Dissipation Kinetics of Glyphosate and Amino Methyl Phosphonic Acid (AMPA) in Soil, their Residue Status in Subsequent Urdbean Crop and Effect on Soil Microbial Population.

A field experiment was conducted to investigate dissipation of glyphosate and its metabolite, aminomethylphosphonic acid (AMPA) in soil and their residual effects on subsequent urdbean crop. Glyphosate was applied at 1.5 and 2.25 kg a.i ha- 1, followed by sowing of urdbean at 7, 14 and 21 days after application. Residues of glyphosate and AMPA were quantified using a validated HPLC-FD method, which showed excellent linearity (R² > 0.99), recoveries of 84.96-95.80% for glyphosate and 84.15-96.32% for AMPA, with relative standard deviations (RSDs) ≤ 4.18% and a limit of quantification (LOQ) of 0.05 µg g⁻¹. Glyphosate persisted in soil up to 45 and 60 days at application rates of 1.5 and 2.25 kg a.i ha- 1, respectively, with corresponding half-lives of 11.40 and 16.09 days, while AMPA quantified up to 75 days. Glyphosate and AMPA residues were below the LOQ in all subsequently grown urdbean plant or grain samples. Glyphosate exhibited distinct and time-dependent responses towards soil microbial populations, with increased bacterial populations, transient stimulatory response of fungal and actinomycetes biomasses, and dynamic soil dehydrogenase activity that stabilized over time. These findings suggest that glyphosate applied at recommended dose for weed control permits safe sowing of urdbean at 7 days after application, without any adverse effect on succeeding crop or soil microbial population.