Appraisal of the crop-rotation effect of rhizobial inoculation on potato cropping systems in relation to soil bacterial communities

Non-legume crops grown in rotation with legumes usually have reduced N-fertilizer requirement, which has both economic and environmental benefits. In this study, we aimed to assess the indirect effect of Phaseolus vulgaris inoculation with two indigenous rhizobia strains on potato growth promotion and disease control in relation to inoculation effects on soil bacterial communities. T-RFLP profiling demonstrated that inoculation significantly increased the phylotype richness of the bacterial communities at the end of P. vulgaris life cycle. A significant difference in richness between simple and dual inoculation was found. Effects on bacterial structure are clearly sensed with both inoculants. Various bacteria like Halomonas, Arthrobacter, Rahnella, Actinobacterium and Frankia were enhanced by inoculation irrespective of the inoculant type. However, other bacteria like Clostridium, Bacillus, Stenotrophomonas and Xanthomonas were enhanced by rhizobial mono-inoculation only and not by co-inoculation with both strains. Some bacteria may potentially behave like plant growth promoting rhizobacteria (PGPR) towards potatoes grown in rotation with common bean, as indicated by the 32% increase observed in potato yield, and also by the 56% decrease in potato wireworm infection. Therefore, rhizobia inoculation may contribute to the rotational benefits of legumes in potato cropping systems not only by providing fixed nitrogen, but also by increasing microbial diversity and structure, potentially stimulating plant growth promoting rhizobacteria and enhancing disease control. However, these effects depend largely on inoculant formulation.