Biogeochemical responses to climate change and anthropogenic nitrogen deposition from a 200-year record from Tianchi Lake, Chinese Loess Plateau

Human activities over the last 100 years have fundamentally changed the biogeochemistry of the global nitrogen cycle. For example, increased nitrogen deposition from industrial and agricultural sources has been linked to lake acidification and nutrient fertilization, and thus it has the potential to significantly influence lake ecosystems. Records of anthropogenic nitrogen deposition from alpine lakes are sparse in China, which limits our understanding of its effects on remote alpine lake ecosystems. In this study, we analyzed multiple geochemical proxies at Tianchi Lake (2430 m a.s.l.), which is part of the Liupan Mountains National Nature Reserve in the central Chinese Loess Plateau (CLP). We measured total nitrogen (TN) concentrations, C/N ratios and nitrogen stable isotope ratios (S15N), as well as a biological proxy (spectrally-inferred chlorophyll a), from a well-dated sediment core spanning the past 200 years. Our aim was to examine anthropogenic nitrogen deposition to the lake and to assess its implications for lake primary production. We found that 815N was stable prior to 1980, but it decreased significantly thereafter. This is consistent with documented changes in anthropogenic nitrogen deposition over the past 200 years, and it indicates that the 815N record of Tianchi Lake likely reflects anthropogenic nitrogen deposition, at least in this region. Prior to 1980, the trend in primary production of Tianchi Lake agrees with the trend of reconstructed regional temperature, when reactive nitrogen emissions in China were very low. This suggests that temperature was likely the main factor driving lake primary production before 1980. Primary production has increased significantly since 1980, consistent with the continued rising temperature and with enhanced nitrogen deposition, indicating that both factors are simultaneous drivers. Although the current data for Tianchi Lake are insufficient to unequivocally determine which of the two stressors is more important after the 1980s, temperature likely played the more significant role in driving primary production. This is because the lake is phosphorus-limited today, and in addition there is a strong association between temperature and chlorophyll a over the past 200 years. Continuing global warming and increasing nitrogen deposition in the future will likely further affect the fragile alpine ecosystems in the region.