Grazing and seasonal variation in precipitation and temperature are important controls of soil and plant processes in grasslands. As these ecosystems store up to 30% of the world's belowground carbon (C), it is important to understand how this variability affects mineral soil C pools/fluxes, and how C cycling might be affected by changes in precipitation and temperature, due to climate change. The aim of this study was to investigate the effects of grazing and differences in soil temperature and moisture on standard organic matter (OM) decomposition rates (cotton cloth) incubated in the top 10 cm soil of grasslands with variable topography in Yellowstone National Park (YNP) during the 2004 growing season. Grazing did not affect soil temperature, moisture, cotton cloth decomposition rates, soil bulk density, soil C and N concentrations, or soil C:N ratios. However, a large spatio-temporal variability in decomposition was observed: cotton cloth decomposition was positively related to soil moisture and soil C and N concentrations, and negatively to soil temperature. Highest decomposition rates were found in wetter slope bottom soils [season averages of decomposition given as rate of decomposition (cotton rotting rate=CRR)=23-26%] and lower rates in drier, hill-top soils (season averages, CRR = 20%). Significantly higher decomposition rates were recorded in spring, early summer and early fall when soils were moist and cool (spring, CRR=25%; early summer, CRR=26%; fall, CRR=20%) compared to mid-summer (CRR=18%) when soils were dry and warm. Our findings suggest that climate-change related decreases in precipitation and increases in temperature predicted for North American grasslands would decrease soil OM decomposition in YNP, which contrasts the general assumption that increases in temperature would accelerate OM decomposition rates.