Linking soils and streams: Chemical composition and sources of eroded organic matter during rainfall events in a Loess hilly-gully region of China

Ascertaining the chemical compositions and sources of eroded organic matter (OM) is of significant importance for understanding the flux and fate of laterally transported OM at the catchment scale. Limited information, however, is available for the sources and dynamics of the eroded OM in bulk and soluble phases in relation to land-use types and rainfall events from an erosion prone landscape. In this study, we collected the top-soils from different land-uses and gully bank in uplands over seven rainfall events in 2016 to address this issue at two contrasting watersheds (with vs. without soil conservation measures) in the Loess hilly-gully region of China. Chemical compositions of eroded OM in bulk sediments and stream waters at the stream channels during rainfall processes were determined by spectroscopic analysis and source partition was inferred with the stable isotopic technique. Our results indicated that OM from forested soils had higher aromaticity, hydrophobic fraction, humification, molecular weight, and proportion of humic-like substance compared with other land-use types. The calculation with isotope mixing model indicated that the eroded bulk OM sources in sediments mainly originated from gully bank, orchard, and forests at the watershed coupled with soil conservation measures. Further, gully banks were the main contributor of eroded OM during rainfall events only at the watershed without conservation practices. Moreover, the principal component analysis suggested that the dissolved organic matter in stream waters were primarily derived from microbial OM sources (i.e. biological production of OM) in deep soils during rainfall events at both contrasting watersheds. Our findings highlight the effects of soil conservation measures and rainfall events on the nature and reactivity of eroded OM in various forms issuing from watersheds.