Response of sedimentary organic matter source to rainfall events using stable carbon and nitrogen isotopes in a typical loess hilly-gully catchment of China

Abstract

Understanding the information on sedimentary organic matter (SOM) source is of great importance for better understanding biogeochemical cycling and terrestrial carbon sequestration on the Earth’s surface. Such information, especially during field rainfall process, however, is unavailable or difficult to assemble. In order to further study the dynamic variation of sedimentary organic matter source during rainfall process, suspended sediment were sampled manually at the outlet of two paired typical hilly-gully watersheds of Loess Plateau in China [Qiaozi East watershed (QEW) and Qiaozi West watershed (QWW)], and source materials included subsoil (i.e., soils from channel bank and gully bank) and topsoil (i.e., surface soils from different slope positions in land uses), along with C3 and C4 plants. In this study reported, natural stable isotopes (13C and 15N) and elemental compositions (TOC and TN), combined with a isotope mixing model (stable isotope analysis in R, SIAR) were successfully used to assemble this information. The results showed that the response of source contributions to SOM displayed difference significantly during different rainfall processes, showing fluctuation with flow. Subsoil was the main source of SOM pool for two similar small watersheds (accounting for 38.50% for QWW and 35.63% for QEW). Minor contribution was sourced from the C3 and C4 plants (< 25%), but only 10% for the C4 plants, indicating that the C3 plants dominated the input of organic matter in superficial soils of this study region. The source of sedimentary organic matter transported by erosion showed different contribution proportions from potential sources in two similar small watersheds with different management practices, indicating that management practice could decrease the soil organic matter loss by reducing the sediment transportation and flow during rainfall process in the Loess Plateau of China.