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Root exudates mediate soil microbiome composition and diversity, which might further influence plant development and health. Vanillic acid from root exudates is usually referred as autotoxin of cucumber, however, how vanillic acid affect soil microbial community diversities and abundances remains unclear. In this study, vanillic acid (VA; 0.02, 0.05, 0.1, and 0.2 μmol g-1 soil) was applied to soil every other day for a total of five applications. We used Illumina MiSeq sequencing, quantitative PCR (qPCR) and PCR-denaturing gradient gel electrophoresis (PCR-DGGE) to test the effects of VA on the total fungi community composition as well as the Trichoderma and Fusarium spp. community abundances and structures in the cucumber rhizosphere. Illumina MiSeq sequencing showed that VA (0.05 μmol g-1 soil) increased the relative abundance of the fungal phylum Basidiomycota while decreasing the relative abundance of Ascomycota (P < 0.05), and not altered the diversity of the soil fungal community. VA (0.05 μmol g-1 soil) also increased the relative abundances of the fungal genera with plant pathogens, such as Conocybe and Spizellomyces spp.(P < 0.05). A qPCR analysis showed that VA (0.05 to 0.2 μmol g-1 soil) exerted promoting effects on Trichoderma spp. community abundance and stimulated Fusarium spp. abundance at low concentrations (0.02 to 0.05 μmol g-1 soil) but inhibited it at high concentrations (0.1 to 0.2 μmol g-1 soil). The PCR-DGGE analysis showed that all concentrations of VA altered the community structures of Trichoderma spp. and that the application of VA (0.02 and 0.05 μmol g-1 soil) changed the band number and the Shannon-Wiener index of the Fusarium spp. community. This study demonstrated that VA changed the total fungal community in the cucumber seedling rhizosphere and that the Trichoderma and Fusarium spp. communities showed different responses to VA.

Intercropping can achieve sustainable agricultural development by increasing plant diversity. In this study, we investigated the effects of tomato monoculture and tomato/potato-onion intercropping systems on tomato seedling growth and changes of soil microbial communities in greenhouse conditions. Results showed that the intercropping with potato-onion increased tomato seedling biomass. Compared with monoculture system, the alpha diversity of soil bacterial and fungal communities, beta diversity and abundance of bacterial community were increased in the intercropping system. Nevertheless, the beta-diversity and abundance of fungal community had no difference between the intercropping and monoculture systems. The relative abundances of some taxa (i.e., Acidobacteria-Subgroup-6, Arthrobacter, Bacillus, Pseudomonas) and several OTUs with the potential to promote plant growth were increased, while the relative abundances of some potential plant pathogens (i.e., Cladosporium) were decreased in the intercropping system. Redundancy analysis indicated that bacterial community structure was significantly influenced by soil organic carbon and pH, the fungal community structure was related to changes in soil organic carbon and available phosphorus. Overall, our results suggested that the tomato/potato-onion intercropping system altered soil microbial communities and improved the soil environment, which may be the main factor in promoting tomato growth.

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