【Objective】Effects of nitrogen (N) application rate on N-fixing microbial communities in rhizosphere soil and maize growth on land under integrated irrigation-fertilization practice were analyzed.

Method N-topdressing with no N (N0 as control), 70% N at 98 kg·hm⁻² (N1), 100% N at 140 kg·hm⁻² (N2), 130% N at 182 kg·hm⁻² (N3) or 150% N at 210 kg·hm⁻² (N4) was applied on a maize growing field. High-throughput sequencing was conducted to compare the differences in the diversity and composition of N-fixing microbial communities in the rhizosphere soil. Growth of maize plants on the lots were correlated to the collected data on the soil at the same lot.

Result Application of N fertilizer significantly increased the soil N as well as the maize biomass and N accumulation at growth stages. N2 yielded the optimal performance among all treatments. The α-diversity of N-fixing microbes in the rhizosphere soil decreased at first as the N application rate increased. It was followed by an increase and another decline. Of all treatments, N2 enhanced the α-diversity but N1 and N4 suppressed it. N-application significantly affected the soil microbial community structure, particularly during the jointing and flowering stages of maize. It also significantly raised the abundance of beneficial species, such as Bradyrhizobium. Notably, N2 enriched the relative abundance of plant growth-promoting bacteria of the unclassified order Burkholderiales. The upregulated modularity index and other topological parameters of the network analysis suggested a more stable community structure under N2 as well. The partial least squares path modeling further indicated that the rate of N-application indirectly stimulated the maize growth by way of altering the N composition in soil.

Conclusion The N-application at a rate of 140 kg·hm² (N2) appeared to optimize the soil N, stabilize the N-fixing microbial community, and promote the maize growth. Consequently, it was recommended for maize farming under the integrated irrigation-fertilization practice.