Synergistic soil-foliar fertilization promotes maize growth under salt stress by shaping rhizosphere metabolites and microbiomes

Both soil and foliar fertilization can alleviate plant salt stress; however, the synergistic mechanism by which combined soil–foliar fertilization alleviates plant salt stress remains understudied. In this study, a pot experiment of synergistic soil–foliar fertilization was conducted in maize, and combined microbiome and metabolome analyses were used to investigate plant–metabolite–microbiome interactions. Random forest and microbial network module analyses revealed that compared with other treatments, synergistic fertilization altered the diversity and composition of rhizosphere metabolites, including phenylpropanoids and polyketides, organic acids and their derivatives, alkaloids and their derivatives, and organic nitrogen compounds, as well as those of microorganisms, including Actinobacteriota, Chloroflexi, Zygomycota. This highlights the potential of synergistic fertilization to shape plant–metabolite–microbe interactions. Correlation and partial least squares path modeling equation analyses indicated that metabolic diversity and composition not only directly reduced harmful substances in plants to promote growth but also positively influenced soil microbial diversity and composition by modulating plant photosynthetic and antioxidant enzyme activities. This dual mechanism collectively contributes to reduced plant toxicity and growth promotion. Overall, this study demonstrated that synergistic soil–foliar fertilization improves the growth of maize under salt stress, providing guidelines for optimal fertilization in saline soil. • Synergistic soil-foliar fertilization (SD-FS) significantly enhances maize growth under salt stress. • SD-FS reshaped rhizosphere metabolite and microbial diversity/composition. • Rhizosphere metabolites directly reduce the levels of plant oxidative markers, increasing plant biomass under salt stress. • Rhizosphere metabolites boost microbes, photosynthesis，antioxidation, reducing damage and increasing biomass under salt stress.