Effects of Soil Fumigant-Mediated Changes in the Microbial Communities of Soil with Continuous Cropping on Tomato Yield and Soil-Borne Diseases

To scientifically evaluate the effects of three environmentally friendly fumigants, dazomet, metham sodium, and calcium cyanamide, on soil microbial community changes, soil from a greenhouse with 7 years of continuous cropping was studied. Soil samples were collected from unfumigated soil (CK), dazomet-fumigated soil (DZ1), metham sodium-fumigated soil (MS1), calcium cyanamide-fumigated soil (CC1) at seedling recovery stage, dazomet-fumigated soil (DZ2), metham sodium-fumigated soil (MS2), and calcium cyanamide-fumigated soil (CC2) at fruiting stage. The effects of different fumigant treatments and treatment periods on soil microbial community changes, tomato yield, and soil-borne diseases were analyzed. The results showed that the application of soil fumigants significantly reduced the richness and diversity of soil bacterial and fungal communities at the seedling recovery stage. Three months after transplanting (at the fruiting stage), the soil microbial diversity gradually recovered. After fumigation, the relative abundance of soil bacteria and fungi at the phylum level changed. Under different fumigation treatments, the trends in the relative abundance changes of species during the seedling recovery and fruiting stages after fumigation exhibited certain differences. Analysis of the dominant soil microbial communities at the genus level revealed that fumigation effectively eliminated pathogenic genera such as Fusarium, but the inhibition rate of pathogens increased during the seedling recovery stage after fumigation. By comprehensively considering microbial community changes during the seedling recovery and fruiting stages after fumigation, as well as indicators such as yield, incidence of soil-borne diseases, and disease index, among the three fumigation treatments, the DZ treatment demonstrated the best overall performance. It increased beneficial genera while reducing the majority of pathogenic genera in the soil. Even after three months, when the plants transitioned from the seedling recovery stage to the fruiting stage, the relative abundance of harmful genera remained suppressed, and the abundance of some beneficial genera increased. However, given the broad-spectrum microbial eradication by fumigants and the time-limited suppression of pathogenic communities, it is essential to supplement beneficial microorganisms promptly after soil fumigation, such as by applying microbial fertilizers in combination.