Interactive effects of soil moisture, organic amendment and herbicide application on fipronil degradation and structure functional potential of soil microbial community in tropical paddy soil

Fipronil degradation followed first-order kinetics. In sterile soil, residues declined slowly from 2.43 to 1.05 µg g-1 over 30 days, with 30 % dissipation and a half-life of 54.15 days. In non-sterile soil, degradation was faster, decreasing from 1.11 to 0.02 µg g-1, corresponding to 79.28 % dissipation and a half-life of 11.53 days. Microbial activity accelerated degradation nearly fivefold, contributing 49 % to total dissipation. Fipronil application initially suppressed bacterial and fungal populations, which recovered within 10–30 days, while actinomycetes were largely unaffected. Organic matter (OM) under flooded conditions enhanced bacterial populations (6.61 × 10⁶ cfu g-1 soil), whereas fungal populations were higher under OM with field capacity moisture (4.04 × 10⁴ cfu g-1 soil) at 30 days after application. Enzymatic activities showed transient inhibition followed by recovery, with higher dehydrogenase activity in flooded + OM soils (105 µg triphenyl formazan (TPF) g-1 soil 24 h-1) and increased urease (39 mg NH₄-N g-1 soil h-1) and phosphatase activities (61 µg p-nitrophenol (PNP) g-1 soil h-1) under field capacity + OM. Herbicide application consistently suppressed microbial and enzymatic activities across all moisture regimes. Recovery of nontarget microbial populations beyond control levels suggests that fipronil and its metabolites may serve as nutrient or energy sources for specific soil microorganisms.