Environmental and Geological Controls on the Soil Carbon Cycle In a Changing World

Soil C is the balance between plant inputs and biological and physical losses. Nearly 20% of anthropogenic CO₂ emissions are from land use changes, but reducing this loss requires understanding the natural and management-specific impacts on soil C. The main control on the soil C cycle is climate. Radiocarbon measurements have shown that decomposition accelerates with increasing temperature and that under projected warming scenarios soils should release CO₂ and provide a positive feedback to warming. Ecosystem-scale C fluxes also reveal a strong temperature effect on soil C residence times. Recently-assessed reductions in soil C storage in Great Britain provide support for the concern about positive feedbacks to warming. Recent research has refined the rates of C erosion and its fate in both cultivated and undisturbed upland ecosystems. Briefly, soil C on actively eroding hillslopes is in a perpetual non-steady state due to ongoing erosion and deposition. Additionally, vegetated upland watersheds act as filters for large annual fluxes of DOC, ultimately releasing small refractory C pools to aqueous systems. In cultivated soils, the changes in soil C are further impacted by climate. Comparative studies of cultivated and non-cultivated soils in the Great Plains and India show that fractional losses of soil C during cultivation are greater under humid climates. Recent ¹⁴C work shows that large-scale soil structure significantly controls soil C storage, and that disruption of this structure by tillage can rapidly release pools of C that have been stored for millennia. While improved and scientifically-informed management of agricultural ecosystems is an important tool in mitigating anthropogenic greenhouse gas emissions, the most significant impact on C emissions can be achieved by reducing the rates of global land conversion.