Zonal characteristics of sediment-bound organic carbon loss during water erosion: A case study of four typical loess soils in Shaanxi Province

The soils of the Loess Plateau of China exhibit zonal characteristics, with declining clay percentages from the southern to northern regions. However, these differences have been largely ignored in studies of soil erosion and the related organic carbon (OC) loss. The goal of this study was to investigate the effect of gradual changes in soil texture on the loss and selective transport of soil and soil organic carbon (SOC) on the Loess Plateau of China. Four typical soils with decreasing clay contents (26.3%, 21.2%, 15.6% and 12.1%) in Shaanxi Province were selected and studied. Rainfall-simulated experiments were performed in 1 m (width) x 5 m (length) plots with a rainfall intensity of 120 mm h(-1) on three slope gradients (15, 20 and 25). The runoff and sediment yields, particle size distributions of the sediments, OC concentrations and enrichment ratios of OC in sediment (ERoc) during rainfall processes were measured and calculated. A comparison of the rainfall characteristics between rainfall events in different soils indicated that a decrease in the clay percentage increased the intensity of rill erosion. The runoff rates of different events decreased with the clay percentage, whereas no general pattern was observed in the sediment yield rate due to differences in the initial timing and intensity of rill erosion. Furthermore, the ERoc values in the sediments of all the events were between 0.64 and 1.44, with 90% of values between 0.80 and 1.20. Similarly, the effective particle size distribution of the sediments exhibited relatively constant values with rainfall duration. Non-significant correlations (P > 0.05) between ERoc and dispersed and non-dispersed sediment particles suggested that the transport of sediment and OC was non-selective. Moreover, significant linear correlations were observed between the amount of SOC loss and sediments in all the events. However, there was no significant difference in total SOC loss between the rainfall events, except for in events with a soil clay percentage of 15.6%. SOC losses in soils with 26.3% and 21.2% clay percentages were mainly due to the high SOC concentrations of the original soils, whereas those in other soils were due to the large amount of lost sediment. However, the variations in soil loss and SOC loss were not associated with changes in the clay content of the soil. These results suggest that zonal soils with decreasing clay contents do not influence the selective transport of OC, but soil textures affect the sediment and SOC loss through complicated mechanisms.