The Conservation Reserve Program (CRP) was originally implemented to protect highly erodible land.  Subsequently, it was extended to protect marginally-productive farmland, establish buffers and sequester soil organic C (SOC). With increasing economic pressure from biofuels, current CRP acreage is expected to decrease and conversion into cropped systems could promote, rather than mitigate, soil C storage in agriculture. Measured values of SOC stocks are crucial as a baseline to predict land-use change impacts on SOC stocks from biofuels conversion.  The objectives of this study were to quantify SOC stocks in CRP versus paired cropped and native grassland sites at 14 locations across the Great Plains and to assess the relative stabilization of C in particulate organic matter (POM) and mineral-associated fractions. Soil C stocks were strongly driven by temperature, with the greatest SOC in cool climates. CRP averaged 151.1 Ma C ha^-1 (0-30 cm), greater than cropped soils (118.3 Ma C ha^-1), but less than native (183.6 Ma C ha^-1).  Greatest loss of SOC under cropped compared to native treatments occurred in cooler climates.  Distribution of C between fractions was influenced by treatment, with mineral-associated C comprising 83% of total SOC stocks under cropping, 79% under CRP and 72% in native sites.  The POM-C increased from 17% under cropped to 21% with restoration under CRP compared to 27% under native. If CRP lands are converted into cropping for production of biofuels, our data suggest that the largest SOC loss will be in cool climates that presently have the greatest SOC stocks.