3/30/09 3:32 PMCARBON AND AGRICULTURE:Carbon Sequestration in Soils -- Schlesinger 284 (5423): 2095 -- Science

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Science 25 June 1999:Vol. 284. no. 5423, p. 2095DOI: 10.1126/science.284.5423.2095

POLICY FORUM

CARBON AND AGRICULTURE:Carbon Sequestration in SoilsWilliam H. Schlesinger*

Maintaining and increasing soil organic matter (SOM) adds to soil fertility, waterretention, and crop production. Recently, many soil scientists have suggested that thesequestration of atmospheric carbon dioxide in SOM could also contribute significantlyto attempts to adhere to the Kyoto Protocol. Conversion of large areas of cropland toconservation tillage, including no-till practices, during the next 30 years couldsequester all the CO2 emitted from agricultural activities and up to 1% of today's fossilfuel emissions in the United States (1). Similarly, alternative management ofagricultural soils in Europe could potentially provide a sink for about 0.8% of theworld's current CO2 release from fossil fuel combustion (2). Beyond conservationtillage, however, many of the techniques recommended to increase carbonsequestration in soils contain hidden carbon "costs" in terms of greater emissions ofCO2 into the atmosphere.

Application of nitrogen fertilizer is often recommended to increase SOM, particularlyon lands that have already experienced significant losses in SOM as a result ofcultivation (3). At 100% efficiency, the stoichiometry of the Haber-Bosch process forthe industrial production of ammonia indicates an emission of 0.375 moles of C permole of N produced:

3CH4 + 6H2O 3CO2 + 12H2

4N2 + 12H2 8NH3

Ismail et al. (4) found a sequestration of 1250 grams of carbon per square meter(gC/m2) in soils under conventional tillage, and 1740 gC/m2 under no-till practices inKentucky, during 20 years of continuous cultivation of corn receiving nitrogen fertilizerat an annual rate of 336 kg/ha. However, the CO2 released during the production ofthe fertilizer was equivalent to 334 gC/m2, or 19 to 27% of the carbon sequestration(5). Additional carbon emissions are incurred in the manufacture, transport, and

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application of fertilizer. A factor of 1.436 moles of CO2-C released per mole of N moreaccurately reflects the full carbon cost of N fertilizer (6). This factor would effectivelynegate any net carbon sink as a result of the application of the fertilizer.

Increasing the production of plants on marginal, semiarid lands is another methodfrequently proffered to increase the storage of carbon in soils. In most cases,increasing plant production on these lands will require irrigation, yet irrigation ispotentially associated with large CO2 emissions. The fossil fuel-derived energy used inpumping irrigation water amounts to 22 to 83 gC/m2 per year for irrigated lands in theUnited States (7). This emission is likely to exceed any net carbon sequestration onirrigated agricultural lands. Moreover, groundwaters of arid regions often contain asmuch as 1% dissolved Ca and CO2 versus 0.036% in the atmosphere (8). When suchwaters are applied to arid lands, CO3 is released to the atmosphere and CaCO3precipitates:

Ca2+ + 2HCO3- CaCO3 + H2O + CO2

For example, taking the water-use efficiency of arid-land plants as 1428 g of H2O lostper gram of biomass produced (9) and assuming a 50% C content in plant tissue,0.042 gC is released as CO2 per gram of C fixed in plant biomass. Thus, if irrigationwater containing dissolved Ca at 50 mg/liter is used to increase plant production by200 gC/m2 per year on semiarid land, the net CO2 released from the formation of soilcarbonate would be 8.4 gC/m2 per year. If only 1% of added plant productioncontributes to long-term carbon sequestration in the soil (10), irrigation actuallytransfers CO2 from soils to the atmosphere.

Plants grown under high CO2 concentrations use water more efficiently. Many workershave thus suggested that plant production and soil carbon storage may increase as theconcentration of CO2 rises in Earth's atmosphere. Wood et al. (11), using free-air CO2enrichment (FACE) technology on arid agricultural lands in Arizona, reported smallincreases in soil organic carbon as a result of the growth of cotton under high CO2concentrations. On their wet plot treatments, application of ~1.0 m of irrigation water(12), likely containing Ca concentrations as high as 50 mg/liter, would annually release15 gC/m2 to the atmosphere from the formation of pedogenic carbonate.

Applications of manure are often assumed to increase carbon sequestration in soils(2), but manure is not likely to yield a net sink for carbon in SOM. Buyanovsky andWagner (13) showed that SOM increased with increasing C input from residues andmanure in the Sanborn plots in Missouri. Manure was applied at an annual rate of 1340g/m2 to fields of corn and wheat. The highest levels of production were for corn, up to1100 g/m2 per year. If all of this crop were used for silage, and assuming thedigestion efficiency of livestock is 60% (14), then the production of manure would be440 g/m2. Thus, the entire above-ground plant production on 3.0 ha of land wasrequired to supply the manure to each hectare of manured land. Greaterconcentrations of SOM in manured fields can thus be expected to be associated withdeclining SOM on a proportionally larger area of off-site lands. Manuring has a numberof practical applications, but net carbon sequestration is not one of them.

A substantial sink for carbon in soils may derive from the application of conservationtillage and the regrowth of native vegetation on abandoned agricultural land.Applications of fertilizer, irrigation, and manuring are important agricultural practices,but we should not be overzealous in estimating their contributions to the Kyoto

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Protocol. Treaty negotiators must keep in mind the complexities of a full accounting ofthe carbon emissions and sinks associated with various human activities.

References and Notes

1. J. S. Kern and M. G. Johnson, Soil Sci. Soc. Am. J. 57, 200 (1993).2. P. Smith et al., Global Change Biol. 3, 67 (1997).3. P. E. Rasmussen and C. R. Rohde, Soil Sci. Soc. Am. J. 52, 1114 (1988).4. I. Ismail, R. L. Blevins, W. W. Frye, ibid. 58, 193 (1994).5. A table containing this information resides at

http://www.sciencemag.org.ezproxy1.lib.asu.edu/feature/data/1042216.shl6. R. C. Izaurralde et al., in Management of Carbon Sequestration in Soil, R. Lal et al.,

Eds. (CRC Press, Boca Raton, FL), pp. 433-446. The Intergovernmental Panel onClimate Change recommends a factor of 0.58 for the manufacturing process alone[Revised 1996 Guidelines for National Greenhouse Gas Inventories, ReferenceManual, Organization for Economic Cooperation and Development, Washington, DC(1996)].

7. A. C. Morris, thesis, Princeton University (1999); R. J. Maddigan, W. S. Chern, C. G.Rizy, Am. J. Agr. Econ. 64, 673 (1982).

8. W. W. Wood and M. J. Petraitis, Water Resourc. Res. 20, 1193 (1984).9. R. A. Fischer and N. C. Turner, Annu. Rev. Plant Physiol. 29, 277 (1978).

10. W. H. Schlesinger, Nature 343, 232 (1990).11. C. W. Wood et al., Agr. Forest Meteorol. 70, 103 (1994).12. J. R. Mauney et al., ibid., p. 49.13. G. A. Buyanovsky and G. H. Wagner, Biol. Fert. Soils 27, 242 (1998).14. National Research Council, Nutrient Requirements of Beef Cattle (National Academy

of Sciences, Washington, DC, ed. 7, 1996).15. This paper was prepared while I was a visiting professor at the California Institute

of Technology.

The author is in the Department of Botany and Division of Earth and Ocean Sciences,Duke University, Durham, NC 27708, USA. E-mail: schlesin@duke.edu

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