Carbon emissions from cropland expansion in the United States

After decades of decline, croplands are once again expanding across the United States. A recent spatially explicit analysis mapped nearly three million hectares of US cropland expansion that occurred between 2008 and 2012. Land use change (LUC) of this sort can be a major source of anthropogenic carbon (C) emissions, though the effects of this change have yet to be analyzed. We developed a data-driven model that combines these high-resolution maps of cropland expansion with published maps of biomass and soil organic carbon stocks (SOC) to map and quantify the resulting C emissions. Our model increases emphasis on non-forest-i.e. grassland, shrubland and wetland-above and belowground biomass C stocks and the response of SOC to LUC-emission sources that are frequently neglected in traditional Caccounting. These sources represent major emission conduits in the US, where new croplands primarily replace grasslands. We find that expansion between 2008-12 caused, on average, a release of 55.0 MgCha(-1) (SDspatial = 39.9 MgCha(-1)), which resulted in total emissions of 38.8 TgC yr(-1) (95% CI = 21.6-55.8 TgC yr(-1)). We also find wide geographic variation in both the size and sensitivity of affected Cstocks. Grassland conversion was the primary source of emissions, with more than 90% of these emissions originating from SOC stocks. Due to the long accumulation time of SOC, its dominance as a source suggests that emissions may be difficult to mitigate over human-relevant time scales. While methodological limitations regarding the effects of land use legacies and future management remain, our findings emphasize the importance of avoiding LUC emissions and suggest potential means by which natural C stocks can be conserved.