Exploring particle size distribution and organic carbon pools mobilized by different erosion processes at the catchment scale

Purpose Selective water erosion of soil organic carbon (OC) has been reported to occur in small plots under field and laboratory conditions. However, little is known about how the process of OC erosion happens at larger scales. This study was conducted to assess the differences in the selectivity of OC between different erosion processes taking place within the same catchment. Materials and methods A geomorphological survey of the hillslope-streambed connections within a catchment in SE Spain was conducted to determine and select the dominant erosion forms to be studied. Undisturbed sediment samples were taken from deposits derived from four different erosion processes (gully, interrill, bank and tillage erosion) and from the streambed sediment bars. Soils of the three main land uses (forest, shrubland and agricultural fields) of the drainage area were also sampled to establish comparisons with the sampled sediments and determine the enrichment ratios. Different organic carbon pools (size fractions) and particle size distribution, both dispersed and non-dispersed, were studied in soils and sediments. Results and discussion All sediments were impoverished in OC compared to the source soils of the catchment. The largest differences were found between bank erosion and tillage erosion processes. Particle size distribution analysis indicated that soil particles were mainly being transported as aggregates. In addition, aggregates in the 250-63 mu m size fraction seemed to contain primary silt and clay particles, which were positively correlated to total organic carbon. Bar sediments downstream in the streambed were impoverished in mineral-associated organic carbon, suggesting that this OC fraction can be carried further downstream by remaining in suspension longer due to its binding to smaller mineral particles. Conclusions In general terms, there are indications that different erosion processes can result in different organic carbon enrichment ratios. Although selectivity during detachment, transport and deposition results in a complex set of interactions that need further research and field-based data, in this study, the nature of the erosion process, the characteristics of the source material and the level of aggregation of soil particles were found to be important variables.