Soil carbon and nitrogen sources and redistribution as affected by erosion and deposition processes: A case study in a loess hilly-gully catchment, China

Understanding how organic carbon (C) and nitrogen (N) move with soil along the fluvial system and where eroded organic C and N sources occur over catchment landscape is of significant importance for evaluating accurately carbon or nitrogen budget at the catchment scale and designing proper management practice in the fragile ecosystem. In this study, we selected a dam-controlled catchment in the southwestern hilly-gully region of the Chinese Loess Plateau with severe soil erosion, and explored the catchment scale carbon and nitrogen redistribution by erosion and deposition processes as well as organic carbon and nitrogen sources in sediments retained by check dam. The physio-chemical characteristics, stable isotopic signatures (13C, 15N), total organic C (TOC) and total N (TN) of soils and sediments in studied catchment were determined and an isotopic mixing model was employed to quantify the relative contributions of each source type (ie., forests, cropland, and gully) to eroded sediment C and N. The results showed that the check dam intercepted 309.6 Gg of eroded soil, 1405.1 Mg of TOC, and 153.5 Mg of TN. The sediment the eroded TOC was mainly sourced from cropland, accounting for 53.54%, followed by gully (29.28%) and then forests (17.18%), respectively. Eroded TN sources was similar to C sources, showing that cropland contributed 53.53%, with gully and forests contributing 30.86% and 15.61%, respectively. Moreover, the forests contributions to eroded C and N gradually decreased in the direction of the runoff pathway at the check dam, and the C and N contributions of cropland and gullies showed the orders of mid-check dam > post-check dam > pre-check dam and pre-check dam > post-check dam > mid-check dam, respectively. Soil erosion and deposition processes induced 1569.8 Mg TOC and 146.7 Mg TN losses, with an average soil C and N erosion rate of 0.051 Mg C ha−1 yr−1 and 0.005 Mg N ha−1 yr−1 during the period (i.e., from 2004 to 2016), accounting for approximately 52.8% and 48.87% of the total amount of eroded TOC and TN, respectively. The results indicated that although the check dam served as a carbon and nitrogen storage and sequestration structure in the loess hilly-gully region, erosion-induced carbon or nitrogen redistribution might still act as a major source for atmospheric CO2 or nitrogen oxide in our studied catchment.