Effects of co-inoculation of indole-3-acetic acid- and ammonia-producing bacteria on plant growth and nutrition, soil elements, and the relationships of soil microbiomes with soil physicochemical parameters

Abstract To cope with the problem of the reduction in agricultural productivity, the massive usage of chemical fertilizers for boosting agricultural productivity is popular across the world. In return, it thereby declines soil quality and biodiversity. We therefore determined the potential of co-inoculation of the indole-3-acetic-acid- and ammonia-producing bacteria, Pseudomonas sp. strain NK2/7 and Bacillus sp. strain NK1/19, immobilized in agar to promote mustard greens growth and investigated the effects of co-inoculation of both strains on plant and soil elements as well as soil bacterial community structure, in comparison with a chemical fertilizer and single-strain inoculations. The knowledge on the relationships of both species with plant and soil microbiota in field conditions is limited. The results show that the co-inoculation of both strains immobilized in agar increased plant dry weight up to 62.02% and plant elements, including N (38.46%), P (28.37%), K (203.16%), Ca (10.89%), Mg (25.8%), and Na (70.3%). The co-inoculation also conferred the highest organic matter (OM) (by 429.45%) and increased soil elements including Fe (92.74%), Mn (126.88%), Cu (12.05%), and Zn (107.46%). Illumina next-generation sequencing demonstrated that the establishment of both strains affected the soil bacterial community by reducing bacterial diversity and richness. Bacterial classes Bacteroidia, alpha-Proteobacteria, Vicinamibacteria, Gemmatimonadetes, Verrucomicrobiae, Polyangia, Nitrososphaeria, and Blastocatellia were significantly decreased, whereas the amounts of gamma-Proteobacteria and Bacilli were significantly increased. Soil parameters, including pH, OM, total Fe, Mn, Cu, and Zn, were the major factors influencing the soil bacterial community structure.