Coupled and decoupled antimony-arsenic mobility along environmental gradients in a mining soil: Links to geochemical contexts and microbial functional potential.

Distinct redox behavior, surface-binding affinities, and microbial transformation pathways of antimony (Sb) and arsenic (As) complicate sustainable remediation of mining soils. Understanding how these differences shape field mobility is essential for nature-based remediation that avoids unnecessary inputs. Yet whether natural gradients produce generalizable Sb-As mobility patterns and mechanisms remains unclear. Here, we integrated physicochemical, metagenomic, and statistical analyses to determine how field gradients shaped microbial functional potential and Sb-As mobility at a single Sb smelting site. We identified two Sb-As mobility modes, further supported by external validation across 60 published studies. In the decoupled mode, along resource-driven gradients including moisture, soil organic carbon, and slope-associated light exposure, water-extractable Sb increased whereas As decreased. In the coupled mode, along pH-redox gradients, especially in alkaline and oxic soils, water-extractable Sb and As increased together. The decoupled mode was generally characterized by Sb(V) and As(V) species with Sb redistribution into more labile pools but As retention in stable fractions. Metagenomic profiles linked this decoupling to heterogeneous C-N-S-Fe geochemical contexts that reshaped microbial functional potential related to electron transfer and mineral-associated reactions. By comparison, the coupled mode was characterized by redistribution of Sb and As into reactive fractions under direct pH/Eh control. Hierarchical partitioning identified environmental factors as the dominant explanatory domain for Sb and As (62.5% and 53.1% of adjusted-R2-based relative importance), with metagenomic functional variables contributing 18.5% and 9.3%, respectively. These contrasting mobility modes help distinguish when Sb and As may require joint or separate management, thereby supporting targeted nature-based remediation.