Response of bacteria harboring nirS and nirK genes to different N fertilization rates in an alkaline northern Chinese soil

- The abundances of nirS genes were higher than that of nirK genes.
- The N fertilization rates strongly affected the nirS-denitrifiers community composition.
- Soil pH and TN were associated with shifts of the nirS- and nirK-denitrifiers communities.

The impact of N fertilization rates on the abundance and diversity of bacteria with a potential for denitrification as indicated by presence of either nirS or nirK genes was investigated in an alkaline wheat field in northern China. Four treatments were compared: no N, 0 kg N ha−1, CK; low N rate, 75 kg N ha−1, LN; moderate N rate, 150 kg N ha−1, MN; and high N rate, 225 kg N ha−1, HN. The nirS and nirK genes were quantified by real-time PCR and their diversity estimated by terminal restriction fragment length polymorphism (T-RFLP) and DNA sequencing of clone libraries. The results showed higher abundances of nirS gene in all treatments, with nirS to nirK ratios ranging from 9.95 to 2.31. The abundance of nirK gene increased with N fertilization increments; however, significantly lower numbers of nirS gene were detected in the treatments with more N fertilization (P < 0.05). A remarkable difference of nirS-harboring bacteria in response to N fertilization rates was observed compared to those with nirK genes. Shifts in nir abundance and community composition of bacteria with nirS and nirK genes were correlated with the changes in soil pH, total nitrogen, and ammonium or nitrate. The results of this study demonstrate that N fertilization rates can influence the abundance and community composition of bacteria with nirS and nirK genes and that soil pH as well as total nitrogen can be dominant factors controlling their community composition.