Seasonal variation and provenance of organic matter in the surface sediments of the three gorges reservoir: Stable isotope analysis and implications for agricultural management

Heng Xiao; Changping Mao; Shuai Wang; Zhimin Jia; Wenbo Rao

doi:10.1016/j.scitotenv.2023.161886

Seasonal variation and provenance of organic matter in the surface sediments of the three gorges reservoir: Stable isotope analysis and implications for agricultural management

Sci Total Environ. 2023 Apr 20:870:161886. doi: 10.1016/j.scitotenv.2023.161886. Epub 2023 Jan 31.

Authors

Heng Xiao¹, Changping Mao², Shuai Wang³, Zhimin Jia¹, Wenbo Rao¹

Affiliations

¹ School of Earth Sciences and Engineering, Hohai University, Nanjing 211100, China.
² School of Earth Sciences and Engineering, Hohai University, Nanjing 211100, China. Electronic address: maochangping@hhu.edu.cn.
³ Yellow River Institute of Eco-Environmental Research, Zhengzhou 450004，China.

PMID: 36731557
DOI: 10.1016/j.scitotenv.2023.161886

Abstract

The construction of the Three Gorges Dam has altered the hydrology and increased the trapping of sediment in the reservoir. This has also changed the composition and export of particulate organic matter in the Yangtze River. To understand the seasonal variations and sources of organic matter in sediments, total organic carbon (TOC), total nitrogen (TN), δ¹³C_org and δ¹⁵N in surface sediment samples from the mainstream and tributaries of the Three Gorges Reservoir were measured in the summer (July) and winter (December) of 2017, respectively. The results showed that the concentrations of TOC and TN in the surface sediments of the Three Gorges Reservoir were 0.79 %-1.46 % and 0.07 %-0.13 %, respectively. The ranges of δ¹³C_org and δ¹⁵N were - 26.35 ‰ to-24.70 ‰ and 2.59 ‰ to 5.67 ‰, respectively. According to δ¹³C_org and the TOC/TN ratio, the source range of organic matter was determined, and the contributions of different organic matter sources were quantified using a Bayesian mixed model. The results showed that soil organic matter and river plankton were the main sources of surface sediment organic matter in summer, whereas soil organic matter and aquatic vascular plants were the main sources in winter. The source of organic matter is related to biological factors in summer, whereas it is mainly caused by hydrodynamic conditions in winter. The analysis of δ¹⁵N further reveals that there are obvious external pollutants in the Three Gorges Reservoir, mainly related to artificial nitrogen fertiliser and domestic sewage. This study highlights the influence that soil nitrogen loss may be an important reason for the impact of agricultural non-point source pollution in the reservoir area, showing seasonal differences which were mainly affected by rainfall in summer and controlled by impoundment in winter. Hence, fine nitrogen management is required to reduce pollution in the Three Gorges Reservoir.

Keywords: Carbon; Nitrogen; Organic matter sources; Stable isotopes; Surface sediment; Three gorges reservoir.