Geomorphologic controls and anthropogenic impacts on dissolved organic carbon from mountainous rivers: insights from optical properties and carbon isotopes

Abstract. Mountainous rivers are critical in transporting dissolved organic carbon (DOC) from terrestrial environments to downstream ecosystems. However, how geomorphologic factors and anthropogenic impacts control the composition and export of DOC in mountainous rivers remains largely unclear. Here, we explore DOC dynamics in three subtropical mountainous catchments (i.e., the Yinjiang, Shiqian, and Yuqing catchments) in southwest China, which are heavily influenced by anthropogenic activities. Water chemistry, stable and radioactive carbon isotopes of DOC (δ13CDOC and Δ14CDOC), and optical properties (UV absorbance and fluorescence spectra) were employed to assess the biogeochemical processes and controlling factors on riverine DOC. The radiocarbon ages of DOC in the Yinjiang River varied widely from 928 years BP to the present. Stepwise multiple regression analyses and partial least square path models revealed that geomorphology and anthropogenic activities were the major drivers controlling DOC concentrations and DOM characteristics. Catchments with higher catchment slope gradients were characterized by lower DOC concentrations, enriched δ13CDOC and Δ14CDOC, and more aromatic dissolved organic matter (DOM), which were opposite to catchments with gentle catchment slopes. Variabilities in DOC concentrations were also regulated by land use, with higher DOC concentrations in urban and agricultural areas. Furthermore, DOM in catchments with a higher proportion of urban and agricultural land uses was less aromatic, less recently produced, and exhibited a higher degree of humification and more autochthonous humic-like DOM. This research highlights the significance of incorporating geomorphologic controls on DOC sources and anthropogenic impacts on DOM composition into the understanding of DOC dynamics and the quality of DOM in mountainous rivers, which are globally abundant.