Dissipation dynamics of fipronil in tropical paddy (Oryza sativa L.) soil: unravelling its persistence and metabolism under varying moisture regime.

backgroundFipronil, a phenylpyrazole insecticide, is widely used for the management of rice pests; however, its environmental fate and potential risks in tropical paddy ecosystems to food safety under varying soil conditions remain insufficiently understood.

methodsA field study was conducted in a tropical paddy ecosystem with sandy loam soil to evaluate the uptake, translocation, and dissipation dynamics of fipronil under different soil moisture regimes (flooded, saturated, and field capacity) with and without organic matter amendment. Fipronil and its metabolites in soil and plant samples were quantified using LC-MS/MS following the QuEChERS extraction method validated using standard method validation parameters.

resultsFipronil applied at 75 g a.i. ha⁻¹ (0.3G formulation) at 20 days after transplanting showed a progressive decline in soil residues from 0.70, 0.79, and 1.37 µg g⁻¹ (day 1) to 0.01, 0.02, and 0.03 µg g⁻¹ (30 DAA) under flooded, saturated, and field capacity conditions, respectively, indicating faster dissipation under flooded conditions. The dissipation of fipronil was 1.10% and 0.76% higher under flooded than saturated and field capacity conditions respectively. Organic matter-amended Fipronil soils showed higher persistence, with half-lives of 11.87 days (flooded without organic matter) to 13.75 days (field capacity with organic matter) in cropped soils and 20.82 days in non-cropped soil, indicating prolonged residue retention and potential environmental risks. Plant uptake was rapid, peaking at 2 days after application, followed by a gradual decline. Residue levels in plants were lower under organic amendment, indicating reduced bioavailability. Fipronil predominantly accumulated in roots, followed by stem and leaves, facilitating translocation to aerial parts. Among metabolites, Fipronil sulfone dominated, reaching 0.82 µg g⁻¹ in field capacity, 0.63 µg g⁻¹ in saturated, and 0.60 µg g⁻¹ in flooded soils at 15 DAA, indicating oxidative persistence and higher toxicity. Sulfide (0.10 µg g⁻¹ at 15 DAA), reflecting anaerobic degradation, while desulfinyl remained below 0.01 µg g⁻¹. Harvest residues remained below MRL (< 0.01 µg g⁻¹), indicating low health risk.

conclusionsThese findings provide important scientific evidence for developing irrigation- and organic amendment-based pesticide management strategies to support safer fipronil use, residue regulation, and sustainable rice production in tropical paddy ecosystems.