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Growth, Nodulation and Mycorrhizal Colonization in Bean Plants Improved by Rhizobial Inoculation with Organic and Chemical Fertilization

ABSTRACT The poor nutrition of organic crops during the early growth stage has frequently resulted in a low productivity. Nutrients from organic fertilizers are not available to crops unless they are mineralized and a low microbial population can further delay the decomposition processes. We hypothesized that increasing microbial population, such as rhizobia in the rhizosphere of the organic crops might offset the problem of low productivity by increasing the rate of mineralization of organic fertilizers. Accordingly, the current study was aimed at evaluating the responses of potted beans (Phaseolus vulgaris L. cultivar Hoseki) to rhizobial (Rhizobium leguminosarum cv. Phaseoli) inoculation in organically fertilized (OF) and chemically fertilized (CF) normal soils. Inoculation resulted in a significant increase in the pod yield (P < 0.05) with organic fertilizer, yet not with chemical fertilizer. Inoculation did not increase the shoot and root dry masses in the CF plants, yet did increase the shoot dry mass in the OF plants. Inoculation positively influenced nodulation in both the OF and CF plants, and the effect was more pronounced in the OF plants. The Arbuscular Mycorrhizal infection rates and frequency of arbuscule formation were always higher in the OF plants than in the CF plants, and moreover, the inoculation increased AM infectivity in the OF plants but not in the CF plants. The inoculated OF plants had more shoot N and P compared to un-inoculated plants, while the CF plants were unaffected. Interestingly, AM infection rates showed a significant positive correlation to the shoot P or N in the OF plants, yet the relation was not significant in the CF plants, indicating greater dependency of organic plants than the chemical plants on the symbiotic fungi. Therefore, the present results demonstrate importance of rhizobial inocula as a potential biofertilizer in organic farming.