Root and soil carbon distribution at shoulderslope and footslope positions of temperate toposequences cropped to winter wheat

• Shoot and macro-root DM biomass is higher in footslope than in shoulderslope soil.
• Soil C at shoulderslope positions mainly originates from macro-root biomass.
• Management should aim at increasing soil C stocks especially in shoulderslope soil.

Crop root residues are an important source of soil organic carbon (SOC) in arable systems. However, the spatial distribution of root biomass in arable systems remains largely unknown. In this study, we determined the spatial distribution of macro-root and shoot biomass of winter wheat at shoulderslope and footslope positions from four cultivated slopes within an arable field in western Denmark. In addition, soils from the shoulderslope and footslope positions of four slopes were characterized for physical and chemical properties. Root biomass dry matter (DM) was marginally higher (P = 0.06) at footslope (1.2 Mg DM ha- 1) than at shoulderslope positions (0.9 Mg DM ha− 1), in particular in the subsoil. Likewise shoot biomass was higher (P = 0.03) at footslope (10.3 Mg DM ha− 1) compared to shoulderslope (7.1 Mg DM ha− 1) positions. Soil bulk density increased with depth at shoulderslope positions, but was more variable with depth at footslope positions. Root C was significantly correlated with SOC in shoulderslope soils (r = 0.98), but not in footslope soils. We conclude that, at shoulderslope positions, SOC originated mainly from root residues whereas at footslope positions, SOC was derived from both root residues and likely soil redistribution processes. Management practices that increase C input at shoulderslope positions potentially enhance soil carbon storage and increase crop productivity, which would probably not be the case for C rich footslope soils. These findings imply that models used to simulate or predict C dynamics and crop productivity should consider topography-controlled variations in root C input and SOC redistribution as well as their effects on soil properties, root growth and crop productivity.