Stability of organic matter in soils of the Belgian Loess Belt upon erosion and deposition

Soil erosion has significant impacts on terrestrial carbon (C) dynamics. It removes C-rich topsoil and deposits it in lower areas, which might result in its stabilization against microbial decay. Subsequently, C-poor deeper horizons will be exposed, which also affects C stabilization. We analysed factors governing soil organic C (SOC) mineralization in topsoil (5-10 cm) and subsoil (75-100 and 160-200 cm) horizons from two contrasting sites (up-slope compared with down-slope) in the Belgian Loess Belt; we refer to these as eroding and depositional sites, respectively. Deposition of eroded soil material resulted in significantly increased SOC contents throughout the entire soil profile (2 m) and microbial biomass C in the topsoil. In a 28-day incubation experiment we studied effects of O2 concentrations (0, 5 and 20%) and substrate (glucose) availability on C mineralization, soil microbial biomass and CaCl2-extractable C. Carbon enrichment at the depositional site was accompanied by weak mineralization rates and small contents of water-extractable organic C. Addition of glucose stimulated microbial growth and enhanced respiration, particularly in the subsoil of the depositional site. Availability of O2 showed the expected positive relationship with C mineralization in topsoils only. However, small O2 concentrations did not decrease C mineralization in subsoils, indicating that controls on C dynamics were different in top- and subsoils. We conclude that reduced C mineralization contributed to C accumulation as observed at depositional sites, probably because of poor availability of C in subsoil horizons. Limited availability of O2 in subsoils can be excluded as an important control of soil C accumulation. We hypothesize that the composition of the microbial community after burial of the organic-rich material might play a decisive role.