Erosion, deposition and replacement of soil organic carbon in Mediterranean catchments: a geomorphological, isotopic and land use change approach

Determination of whether soil erosion can constitute a net terrestrial carbon dioxide (CO2) sink continues to suffer from lack of sufficient focused studies and field data. Two of the major gaps in our understanding of the erosion induced terrestrial carbon sink issue include rate of eroded soil organic carbon replacement by production of new photosynthate and stability of eroded organic carbon (OC) post deposition. Here we examined the effect of erosion processes and land use change on the stock, type, and stability of OC in two medium-sized subcatchments (18 and 50 ha in size) in SE Spain. We analysed soil samples from drainage areas and depositional settings for stock and isotopic composition of OC (C-14 and C-13), and particle size distribution. In addition, we conducted land use change analysis for the period 1956-2008 and a geomorphological survey of the current erosion processes taking place in the slope-streambed connections. Our findings demonstrate that land use change influenced the dominating erosion processes and, thus, the source of eroding sediments. Carbon isotopes used as tracers revealed that in one of the subcatchments the deposited sediments were derived from deep soil (average delta C-14 of -271.5 parts per thousand) through non-selective erosion processes and channel incision. In the other subcatchment, topsoil material was predominantly eroded and the average delta C-14 in sediments was -64.2 parts per thousand. Replacement of eroded soil OC was taking place in the analysed soil profiles in the slopes suggesting that erosion processes do not necessarily provoke a decrease in soil OC stock over time.