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Reforestations and check-dams are two commonly used measures to reduce soil erosion rates and sediment export from highly eroding catchments. Here, we evaluated the impact of the construction of 94 check-dams and land use changes (caused by agricultural abandonment and reforestations) on sediment yield in the Upper Taibilla catchment (320 km 2 , SE Spain) from 1956 to 2000. We combined land use change analysis, field surveys, and application of the WaTEM-SEDEM erosion and sediment yield prediction model for nine scenarios combining land use maps (1956, 1987, and 2000) and different numbers of check-dams throughout the catchment. Land use changes alone reduced sediment yield up to 14%, but in combination with check-dams, the reduction in sediment yield reached 44 ± 6%. Sediment yield reduction was higher in smaller sub-catchments, with, on average, a higher transport capacity than larger catchments, illustrating the scale dependency of human impacts on sediment fluxes and the buffer capacity of larger catchments. From an economical perspective, the construction of check-dams was estimated to be more expensive than reforestation programs in the studied catchment, while adding more check-dams did not always result in a proportional reduction of sediment yield. This indicates that optimizing check-dam distribution relative to land use patterns is crucial to decrease catchment sediment yield. Check-dams have a large and instantaneous impact on sediment yield over a restricted time period, while reforestations have important sustained effects at a lower economic cost. These contrasting effects require a careful evaluation for optimal effective catchment management.

Abstract. Determination of whether soil erosion can constitute a net terrestrial carbon dioxide (CO 2 ) 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 ( 14 C and 13 C), 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 14 C of −271.5 ‰) through non-selective erosion processes and channel incision. In the other subcatchment, topsoil material was predominantly eroded and the average 14 C in sediments was −64.2 ‰. 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.

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 μm 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.