Direct biological fixation provides a freshwater sink for N₂O.

Abstract

Nitrous oxide (N₂O) is a potent climate gas, with its strong warming potential and ozone-depleting properties both focusing research on N₂O sources. Although a sink for N₂O through biological fixation has been observed in the Pacific, the regulation of N₂O-fixation compared to canonical N₂-fixation is unknown. Here we show that both N₂O and N₂ can be fixed by freshwater communities but with distinct seasonalities and temperature dependencies. N₂O fixation appears less sensitive to temperature than N₂ fixation, driving a strong sink for N₂O in colder months. Moreover, by quantifying both N₂O and N₂ fixation we show that, rather than N₂O being first reduced to N₂ through denitrification, N₂O fixation is direct and could explain the widely reported N₂O sinks in natural waters. Analysis of the nitrogenase (nifH) community suggests that while only a subset is potentially capable of fixing N₂O they maintain a strong, freshwater sink for N₂O that could be eroded by warming.