Soil Carbon Dynamics in US Agricultural Ecosystems

Historically, a significant C emission source, agricultural soils in the US are now largely in a state of `recovery' with respect to soil C stocks. Estimates from national inventory methods, employing either empirical-based models or dynamic simulation approaches, show soil C accumulations occurring on most agricultural soils, whereas a relatively small area of cultivated histosols(< 1 Mha) are a net source of CO2. Net soil C storage is estimated at around 10-20 Tg C /year, where mineral soils are accumulating 15-25 Tg/year and organic soils (histosols) are emitting 5-10 Tg C per year. C sink and source strengths vary considerably across the continental US. Regional differences in land use and management practices and climatic conditions are the main drivers of the spatial heterogeneity in sinks/sources. The availability, quality and limitations of the driving variables needed for national-scale estimates of agricultural C dynamics are discussed along with methods and results of uncertainty analyses.