The Second State of the Carbon Cycle Report: The Persistence and Vulnerability of Carbon in Soil

Soils are a critical reservoir for the global C cycle, containing approximately 3 times the atmospheric C load, and more than twice the amount of C cycling in the surface oceans. Yet soil is not a stagnant reservoir, as evidenced by traditional classifications of different soils as C "sinks" or "sources." Soil C is highly dynamic and the soil C cycle shifts in response to land use, and to climate and weather perturbations. These shifts from nominally "stable" pools to "active" are likely a function of new litter inputs, microbial decomposition, and physical and chemical protection mechanisms; net losses include gas fluxes and run-off, both of which will significantly affect other global C compartments. These traits, and others, are being used to frame and advance soil C cycle models and also global C models to more accurately predict feedbacks from the soil to the physical Earth system via greenhouse gas emissions. The Second State of the Carbon Cycle Report comprehensively addresses the role of soils in the global C cycle for the United States, Canada, and Mexico.