Salt/alkali‑tolerant Streptomyces luteus TRM 45540 improves pepper growth and soil quality in acidic and alkaline soils

This study evaluated the potential of the salt-alkali-tolerant actinobacterium Streptomyces luteus TRM 45540, isolated from Lop Nur saline-alkali soil in Xinjiang, to alleviate pH-related iron deficiency and promote pepper growth. We hypothesized that this strain could mobilize and compete for soil iron via siderophore production, thereby benefiting plants under variable-pH conditions. To test this hypothesis, we compared its effects with conventional iron fertilizers (EDDHA-Fe₆, ferrous sulfate) and compound microbial fertilizer on pepper growth, soil physicochemical properties, and rhizosphere microbial communities in acidic (pH 5) and alkaline (pH 7.69) soils. TRM 45540 exhibited good stress tolerance, growing well at 3%–9% NaCl and pH 9–12. In both soil types, it significantly promoted pepper growth: fresh weight was increased by over 44% in acidic soil relative to the control, while root and stem lengths were elevated by 26%–91% in alkaline soil compared with conventional amendment groups. The strain increased the content of indigenous soluble iron in soil, raising soluble iron to 42 mg/kg − 1 in acidic soil without exogenous iron addition, neutralized acidic soil toward neutral pH, increased total nitrogen to 6.5 g/kg − 1 , and enhanced phosphorus and potassium availability. Redundancy analysis identified pH and total iron as the dominant factors shaping microbial communities in acidic and alkaline soils, respectively. KEGG pathway enrichment revealed significant changes in organic pollutant degradation, nutrient metabolism, and stress response pathways following TRM 45540 inoculation. The strain enriched functional microorganisms related to siderophore secretion and nitrogen fixation, thereby improving soil microbial diversity and richness, while compound microbial fertilizer was associated with relatively lower microbial community activity. These findings demonstrate that S. luteus TRM 45540, with cross-pH adaptability and functional stability, enhances pepper growth and soil quality via synergistic effects of stress tolerance, nutrient activation (especially iron mobilization), and microbial community regulation. This strain provides a promising microbial inoculant for sustainable pepper production in variable-pH soils, especially saline-alkali soils.