Ferrous iron fertilizer triggers a time-dependent switch in PFAS bioavailability via soil organic matter remodeling

Per- and polyfluoroalkyl substances (PFAS) are persistent and pervasive in agricultural soils. Ferrous iron [Fe(Ⅱ)] fertilizers are applied to alleviate crop Fe-deficient. However, whether and how Fe fertilization dynamically alters PFAS fate in soil remains unknown. Using an emerging PFAS alternative 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFESA) as a model compound, we revealed a time-dependent trade-off in its soil bioavailability after Fe(Ⅱ) addition. Within five days, high-dose Fe(Ⅱ) triggered substantial reactive oxygen species production, which preferentially mineralized the water-soluble soil organic matter (SOM), including chromophoric dissolved organic matter and aromatic components. This process released the SOM-associated 6:2 Cl-PFESA and transiently increased its bioavailability, as confirmed by dialysis tests. Upon prolonged aging, Fe(Ⅱ) stimulated the activity of Pseudomonadota microbes, driving the neo-formation of protein- and polysaccharide-like organic matter and the transformation of SOM into stable well-crystallized mineral-associated organic carbon. These newly formed binding domains sequestered 6:2 Cl-PFESA, overriding the initial mobilization and substantially reducing its bioavailability. Although amorphous Fe oxides adsorbed 6:2 Cl-PFESA, their contribution declined during mineral recrystallization. Instead, Fe(Ⅱ)-mediated SOM compositional remodeling governed PFAS bioavailability, highlighting strategic Fe(Ⅱ) fertilization with adequate aging as a promising approach to mitigate PFAS accumulation in agricultural soils.