Technical Abstract: Grazing lands represent the largest and most diverse single land resource in the world, and as such, are an important terrestrial sink for carbon (C). Native grazing lands of the western U.S. (including Alaska and Hawaii) occupy over 300 Mha, more than twice the land area that is occupied by annual and perennial crops in the U.S. However, few data exist quantifying C emissions and storage in these native ecosystems. Accurate long-term field measurements of both organic and inorganic forms of rangeland soil C are needed to develop and validate empirical model estimates of C stocks and management-induced changes in C sequestered. Measurement accuracy is particularly important in semi-arid and arid ecosystems where annual changes in soil C, as the result of changes in management, frequently are very small relative to the spatial heterogeneity of C stocks. Critical factors influencing the magnitude of variance in soil C estimates include horizontal and vertical C distribution patterns, and consistent sample collection and processing protocols. By careful stratification of soil sampling both across landscape and within the soil profile, significant management-induced differences in soil C pools have been detected in northern mixed-grass prairie, short-grass steppe and sagebrush steppe.