Soil bacteria and symbiotic rhizobia synergistically promote nitrogen fixation and biomass production of alfalfa (Medicago sativa L.) plants even under water shortage conditions without altering the native rhizospheric microbiota

In modern and sustainable agriculture, the exploitation of biological nitrogen fixation (BNF) is an alternative to synthetic nitrogen fertilizers which can lead to groundwater contamination, reduction of soil quality, and greenhouse gas emissions. This study aimed to exploit the synergistic effects of nitrogen-fixing symbiotic rhizobia and nonsymbiotic soil bacteria on alfalfa (Medicago sativa L) yield in low-input farming. Two microbial consortia (Mix1 and Mix2) containing a nitrogen-fixing alfalfa symbiont Sinorhizobium meliloti were tested along with a different number of soil bacteria whose PGP (Plant Growth Promoting) characteristics (IAA production, nitrogen fixation, and phosphate solubilization) led to synergistic effect. The two consortia and the S. meliloti strain were tested under increasingly complex conditions, through in vitro strain culture testing, growth chamber, pot trials, and field trials. In the laboratory growth chamber, the alfalfa plants inoculated with the consortium Mix2, containing more strains with different activities, showed increased nitrogenase activity compared to those inoculated with S. meliloti alone. Field experiments showed that biomass production and assimilation of nitrogen due to nitrogen fixation (¹⁵N test) of Mix2-inoculated plants was significantly increased compared to Mix1-inoculated and uninoculated ones (native microbiota), even under water shortage. Moreover, no significant impact of consortia on the native rhizospheric microbiota was detected. Our findings indicated that consortia containing both rhizobia and non-rhizobia PGP strains, which don't alter soil ecology, could be used for the enhancement of growth and nitrogen fixation of legume plants and that diversity and synergistic interaction of the consortium could be a good index for predicting success in field trials.