Chronic Nitrogen Fertilization Modulates Competitive Interactions Among Microbial Ammonia Oxidizers in a Loess Soil

Nitrogen (N) application may lead to niche segregation of soil ammonia-oxidizing archaea (AOA) and bacteria (AOB), thereby reducing the competitive interactions between AOA and AOB due to higher ammonium substrate availability. However, the adaptive mechanisms of AOA and AOB under N enrichment remain poorly understood. Stable isotope probing (SIP) microcosm incubation was employed to reveal community changes of active AOA and AOB in a loess soil from a field experiment growing potatoes that received no N (control, CK), low N (LN, 75 kg N ha^?1), and high N (HN, 375 kg N ha^?1). The results showed that the soil potential nitrification rate (PNR) was measured by culturing of the soil samples from the field experiment. Soil PNR was significantly increased in HN by 87.5% and 67.5% compared with CK and LN, respectively. Compared with CK, the ¹³C-amoA genes of soil AOA and AOB in HN had 2.58 × 10⁴ and 1.55 × 10⁶ copies, representing 1.6- and 16.2-fold increase respectively. It was indicated that AOB dominated soil ammonia oxidation. A phylogenetic analysis of the ¹³C-amoA gene showed that N application significantly increased the proportion of 54d9-like AOA up to 90% in HN, while the Nitrososphaera gargensis-like and Nitrososphaera viennensis-like AOA were inhibited and completely disappeared. Nitrogen application also resulted in the community shift of active AOB-dominant group from Nitrosospira briensis-like to Nitrosospira sp. TCH711-like. Our study provides compelling evidence for the emergence and maintenance of active nitrifying communities under the intensified N input to an agricultural ecosystem.