Climate Change Impacts on Soil Carbon Along Ecological Trajectories at Continental Scale (U.S.).

Over the past decades a changing climate, land use shifts, intensification of management, and disturbances, such as wildfires, have had a tremendous impact on the spatial and temporal variation of soil carbon across the United States. How soil carbon interacts with such changing environmental forcings at the continental scale is still poorly understood. This motivated us to synthesize historic to current (~1925 to 2012) soil organic carbon (SOC) trajectories and 750+ geospatial temporal environmental datasets for the U.S.. The specific objectives were to: (1) Assess spatial and temporal trends in SOC stocks, (2) Identify the interrelationships between climatic factors, land use/land cover, and SOC, and (3) Elucidate on the impact of environmental forcings – specifically climate and moisture, land use/land cover, and humans – on SOC stocks across the U.S. We present an integrated SOC assessment in a soil-landscape about 9.8 million square kilometer in size which covers diverse climatic zones (from semiarid to wet tropical), a multiplicity of biomes, land uses, and management types that have generated a diverse mosaic in SOC from less than 10 Mg ha^-1 to more than 3000 Mg ha^-1 (0-100 cm soil profile). Climate variations (1979 – present) ranged from monthly averages of -12.2 °C to 32.9 °C with profound spatial and temporal variations. Various statistical and mixed statistical/geospatial methods were used to investigate our objectives. The independent validation of SOC models included assessment of accuracy, bias, and model uncertainty. The characterization of SOC stocks along lithologic, pedogenic, topographic, biotic, and climatic trajectories provides new insight into possible causes and effects of soil carbon variation and their role in regional, continental and global carbon cycles.