Transport mechanisms and fate of neonicotinoids in the soil-water systems under the effects of wetting-drying cycles and rice cultivation

Neonicotinoids (NEOs), the most widely used class of insecticides in global agriculture, pose environmental risks due to their persistence and mobility. Despite their extensive application, the transport mechanisms and fate of NEOs in agroecosystem remain unclear. This study investigated the spatial-temporal dynamics mechanisms of six typical NEOs (i.e., clothianidin (CLO), imidacloprid (IMI), acetamiprid (ACE), thiamethoxam (THM), dinotefuran (DIN), and nitenpyram (NIT)) under the effects of different water managements and rice cultivation in soil-water systems. After 61 days incubation, the dissipation in the soil-water systems were the dominated fate of NEOs (accounting for 96.3 ± 3.1 %), followed by storage in soils (2.6 ± 1.2 %), dissolution in interstitial water (0.7 ± 0.5 %) and overlying water (0.1 ± 0.1 %), and leakage in leakage water (0.3 ± 0.3 %). Both wetting-drying cycles and rice cultivation accelerated the NEOs dissipation. The possible reasons for this phenomenon were the wetting-drying cycles and rice cultivation stimulated NEO-degrader (i.e., Pseudomonas) growth in topsoil. Compared to flooding no rice treatment (half-life = 15.5 ± 4.4 days), the severe wetting-drying cycle with rice cultivation reduced NEO half-lives to 10.5 ± 3.1 days. The residual amounts of NEOs in each treatment were negatively (p < 0.05) correlated with the water solubility (CLO > IMI > ACE > THM > DIN > NIT), with the topsoil acting as the primary NEOs sink (41.6 % of total residues). The concentration of NEOs in the water phase decreased by 74.5 % from overlying water to leakage water, which due to the filtering effect of soils. The findings provide a scientific theoretical basis for the further prevention, control, and remediation of NEO pollution in agroecosystem.