Decrease of nitrogen cycle gene abundance and promotion of soil microbial-N saturation restrain increases in N₂O...

Increases in soil available nitrogen (N) influence N-cycle gene abundances and emission of nitrous oxide (N₂O), which is primarily due to N-induced soil acidification in forest. Moreover, the extent of microbial-N saturation could control microbial activity and N₂O emission. The contributions of N-induced alterations of microbial-N saturation and N-cycle gene abundances to N₂O emission have rarely been quantified. Here, the mechanism underlying N₂O emission under N additions (three chemical forms of N, i.e., NO₃^--N, NH₄⁺-N and NH₄NO₃-N, and each at two rates, 50 and 150 kg N ha^-1 year^-1, respectively) spanning 2011-2021 was investigated in a temperate forest in Beijing. Results showed N₂O emissions increased at both low and high N rates of all the three forms compared with control during the whole experiment. However, N₂O emissions were lower in high rate of NH₄NO₃-N and NH₄⁺-N treatments than the corresponding low N rates in the recent three years. Effects of N on microbial-N saturation and abundances of N-cycle genes were dependent on the N rate and form as well as experimental time. Specifically, negative effects of N on N-cycle gene abundances and positive effects of N on microbial-N saturation were demonstrated in high N rate treatments, particularly with NH₄⁺ addition during 2019-2021. Such effects were associated with soil acidification. A hump-backed trend between microbial-N saturation and N₂O emissions was observed, suggesting N₂O emissions decreased with increase of the microbial-N saturation. Furthermore, N-induced decreases in N-cycle gene abundances restrained N₂O emissions. In particular, the nitrification process, dominated by ammonia-oxidize archaea, is critical to determination of N₂O emissions in response to the N addition in the temperate forest. We confirmed N addition promoted soil microbial-N saturation and reduced N-cycle gene abundances, which restrained the continuous increase in N₂O emissions. It is important for understanding the forest-N-microbe nexus under climate change.