Analyzing the contribution of climate change to long-term variations in sediment nitrogen sources for reservoirs/lakes

We applied a mixing model based on stable isotopic δ¹³C, δ¹⁵N, and C:N ratios to estimate the contributions of multiple sources to sediment nitrogen. We also developed a conceptual model describing and analyzing the impacts of climate change on nitrogen enrichment. These two models were conducted in Miyun Reservoir to analyze the contribution of climate change to the variations in sediment nitrogen sources based on two ²¹⁰Pb and ¹³⁷Cs dated sediment cores. The results showed that during the past 50 years, average contributions of soil and fertilizer, submerged macrophytes, N₂-fixing phytoplankton, and non-N₂-fixing phytoplankton were 40.7%, 40.3%, 11.8%, and 7.2%, respectively. In addition, total nitrogen (TN) contents in sediment showed significant increasing trends from 1960 to 2010, and sediment nitrogen of both submerged macrophytes and phytoplankton sources exhibited significant increasing trends during the past 50 years. In contrast, soil and fertilizer sources showed a significant decreasing trend from 1990 to 2010. According to the changing trend of N₂-fixing phytoplankton, changes of temperature and sunshine duration accounted for at least 43% of the trend in the sediment nitrogen enrichment over the past 50 years. Regression analysis of the climatic factors on nitrogen sources showed that the contributions of precipitation, temperature, and sunshine duration to the variations in sediment nitrogen sources ranged from 18.5% to 60.3%. The study demonstrates that the mixing model provides a robust method for calculating the contribution of multiple nitrogen sources in sediment, and this study also suggests that N₂-fixing phytoplankton could be regarded as an important response factor for assessing the impacts of climate change on nitrogen enrichment. - Highlights: • A mixing model was built to analyze sediment N sources of lakes/reservoirs. • Fertilizer/soil and macrophytes showed decreasing trends during the past two decades. • Contribution of phytoplankton source to sediment N increased during the past 50 years. • Impacts of climate change on variations in sediment N sources were quantified. • N₂-fixing phytoplankton can be used as a response factor to climate change