Soil thickness changes across the conterminous USA from 1950 to 2016 shows erosion and conservation effects

It is widely accepted that centuries of soil loss from human-induced erosion and natural causes have reduced the soil resource in the US. In this study, we compiled soil A horizon (n=33,498) and solum (n=19,737) thickness data from 1950 to 2016 across the conterminous US (CONUS) and fitted generalized additive models (GAMs) to quantify the spatial-temporal variations in thickness and their driving factors. The A horizon was the shallowest in the western US including the desert and Rocky Mountain regions, and deepest in the Mississippi River Basin and at the West Coast. The solum was the shallowest in the southwest and Nebraska Sandhills, and deepest in the southeast along the Gulf of Mexico. At the national scale, the A horizon thickness increased by 2.4 cm from 1950 to 1966, then gradually decreased by 3.1 cm until 1993 and leveled with slight increase until 2016. Changes in the A horizon and solum thickness varied in different eco-climatic domains. Climatic drivers determined the spatial variation of soil thickness, in which soil moisture regulated the formation and thickening of A horizon, while temperature dominated the development of the solum. At regional scales, topography, parent materials, and land use controlled the spatial variation of soil thickness. The temporal variation of soil thickness was affected by land use change and erosional process at the regional scale. Conservation practices helped to reduce soil losses since the 1990s. It is concluded that spatial and temporal variation of soil thickness can be explained by environmental factors controlling soil formation and erosion at the national and regional scale.