Selective uneven enrichment of soil organic carbon among different-sized sediments under a rain-induced overland flow: 13 C stable isotope evidence

Soil organic carbon (SOC) enrichment varies among sediments of different sizes during rain-induced overland flow erosion. This selective transport of SOC is complex in conjunction with the exposure of labile and stable organic carbon (OC), accompanied by heterogeneous aggregate disintegration under raindrop effects. Utilizing the variations in δ C values of SOC fractions, we traced this selective transport, linking it to aggregate-wrapped SOC changes during erosion. A modified soil pan facilitated the simultaneous monitoring of splash and sheet erosion via artificially simulated rainfall, with control over the intensity and slope. Aggregate composition, SOC distribution, and δ C values in the erosion samples were analyzed. The results indicated that distinct sorting existed within the aggregate fragments. Along with SOC variation among different sediment sizes, the proportions of clay and fine silt within sediment aggregates increased as a function of slope and rainfall intensity, whereas particulate OC within aggregates decreased. The SOC enrichment ratios (ERocs) and δ C values in splash-eroded sediments were positively correlated with those in sheet-eroded sediments. The ERocs in splash-eroded sediments were lower than those in sheet-eroded sediments, but δ C values were the opposite. Moreover, δ C values of SOC enriched in sediment particles of all sizes from aggregate stripping were lower than those of the original soil. This indicates that raindrop hits promote heavy C loss during sheet erosion, which is different for mineral-associated and particulate OC. As the slope and rainfall intensity increased, δ C values for all sediment sizes decreased over the course of erosion. Interestingly, the highest δ C values were observed under a rainfall intensity of 60 mm h , whereas the highest SOC concentrations were noted on a 5° slope. These observations suggest divergent mechanisms affect δ C values and SOC concentrations in eroded sediments. All these results verified that selective sorting existed for the light SOC fraction. Finally, the internal selective transport of one SOC fraction may explain the enhanced mineralization and reaggregation capacity of the deposited sediments.