Little is known about the exchange of gaseous nitrogen (N₂) with the atmosphere in freshwater systems. Although the exchange of N₂, driven by excess or deficiencies relative to saturation values, has little relevance to the atmospheric N₂ pool due to its large size, it does play an important role in freshwater and marine nitrogen (N) cycling. N-fixation converts N₂ to ammonia, which can be used by microbes and phytoplankton, while denitrification/anammox effectively removes it by converting oxidized, inorganic N to N₂. We examined N₂ 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 N₂ (>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.

_{关于淡水系统中气态氮 (N 2} ) 与大气的交换知之甚少。尽管 N ₂的交换是由相对于饱和值的过量或不足驱动的，但由于其较大的尺寸，它与大气 N _{2库几乎没有相关性，但它确实在淡水和海洋氮 (N) 循环中发挥着重要作用。}固氮作用将 N ₂转化为氨，可供微生物和浮游植物利用，而反硝化/厌氧氨氧化则通过将氧化的无机 N 转化为 N ₂来有效去除氨。我们检查了 N ₂饱和度来推断纬度 5° 的 34 个湖泊中的净生物氮过程，这些湖泊的营养状态、混合状态和水深不同。_{在这里，我们报告说，几乎所有在美国中西部上部检查的湖泊都被 N 2}过饱和（>85% 的样本，n = 248），这表明湖泊不断向大气中释放氮。传统的范式是淡水通过固氮来补偿氮的限制，但这些结果表明，湖泊通过反硝化和/或厌氧氨氧化不断地将氮流失到大气中，这表明需要陆地氮输入来平衡内部氮循环。