Aging of methabenzthiazuron, imidacloprid, and N,N-dimethylsulfamide in silty soils and effects on sorption and dissipation.

Differences in soil properties can influence the fate of plant protection agents in the environment. The present study aims to investigate the sorption behavior and related aging processes of imidacloprid (IMI; insecticide), methabenzthiazuron (MBT; herbicide), and N,N-dimethylsulfamide (DMSA; degradate of the fungicide tolylfluanid) in six soils of silty texture but otherwise varying properties. The sorption behavior of these ¹⁴C-labeled compounds exhibiting different physicochemical properties was characterized by applying a three-step sequential extraction procedure. After 119 d, MBT revealed strongest sorption (K'(tot) 47.4-200.4 L/kg), followed by IMI (K'(tot) 11.7-30.6 L/kg), and DMSA with K'(tot) close to zero. Aged sorption factors (AFs) were calculated to characterize aging processes over time exhibiting a 2.6-3.5-fold (IMI), a 1.8-4.5-fold (MBT), and no (DMSA) increase of sorbed amounts within 84 d. Sorption and aging varied widely in the group of silty soils, which differed with respect to organic matter content, C/N-ratio, and microbial soil parameters. The time-dependent increase of adsorption of MBT and IMI was more pronounced in those soils that had a lower organic carbon and low microbial biomass content. Concomitantly, MBT and IMI degradation decelerated, presumably because of aged sorption at inner binding sites leading to a lower accessibility. In contrast, in the soils with a higher organic carbon content a strong initial (but later still reversible) sorption of MBT and IMI, occurring presumably at outer surface sites, reduced the extent of time-dependent diffusion toward inner binding sites.