The decrease of soil microbial community diversity and network complexity results in the increase of soil-borne diseases with mono-cultivation years in greenhouse tomato production systems

Excessive input of water and fertilizer and long-term monoculture in greenhouse vegetable fields caused soil quality decline and soil microflora imbalance, which may be one of the reasons for the aggravation of soil-borne diseases. In order to investigate the relationship between soil microbial community composition and disease-suppressing microorganisms in greenhouse vegetable fields with different planting years, soil samples were collected respectively from the greenhouse fields which had been continuously planted tomatoes for 5, 10 and 20 years as well as the adjacent maize-wheat fields (control) in Shouguang county, Shandong Province of China. Soil physicochemical properties, microbial community composition, plant pathogens and antagonistic bacteria were analyzed. Our results showed (1) the diversity and richness index of soil bacteria and fungi decreased significantly with the extension of cultivation years. (2) Bacterial and fungal diversity index was negatively correlated with soil nutrient content and positively correlated with soil pH. The increase of soil NO3--N content was the main reason for the decline of soil microbial diversity index. (3) With the extension of planting years, the complexity of soil bacterial network increased first and then decreased, reaching the highest value after 5 years of continuous mono-cultivation. The complexity of fungal network decreased gradually with the extension of planting years. (4) The relative abundance of Fusarium, Alternaria and Cladosporium, pathogens of tomato wilt and leaf mold diseases, increased significantly with the extension of cultivation years. In contrast, the relative abundance of the three antagonistic bacteria Bacillus, Paenibacillus and Streptomyces decreased significantly. In conclusion, the diversity of soil microbial communities and the number of antagonistic bacteria were significantly reduced after 10 years of continuous mono-cultivation, and the proliferation of pathogenic fungi was induced, which was an important reason for the aggravation of soil-borne diseases, which seriously threatened the sustainability of greenhouse vegetable production in China.