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

To scientifically assess the effects of environmentally friendly fumigants on soil microbial communities, soils from a 7-year continuous cropping tomato greenhouse were studied, with unfumigated soil used as the control (CK). Rhizosphere soil samples treated with dazomet (DZ), metam sodium (MS) and calcium cyanamide (CC) were collected at the seedling recovery and fruiting stages. The influences of different fumigants and growth stages on soil microbial communities, tomato yield and soil-borne diseases were investigated. The results indicated that soil fumigation significantly decreased microbial community richness and diversity at the seedling recovery stage, which gradually recovered at the fruiting stage. The variation trends of microbial relative abundance at the phylum and genus levels differed among the treatments at both stages. At the phylum level, Actinobacteria and Proteobacteria were the dominant bacterial phyla, and Ascomycota was the dominant fungal phylum. Genus-level clustering revealed that the bacterial communities under MS and CC were similar to those under CK at the fruiting stage, whereas the fungal communities under all the fumigation treatments were significantly distinct from those under CK. Fumigation effectively inhibited pathogenic genera, including Amesia, Fusarium, Rhizopus and Ascobolus, at the seedling recovery stage, but some pathogens recovered at the fruiting stage. The relative abundance of Fusarium in the MS treatment increased to 8.25%. DZ treatment performed optimally: it increased beneficial genera such as Bacillus and Streptomyces at the seedling recovery stage, suppressed harmful genera, including Amesia and Fusarium, and further enriched Remersonia at the fruiting stage. Fumigation significantly improved tomato yield and reduced the incidence of soil-borne diseases. The yield of CC was the highest, at 35.41% greater than that of CK, but it was not significantly different from that of DZ in terms of cost. In conclusion, the DZ treatment had the best overall effect.