期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：29
页码：17063-17067
DOI：10.1073/pnas.1921689117
出版社：The National Academy of Sciences of the United States of America
摘要：Little is known about the exchange of gaseous nitrogen (N 2 ) with the atmosphere in freshwater systems. Although the exchange of N 2 , driven by excess or deficiencies relative to saturation values, has little relevance to the atmospheric N 2 pool due to its large size, it does play an important role in freshwater and marine nitrogen (N) cycling. N-fixation converts N 2 to ammonia, which can be used by microbes and phytoplankton, while denitrification/anammox effectively removes it by converting oxidized, inorganic N to N 2 . We examined N 2 saturation to infer net biological nitrogen processes in 34 lakes across 5° latitude varying in trophic status, mixing regime, and bathymetry. Here, we report that nearly all lakes examined in the upper Midwest (USA) were supersaturated with N 2 (>85% of samples, n = 248), suggesting lakes are continuously releasing nitrogen to the atmosphere. The traditional paradigm is that freshwaters compensate for N-limitation through N-fixation, but these results indicate that lakes were constantly losing N to the atmosphere via denitrification and/or anammox, suggesting that terrestrial N inputs are needed to balance the internal N cycle.
