Response of continuous cropping cowpea and root-zone soil to composite microbial agents: plant growth promotion and nitrogen-fixing bacterial community regulation

Continuous cropping obstacle of cowpea is an urgent problem to be solved for its sustainable production. A field experiment was performed from April to June 2024 to study the effects of four commercially microbial agents on the growth and yield of continuously cropping cowpea plants and nitrogen-fixing bacterial community structure in root-zone soil by high-throughput sequencing technology. Four commercially microbial agents used were BS (Bacillus subtilis), TH (Trichoderma harzianum), AB (Bacillus amyloliquefaciens, Brevibacillus laterosporus, Bacillus mucilaginosus and Enterobacter ludwigii), and CP (T. harzianum, B. amyloliquefaciens, Bacillus licheniformis and B. laterosporus). The results showed that four microbial agents enhanced soil organic matter, nitrate nitrogen and available potassium content in soil. The application of BS resulted in an increase in the relative abundance of Proteobacteria in root-zone soil of cowpea, while Actinobacteria and Firmicutes showed a downward trend compared to the control (CK). The treatments of BS, TH and AB significantly increased the relative abundance of Bradyrhizobium in soil by 109.4%, 38.5%, and 20.6% compared with CK, respectively. Furthermore, TH and CP increased the relative abundance of Halorhodospira in soil by 47.4% and 98.5%, respectively. The yield of cowpea plants increased by 9.6% with CP treatment compared to CK (P < 0.05). The BS treatment significantly promoted the main stem width of cowpea plants. The AB treatment enhanced protein, vitamin C and organic acid contents in cowpea plants. Overall, CP treatment is a better choice, as it has potential advantages over other agents in terms of cowpea growth and yield, and can improve the structure of the soil nitrogen-fixing bacterial community.