Agricultural amendments enhanced the redox cycling of iron species and hydroxyl radical formation during redox fluctuation of paddy soil.

Hydroxyl radical (OH) plays a critical role in accelerating organic contaminant attenuation during water-table decline in paddy soil, but the impacts of widely applied agricultural amendments (e.g., organic manure, rice straw, and biochar) on these processes have been rarely explored. Hence, the effects of agricultural amendments on OH formation and pollutant degradation were examined based on field experiments. Compared with control, organic fertilizer (supplying more organic carbon (OC) and bioavailable elements that promoted Fe(II) formation by microorganisms) enhanced OH production by 0.8-1.3 times, while straw returning and biochar have negligible effects, probably due to the decreased pH and inhibition of microorganisms. The increased oxidation of active Fe(II) species (e.g., exchangeable Fe(II) and Fe(II) in lower-crystallinity minerals) mainly contributed to OH production. Further analyses showed that organic fertilizers significantly enhanced the redox cycling of Fe species mainly through increasing the contents of soil organic carbon and relative abundances of Fe(III)-reducing microorganisms. In addition, the increased OH formation markedly enhanced imidacloprid degradation by 24.3-42.4 %, with the toxicity of intermediates increased versus the parent compound. This study systematically examined the effects of typical agricultural amendments on the OH formation and organic contaminant attenuation in paddy soil, which probably provides promising strategies for regulating contaminant remediation in agricultural fields.