Responses of Alpine Soil Nitrification and Denitrification Rates to Nitrogen Addition Gradient—The Role of Functional Genes

Increases in nitrogen (N) deposition affect the nitrification and denitrification processes that are regulated by microorganisms. With the aim of understanding what happened to the soil nitrification and denitrification rates under the N application gradients, we set a field experiment treated with N at 6 different rates (0, 2, 4, 8, 16, and 32 g N·m −2 ·yr −1 ) in 2014. We determined the physico-chemical properties, abundances and community structures of the nitrifiers and denitrifiers, the net nitrification rate (NNR) and the potential denitrification rate (PDR) of soil samples that were collected in 2020. We found that the abundances of ammonia-oxidizing bacteria amoA (AOB amoA ), the sum of ammonia-oxidizing archaea amoA (AOA amoA ) and AOB amoA increased with the increase in N application rate, but the abundances of nosZ decreased with that. The microorganisms that dominate the ammonia oxidation process could shift from AOA to AOB under high N application rates. Furthermore, the soil microorganisms respond to the N addition preferentially with the abundance changes rather than the community composition changes. Moreover, the NNR increased with the N input, while the decrease in the PDR was due to the decrease in the pH value caused by high N application. The results also showed that the amoA gene abundance explained most (46.3%) of the variation in the NNR. Moreover, the soil moisture and pH explained 44.0% and 27.1% of the variation in the PDR, respectively. The results demonstrated that the NNR and PDR were mainly explained by functional genes abundances and environmental factors, respectively, in alpine meadow soil under sustained N deposition.