Carbon cycling in eroding landscapes: geomorphic controls on soil organic C pool composition and C stabilization

Recent studies have highlighted the tight coupling between geomorphic processes and soil carbon (C) turnover and suggested that eroding landscapes can stabilize more C than their non-eroding counterparts. However, large uncertainties remain and a mechanistic understanding of geomorphic effects on C storage in soils is still lacking. Here, we quantified the soil organic carbon (SOC) stock and pool distribution along geomorphic gradients and combined data derived from soil organic matter fractionation and incubation experiments. The size and composition of the SOC pools were strongly related to geomorphic position: 1.6 to 6.2 times more C was stabilized in the subsoils (25100cm) of depositional profiles than in those of eroding profiles. Subsoil C of depositional profiles is predominantly associated with microaggregates and silt-sized particles which are associated with pools of intermediate stability. We observed a significantly higher mean residence time for the fast and intermediate turnover pools of buried C at depositional positions, relative to non-eroding and eroding positions, resulting from the physical protection of C associated with microaggregates and silt particles. Conversely, significant amounts of C were replaced at eroding positions but the lower degree of decomposition and the lack of physically protected C, resulted in higher respiration rates. By considering C cycling at non-eroding, eroding and depositional positions, we found that the eroding landscapes studied store up to 10% more C due to soil redistribution processes than non-eroding landscapes. This is the result of the stabilization of C in former subsoil at eroding positions and partial preservation of buried C in pools of intermediate turnover at depositional positions. However, the sink strength was limited by significant losses of buried C as only a small fraction of the C was associated with stable pools.