Co-application of spent mushroom substrate and PGPR alleviates tomato continuous cropping obstacle by regulating soil microbial properties

Abstract

Continuous cropping of tomato (Solanum lycopersicum L.) in China has contributed to the decline of crop quality and yield. The aims of this study were to evaluate the integrated effect of plant growth-promoting rhizobacteria (PGPR) Bacillus sp. HW27 and spent mushroom substrate (SMS) on tomato plant growth and microbiological properties of rhizosphere soil. Bacillus sp. HW27 showed indole-3-acetic acid (IAA) and siderophore production, phosphate solubilization, and root colonization capacity in vitro assays. A pot experiment was carried out with four treatments: (1) CK: unamended control, (2) PGPR, (3) SMS, and (4) PGPR + SMS: combined application of strain HW27 and SMS. The enzyme activities, the abundances of beneficial microbes Bacillus and Pseudomonas and soil-borne pathogenic Fusarium oxysporum (using real-time PCR), the microbial functional diversity (using Biolog Ecoplates), and microbial community (using phospholipid fatty acid (PLFA) analysis) in rhizosphere soil were studied 20, 40 and 60 days after transplantation. At the end of the 60-day pot experiment, the results showed that the PGPR + SMS treatment performed best in improving tomato plants growth, which significantly (p < 0.05) increased the shoot length, root length, shoot dry weight and root dry weight by 34.4, 78.4, 93.5 and 82.4%, respectively, compared to the CK treatment. The enzyme activities of β-1,4-glucosidase, β-d-cellobiosidase, β-1,4-N-acetyl-glucosaminidase, protease and acid phosphatase were higher in the SMS and/or PGPR application treatments than in the CK treatment in the order PGPR < SMS < SMS + PGPR. The SMS + PGPR treatment showed the highest abundances of Bacillus and Pseudomonas, and the lowest abundance of Fusarium oxysporum. Furthermore, the application of SMS and/or PGPR stimulated microbial functional diversity and microbial growth (total PLFAs, bacterial, and Gram (+) and Gram (−) bacterial PLFAs), in which SMS + PGPR treatment had the greatest effect. Consequently, this study showed that co-application of PGPR and SMS expressed synergistic effect to increase tomato plant growth, enzyme activities and microbial biomass in continuous cropping greenhouse soil.