Microbial mechanisms responsible for diurnal dynamics of N ₂ O 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 N ₂ O emissions. A significant yet often overlooked phenomenon in the Arctic is the diurnal variation of N ₂ O 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 N ₂ O exchange in nutrient-poor subarctic permafrost peatland, where higher atmospheric N ₂ O consumption occurs at night than during the day. By quantitative PCR, we assessed the abundance and expression of denitrifier genes related to N ₂ O production ( nirK and nir S) and consumption ( nos Z clade I and II). Although all four genes were present, only gene nir K and nos Z clade I were expressed. Remarkably, we found that the diurnal pattern of nir K/ nos Z gene expression ratio closely aligns with the diurnal pattern of N ₂ O flux data, with dominance of N ₂ O production rather than N ₂ O reduction activity driving the day-to-night variation in N ₂ O 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 N ₂ O 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 N ₂ O 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.