Autochthonous sources and biogeochemical processes drive the spatiotemporal variation of DOM composition and optical indicators in a karst river

Dissolved organic matter (DOM), as a large carbon pool, is key to biogeochemical cycles, especially in some unique areas. Karst rivers typically exhibit unique biogeochemical characteristics and different human inputs; whether and how these special differences affect DOM distribution, transport and composition are not clear. Here, 134 samples were obtained from the Daning River, a typical karst river located in the mountain landscapes of the Yangtze River, during both the dry and wet seasons. Multiple methods including UV–visible absorption, three-dimensional fluorescence spectroscopy parallel factor analysis (PARAFAC), principal component analysis (PCA) and correlation analysis were used to research the molecular compositions, optical properties and origins of DOM. During the wet season, there was higher aromaticity and intensity of UV absorbing functional groups in DOM. The higher Fmax of the protein-like component in the wet season suggested more biological sources in DOM. PCA indicated that autochthonous sources and biodegradation were the main sources of DOM during the two seasons. During the dry season, forest showed the opposite effect on DOM quality when compared to farmland and urban. The findings demonstrated that monsoonal climate and biogeochemical processes were the primary reasons for the differences in DOM components and sources, which benefits the understanding of carbon biogeochemical processes in the karst rivers. More importantly, the analysis of DOM composition and sources can provide valuable insights into identifying potential contaminants and understanding water quality variations, which is beneficial to the concrete implementation of water protection.