Fate, transport, and transformation of mesosulfuron-methyl and iodosulfuron-methyl in organically amended agricultural soils.

Herbicide residues in agricultural soils pose increasing concerns for environmental sustainability, food safety, and soil ecosystem functioning, particularly with the widespread use of sulfonylurea herbicides in intensive agriculture. This study investigated the fate of two sulfonylurea herbicides, mesosulfuron-methyl and iodosulfuron-methyl, with emphasis on the influence of organic amendments (OAs) on their degradation kinetics, leaching behavior and associated soil biochemical and microbial activity responses. The incorporation of OAs significantly enhanced the degradation rates of both herbicides compared with unamended soil, with half-life (DT50) values reduced from 10.72 to 22.47 days in unamended soil to 3.58 to 16.24 days in amended soils. Among the tested amendments, the fastest dissipation was observed in press mud (PM)-amended soil, followed by farmyard manure (FYM), vermicompost (VC), poultry manure (POM), dextrose (D), and cyclodextrin (CD). Increased degradation in amended soils was associated with higher dehydrogenase activity, indicating enhanced microbial activity. Several transformation products were identified during degradation, including metabolites M1, M2, M4, and M5 from mesosulfuron-methyl and I1, I2, I3, and I4 from iodosulfuron-methyl, which appeared earlier in amended soils. OAs significantly reduced herbicide leaching, with PM and FYM decreasing leachate concentrations by up to 70 to 100% compared to unamended soil. Bioassay results indicated reduced phytotoxicity, with IC50 values for Brassica juncea ranging from 0.0514 to 0.1089 and faster recovery observed in amended soils (10 to 45 days) compared to unamended soil (30 to 90 days). These results demonstrate that organic amendments, particularly PM and FYM, can enhance herbicide dissipation, reduce leaching losses, mitigate phytotoxic risks, and contribute to improved soil health and sustainable agricultural systems.