Effects of Tomato-Rice Rotation on Physicochemical Properties and Microbial Communities of Soil with Continuous Cropping Obstacles in Cangnan, Zhejiang

Long-term continuous cropping of tomato in Cangnan, Zhejiang Province leads to a series of soil continuous cropping obstacles, such as soil acidification, secondary salinization, soil compaction and serious soil-borne diseases. In this paper, the effects of tomato-rice rotation on soil physicochemical properties, enzyme activity, microbial biomass and microbial community structure under long-term continuous cropping were analyzed, aiming to provide a method to alleviate soil continuous cropping obstacles and improve soil environment.

Three treatments were set up and soil samples were collected. T1 was tomato continuous monoculture for 18 years, T2 was tomato continuous monoculture for 17 years followed by one year of tomato-rice rotation, and T3 was tomato continuous monoculture for 15 years followed by three years of tomato-rice rotation. Soil pH and EC values were measured by pH meter and conductivity meter. Soil physicochemical properties such as total carbon, total nitrogen and ammonium nitrogen were determined by high temperature combustion method, Kjeldahl nitrogen fixation method and potassium chloride solution leaching method. Soil enzyme activities were determined using enzyme activity kits. Total and harmful microorganism concentrations were detected using qPCR, and soil culturable microorganisms were counted using selective medium plate smear counting method. The MiSeq PE3000 high-throughput sequencing platform was used for sequencing, splicing and assembly, sequence comparison and functional annotation of the soil microgenome.

There were significant differences in soil physicochemical properties among different treatments. One and three years of tomato-rice rotation treatments increased soil pH from 5.20 to 6.04 and 6.73, and reduced EC from 558 μS·cm^-1 to 417 and 445 μS·cm^-1, and increased C∶N ratios from 9.16 to 10.45 and 10.74. Soil enzyme activities were increased in the rotations, with catalase, urease, polyphenol oxidase activities increased from 11.72 μmol·d^-1·g^-1, 10.76 μg·d^-1·g^-1, and 22.67 mg·d^-1·g^-1 to 58.58 μmol·d^-1·g^-1, 142.48 μg·d^-1·g^-1, and 37.10 mg·d^-1·g^-1. After crop rotation, the microbial population and community structure changed, harmful microorganisms decreased. The content of soil culturable actinomycetes increased, the fungi/bacteria ratio decreased. Chloroflexi, Acidobacteria, and Proteobacteria increased significantly, and Actinobacteria decreased significantly. The contents of Ralstonia solanacearum, Pectobacterium carotovorum subsp. carotovorum, P. c. subsp. brasiliensis and Fusarium sp. decreased to 1.76×10³, 7.28×10², 3.94×10³ and 3.07×10³ copies/g in the three years of crop rotation. The potential functions of soil microorganisms changed after crop rotation, with an increase in the abundance of genes related to carbohydrate metabolism, energy metabolism, and other related pathways. Specifically, genes associated with carbohydrate metabolism in metabolism were up-regulated.

Tomato-rice rotation can improve soil acidification, salinization and soil element imbalance caused by long-term monoculture, increase soil C∶N ratio and enzyme activity, reduce the occurrence of soil-borne diseases, transform the soil from fungal type to bacterial type, change the structure of soil microbial community, and promote soil microbial carbohydrate metabolism, which is important for improving soil continuous cropping obstacles.