Response of ammonia-oxidizing archaea to experimental warming in high-N2O-emitting permafrost peatland habitats

High nitrous oxide (N2O) emissions from bare peat surfaces in permafrost peatlands are mainly produced by denitrification but depend on nitrate (NO3–) supply through nitrification. Here, we investigated the effect of experimental warming on ammonia oxidizers , who catalyze the rate limiting step of nitrification, in bare peat surfaces. We determined the abundance and community composition of amoA genes in warmed and control plots, and combined these results with gross nitrification rates and previously reported data on mineral nitrogen (N) pools, soil N2O concentrations and surface emissions. Ammonia oxidizer communities comprised only a few closely related amplicon sequence variants (ASV) of ammonia-oxidizing archaea (AOA). Contrary to our expectations, warming did not cause significant changes in the archaeal amoA abundance but shifted their community composition: a single ASV of the Zeta (ζ) clade of Nitrososphaera became highly dominant in subsurface peat, where a tendency, towards higher gross nitrification rates, although not significant, was observed. High NO3– concentrations and gross nitrification rates indicated that denitrification was not limited by NO3– availability. Thus, increased N2O emissions with warming were not caused by enhanced nitrification. However, the sensitivity of the nitrifier community to the mild warming treatment suggests that compositional shifts are expected in Arctic soils under a future warmer climate, which may have implications for N turnover.