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

Methods 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 in Ningxia desert grassland. Pearson correlation analysis and redundancy analysis were used for environmental factor correlation analysis.

Results (1) There were differences in the abundance of nifH gene and the diversity of nitrogen fixing microorganisms in soils of the four scrubland. The abundance of nifH in rhizospheric soil of Caragana korshinskii was significantly higher than that in soils of Ammopiptanthus mongolicus, Oxytropis aciphylla and Caragana tibetica (P < 0.05). The richness of nitrogen fixing microbial communities in soils of C. korshinskii and A. mongolicus were significantly higher than that in the O. aciphylla and C. tibetica soils (P < 0.05). The communities diversity was significantly higher in C. korshinskii soil than that of A. mongolicus soil (P < 0.05). However, there were no significant differences in the diversity of nitrogen fixing microbial communities in soils of O. aciphylla , C. tibetica and A. mongolicus. (2) A total of 5 phyla, 15 families and 19 genera of nitrogen-fixing microorganisms were detected in soils of the legume scrubland. Proteobacteria was predominant phylum with relative abundance ranging from 91.13% to 97.79%. Rhodospirillaceae was common predominant family in rhizospheric soils of the four shrubs, relative abundance accounting for 59.56% -79.68%. At the genus level, the genera Skermanella, Azospirillum, and Azohydromonas were the common dominant genera. The composition of nitrogen fixing microorganisms in soil of A.mongolicus differs 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, the richness and diversity of nitrogen fixing microorganisms in rhizospheric soil of C. korshinskii were higher than those in other scrubland soils. Soil microbial functional communities of C. korshinskii displayed stronger nitrogen accumulation and transformation abilities.