Unraveling the microbial mechanism responsible for diurnal patterns in N2O fluxes in a subarctic permafrost peatland

Nitrous oxide (N₂O) is a critical greenhouse gas, ranking third in prevalence and serving as the leading contributor to ozone depletion in the twenty-first century. Its global warming potential is 298 times higher than that of carbon dioxide (CO₂) over a 100-year timeframe. Current estimates suggest that global N₂O emissions range from 8.1 to 30.7 teragrams (Tg) per year. Alarmingly, about two-thirds of these emissions stem from natural terrestrial sources, mainly related to microbial processes in soils. While significant research has focused on microbial mechanisms driving N₂O emissions in nutrient-rich ecosystems, there is an urgent need to address the limited knowladge on N₂O fluxes and underlying microbial mechanism in low-nutrient regions, such as Arctic ecosystems.

Many Arctic soils hold very low amounts of available nitrogen, leading to the assumption that they do not produce N₂O in measurable quantities. However, recent advances with portable gas analyzers have made it possible to successfully capture low fluxes, which are important for the nitrous oxide budget of areas covering vast tundra landscapes, challenging this perception. Additionally, diurnal variations of nitrous oxide fluxes are rarely taken into account. In our study site, the Stordalen palsa mire (Abisko, Sweden), a diurnal variation in N₂O emissions has been observed with chamber methods, with notably higher net emissions during the daytime and atmospheric N₂O consumption in the night. This study aims to delve deeper into the geochemical and microbial mechanisms behind this intriguing phenomenon.

We conducted round-the-clock microbiological and geochemical sampling, along with N₂O flux measurements, in the Stordalen Mire, Abisko, Sweden. Our research reveals significant insights into the availability of soluble nutrients, which were markedly higher during the daytime than at night. In our exploration of N₂O consumption and production, we meticulously quantified the activity of N₂O exchange-related genes responsible for N₂O flux at the transcript level—specifically the denitrification genes (nirK and nirS) and N₂O consumption genes (nosZ)—and analyzed their association with the day-night N₂O flux phenomenon, followed by RNA sequencing. Remarkably, we found that microbial gene expression patterns closely correlate with the flux data and geochemical trends. Consequently, our day-night flux measurements, paired with thorough geochemical and microbiological analyses, provide critical undestanding into the diurnal variations in N₂O fluxes within Arctic ecosystems. Our findings provide new insights into how microbes mediate complex N₂O flux dynamics in nutrient-poor Arctic ecosystems during day and night, underscoring their significance in the global nitrogen cycle.