The Decrease of Soil Microbial Community Diversity and Network Complexity Results in the Increase of Soil‐Borne Diseases With Monocultural Years in Greenhouse Tomato Production Systems

The excessive use of water and fertiliser, along with long-term monocultivation in greenhouse vegetable fields, has led to a decrease in soil quality and an imbalance in the soil microflora, which may contribute to worsening soil-borne diseases. To examine the associations between the soil microbial community composition and disease-suppressing microorganisms under different years of cultivation shifting from croplands to greenhouses, we collected soil samples from fields continuously planted with tomatoes for 5, 10 and 20 years, as well as from nearby maize-wheat fields (MW). The soil physicochemical properties, microbial community composition, putative plant pathogens and putative antagonistic bacteria were analysed. The results indicated that (1) the diversity and richness of soil bacteria and fungi significantly decreased with longer cultivation years; (2) bacterial and fungal diversity indices were negatively associated with soil nutrient content and positively associated with soil pH and NO₃ ^--N content, the latter being identified as the main factor contributing to the decline in microbial diversity; (3) the complexity of the soil bacterial network initially increased, peaking after 5 years of continuous monoculture, and then decreased, whereas fungal network complexity gradually declined over time; (4) the relative abundance of putative pathogens such as Fusarium, Alternaria and Cladosporium (associated with tomato wilt and leaf mould) increased significantly with longer cultivation, whereas the relative abundance of the bacterial genera associated with putative antagonism Bacillus, Paenibacillus and Streptomyces decreased. In conclusion, after 10 years of continuous monocultivation, a marked reduction in soil microbial diversity and putative antagonistic bacteria was observed, along with an increase in putative pathogenic fungi. These changes likely contributed to the worsening of soil-borne diseases, threatening the sustainability of greenhouse vegetable production.