Modeling long-term carbon dynamics in colluvial soils

A significant part of the soil organic carbon that is eroded in uplands is deposited and buried in colluvial settings. Understanding the fate of this deposited SOC is of key importance for the understanding of the role of (accelerated) erosion in the global C cycle: the residence time of the deposited carbon will determine if, and for how long, accelerated erosion due to human disturbance will induce sequestration of SOC from the atmosphere to the soil. Experimental studies may provide useful information, but, given the time scale under consideration, the response of the colluvial SOC can only be simulated using numerical models. In this study, we present a depth explicit SOC model including soil profile evolution due to sedimentation (ICBM-SE) to simulate the long-term C dynamics in colluvial soils. The SOC profile predicted by our model is in good agreement with field observation. Simulations show that the C storage within a given soil depth in colluvial soils is determined by the sedimentation rate with higher colluvial SOC storages associated with higher deposition rates. It takes centuries for the SOC in the deposited sediments to reach its equilibrium C burial efficiency (the ratio of current C content of the buried sediments to the original C content at the time of sedimentation). Also, the equilibrium C burial efficiency is positively correlated with the sedimentation rate. With increasing sedimentation rate, the contribution from originally associated SOC at the time sedimentation to the bulk SOC (including originally associated SOC and assimilated SOC from input after sedimentation) increases, while that from C input increases with decreasing sedimentation rate. The reason why sedimentation rate is crucial in the long-term dynamics of the deposited SOC is due to the fact that sedimentation rate determines the duration of the sediments at a given depth while the C input and decomposition rate decrease with depth due to the vertical variation of root distribution and soil environmental factors such as humidity, temperature and aeration condition. A better understanding of the long-term C dynamics in colluvial soils is a necessity in order to evaluate the role of soil erosion in global C cycles.