Cultivable diazotrophic community in the rhizosphere of three desert shrubs and their nitrogen-fixation and siderophore-producing capabilities

Abstract:

[Objective] To reveal the cultivable diazotrophic communities in the rhizosphere of three relict shrubs of Tetraena mongolica, Ammopiptanthus mongolicus and Nitraria tangutorum in Western Ordos and the bacterial capabilities for nitrogen-fixation and siderophore-producing, and finally to provide basis for understanding and utilizing the desert plant growth-promoting rhizobacteria. [Methods] Using Ashby medium nitrogen-free, the nitrogen-fixation bacteria strains were isolated and purified by the plate spread and streak technique; 16S rRNA gene was applied to classify the isolates and reveal the community composition. The nitrogenase activities of rapid-growing strains were analyzed through acetylene reduction assay. To screen siderophore-producing strains and their capability of siderophore production, chrome azurol S (CAS) blue plate and spectrophotometer were used for qualitative screening the strains and quantitative determination of their products, respectively. [Results] A total of 22 rhizospheric diazotrophs were isolated and classified into 3 phyla and 9 genera and they were predominated by the phylum Proteobacteria (82%) and the genus Pseudomonas (27.27%). The genus Rhizobium and Bacillus was solely found associated with Ammopiptanths mongolicus and Teraena mongolica respectively, whereas Enterobacter, Stenotrophomonas and Paenibacillus only with Nitraria tangutorum. Ten strains could rapidly grow on the nitrogen-free medium and they possessed nitrogenase activity in the range of 871.71 to 3 383.09 nmol C2H4/(H?culture). Seven of the 10 strains could produce siderophore and the producing capability (As/Ar value) was ranged from 0.35 to 0.79. [Conclusion] the cultivable rhizospheric diazotroph in the rhizosphere soil of the relict shrubs in Ordos desert was highly diverse and markedly different between the shrubs. Most of these diazotrophs have both high nitrogen-fixation and siderophore-producing capability. Thus, they could be useful as plant growth-promoting rhizobacteria.

Key words:Diazotrophs, Acetylene reduction assay, Siderophore producing ability