Inoculation of Ensifer fredii strain LP2/20 immobilized in agar results in growth promotion and alteration of bacterial community structure of Chinese kale planted soil

In our former research, we succeeded in using agar, alginate, and perlite as immobilization materials to maintain long-term survival of the inoculant, Ensifer fredii LP2/20, in a controlled glasshouse. Therefore the information on the establishment and activity of the inoculant to promote plant growth under field conditions, the effects of the inoculant on the soil microbial communities and specific microbial taxa, and the association between the inoculant and soil elements merit further studies. Here, we found that agar was the most suitable material that supported the establishment of the inoculant under field conditions. RNA-based analysis showed that E. fredii LP2/20 immobilized in agar was still metabolically active at day 50 after being introduced into soil. Inoculation of E. fredii LP2/20 immobilized in agar conferred the highest plant dry weight (up to 89.94%) and all plant elements including total N (9.55%), P (17.94%), K (68.42%), Ca (39.77%), Mg (30.76%), Fe (29.85%), and Zn (22.44%). Inoculation of E. fredii LP2/20 immobilized in agar increased soil chemicals including soil organic matter (99.02%), total N (272.48%), P (31.75%), K (52.74%), Fe (51.06%), and Zn (63.10%). High-throughput next-generation sequencing of bacterial 16S rRNA amplicons showed that the Proteobacteria, Acidobacteria, Bacteroidetes, and Firmicutes were dominant phyla in Chinese kale field soil. Inoculation of E. fredii LP2/20 significantly affected the soil bacterial community structure by decreasing total bacterial richness and diversity. The numbers of alpha- and gamma-Proteobacteria were significantly increased while the number of delta-Proteobacteria was significantly decreased due to E. fredii LP2/20 establishment. Soil total P, K, and Ca and soil pH were the important factors that shaped the soil bacterial community composition.