Linking soil organic matter dynamics and erosion-induced terrestrial carbon sequestration at different landform positions

Recently, the potential for terrestrial carbon (C) sequestration by soil erosion and deposition has received increased interest. Erosion and deposition constitute a sink for atmospheric carbon dioxide relative to a preerosional state or a noneroding scenario, if the posterosion watershed C balance is increased due to (1) partial replacement of eroded C by new photosynthate in the eroded site; and (2) preservation from decomposition of at least some eroded soil organic carbon (SOC) arriving in depositional settings. Little is known, however, about differences in C dynamics at different erosional and depositional landform positions within the same eroding system. We determined the contribution of different landform positions to erosion-induced terrestrial C sequestration by measuring rates of net primary productivity (NPP), replacement of eroded C, and decomposition of organic matter (OM) at four categorically different landform positions within a naturally eroding toposequence in northern California. We found that eroded C is replaced by NPP 15 times over in the summit of the site studied and 5 times over in the slope. Profile-averaged, long-term rate constant for SOM decomposition was 2 to 14 times slower in the depositional settings compared with that in eroding slopes. As a result, the inventory of C in the depositional settings was 2 to 3 times larger than that of the eroding positions. Owing to both C replacement at eroding sites and reduced rates of OM decomposition in depositional sites, soil erosion constitutes a C sink from the atmosphere at our study site.