Isolation, identification and growth-promoting effects of culturable nitrogen-fixing bacteria and ammonifying bacteria in rhizosphere soil of Mikania micrantha.
Objective: Nitrogen-fixing bacteria and ammonifying bacteria are the key initial links of nitrogen cycle to produce bioavailable nitrogen, which directly affect the growth and spread of invasive alien plants. However, the culturable nitrogen-fixing bacteria and ammonifying bacteria in the rhizosphere of typical invasive plant Mikania micrantha H.B.K. have been rarely reported, which restricts our understanding of the efficient nitrogen transformation mechanism therein. Methods: The culturable nitrogen-fixing bacteria and ammonifying bacteria in the rhizosphere soil of M. micrantha were isolated and purified by traditional culture method, and the inoculation experiment was carried out for verification. Results: The density and nitrogen transformation rate of nitrogen-fixing bacteria and ammonifying bacteria and their nitrogen fixation efficiency and organic nitrogen mineralization efficiency were all higher than those of two co-occurring native competitors, Persicaria chinensis and Paederia scandens. Phylogenetic tree indicated that the nitrogen-fixing bacterial strains in the rhizosphere of M. micrantha were classified into five genera, including Burkholderia,Enterobacter,Phytobacter, Kosakonia and Rhizobium, and ammonifying bacterial strains fell into seven genera, including Serratia, Acinetobacter, Pseudomonas, Bordetella, Stenotrophomonas, Ochrobactrum and Chryseobacterium. Both Burkholderia and Rhizobium of the nitrogen-fixing bacteria and Serratia of the ammonifying bacteria were the dominant functional bacteria of M. micrantha. The greenhouse pot experiments of inoculation with the culturable functional strains demonstrated that the two groups of bacteria significantly promoted the growth of M. micrantha seedlings. Amongst them Rhizobium YHAzMm-21 of nitrogen-fixing bacteria and Ochrobactrum YHAmMm-14 of ammonifying bacteria had the best growth-promoting effects, which were expected to be developed into microbial fertilizers or engineering microorganisms. Conclusion: The density and nitrogen transformation efficiency of the culturable nitrogen-fixing bacteria and ammonifying bacteria in the rhizosphere soil of M. micrantha outweighed those of the native species, which promoted the plant growth. These results clarified the contribution of the culturable flora in the rhizosphere soil of M. micrantha to nitrogen cycle, and provided an ideal material for screening functional strains with high nitrogen cycle rates.