Abstract
Deep soils are located in most continents of the world. Soil carbon measurements are invariably made from the surface horizons, whereas much larger carbon stores occur to depths of many meters, with plant roots providing the main source of carbon. This root biomass persists long after land is deforested for agricultural and other pursuits or forests are killed by pests and fires and may represent a considerable carbon store at the global scale. The impacts on these carbon stores of reforestation or climate change are mostly unknown as the estimation of root biomass and carbon dynamics is challenging in deep soils. This chapter explores deep soil carbon from the perspectives of its definition, source, and persistence; methodologies available to study deep soil carbon; and the effect of land-use change on this carbon store.