Response of Soil Enzymatic and Microbial Activities to Mixture Formulation of Mesosulfuron Methyl and Iodosulfuron Methyl and Its Degradation in Soil

The present study investigated the effect of application rate, soil type and temperature on the degradation dynamics of mesosulfuron methyl and iodosulfuron methyl and their effect on soil enzymatic and microbial activity under laboratory conditions. Mixture of mesosulfuron methyl + iodosulfuron methyl was applied to soils at application rates of 14.4, 28.8, 43.2 and 57.5 g ha(-1). Residues of herbicides were extracted using ultrasonic-assisted extraction method and analyzed with high-performance liquid chromatography. Degradation was dependent on soil type and application rate of herbicides with DT50 (50% degradation time) and DT90 (90% dissipation time) of 8.71 to 47.08 and 28.95 to 156.43 days for mesosulfuron methyl while 7.17 to 38.24 and 20.81 to 127.07 days for iodosulfuron methyl. Increase in temperature enhanced the dissipation of herbicides. The fastest dissipation was observed in loam 1 soil probably because in acidic soil abiotic hydrolysis was a predominant contributor in degradation. Among both herbicides, mesosulfuron methyl was more persistent in studied soils. Various metabolites of mesosulfuron methyl and iodosulfuron methyl, viz. mesosulfuron (M1) and 2-[3-(4-hydroxy-6-methoxypyrimidin-2-yl) ureidosulfonyl] - 4 - methane sulfonamidomethyl benzoate (M2), 4,6-dimethoxypyrimidine-2-yl-urea (M4), 2-amino-4,6-dimethoxypyrimidine (M5), metsulfuron methyl (I1), methyl-2-amino-sulfonyl benzoate (I2), 4-methoxy-6-methyl-1,3,5-triazine-2-amine (I3) and saccharin (I4), were also detected in the studied soils and their appearance and disappearance varied with soil type and application rate. The application of herbicides showed short-lived harmful effects on dehydrogenase, alkaline phosphatase and acid phosphatase activity and soil microbial activity while urease activity remained unaffected. The results indicated that studied herbicides have low to moderate persistence and transitory adverse effects on soil microbial activity depending on pH, organic matter content and temperature conditions.