Impact of soil moisture changes on nitrogen cycle microbiome during the experiment of thaw-freeze cycle in a drained peatland forest

Freezing and thawing are common phenomena and potential sources of N2O emissions in ecosystems at high latitudes. Earlier it was hypothesized that the frozen soil layer might trap the underlying production of N2O and release this as the top layer is thawed. However, newer research has found other factors playing role in the de novo emissions such as fluctuating availability of organic matter, nitrates, and ammonia, microbial activity, and changing oxic conditions of the soil. But, the variation in the abundance of genes involved in the nitrogen cycle during these events is rarely explained thus, a generally accepted theory of the impact of freeze-thaw on N2O fluxes is still missing.
To further investigate the relationships between physical, chemical, and microbial parameters with N2O emissions, we conducted a two-week experiment of three thaw-freeze events in March 2022 using artificial heating with electrical cables installed in collars of greenhouse gas sampling chambers conducted in a drained Downy birch peatland forest. Gas and soil samples were obtained on three non-consecutive days from these collars. Soil temperature, soil water content (SWC), NH4-N, and NO3-N were measured in the soil. Also, the abundances of functional genes involved in the nitrification (bacterial, archaeal, and comammox (complete ammonia oxidation) amoA) and denitrification (nirS, nirK, nosZI, and nosZII) were known using qPCR.
Our results show that artificial heating induced the thawing of the frozen top layer of soil during our experiment. The increase in soil surface temperature positively correlated with the soil water content in the top layer (R=0.58, p<0.01). N2O emissions also increased with heating and correlated with SWC (R=0.38, p<0.01). Ammonia in soil decreased and was negatively associated with N2O emissions (R=−0.28, p<0.05), suggesting active nitrification as the amount of nitrates also increased during heating. The abundance of all functional genes significantly increased during the heating except for those responsible for the consumption of N2O (nosZ genes) during
denitrification. Although we found evidence of both active nitrification and denitrification, the multiple regressions between N2O emissions and the proportion of different functional genes suggest that the nirK-type denitrifiers dominated in the denitrification as well as in the overall production of the N2O (p<0.001). Meanwhile, the inactivation of N2O consumers (nosZ) at thawing temperatures resulted in the emission of N2O during the thawing events in the drained peatland’s nitrogen-rich soil.