Anthropogenic nitrogen inputs favour increased nitrogen and organic carbon levels in Qinghai–Tibetan Plateau lakes: Evidence from sedimentary records

Abstract

The water quality of lakes on the Qinghai‒Tibetan Plateau (QTP) is critical, as the QTP serves as a water tower for billions of people in Asia. However, the lack of long-term successive monitoring data has hampered the knowledge of the current patterns of variation in the nitrogen content in QTP lakes, as well as their future trends. In this study, the variations in the nitrogen content in sediment records from Lake Hurleg and Lake Toson and collected datasets from other lakes on the QTP were analysed. In addition, the responses of the fates of nitrogen and organic carbon in lake sediments to climate change and human activities were also considered. The results revealed that the TN contents in Lake Hurleg and Lake Toson sediments have increased by 34 % and 23 %, respectively, since the 1960s compared with the pre-1960s. The nitrogen isotopic composition revealed that anthropogenic nitrogen inputs were the critical source of the increase in TN in the lake sediment and that climate change facilitated the assimilation process to alter the N level. The combined dataset with other QTP lakes revealed that TN (TOC) in QTP lake sediments increased at a rate of 0.026 %/decade (0.19 %/decade) from the 1890s to the 1990s, mainly from endogenous lake sources, and its contribution increased owing to anthropogenic nitrogen inputs. The nitrogen input increased the TOC content of the QTP lake sediments, resulting in lower TOC/TN ratios. Changes in nitrogen in QTP lake sediments were controlled mainly by climate change before the 1920s, by a combination of climate change and human activities during the 1920s–1960s, and mainly by human activities after the 1960s. These results highlight that anthropogenic nitrogen inputs are critical for increasing nitrogen levels in QTP lakes.