Assessment of pesticide residues in rice-wheat based conservation agriculture fields using validated LC-MS/MS methodology.

Pesticide residues in soil are a significant environmental concern in intensive rice-wheat cropping systems, especially under conservation agriculture (CA), where reduced soil disturbance and residue retention may alter pesticide dissipation and vertical distribution. This study developed and validated a robust Liquid Chromatography tandem Mass Spectrometry (LC-MS/MS) method for the simultaneous detection of 93 pesticides in soil and applied it to long-term CA fields of the rice-wheat system. A modified QuEChERS method, involving soil hydration, acetonitrile extraction, buffer salt addition, and dispersive-Solid Phase Cleanup (dSPE) cleanup, was optimized for soil samples. Method validation was performed according to SANTE/11312/2021 guidelines, demonstrating satisfactory specificity (0-26% interference), linearity (R2 > 0.99), recovery within 70 to 120%, precision (Relative Standard Deviation [RSD]) ≤ 20%, HorRat between 0.15 and 2.0), Limit of Quantification (LOQ) ranging from 0.01 to 0.10 mg/kg depending on the analyte and measurement uncertainty (50%) was within acceptable limits. Matrix effects varied across pesticides, with most showing medium suppression (20-50%), and matrix-matched calibration was used for accurate quantification. Application of this validated method to field soils revealed that few pesticides, including chlorantraniliprole, metribuzin, pendimethalin, pretilachlor, and tebuconazole, primarily accumulated at a depth of 0-5 cm under CA practices. Seasonal variation showed higher pesticide persistence during the Rabi season. Long-term CA affects the persistence and surface accumulation of selected pesticides, emphasizing the need for careful pesticide management in CA-based rice-wheat systems.