Objective This paper explores the diversity of soil nitrogen-fixing microorganisms, community structure composition, and environmental influencing factors in desert leguminous scrubland at the community level.

Method Fluorescence quantitative PCR and Illumina high-throughput sequencing methods were used to study the abundance of nifH gene, nitrogen-fixing microbial diversity and community composition in soils of four typical legume scrubland (Ammopiptanthus mongolicus, Oxytropis aciphylla, Caragana korshinskii, and Caragana tibetica) in Ningxia desert grassland of northwestern China. Pearson correlation analysis and redundancy analysis were used for environmental factor correlation analysis.

Result (1) There were differences in the abundance of nifH gene and diversity of nitrogen-fixing microorganisms in soil of four types of scrubland. The abundance of nifH in rhizospheric soil of Caragana korshinskii was significantly higher than that in soil of Ammopiptanthus mongolicus, Oxytropis aciphylla and Caragana tibetica forestland (P < 0.05). The richness of nitrogen-fixing microbial communities in soil of C. korshinskii and A. mongolicus forestland were significantly higher than that in O. aciphylla and C. tibetica soil (P < 0.05). The community diversity was significantly higher in C. korshinskii soil than that of A. mongolicus soil (P < 0.05). However, there were no significant differences in diversity of nitrogen-fixing microbial communities in soil of O. aciphylla , C. tibetica and A. mongolicus forestland. (2) A total of 5 phyla, 15 families and 19 genera of nitrogen-fixing microorganisms were detected in soil of legume scrubland. Proteobacteria was predominant phylum with relative abundance ranging from 91.13% to 97.79%. Rhodospirillaceae was common predominant family in rhizospheric soil of the four shrubs, relative abundance accounting for 59.56%−79.68%. At genus level, Skermanella, Azospirillum and Azohydromonas were common dominant genera. The composition of nitrogen-fixing microorganisms in soil of A. mongolicus differed significantly from the other three types of soil, with a large number of distribution of genus Azotobacter (accounting for 25.09%). (3) Correlation analysis showed that the abundance of nifH gene, richness index, and β diversity of nitrogen-fixing microorganisms were significantly positively correlated with soil pH. Soil available phosphorus, available potassium, pH, total nitrogen, and total phosphorus significantly affected the composition of nitrogen-fixing microbial communities.

Conclusion The abundance of nifH gene, richness and diversity of nitrogen-fixing microorganisms in rhizospheric soil of C. korshinskii are higher than those in other scrubland soils. Soil microbial functional communities of C. korshinskii display stronger nitrogen accumulation and transformation abilities.