Rangelands are also importantpools of soil organic carbon stored in soil and vegetation(figure A-4). On a global basis, 9.1 billion ac (3.7 billion ha)of rangeland stores about 20–25percentof the total global terrestrial carbon (306–330 Petagramsof organic carbon and 470–550 Petagramsof inorganic carbon) (A petagram (Pg) is a unit of mass equal to 1015grams) (Batjes 1996; Kimble et al. 2001). On rangelands, carbon sequestration dynamics are quite complex, and estimation of rates and amounts are systematically more difficult than cultivated croplands (Schuman et al. 2002). This is because rangelands havemore heterogeneous soilcharacteristics, wide daily temperature fluctuations, intermittent precipitation, and diverse vegetation life and growth forms(productivity,root-shoot ratios, herbivore use, and imposed disturbance and management practices).(ii) Globally, forests (1.2–1.4 Pg Carbonyr-1) and cropland (0.4–1.2 Pg Carbonyr-1) have the largest potentials for sequestering carbon,although grazinglands (range and pasturelands) can contribute up to 10percentof the overall terrestrial sink capacity. On a global perspective, rangelands occupy about half of the world’s land area, 10percentof the terrestrial biomass, and 10–30percentof the soil organic carbon (Schlesinger 1997). An average estimate of globallysequestered soil carbon on rangelands is 0.5 Pg Carbonyr-1(Schlesinger 1997; Scurlock and Hall 1998). Table A-3shows global and U.S. potential carbon storage for varied terrestrial biomes.