Organic Carbon in Soil and the Global Carbon Cycle

Abstract

Soil organic matter is, simultaneously, the most inert carbon cycle component of terrestrial ecosystems, and the most dynamic component of terrestrial geologic systems placing it in a pivotal position in the biogeochemistry of carbon. The large size and potentially long residence time of the soil organic matter pool make it an important component of the global carbon cycle (Schlesinger 1977, Post et al. 1982, 1985). Figure 1 shows the relationship of the soil organic matter pool to other components of global terrestrial ecosystems with both significantly longer and shorter turnover times. Net terrestrial primary production of about 60 Pg C·yr^-1 is, over a several-year period of time, balanced by an equivalent flux of litter production and subsequent decomposition of detritus and soil organic matter (Post et al. 1990). Using estimates in Figure 1, the turnover time, T _t , of all organic matter (litter and soil of Figure 1) globally is

$$T_t=\frac{litter+soil}{litterfall}=\frac{1372}{62}=22 years$$

(1)

However, the input rates and decomposition rates for different terrestrial ecosystems vary over several orders of magnitude resulting in widely different amounts and turnover rates of soil organic matter. The amounts of carbon stored in soils and the rates of exchange of soil carbon with the atmosphere depend on many factors related to the chemistry, biology, and physics of soil and soil organic matter.

Research sponsored by the U.S. Department of Energy, Carbon Dioxide Research Program, Atmospheric and Climate Research Division, Office of Health and Environmental Research, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Publication No. 3876, Environmental Science Division, Oak Ridge National Laboratory.