Abstract: Dissolved organic carbon (DOC) represents a crucial carbon reservoir in the global carbon cycle, serving as a carbon source for microorganisms and aquatic plants. Its concentration can reflect the input and transformation of organic pollutants, while its isotope compositions provide insights into the impacts of precipitation and land-use changes on aquatic ecosystems. Hanjiang River is the primary water source of the Danjiangkou Reservoir, which plays a pivotal role in regional water quality. Yet, the sources and transformation processes of DOC in the water bodies of the Hanjiang River Basin remain poorly understood. We collected water samples from the mainstream, tributaries, and adjacent groundwater in the Hanjiang River Basin during the wet season. DOC concentrations and δ¹³C_DOC values of water samples were analyzed. Combined with hydrochemical parameters and stable isotopes of water hydrogen and oxygen (δD and δ¹⁸O), we examined the influence of natural processes and anthropogenic activities on the DOC in the river basin. The results showed that: (1) Mean DOC concentrations were 2.34 mg·L^-1 (n=28) in the mainstream, 2.59 mg·L^-1 (n=30) in tributaries, and 1.56 mg·L^-1 (n=17) in groundwater, while mean δ¹³C_DOC values were -28.3‰, -28.9‰, and -25.6‰, respectively. (2) Soil organic matter decomposition was the primary source of DOC in the upstream natural background areas, contributing 64%-91% (with a mean of 72%) to the mainstream DOC. In contrast, domestic sewage input and internal release increasingly exerted influences on DOC sources in the middle-lower plain regions. (3) DOC degradation occurred in the reducing environment of groundwater, leading to reduced DOC concentrations and enriched δ¹³C_DOC values. These results demonstrate that the DOC sources in the upstream areas of the Danjiangkou Reservoir are predominantly controlled by natural processes, with minimal influence from anthropogenic activities and internal release, providing scientific support for water resource management in the Hanjiang River Basin.

Key words: Hanjiang River Basin, dissolved organic carbon, carbon isotope, natural process, anthropogenic activity