Metabolic potential and contributions of ammonia-oxidizing microorganisms and comammox to soil nitrification in upland soils of northern China

Ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and complete ammonia oxidizers (comammox) play a crucial role in nitrification, which is an essential process in terrestrial nitrogen cycling. However, their metabolic potential and relative contributions to soil nitrification in upland soils remain unclear. In this study, the metabolic pathways and the interaction of AOA, AOB, and comammox of upland soil in northern China was conducted by combination of 13C-stable isotopes and multiple inhibitor methods. Meanwhile, under acetylene inhibition, the urea treatment in WI2 soil exhibited remarkably high 13C labeling proportions in AOA and comammox, reaching 80.3% and 64.2%, respectively, compared with the other soils. This suggests that long-term application of organic liquid fertilizer may induce shifts in microbial community composition and functional potential. Furthermore, inhibition of AOA reduced the 13C-labeled abundance of comammox, indicating that comammox may depend on metabolites produced by AOA. Across all soil treatments (CK, WI1, and WI2), the nitrification potential of AOA and AOB acting together was lower than the sum of their respective nitrification potentials when operating independently. However, in the WI2 treatment, the co-occurrence of AOA and AOB increased the nitrification potential of comammox from 22%–26% to 29%–46%, indicating that the presence of AOA and AOB can enhance the nitrification potential of comammox in upland soils with high organic matter content. The results of this study provide the knowledge of the genetic and metabolic associations among soil nitrifying microbial communities.