Deep Soil Carbon and Its Vulnerability to Global Environmental Change: Process Understanding and Representation in Earth System Models

Abstract:

Because more than 80% of the world's soil organic carbon (SOC) is found below 20 cm depth, deep SOC has the potential to form large positive feedbacks to climate change. According to climate projections, temperature changes at 1 m depth will largely keep pace with warming air temperatures over the next century. While surface SOC decomposes exponentially faster with warming, little is known about the temperature response of deeper carbon, or the modes of stabilization of deeper SOC. Most biogeochemical studies have been limited to the surface soil. Likewise, soil biogeochemical models in Earth System Models are parameterized for surface soil and lack mechanisms important for stabilization of deep SOC, such as organo-mineral associations. Radiocarbon observations in different soil types show that SOC residence of time of SOC increases with depth, with residences times over 10,000 years in many soils by 1 m deep. That means this SOC has accumulated slowly over time; if stabilization mechanisms are disrupted, leading to large changes in decomposition rates, SOC losses are unlikely to be compensated for by increased in plant inputs. Therefore, we suggest that a critical challenge for ESMs is to achieve process-level understanding and the ability to predict whether, and how, the large stores of old SOC can be made rapidly accessible to microbial transformation.