Meltwater of freeze-thaw cycles drives N2O-governing microbial communities in a drained peatland forest soil

Soil freeze-thaw cycles affect N₂O fluxes in high- and mid-latitude regions, but understanding microbial processes behind N₂O will help clarify the long-term impact of freeze-thaw on climate change. The aim of this study was to investigate the impacts of freeze-thaw cycles on microbial abundances and N₂O emissions in a hemi-boreal drained peatland forest. The soil freeze-thaw experiment involved artificial heating to thaw the topsoil after freezing. Results showed that thawing of the 5 cm topsoil increased soil water content (SWC) and N₂O emissions. Microbial analysis demonstrated that the abundance of soil prokaryotes increased with thawing. N₂O emissions were negatively correlated with NH₄⁺-N while ammonia-oxidizing archaea and bacteria, including complete ammonia oxidizers, increased their abundance. This indicates a potential nitrification pathway. The abundance of nitrite reductase genes (nirK and nirS) showed a positive correlation with N₂O fluxes, while nosZ genes did not increase. The results provide an insight into the impact of soil freeze-thaw cycles on N₂O fluxes and the underlying microbial processes. The dynamics of SWC during the thawing period were the most direct driver of the increase in N₂O emissions. Incomplete denitrification was the dominant process for the N₂O emissions during the thaw. More than 80% of produced N₂O was denitrified to inert N₂, as shown by high potential N₂ emissions. The frequency of freeze-thaw events is expected to increase due to climate change; therefore, determining the underlying microbial processes of the N₂O emissions under freeze-thaw is of great importance in predicting possible impacts of climate change in forests.