Microbial mechanisms responsible for diurnal dynamics of N2O fluxes in a nutrient-poor subarctic permafrost environment

Arctic soils affected by permafrost are generally low in nutrients but highly sensitive to climate warming, which may enhance reactive N availability and subsequently N2O emissions. A significant yet often overlooked phenomenon in the Arctic is the diurnal variation of N2O fluxes. This variation is a well-documented occurrence in other ecosystems, however, the underlying microbial mechanisms that drive it remain entirely unknow. Here, we studied the microbial factors behind the daily variations in N2O exchange in nutrient-poor subarctic permafrost peatland, where higher atmospheric N2O consumption occurs at night than during the day. By quantitative PCR, we assessed the abundance and expression of denitrifier genes related to N2O production (nirK and nirS) and consumption (nosZ clade I and II). Although all four genes were present, only gene nirK and nosZ clade I were expressed. Remarkably, we found that the diurnal pattern of nirK/nosZ gene expression ratio closely aligns with the diurnal pattern of N2O flux data, with dominance of N2O production rather than N2O reduction activity driving the day-to-night variation in N2O flux. Our analysis of 1,635 metagenome-assembled genomes (MAGs) sites indicates that partial denitrifiers followed by complete denitrifiers are key players in driving diurnal N2O variations within this acidic Arctic ecosystem, rather than nitrification processes. To the best of our knowledge, this is the first study to uncover the microbial mechanisms underlying the diurnal dynamics of N2O flux. Our findings not only advance our understanding of nitrogen cycling in Arctic ecosystems but also underscore the critical need for similar research in diverse ecological contexts to fully grab the implications of climate change on greenhouse gas emissions.