Study on soil improvement and growth-promoting effect of compound microbial agents in tomato cultivation

Using microorganisms to improve and repair soil and assist plant growth is an effective ecological restoration method. The compound microbial inoculant prepared by mixing several microorganisms with different functions is one of the hot spots in the current research of agricultural green development and food security. In this study, four treatments were set up: no microbial agent (CK), Trichoderma harzianum agent (H), Bacillus subtilis agent (K), and compound microbial agent (K + H). The effects of single and compound microbial agents on tomato growth, rhizosphere soil physical and chemical properties, enzyme activity and microbial community structure and function were investigated, and the correlation between soil nutrients and microbial community was analyzed. The results showed that the application of microbial agents significantly promoted the growth of tomato, and the effect of K + H treatment was the best. The fresh weight, dry weight, plant height and stem diameter of tomato were 25.06%, 17.61%, 3.05% and 16.62% higher than CK, respectively. Microbial agents changed the physical and chemical properties and enzyme activities of rhizosphere soil, significantly increased the content of alkali-hydrolyzable nitrogen and significantly decreased the activity of catalase, and the activity of alkaline phosphatase increased the most under K treatment. High-throughput sequencing showed that microbial agents had a significant effect on the diversity and structure of microbial community in rhizosphere soil. In terms of microbial community diversity, K + H treatment significantly increased the Simpson index of bacteria, and significantly reduced the Chao1, ACE richness index and Shannon, Simpson diversity index of fungi. In the microbial community structure, the dominant phylum of the bacterial community is Proteobacteria, Gemmatimonadota, Actinobacteriota and Acidobacteriota, and the fungi are Ascomycota and Basidiomycota. After microbial inoculant treatment, the relative abundance of Gemmatimonadota, Acidobacteriota, Chloroflexi and Ascomycota increased significantly, while the relative abundance of Actinobacteriota, Olpidiomycota and Chytridiomycota decreased. At the genus level, the abundance of KD4-96, SC-I-84, Penicillium and Trichoderma spp. changed significantly. Microbial agents also regulated microbial functions, and bacterial amino acid transport and metabolic functions were the most active. K + H treatment increased the relative abundance of fungal saprophytic nutrients to 52.37%, which was the highest in each treatment. Redundancy analysis showed that soil alkaline phosphatase, catalase and alkali-hydrolyzable nitrogen were the key environmental factors affecting microbial community structure, which were significantly correlated with the dominant genera of bacteria and fungi. This study confirmed that Bacillus subtilis and Trichoderma harzianum compound inoculants had a good synergistic effect in promoting tomato growth and improving soil microecology, which provided theoretical and practical basis for the development and application of microbial inoculants in tomato production.