggest that the seques-tration of atmospheric carbon dioxide, derived from fossil fuel combustion, should be added to this list of benefits, contributing to the Kyoto protocol (Bruce et 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: schlesin@duke.edu (W.H. Schlesinger).
which changes in soil C might be included in national carbon accounts.
In the US, lands set aside under the conserva-tion reserve program (CRP) have been small sinks for atmospheric CO2, accumulating C at rates up
to 110 g/m2/yr, or 17×1012g C/yr, during the past decade (Gebhart et al., 1994). Conservation tillage, including no-till, is also an effective process to se-quester C in some agricultural soils (Rasmussen and Collins, 1991; Reeves, 1997; Lal, 1997; Paustian et al., 1998), although its success varies with soil tex-ture (Needelman et al., 1999), and increases in SOM in the surface layers are sometimes matched by losses at depth (Angers et al., 1997; McCarty et al., 1998; Campbell et al., 1999; Six et al., 1999). By reducing the frequency of cultivation, conservation tillage also reduces the emissions of CO2 from fossil fuel use in
the agricultural sector (Frye, 1984). Kern and Johnson (1993) calculate that the conversion of large areas of cropland to conservation tillage during the next 30 years could sequester all of the CO2 emitted from
agricultural activities and up to 1% of the total annual
fossil fuel emissions (at today’s levels) in the US. Similarly, improved management and alternative land use for agricultural soils in Europe could potentially provide 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 in soils contain hidden carbon “costs” in terms of greater emissions of CO2and other “greenhouse” gases to the
atmosphere. The objective of this paper is to examine several agricultural practices frequently recommended to increase C sequestration in soils and estimate how much we should “discount” their net contribution to soil C storage as a result of considering the ancillary C emissions associated with each practice.
2. Nitrogen fertilizer
Applications of nitrogen (N) fertilizer are often recommended to increase SOM, particularly on lands that have already experienced significant losses of SOM as a result of cultivation. Rasmussen and Ro-hde (1988) show a direct linear relationship between long-term N additions and the accumulation of soil
Table 1
Gross 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” of
N-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 +101.2
15–30 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
CO2released from fertilizer as a
proportion of sequestration (%)
465% nme nm nm 413% nm 128%
aC: corn; SB: soybean; SG: sorghum.
bCombinations of these indicate 2- and 4-year rotations involving these crops and clover (OCL). cCalculated using a factor of 1.4 mol C released as CO
2 per mole of N fixed in fertilizer. dAssumes 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–Bosch process for the industrial production of ammonia in-dicates an emission of 0.375 mol of C per mole of N produced:
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 of C released as CO2 per mole of N fixed in fertilizer
production, and a factor of 1.4 best represents a full accounting of the emissions of CO2associated with the
manufacture, 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 reported by Varvel (1994) for the continuous and rotational cropping of corn (Zea mays), soybean (Glycine max), and sorghum (Sorghum bicolor) in Nebraska, the reported increases in soil organic C in the 0–30 cm layer are not sufficient to balance the CO2emissions
associated with the use of N fertilizer. At low fertil-izer applications, as much as 465% of the apparent C sink in soils was released as CO2 during fertilizer
Table 2
Gross and net soil carbon sequestration under different crop rotations and high fertilizer applicatons (from Varvel, 1994) (abbreviation as in 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” of
N-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 +126.1
15–30 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
CO2released from fertilizer as a proportion
of sequestration (%)
151% nmc nm nm 100% 136% 105%
aCalculated using a factor of 1.4 mol C released as CO
2per mole of N fixed in fertilizer. bAssumes 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 high fertilizer applications (Table 2). A slightly more fa-vorable balance is calculated from the data of Ismail et al. (1994), in a 20-year study of corn receiving N fertilizer in Kentucky (Table 3). The annual rates of C sequestration in these soils (62.5 and 87 g C/m2/yr, respectively) are significantly higher than the rate of formation of humic compounds seen in most natural soils (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/yr treatment and the 336 kg N/ha/yr treatment. Under
Table 3
Gross 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
CO2released from fertilizer as a proportion of sequestration (%) 27.1 53.4 64.5 0 21.2 40.1 46.3 aCalculated using a factor of 1.4 mol C released as CO
2per mole of N fixed in fertilizer production, following IPCC guidelines.
no-till practices, the application of N fertilizer at 168 kg N/ha/yr produced no net gain in C sequestra-tion over implementasequestra-tion of no-till without any N fertilizer.
