Microclimate and soil-type effects on chlorpyrifos degradation kinetics in tropical agricultural soils

Microclimate and soil properties jointly control pesticide persistence in the tropics, yet their combined effects are rarely quantified. We investigated chlorpyrifos dissipation in tropical Ultisol and peat soils under light and dark conditions for 29 days (n = 3). Residues were quantified by GC-FPD-P (LOQ = 0.05 mg kg−1; recovery 75–91%). First-order models adequately described dissipation kinetics (R² = 0.92–0.99), with residual diagnostics supporting model suitability. Half-lives were 6.47 d (Ultisol–light), 7.07 d (Ultisol–dark), 53.3 d (peat–light), and 57.8 d (peat–dark), indicating non-persistence in mineral soil but moderate persistence in peat. Exposure-normalized kinetics using cumulative photolytic time (τ) slightly improved linearity in Ultisol–light (R²τ = 0.935 vs 0.922), whereas gains were minimal in peat, consistent with strong sorptive buffering by organic matter. Interval-wise rates (k̂) showed directional associations with a Dry–Sunny Index (DSI), although relationships were not statistically significant (p ≥ 0.07). Decision-oriented metrics further differentiated treatments: AUC₀–₂₉ ranged from 6.48–13.4 mg kg−1·d in Ultisol and 38.5–41.1 mg kg−1·d in peat, while projected tLOQ was ~25–34 d in Ultisol and ~259–288 d in peat. These results support soil-specific risk management and highlight the context-dependent role of microclimatic forcing in tropical pesticide dissipation.