Nitrogen, season, and tillage management influence ammonia oxidizing bacterial communities in long-term maize

Nitrification is the biological oxidation of NH₃ to NO₂ ⁻ and NO₃ ⁻ for which ammonia-oxidizing bacteria (AOB) are the functionally dominant group in cropland responsible for the rate-limiting step of NH₃ oxidation. Different AOB species have functional differentiation and do not equally contribute to nitrification. This investigation explored if long-term N fertilization and tillage influenced AOB community structure. The study site was a long-term (>40 years) continuous maize (Zea mays L.) experiment with three N fertilization rates (0, 168, and 336 kg ha⁻¹) and either no-tillage (NT) or moldboard plow tillage (PT). We used denaturing gradient gel electrophoresis (DGGE) to analyze PCR-amplified bacterial ammonia monooxygenase genes and detect changes among NH₃-oxidizing bacteria. Tillage, fertilization, and sample season all significantly influenced the AOB community. The AOB were more diverse in NT than PT. The AOB became more diverse with increasing N input and were more diverse in summer than winter samples. Nitrosomonas-like and Nitrosospira-like groups were identified based on gel migration patterns. Unique bands occurred in different treatments, manifesting environmental selection. The long-term field trial showed soil management consistently influenced AOB communities. Significantly, AOB diversity differed at a small scale within a relatively uniform landscape, even in the presence of long-term management practices.