Effect of nitrogen fertilizer level on bacterial community and N₂O emission in paddy soil

Microbial community structures are relevant for the utilization of nitrogen (N) and the emission of nitrous oxide (N₂O) in paddy soils. Nitrification and denitrification are the main ways to produce N₂O in soils and nitrification bacteria and denitrifying bacteria respectively drive the processes of nitrification and denitrification. In this study, changes in bacterial communities and the abundance of nitrification bacteria or denitrifying bacteria under different nitrogen fertilizer levelsCK (no N fertilization), 1/3N (N application of 60 kg·hm^-2), 2/3N (N application of 120 kg·hm^-2) and N (N application of 180 kg·hm^-2 in paddy soils were analyzed respectively by high-throughput sequencing and real-time PCR. The analysis in abundance of nitrification bacteria or denitrifying bacteria was based on ammonia-oxidizing bacterial amoA gene and denitrifying bacterial nirS gene or qnoB gene or nosZ gene. The results showed that increase in application of nitrogen fertilizer enhanced Chao1 index and Shannon diversity index of bacterial communities and changed the composition of bacterial communities in paddy soil. The relative abundance of Nitrospirae and Acidobacteria increased with improvement of nitrogen fertilizers, while that of Methylosinus decreased with improvement of nitrogen fertilizers in paddy soils. There was greater impact of increased use of nitrogen fertilizer on the abundance of amoA gene in paddy soils. The abundance of amoA gene increased with increasing nitrogen application in the 0-5 cm or 10-20 cm depths of soil. The abundances of nirS gene, qnoB gene and nosZ gene in no-fertilizer soil (CK) were significantly lower than those in fertilizer soils (1/3N, 2/3N and N) (P < 0.05). There was no significant difference in the abundance of nirS gene among 1/3N, 2/3N and N treatments. However, there was an increasing tendency in the abundance of qnoB gene and nosZ gene with increasing application of nitrogen fertilizer in the 0-5 cm depth of soil. The abundance of nosZ gene in the 10-20 cm depth of soil under both 2/3N and N treatments were significantly higher than that in 1/3N treatment (P < 0.05). At the same time, the emission of N₂O under N treatment was significantly higher than that under 2/3N or 1/3N treatment (P < 0.05), and the latter two were also higher than that under CK (P < 0.05). Correlation analysis showed that the emission of N₂O was markedly correlated with the relative abundance of Nitrospirae in the 0-5 cm depth of soil and the abundance of amoA gene in the 10-20 cm depth of soil (P < 0.05, n=10). In summary, increasing nitrogen application improved the diversity of bacterial communities and the emission of N₂O in the studied paddy soils. Also there was a closer correlation between changes in abundance of nitrification bacteria and the emission of N₂O. The results suggested that the influence of nitrification bacteria on the emission of N₂O in the studied paddy soils was greater than that of denitrifying bacteria.