Root Exudate Metabolite Signatures Drive Microbial Succession and Saline–Alkali Soil Recovery During Long-Term Phytoremediation by Elaeagnus angustifolia L.

Background: Saline–alkali soil stress threatens agriculture and ecosystems. Plants actively regulate rhizosphere microenvironments, which is the core mechanism for biological remediation. However, how root exudates from Elaeagnus angustifolia L.—an important salt-tolerant tree species in arid regions—drive rhizosphere biochemical processes remains unclear. Methods: This study examined 5-, 10-, 20-, and 40-year-old E. angustifolia and used non-targeted metabolomics to analyze dynamic changes in root exudates during long-term growth. By integrating soil physicochemical and microbial data, we investigated the ecological roles of these exudates in regulating microbial assembly and driving saline–alkali soil remediation. Results: Exudate profiles differ significantly depending on tree age. Under early high salt stress, E. angustifolia root exudates were enriched in organic acids, lipids, and amino acids, which were associated with an enrichment of Proteobacteria and Desulfobacterota and a subsequent reduction in soil salinity. In the mid-growth stage, as stress declined, metabolite levels decreased while microbes continued nutrient cycling. During the late-growth stage, EC/salinity increased slightly, and E. angustifolia secreted flavonoids and antibiotics, cooperating with stress-resistant microorganisms (including Firmicutes and Ascomycota) for antioxidant defense. Conclusions: Age-dependent changes in E. angustifolia root exudates drive distinct rhizosphere microbial associations and resource allocation strategies, advancing the biological restoration of saline–alkali soils. This study advances the theoretical understanding of regional biological restoration of saline–alkali soils and sheds light on addressing this global challenge.