Response of bacterial communities and nitrogen-cycling genes in newly reclaimed mudflat paddy soils to nitrogen fertilizer gradients.

Conversion of coastal mudflats into paddy soils is an effective measure to alleviate the pressures on land resources. However, few studies have evaluated the effects of nitrogen (N) fertilizers on bacterial communities in newly reclaimed mudflat paddy soils. We performed a field plot experiment with six N fertilizer rates (0, 210, 255, 300, 345, and 390 kg N ha) in a newly reclaimed mudflat paddy for 2 consecutive years and used Illumina sequencing and qPCR to investigate the effects of N fertilizers on bacterial communities and N-cycling genes. Results showed that high N fertilization (above 300 kg N ha) increased the contents of organic matter (OM), total N (TN), ammonium (NH), and nitrate (NO) and significantly decreased the diversity and richness of bacteria. Furthermore, high N fertilization had a stronger effect on bacterial communities than low N fertilization, probably due to high concentrations of NH, OM, and NO. Additionally, in paddy soils with high N fertilizer application, the relative abundances of Bacteroidetes, γ-proteobacteria, and Actinobacteria increased significantly, but the reverse was true for those of Chloroflexi, Firmicutes, δ-proteobacteria, α-proteobacteria, Acidobacteria, and β-proteobacteria. The results of qPCR indicated that high N fertilization significantly increased the relative abundance of nifH genes involved in N fixation and decreased that of amoA-archaea involved in ammonia oxidation, nirS genes involved in nitrite reduction, and nosZ genes involved in nitrous oxide reduction, which suggested that high N fertilization increased the potential of available N retention and reduced the potential of nitrous oxide emission. Overall, N fertilizers with an N fertilizer rate of above 300 kg N ha significantly altered the bacterial communities and N-cycle of mudflat paddy soils.