[Nitrogen Distribution and Inorganic Nitrogen Diffusion Flux in a Shallow Lake During the Low Temperature Period: A...

The distribution of inorganic nitrogen across the sediment-water interface in Baiyangdian Lake, the largest shallow lake in North China, was examined during the low temperature period. Furthermore, the influence of inorganic nitrogen diffusion flux in sediment porewater on the overlying water quality was analyzed. The results showed that the mean TN concentration in the surface water ranged from 4.83 to 8.23 mg·L^-1, the mean NH₄⁺-N concentration ranged from 0.21 to 0.34 mg·L^-1, and the mean NO₃^--N concentration ranged from 0.01 to 2.75 mg·L^-1. TN exceeds the Class V water quality standard for surface water, indicating serious pollution. The mean TN content of the sediments ranged from 681 to 4365 mg·kg^-1, of which TON was the main form of TN, accounting for 61.6%-93.1%. NH₄⁺-N was the main form of TIN, with a mean content ranging from 28.9 to 116.3 mg·kg^-1, and a mean NO₃^--N content of between 5.2 and 23.7 mg·kg^-1, which is relatively low. The NH₄⁺-N concentration in the porewater was 3 to 16 times that of the overlying water, showing a gradual accumulation trend. The diffusion flux ranges of NH₄⁺-N, NO₃^--N, and NO₂^--N across the sediment-water interface were -0.55-4.09, -1.44-3.67, and -0.88-0.04 mg·(m²·d)^-1, respectively. There was still a potential release risk during the low temperature period. A large amount of NH₄⁺-N accumulated in the sediments during the low-temperature period, which may affect the quality of the overlying water after the temperature rises. Therefore, studying the distribution of nitrogen across the sediment-water interface, and the risk of internal release of inorganic nitrogen during the low temperature period, is of great significance for improving water quality in Baiyangdian Lake and understanding the internal nitrogen pollution of shallow lakes.