Long-term dynamics of buried organic carbon in colluvial soils

Colluvial soils are enriched in soil organic carbon (SOC) in comparison to the soils of upslope areas due to the deposition and subsurface burial of SOC. It has been suggested that the burial of SOC has important implications for the global carbon cycle, but the long-term dynamics of buried SOC remains poorly constrained. We address this issue by determining the SOC burial efficiency (i.e., the fraction of originally deposited SOC that is preserved in colluvial deposits) of buried SOC as well as the SOC stability in colluvial soils. We quantify the turnover rate of deposited SOC by establishing sediment and SOC burial chronologies. The SOC stability is derived from soil incubation experiments and the δ13C values of SOC. The C burial efficiency was found to decrease exponentially with time reaching a constant ratio of approximately 17%. This exponential decrease is attributed to the increasing recalcitrance of buried SOC with time and a less favourable environment for SOC decomposition with increasing depth. Buried SOC is found to be more stable and degraded in comparison to SOC sampled at the same depth at a stable site. This is due to preferential mineralization of the labile fraction of deposited SOC resulting in enrichment of more degraded and recalcitrant SOC in colluvial soils. In order to better understand the long-term effects of soil erosion for the global C cycle, the temporal variation of deposited SOC and its controlling factors need to be characterized and quantified.