Pesticide and metabolite residue mixtures in subtropical agroecosystem soils: The inconvenient truth

The presence and accumulation of multiple pesticide residues in soil threaten biodiversity and soil health, yet depth-resolved evidence on pesticide-metabolite mixtures in subtropical agroecosystems remains underexplored. Here, we present the most comprehensive paired-depth assessment of parent pesticides and metabolites in Indian soils to date. We analyzed 181 compounds (63 insecticides, 41 fungicides, 32 herbicides, 45 metabolites) in 140 paired surface (0-5 cm) and subsurface (15-20 cm) soil samples collected during the 2024 growing season from 70 sites representing major land uses in the Hindon River Basin. Seventy-two residues were detected across samples, and almost all (98.6%) surface soils were contaminated, including organically managed and forest reference sites. Pesticide-metabolite mixtures were ubiquitous in agricultural soils, with up to 24 co-occurring compounds per site. Insecticides and metabolites dominated detections, with clothianidin, chlorantraniliprole, and fipronil sulfone present in > 80% of samples. Metabolites of fipronil, neonicotinoids, atrazine, and DDT were frequently detected and often exceeded their parent levels, suggesting persistent and potentially underestimated soil health risks. Subsurface residues were detected at 93% of sites, demonstrating widespread shallow vertical redistribution. Multivariate analyses showed that shallow vertical redistribution was governed primarily by chemical persistence and mobility, amplified by land-use intensity, whereas soil properties played a secondary role. Theoretical risk-quotient-based assessment revealed high risk for earthworms at 80% of sites, particularly in sugarcane and orchard systems. Our findings uncover widespread contamination in subtropical regions, add information on vertically distributed pesticide cocktails, expose limitations of topsoil-only monitoring, and call for mixture-aware, metabolite-inclusive, depth-resolved regulation to protect soil health.