Comammox dominate soil nitrification under different N fertilization regimes in semi-arid areas of Northeast China

The newly identified complete ammonia oxidation (Comammox), which is capable of oxidizing ammonia directly to nitrate, has complemented the knowledge of nitrification in the global nitrogen (N) cycle. Knowledge of the community compositions and contributions to nitrification of autotrophic ammonia-oxidizing archaea (AOA) and bacteria (AOB) and Comammox remain void. In this study, the abundance, community compositions, and con-tributions to nitrification of AOA, AOB, and Comammox were observed in five N gradients (0, 90, 150, 210, and 270 kg ha-1) in a semi-arid area of Northeast China. The results indicated that, compared with low N application rates, higher N application rates significantly improved the soil ammonia monooxygenase (AMO) and the hy-droxylamine oxidase (HAO) activities while total nitrogen (TN) was noted to be the main factor driving AMO and HAO activities. Soil potential nitrification rate (PNR) significantly increased with the increase in N application rate, and soil PNR was positively correlated with TN, nitrate nitrogen (NO3 ?-N), soil organic matter (SOM), and ammonium nitrogen (NH4+-N). The structural equation model (SEM) showed that TN was the main factor driving the community composition of Comammox; SOM had a greatest effect on the community composition of AOA while NH4+-N had a greatest effect on the community composition of AOB. The soil PNR had a direct effect on the yield. Overall, the findings of this study highlighted that N application was more conducive to the reproduction of soil nitrifying microorganisms while Comammox was the dominant contributor to nitrification under N fertilization regimes in the semi-arid area of Northeast China.