Soil pH modulates the impact of ammonia-based fertilizers on N-cycling microbial genes in terrestrial ecosystems

The increase in available nitrogen (N) in terrestrial ecosystems influences N-cycling processes by affecting the abundance of N-cycling microorganisms. However, the responses of N-cycling genes to different N forms and soil pH conditions remain inconsistent. In this study, we conducted a meta-analysis to evaluate the responses of the abundances of functional genes (archaeal amoA, bacterial amoA, narG, nirK, nirS, and nosZ) involved in nitrification and denitrification to exogenous N inputs. Our analysis included 325 observations from 116 publications on terrestrial ecosystems. Elevated N inputs significantly increased the abundances of AOA (+24.6 %), AOB (+103 %), nirS (+31.2 %), nirK (+26.4 %), and nosZ (+24.0 %) genes. The effects of different N forms on N-cycling genes were modulated by soil pH. The response of AOB to N addition was significant across various N forms and a wide range of soil pH levels. In contrast, the strongest effects of inorganic N and urea on AOA were observed in alkaline and neutral soils, respectively, while organic fertilizers had the greatest impact on AOA in acidic soils. The response of the nirS gene to organic N addition was evident across a broad range of soil pH, whereas the nirK gene responded primarily in acidic and neutral soils. In neutral soils, inorganic N similarly influenced the abundances of nirS and nirK genes; however, in acidic soils, inorganic N significantly reduced nirS gene abundance while increasing that of nirK. Urea significantly enhanced the abundances of nirS and nirK genes in acidic and neutral soils, respectively. Moreover, organic and inorganic N markedly increased nosZ gene abundance in neutral and alkaline soils, whereas urea had no significant effect on nosZ. This meta-analysis provides a comprehensive understanding of the responses of N-cycling genes to various N forms across soils with differing pH levels, offering valuable insights to refine strategies for reducing greenhouse gas emissions and improving N management in terrestrial ecosystems.