Irrigation frequency alters the abundance and community structure of ammonia-oxidizing archaea and bacteria in a northern Chinese upland soil

- Irrigation significantly enhanced soil potential nitrification activity (PNA).
- Irrigation significantly increased AOA abundance but decreased AOB abundance.
- Soil PNA was correlated positively with AOA abundance.
- Irrigation greatly altered the composition of the AOA and AOB communities.

Ammonia oxidation, the first and rate-limiting step in nitrification, is a key process in biogeochemical nitrogen cycling. However, little is known about how irrigation affects ammonia oxidizing microorganisms in agroecosystems. In this study, effects of irrigation frequency on soil potential nitrification activity (PNA), the abundance and composition of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were investigated using real-time PCR (qPCR), terminal restriction fragment length polymorphism (T-RFLP), and clone libraries for characterizing the ammonium monooxygenase genes (amoA) in a northern Chinese soil. Results showed that irrigation significantly increased soil PNA and archaeal amoA gene abundance but decreased bacterial amoA gene abundance. Soil PNA was positively correlated with archaeal amoA gene abundance and negatively correlated with bacterial. T-RFLP and PCA results showed that irrigation greatly changed the AOA and AOB community structures by altering the relative abundances of four T-RFs of AOA and four other T-RFs of AOB, respectively. Phylogenetic analysis revealed that all archaeal sequences fell into Group 1.1b, and the bacterial clones were dominated by Nitrosospira-like sequences within Cluster 3a.2. Irrigation induced the appearance of two archaeal and two bacterial OTUs and increased the relative abundance of another two archaeal and four bacterial OTUs, respectively. Additionally, soil moisture, soil pH, NH4+-N, and NO3−-N correlated significantly with the AOA community, and soil pH, total nitrogen (TN), and NO3−-N were significantly correlated with the AOB community. Our results demonstrate that irrigation greatly affected the abundance and community structure of AOA and AOB and that AOA appeared to play a more important role in nitrification in the study soil.