Bacillus velezensis activates beneficial soil microorganisms to promote the growth and health of cucumber after soil fumigation

Cucumber root rot, primarily caused by Fusarium solani, threatens global cucumber production through soil-borne infection, vascular wilt, and yield loss. Soil fumigation with Calcium cyanamide (CaCN₂) effectively suppresses soil-borne pathogens but may impair beneficial soil microbiota. This study investigated the synergistic effects of Bacillus velezensis (ZF336) inoculation on reactivating beneficial soil microbes after CaCN₂ fumigation and its role in controlling cucumber root rot caused by F. solani. The physicochemical properties and microbial community structure of the soil and the health of the cucumber plants were investigated. Our findings revealed the following. (Ⅰ) The potential for hydrogen (pH) and the contents of organic matter (OM), nitrate nitrogen (NO₃—N) and ammonium nitrogen (NH₄—N) significantly increased after combined CaCN₂ and ZF336 treatment. (Ⅱ) Combined CaCN₂ and ZF336 treatment reduced the relative abundance of Fusarium and increased the relative abundances of the potentially beneficial bacteria Trichoderma and Penicillium in the soil. In addition, B. velezensis promoted the recovery of microbial communities after CaCN₂ disinfection. (Ⅲ) Combined CaCN₂ and ZF336 treatment improved the activities of carbohydrate enzymes, effectively promoting soil carbon metabolism and increasing the cellulase, hemicellulose and lignin decomposition abilities. In conclusion, CaCN₂ combined with B. velezensis stimulated the soil microbial community structure and the activities of specific enzymes and is an effective way to control cucumber root rot disease and improve yield and soil quality.