Atmospheric CO2 concentration is increasing as a result of changes in land use and burning of fossil fuels (Watson et al., 1996). Numerous attempts have been made to identify the sizes of C sources and sinks (Rastetter et al., 1992; Schimel, 1995; Fan et al., 1998). Terrestrial grassland ecosystems, which comprise 32% of the earth’s natural vegetation (Adams et al., 1990), play a significant role in uptake of atmospheric CO2 and the global C cycle (Batjes, 1998; Sundquist, 1993). Dixon et al. (1994) estimated that tropical forests contain about 40% of the C stored in terrestrial biomass. Others concluded that temperate regions’ ecosystems also may function as an important C sink that could contribute to balancing the global C budget (Fan et al., 1998; Keeling et al., 1996; Schimel, 1995; Rastetter et al., 1992; Gifford, 1994).