Soil Microbial Mechanisms to Improve Pear Seedling Growth by Applying Bacillus and Trichoderma-Amended Biofertilizers

Bacillus velezensis SQR9 or Trichoderma harzianum NJAU4742-amended bioorganic fertilizers might significantly improve the soil microbial community and crop yields. However, the mechanisms these microorganisms act are far away from distinctness. We combined amplicon sequencing with culturable approaches to investigate the effects of these microorganisms on pear tree growth, rhizosphere nutrients and microbial mechanisms. The SQR9 and T4742 treatments increased the total biomass of pear trees by 68% and 84%, respectively, compared to the conventional organic fertilizer treatment (CK). SQR9 tends to increase soil organic matter and available phosphorus, while T4742 more effectively enhances nitrogen, potassium, iron and zinc levels. These effects were primarily linked to changes in the microbial community. T4742 treatment enriched twice as many differential microbes as SQR9. SQR9 significantly enriched Urebacillus, Streptomyces and Mycobacterium, while T4742 increased the abundance of Pseudomonas, Aspergillus and Penicillium. In vitro experiments revealed that secondary metabolites secreted by B. velezensis SQR9 and T. harzianum NJAU4742 stimulate the growth of key probiotics associated with their respective treatments, enhancing soil fertility and plant biomass. The study revealed the specific roles of these bioorganic fertilizers in agricultural applications, providing new insights for developing effective and targeted bioorganic fertilizer products and promoting sustainable agriculture.