Direct soil moisture controls of future global soil carbon changes: An important source of uncertainty

The nature of the climate-carbon cycle feedback depends critically on the response of soil carbon to climate, including changes in moisture. However, soil moisture-carbon feedback responses have not been investigated thoroughly. Uncertainty in the response of soil carbon to soil moisture changes could arise from uncertainty in the relationship between soil moisture and heterotrophic respiration. We used twelve soil moisture-respiration functions (SMRFs) with a soil carbon model (RothC) and data from a coupled climate-carbon cycle general circulation model to investigate the impact of direct heterotrophic respiration dependence on soil moisture on the climate-carbon cycle feedback. Global changes in soil moisture acted to oppose temperature-driven decreases in soil carbon and hence tended to increase soil carbon storage. We found considerable uncertainty in soil carbon changes due to the response of soil respiration to soil moisture. The use of different SMRFs resulted in both large losses and small gains in future global soil carbon stocks, whether considering all climate forcings or only moisture changes. Regionally, the greatest range in soil carbon changes across SMRFs was found where the largest soil carbon changes occurred. Further research is needed to constrain the soil moisture-respiration relationship and thus reduce uncertainty in climate-carbon cycle feedbacks. There may also be considerable uncertainty in the regional responses of soil carbon to soil moisture changes since climate model predictions of regional soil moisture changes are less coherent than temperature changes.