Increased soil microbial AOB amoA and narG abundances sustain long-term positive responses of nitrification and denitrification to N deposition

Nitrification and denitrification driven by microbes are important processes mediating soil N availability and N2O emission. Enhanced N deposition usually stimulates both nitrification and denitrification rates, but whether these stimulation effects are generally persistent or not, and the underlying biological mechanism remain unclear. To clarify these issues, we compiled the responses of soil nitrification rate, denitrification rate, and microbial functional gene abundances as well as corresponding N2O emission to N deposition at global scale. The results showed that both nitrification rate and AOB amoA abundance were stimulated by N deposition on average, while denitrification rate was stimulated without any change in denitrification related functional gene abundance. When the data set was partitioned by N addition duration, there were larger significant positive responses of nitrification rate and AOB amoA abundance as well as those of denitrification rate and narG abundance with longer duration. The response ratio of nitrification rate was positively correlated with that of AOB amoA abundance (p < 0.01). The response ratio of N2O emission was positively correlated with that of AOB amoA abundance (p = 0.04) as well as that of narG abundance (p < 0.01). These results indicated the crucial role of AOB amoA and narG abundances for the responses of nitrification and denitrification rates to N deposition, as well as the response of N2O emission. The larger gene abundances of AOB amoA and narG with longer term N deposition sustain long-term positive responses of nitrification and denitrification to N deposition. This challenges the traditional cognition that long term N deposition diminished the positive response of N2O emission to N deposition by dampening the microbial activity. The responses of AOB amoA and narG abundances to N deposition should be considered to better predict the dynamic of nitrification rate, denitrification rate or N2O emission in the context of enhanced N deposition.