Transforming iron ore tailings into technosols: Highly biodegradable plant mulch accelerates mineral weathering and organo-mineral association

Organic matter amendments are required in accelerating mineral transformation and early soil-forming processes in tailings, by enhancing microbially driven mineral weathering, geochemical stabilization and aggregate formation, but the role of mulch biodegradability has remained unclear. In this study, we evaluated how mulch biodegradability regulates these processes in alkaline Fe-ore tailings. Highly biodegradable lucerne hay (LH) and sugarcane mulch (SM) rapidly altered tailings' mineralogical, geochemical, and microbial properties, producing far stronger responses than lignin-rich pinewood chips (PC). LH and SM significantly increased prokaryotic species richness (16S rDNA) and shifted microbial community composition, driving faster mulch decomposition and rapid porewater acidification (pH 9.1 to 5.9–6.4 within 24 h), with sharp increases in dissolved organic carbon and nitrogen. Elevated organic acid production—dominated by acetic acid—further lowered solid-phase pH to 8.1–8.5, accelerated weathering of biotite-like phyllosilicates, and releasing substantial quantities of soluble cations, particularly K (>3000 ppm). Enhanced mineral dissolution in the LH and SM treatments promoted Fe-oxalate formation and organo-mineral association, leading to pronounced microaggregate development. In contrast, PC induced weaker microbial activity, organic acid production, and aggregate development, than LH and SM. These findings demonstrate that mulch biodegradability governs microbial growth and activity and mineral weathering, thereby governing the rate of early soil formation in Fe-ore tailings. This helps to inform the design of mulch mixtures that optimise tailings' transformation and soil development while mitigating risks such as acute K phytotoxicity during plant establishment. • Mulch biodegradability controls microbial activity and mineral weathering. • Lucerne and sugarcane mulches rapidly acidified porewater and boosted DOC/DN. • High biodegradability accelerated biotite weathering and K release (>3000 ppm). • Lucerne and sugarcane enhanced Fe-oxalate and microaggregate formation.