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. Existing studies have primarily focused on the burial of organic carbon in lake and reservoir sediments. Due to the lack of effective methods for quantitative estimation, research on the decomposition of endogenous POC during particulate matter sinking in (sub-)deep lakes and reservoirs has been limited. This data gap significantly hinders the accurate scientific estimation of OC burial in inland water sediments, as well as the proper assessment of the role of lakes and reservoirs as carbon sinks under the growing influences of eutrophication.

Recently, a research team led by Professor Chen Jing'an from the Institute of Geochemistry, Chinese Academy of Sciences, selected the Hongfeng Reservoir, an artificial reservoir in a karst basin on the Yunnan-Guizhou Plateau in China, 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.

The innovation of this study lies in the use of the radiocarbon isotope Δ¹⁴C_POC to quantify the decomposition process of endogenous POC in the reservoir, revealing the complexity of reservoirs as carbon sinks. The research results not only provide new methods and data for assessing the carbon sink potential of reservoirs, but also provide directions for future studies on the fate of endogenous organic matter decomposition products. This will contribute to a more comprehensive understanding of the role of reservoirs in the global carbon cycle.

Fig.1. The migration and decomposition process of POC in the water of the Hongfeng Lake basin(Image by IGCAS)

Fig. 2. The possible fate of the decomposed POC(Image by IGCAS)

The study was published online in the journal Water Research.

This work was supported by the National Key R&D Program of China (grant 2023YFF0806000), the National Natural Science Foundation of China (Nos. U2202209, 42177249, 42303018), West Light Foundation of the Chinese Academy of Sciences (CN), the Guizhou Provincial Science and Technology Program (Qiankehe Platform-YWZ[2023]006, GZ2021SIG), the Autonomous Strategy Project of the State Key Laboratory of Environmental Geochemistry (SKLEG2024106).

Contact: 

CHEN Jing'an

Institute of Geochemistry, Chinese Academy of Sciences

Email:chenjingan@vip.skleg.cn

(By Prof. CHEN Jing'an's Group)