Distinguishing the contributions of organic matter and iron (oxyhydr)oxides to zinc availability in paddy soils surrounding lead-zinc mines.

Zinc (Zn) contamination in paddy soils near mining areas poses significant ecological and health risks, yet the mechanisms governing its mobility under redox-fluctuating conditions remain unclear. Here, we combined soil microcosm incubation, sequential Zn fractionation, and process-based kinetic modeling to elucidate Zn transformation pathways in a mining-affected paddy soil subjected to alternating anoxic-oxic phases. Results showed that under anoxic conditions, the concentration of available Zn decreased, and immobilization mainly via organic matter (OM) (45.8%) and Fe (oxyhydr)oxides (42.8%), with sulfate reduction contributing 11.4%. In contrast, oxic conditions enhanced available Zn, dominated by OM (48.6%) and Fe (oxyhydr)oxides (39.3%), with sulfide oxidation accounting for 12.1%. Fulvic acid (FA)-bound Zn was stable across redox transitions, whereas humic acid (HA)-bound and Fe (oxyhydr)oxide-bound fractions were highly redox-sensitive. The process-based kinetic model quantitatively captured Zn speciation changes and their links to Fe, C, and S cycling, enabling prediction of Zn behavior under dynamic redox regimes. These findings identify OM and Fe (oxyhydr)oxides as the key factors controlling Zn mobility and provide a mechanistic basis for redox-based remediation strategies in Zn-contaminated paddy soils near mining regions.