The terrestrial biosphere contains large reserves of carbon (C) — about 1500 Pg of C in the surface meter of soil (Batjes, 1996; Eswaran et al., 1995) and another 600 Pg of C in vegetation (Houghton, 1995; Schimel, 1995). Together, these pools contain three times as much C as the atmosphere and, consequently, a small change in C storage in plants or soils has important implications for atmospheric CO2. This relationship has gained attention with the recognition that atmospheric CO2 content is increasing and with the consequent reevaluation of ways to increase C storage in the biosphere and thereby reduce atmospheric CO2 levels.