J. Agric. Food Chem. 1992, 40, 2500-2504
2500
Degradation of the Sulfonylurea Herbicide [14C]Amidosulfuron(HOE 075032) in Saskatchewan Soils under Laboratory Conditions Allan E. Smith' and Andrew J. Aubin Research Station, Agriculture Canada, 5000 Wascana Parkway, Box 440, Regina, Saskatchewan, Canada S4P 3A2
T h e degradation of the herbicide [14Clamidosulfuron (HOE075032)was studied in three Saskatchewan soils at 0.1 pglg under controlled temperature and moisture conditions. At 85% of field capacity, degradation approximated to first-order kinetics at soil temperatures of 10,20,and 30 O C , with half-life values ranging from 14 f 2 days (mean and standard deviation) in a loamy sand incubated at 30 "C to 231 f 41 days in a clay at 10 O C . In a clay at 20 O C , herbicide degradation followed first-order kinetics and was directly correlated with soil moisture, with half-life values varying from 246 f 21 days at 50% of field capacity to 63 f 7 days at 100% There was negligible herbicide breakdown in air-dried soils incubated at 30 O C for 70 days. During the study period, solvent-extractable 14Cdegradation products accounted for u p to 48% of the applied radioactivity. Solvent-nonextractable radioactivity associated with the soils incubated at 20 O C ranged from 4 to 11% of the initial treatment.
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INTRODUCTION
Hoechst Aktiengesellschaft (Frankfurt am Main, Germany)with a specific activity of 2.57 MBq/mg and a radiochemical purity The sulfonylurea herbicide amidosulfuron, also referred >96% as determined by thin-layer chromatographic (TLC) and to as HOE 075032 [Figure 1, R = CHs; 3-(4,6-dimethoxyradiochemical analysis (see later). The [14C]amidosulfuronwas pyrimidin-2-yl)-l-(N-methyl-N-methylsulfonyl)aminosul- dissolved in 96% ethanol to give a solution with 422 kBq/mL and of amidosulfuron/mL. Analytical samples containing 164 fonylureal is currently being evaluated in Canada at rates (>99% pure) of nonradioactive amidosulfuron and the soil u p to 45 g/ha for the postemergence control of broadleaf degradation product (cf. Figure 1)were also provided by Hoechst weeds in cereal and other crops. Aktiengesellschaft. For TLC purposes, a solution of the former As with other sulfonylurea herbicides (Beyer et al., 1988), was prepared in acetone (2mg/mL),while a solution of the latter the water solubility is pH dependent. In aqueous media, (2 mg/mL) was prepared using a mixture of water and acetonitrile at 20 O C , the solubility of amidosulfuron is 3.3,9.0,and (3:lv/v) as solvent. 13 500 mg/L at pH values of 3, 5.8, and 10,respectively Degradation Studies. Samples (50 g) of all three soils at (Technical Information HOE 075032 Experimental Her85% of their field capacities (FC) were weighed into 175-mL polystyrene foam cartons fitted with plastic lids and incubated bicide, 1990). Amidosulfuronis relatively nonvolatilewith in the dark at 10 f 1,20 f 1, or 30 f 1 O C for 7 days prior to a vapor pressure of 1.3 X Pa at 20 O C (Technical addition of [14Clamidosulfuron. An additionalseries of samples Information HOE 075032Experimental Herbicide, 1990). of the Regina clay at 50,65,and 100% FC were similarly prepared Amidosulfuron undergoes degradation in the soil, preand incubated at 20 1 "C. All incubations were carried out in sumably by both microbiologicaland chemicalmechanisms laboratory incubators (Conviron Products Co., Winnipeg, MB, similar to those reported for other sulfonylureaherbicides Canada). The cartons were weighed every 2 days, and distilled (Joshi et al., 1985;Beyer et al., 1988;Brown, 1990;Cambon water was added as necessary, with stirring, to replace lost et al., 1992),for which t h e demethylated product [Figure moisture. Following the preliminary incubation, [Wlamidosul1, R = H; 3-(6-hydroxy-4-methoxypyrimidin-2-yl)-l-(N- furon (30pL, 12.7 kBq, 4.92 pg of herbicide) was added to each carton. This concentration of 0.1 pg/g, based on moist soil, is methyl-N-methylsulfonyl)aminosulfonylurealhas been approximately equivalent to a field rate of 50 g/ha assuming identified as a breakdown product (Technical Infornuation that, under field conditions, the chemical is distributed in the HOE 075032 Experimental Herbicide, 1990). top 5 cm of soil (Smith et al., 1990). The soils were thoroughly The objectives of the present laboratory studies were t o stirred with a spatula to distribute the herbicide, and the cartons determine the rates of breakdown and fate of [l4C1were capped and reincubated at the required temperatures. Water amidoaulfuron in three Saskatchewan soils at 85% of field was added, by weight with stirring, as necessary, to replace capacity and at 10,20,and 30 O C . T h e rates of breakdown moisture lost by evaporation. For control purposes, to assess of [14C]amidosulfuronin a clay at 20 O C and at 50,65,85, nonbiological degradation and soil extraction recoveries, air-dried and 100% of field capacity were also studied. (
