Direct biological fixation provides a freshwater sink for N 2 O.

Nitrous oxide (N 2 O) is a potent climate gas, with its strong warming potential and ozone-depleting properties both focusing research on N 2 O sources. Although a sink for N 2 O through biological fixation has been observed in the Pacific, the regulation of N 2 O-fixation compared to canonical N 2 -fixation is unknown. Here we show that both N 2 O and N 2 can be fixed by freshwater communities but with distinct seasonalities and temperature dependencies. N 2 O fixation appears less sensitive to temperature than N 2 fixation, driving a strong sink for N 2 O in colder months. Moreover, by quantifying both N 2 O and N 2 fixation we show that, rather than N 2 O being first reduced to N 2 through denitrification, N 2 O fixation is direct and could explain the widely reported N 2 O sinks in natural waters. Analysis of the nitrogenase (nifH) community suggests that while only a subset is potentially capable of fixing N 2 O they maintain a strong, freshwater sink for N 2 O that could be eroded by warming.