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.