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 the CO2emissions associated with fertilizer use during a
30-year period. During 32 years of corn production in Ontario, 62% of the apparent increase in soil organic C was lost via the hidden CO2 costs of N fertilizer
CO2 emissions associated with fertilizer production.
Potter et al. (1997) report no effect of fertilizer on the sequestration of organic C in semiarid soils of the southern Great Plains, and for a dryland cropping sys-tem in Colorado, the fertilizer discount was 65–88% of apparent C sequestration (Halvorson et al., 1999). An even less favorable situation was reported by Jenkin-son (1990) from the continuous wheat (Triticum
aes-tivum) plots at Broadbalk, UK. The production of the
N fertilizer applied during a 140-year period released 905 g C/m2 to the atmosphere, with no apparent increase in soil organic C.
Given the increasing recognition of environmen-tal problems associated with the release of excessive quantities of N to the environment (Vitousek et al., 1997), it would seem difficult to argue for a greater use 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 agriculture
appear to account from most of the increase in this gas in the atmosphere during the past century (Kroeze et al., 1999). Alternatively, the large C “cost” of N fer-tilizer is avoided by agronomic systems that include leguminous 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 production and distribution of phosphorus fertilizer, pesticides and herbicides. Kern and Johnson (1993) estimate that the manufacture and application of herbicides emits the equivalent of 2 g C/m2/yr in no-till systems of the Great Plains. Notwithstanding, in calculating C “credits” to the Kyoto protocol, few workers have provided appropriately discounted estimates of the net C sequestration that can be attributed to the appli-cation of fertilizers and the implementation of no-till practices 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 proffered to 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) on the electricity used to pump irrigation waters, one can calculate that 22.5 g C/m2/yr are released during the irrigation of agricultural lands in a 22-state area of the US. Similarly, Morris (1998) has estimated that the energy used to pump irrigation water amounts to 83 g C/m2/yr for irrigated corn. These emissions are likely to exceed any net C sequestration on irri-gated agricultural lands. For instance, Lueking and Schepers (1985) report increases in soil organic C of 11 g/m2/yr during 15 years of irrigation of croplands in Nebraska’s sandhill region.
Groundwaters are often extracted from subsur-face environments where pCO2is as high as 0.01 vs.
pCO2 = 0.00036 in the Earth’s atmosphere (Wood
and Petraitis, 1984). When allowed to equilibrate at the surface, these waters will degas CO2to the
atmo-sphere. The chemistry of these groundwaters often evolves in equilibrium with calcite in closed-system conditions; thus, the groundwaters, and many surface waters, of arid regions often contain high concentra-tions of dissolved calcium (Ca). On the High Plains of Texas, 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 Ca precipitates in the soil, forming CaCO3and releasing
CO2to the atmosphere, viz.
Ca2++2HCO−3 →CaCO↓ +H2O+CO2↑ (3)
Precipitation of calcite is also favored when large amounts of gypsum (CaSO4·H2O), providing a ready
source of Ca, are used to remediate dryland soils (e.g., Amundson and Lund, 1987).
In experiments with several artificial solutions that were taken to be representative of the irrigation waters used in arid regions, Bower et al. (1965) and Miyamoto et al. (1975) calculated the degassing of CO2and
pre-cipitation of calcite that would occur if these waters were transferred from closed- to open-system condi-tions. Annual application of 1 m of irrigation water at 40 mg Ca/l would liberate 12 g C/m2/yr as CO2during
the precipitation of carbonate. Taking the water-use efficiency of aridland plants as 1428 g H2O lost
production by about 350 g C/m2/yr. If 1% of added plant production (i.e., 3.5 g C/m2/yr) contributes to long-term C sequestration in the soil (Schlesinger, 1990), the extraction and application of Ca-rich irri-gation water actually transfers CO2from soils to the
atmosphere! In such a system, net C sequestration would only be realized through a greater production and storage of C in a crop of woody biomass.
