Microbial community in alfalfa rhizosphere in response to rhizobial inoculation under increased concentrations of...

The rhizosphere is a very active region containing a large number of microorganisms involved in complex biological and ecological processes. The microorganisms can improve the soil conditions, promote plant growth, and alleviate stress in plants under heavy metal contamination. Rhizobial bacterium (Ensifer meliloti) forms endosymbiosis with alfalfa (Medicago sativa L.), providing nitrogen to the plant, and can mitigate the effects of different stress factors.

This study aimed to evaluate the effects of rhizobial inoculation on alfalfa growth and rhizosphere microbiological properties in soils with increased nickel (Ni) concentrations during 2 years and across seasons. Two locations with different heavy metal concentrations, lower and higher, mainly Ni concentrations, but also lead (Pb) and chromium (Cr); six different rhizobial inoculants; and three different sampling time points were tested. The abundance of different groups of culturable bacteria (total number of microorganisms, fungi, actinomycetes, oligonitrophiles, Azotobacter, and ammonifiers), as well as microbial activity, that is, basal soil respiration rates, was evaluated in the rhizosphere soil of each treatment, location, and season.

The inoculation in some treatments significantly increased alfalfa yield in particular cuts, depending on the location (p < 0.05) and treatment (p < 0.05), with up to 38% increase compared to the control non-inoculated plants. Inoculations also influenced the abundance of specific microbial groups and soil respiration rates. Multivariate analysis of variance revealed a significant interaction between the tested factors [rhizobial inoculation (p < 0.05), heavy metal concentrations (p < 0.05), and seasons (p < 0.05)] on all evaluated soil microbiological properties. Generally, the total number of microorganisms was positively influenced by inoculation and varied among different rhizobial treatments within the same field and season. The increased number of rhizosphere bacteria was positively correlated with yield in some cuts.

The results show the potential of inoculation with particular rhizobia in the improvement of alfalfa yield in Ni-contaminated soils and overall microbiological properties and indicate the complexity of the interactions of multiple factors in the environment.