Carbon sequestration in soils: some cautions amidst optimism

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 CO2 from 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 CO2 emissions 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 CO2 that is emitted when energy is used to pump irrigation water and when CaCO3 precipitates 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.