Pioneer plant species shape rhizosphere dissolved organic matter chemodiversity and selective organo-mineral association in bauxite residue

Background and aims Rhizosphere driven organic matter input and stabilization is critical for transforming highly alkaline bauxite residue (BR) into soil-like growth media, yet molecular characteristics of rhizosphere dissolved organic matter (DOM) and its interactions with minerals remain poorly understood. This study investigates whether different pioneer plant species produce distinct DOM molecules that are selectively associated with mineral phases in BR undergoing mineral weathering and neutralization in the early Technosol formation. Methods In a 347-day rhizobox experiment, BR was colonized by one of four haloakali-tolerant pioneer species: Atriplex nummularia , Chloris gayana , Acacia auriculiformis , or Sorghum spp. hybrid cv. Silk, with unplanted BR as a control. High-resolution Orbitrap mass spectrometry, synchrotron-based C 1s near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and synchrotron-based FTIR microspectroscopic mapping were used to resolve DOM chemodiversity and signatures of organo-mineral associations. Results Plant colonization markedly increased DOM abundance, chemodiversity, and oxygenation. The C. gayana treatment spanned the broadest compositional range, A. nummularia enriched oxygenated, carboxyl-rich DOM associated with pronounced de-alkalization, and A. auriculiformis enriched N-containing formulas potentially linked to symbiotic N fixation. Saturation and nominal carbon oxidation state showed species-specific shifts. NEXAFS identified carboxylic C as the dominant solid-phase component, while FTIR mapping indicated preferential spatial association of carboxylic acid and lignin-related signals with secondary mineral phases. Conclusion Species-specific rhizosphere processes diversify DOM and promote selective organo-mineral association in BR. Combining pioneer species with complementary rhizosphere functions may enhance de-alkalization and organic matter retention, thereby supporting early Technosol development and sustainable ecological rehabilitation.