Klebsiella oxytoca P620 and Pseudomonas sp. CFA co-inoculation modulates rhizospheric bacterial communities to alleviate the combined stresses of phenolic acids and nitrate in cucumber

Crops in greenhouses are subjected to the combined stresses of phenolic acids and nitrate; however, approaches to simultaneously mitigate these stresses have not been explored. In this study, p-hydroxybenzoic acid (PHBA)-degrading Klebsiella oxytoca P620 reduced nitrate to ammonium via nitrite through narGHI and nirBD genes. When the strain P620 was mixed with ferulic acid (FA)-decomposing and PHBA-degrading Pseudomonas sp. CFA, the mixture was inoculated into cucumber-planted soil supplemented with FA, PHBA, and nitrate. Compared to the uninoculated strain, co-inoculation with strains CFA and P620 improved cucumber growth and increased the activities of antioxidant enzymes in leaves under FA, PHBA, and nitrate. Meanwhile, co-inoculation decreased the concentrations of FA, PHBA, and nitrate in the soil, enhanced the activities of soil enzymes, including sucrase, urease, and catalase, and altered the abundance of rhizospheric bacterial communities. The mixed strains of CFA and P620 improved cucumber growth in soil containing realistic levels of phenolic acids and nitrate. We proposed that co-inoculation with CFA and P620 affects rhizospheric bacterial communities; consequently, the co-inoculation degraded phenolic acids in soil, reduced soil nitrate, activated plant antioxidant enzymes, and induced soil enzymes, ultimately mitigating the combined stresses of phenolic acids and nitrate in greenhouse cucumbers.