Microbial inoculum improved soil aggregate formation and increased cucumber yield in a greenhouse under secondary...

Greenhouse soil salinization caused by unreasonable fertilizer utilization and irrigation threatens vegetable growth and supply. Inoculation with salt-tolerant and plant growth-promoting microorganisms offers a sustainable solution. We screened eight Bacillus and two Trichoderma strains for salt tolerance using incubation method. Subsequent pot and column experiments were conducted to assess their impact on cucumber growth and soil aggregation. The two promising strains, Bacillus amyloliquefaciens 10225 and Trichoderma harzianum 32527, were selected for further validation in salinized greenhouse soil. Our findings revealed that both strains of B. amyloliquefaciens thrived in 5% NaCl solution and substantially promoted the growth of cucumber seedling growth. Compared with that in the control (non-inoculated treatment), large macroaggregate content increased by 65.12%, 52.60%, and 71.93% with B. amyloliquefaciens 19994, 10225, and T. harzianum 32527, respectively, and aggregate water mean weight diameter (WMWD) increased by 34.47%, 36.55%, and 43.68%. Extracellular polymeric substance (EPS) composition (protein, polysaccharide, and uranic acid contents) varied among strains, with protein and polysaccharide contents increasing with B. amyloliquefaciens 10270. The microbial biomass carbon (MBC) content also increased by 19.26% and 28.53% (p < 0.05) with B. amyloliquefaciens 10225 and 10270, respectively, compared with non-inoculated control. The cucumber yield increased by 14.73%-17.43% under inoculation (p < 0.05). Partial least squares path modeling (PLS-PM) revealed that EPS and MBC promoted soil aggregation under inoculation. Field experiments confirmed that inoculation significantly improved cucumber yield, which was attributed to an increase in soil aggregate stability and a decrease in soil electrical conductivity (EC). The study underscores the critical role of EPS and MBC in maintaining soil aggregation in saline soils post-inoculation with beneficial strains. These findings offer theoretical or practical insights for the use of microbial inoculation as a bio-organic fertilizer or bio-amendment in saline soils.