Many workers have suggested that the higher water-use efficiency of plants grown in high CO2
will be particularly effective in increasing plant pro-duction and soil C storage in aridland soils as CO2
rises in Earth’s atmosphere. Wood et al. (1994) report small increases in soil organic C as a result of the growth of cotton (Gossypium hirsutum) at high CO2,
using free-air CO2enrichment (FACE) technology on
arid agricultural lands in Arizona. On their wet-plot treatments, application of∼1.0 m of irrigation water (Mauney et al., 1994), likely containing as much as 40 mg/l of Ca, would release 12 g C/m2/yr to the at-mosphere by the precipitation of CaCO3-equivalent
to about 14% of the sequestration of organic C in the 0–20 m layer of these soils during a 2-year period.
For a C credit in terms of the Kyoto protocol, the CO2emissions associated with the supply of water and
from the equilibration of those waters in the surface environment must be subtracted from the apparent net sequestration of C in dryland soils.
4. The myth of manure
Since biblical times, farmers have returned animal wastes to farmland as a means of increasing crop yields (Luke 13:8). Applications of manure are often assumed to increase C sequestration in soils (Smith et al., 1997), but manure is not likely to yield a net sink for C in soils, as would be required by the Kyoto protocol. Buyanovsky and Wagner (1998) show in-creasing SOM as a function of inin-creasing C input from residues and manure in the Sanborn plots in Missouri. Manure was applied at a rate of 1340 g/m2/yr to fields of corn and wheat. In the same fields, the highest levels of plant production were found in corn, ranging up to 1100 g/m2/yr. If this crop were all used for silage and the digestion efficiency of livestock is 60% (National Research 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, the annual applications of manure delivered 300 g C/m2/yr to 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 in fresh plant tissue fed to livestock. This is equivalent to 6.25×the net primary production found in control plots. Thus, greater levels of SOM in manured fields can be expected to be associated with lower inputs of plant residues on a proportionally larger area of off-site lands. SOM will decline on those lands, because the return of crop residues to the soil is important to the maintenance of SOM in agricultural systems (Havlin et al., 1990; Robinson et al., 1996; Smil, 1999).
Liang and MacKenzie (1992) report on increases in soil organic C during a 6-year experiment with corn growing 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 C in plant materials were required to produce the 160 g C/m2/yr that was added to the experimental plots as manure. Thus, the total input of organic residues was equivalent to a hypothetical annual plant production of 755 g C/m2 on these fields. Approxi-mately 23.4% (120 g C/m2/yr) of the organic inputs were retained in the soil, or 37 g C/m2/yr derived from manure 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 a number of practical applications in agronomy, but net C 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 of native vegetation on abandoned agricultural land. Any accumulation of SOM on these lands would contribute to a net sink for CO2 that could offset emissions of
CO2from fossil fuel combustion and contribute to the
Kyoto protocol. The rate of accumulation of SOM is often higher on fertilized fields, but this carries a car-bon “cost” that is seldom assessed in the form of CO2
produce a sink for C in plant biomass, but its contri-bution to a sink for C in soils must be discounted by CO2 that is emitted when energy is used to provide
irrigation water and when CaCO3 precipitates in the
soil profile. No net sink for C is likely to accompany the application of manure on agricultural lands, al-though its use as a soil supplement is often preferable to other means of disposal. Many of these activities are important agricultural practices in their own right (Tiessen et al., 1994), but we should not be overzealous in promoting their potential benefits to a sink for C in soils. Intensifying agricultural activity carries substan-tial environmental costs (Matson et al., 1997), which are better weighed against the demand for food by the Earth’s rapidly increasing human population.
Acknowledgements
The genesis of this paper occurred while its author enjoyed the hospitality and facilities of the Califor-nia Institute of Technology, where he was a visiting professor of biogeochemistry in 1998. It has benefited from discussions and data provided from Kris Havstad and Adele Morris, and critical comments by Laurie Drinkwater, Rattan Lal, Bill Parton, Pedro Sanchez, Pete Smith, Pieter Tans and Daniel Yaalon.
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