The rapid expansion of reservoirs, coupled with increasing eutrophication, has profoundly influenced regional and global carbon cycles. To precisely assess the carbon sink potential of reservoirs, it is crucial to quantify the decomposition of endogenous particulate organic carbon (POC) during the deposition and sinking of particulate matter in reservoirs. This is particularly important in the context of rising temperatures and intensified human activities. In this study, the Hongfeng Reservoir, an artificial reservoir in a karst basin on the Yunnan-Guizhou Plateau in China, was selected as a representative reservoir to systematically explore the sources and evolution of endogenous POC in (sub-)deep reservoirs. Particulate matter and water samples were collected from inflowing rivers and reservoir water profiles to analyze the content of POC, stable isotope of POC (δ¹³C_POC), radioisotope of POC (Δ¹⁴C_POC), particulate nitrogen, and chlorophyll concentrations. The results revealed significant differences in POC content and carbon isotope signatures between riverine and reservoir particulate matter, primarily due to distinct POC sources. Riverine particulate matter exhibited C/N ratios of 10.4 to 18.4, δ¹³C_POC values of -29.3 ‰ to -26.1 ‰, and Δ¹⁴C_POC values of -282 ‰ to -183 ‰, in contrast, particulate matter in the reservoir's surface water had C/N ratios of 5.1 to 6.9, δ¹³C_POC values of -34.6 ‰ to -31.3 ‰, and Δ¹⁴C_POC values of -162 ‰ to -143 ‰. From the surface to the bottom of the reservoir water profile, the C/N ratio of particulate matter gradually increased, Δ¹⁴C_POC became increasingly negative, and δ¹³C_POC exhibited varying trends across different water profiles. The combined analysis of chlorophyll and other variables demonstrated that Δ¹⁴C_POC is the most reliable indicator for tracing the source and decomposition process of POC during particulate matter sinking in the reservoir. Quantitative estimates based on Δ¹⁴C_POC indicated that the contribution of endogenous POC decreased from 73–85 % in the surface water to 41–57 % in the bottom water, with 74.7–75.4 % of endogenous POC decomposed during the sinking process, suggesting that only a small fraction of endogenous organic matter could reach the reservoir bottom and was ultimately buried in sediments. Future research should focus on quantifying the fate of endogenous organic matter decomposition products to enhance understanding of reservoirs’ carbon sink potential.