Effect of Soil Conditions on the Degradation of Cloransulam - Methyl

Herbicide efficacy and environmental fate are often controlled by s(il conditions. Aerobic soil laboratory studies were mndertaken to determiine the degradation of the herbicide cloransulam-metliyl IN. (2-carboinethoxy.6-chloro-phenyl)-5-ethoxy-7-fluoroI ,2,41triazolo 115-lpyrimndine-2-sulfonaimdel for a range of soil factors. Ireat. ments included soil temperature (5, Ls, 25, 40, and 508i), moisture (20, 48, and 60% water filed pore space), and soil type, Ihe soils uider study were a )mrmmer silty day loam (fine-silty, nixed, super. active, iueesic 'ypic Endoaquolis) and a Cisne silt lom (fine, smeetitic, nesic Vertic Albaqualfs). Variablity in molecular degradation was investigated using two radiolaleled fonns (lPhenyl-UL_ C] and IPy rimidine-7,9-.4CD). Dissipation of parent compound In soil solution and sorbed phases, fotnatiosn of radiolabeled metaholites, iC mineralizatinm, total microbia respiration, and boud residue formation were measured for up to 120 d& Dissipation of parent and formation of bouad residues in Drnmmer soil increased with greater temperatures. The influence of temperature on '"C mineralization, however, was dependent on position of radiolahel, suggesting that distinct groups of microorganisms degrade different parts of the molecule at higher temperatures. Only 'C mineralization was influenced by moisture, with response depending on soil type. Increasing moisture resulted in more 'C mineralization in Drummer, but not Cisne soil, which was attributed to increased microbial access to pesticide at greater moisture contents in D)runmer soil. Reduced availability, suggested by greater sorption In Drummer sol, may explain persistence of parent in this sroiL Bound residues were more extensive ad exhdbited greater dependence on biological cwivity iu Cisne soil, owing to einhanced dissipation of parent compound in this soil. CLORANSUI.AM-METHYL [N-(2-carbomethoxy-6-chlorophenvl)-S ethoxy-7-fluoro[ 1,2,41triazolo [1,5-ciPyAlison M. Cupples and Ryan P. Hl ltgren, Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois, 1rbana, IL 61801; Geraki K. Sims, USDA-ARS, Urbana, 11 61801; and Steven F. Hlart, Dep. of Planit Scieice, Rutgers Univ., New Brunswick, NJ 08901. Received 26 July 1999. *Corresponding author ([email protected]). published in J. Environ, Quial. 29:7186-'794 (2000). rimidine-2-sulfonarnidel ((AS Registry No. 147150-354), recently developed for the control of broadleaf weeds in sovbean [Givcine max (L.) Merr., is a triazolopyriimiidine sulfonamide herbicide (Fig. 1 ) that functions by inhibition of the plant enzyme acetolactate synthase (ALS). Notable characteristics of the compound include a low application rate (label rates of 17.5 and 44 g ha 1, preplant and preemergent, respectively), low pK, (4.81), a short half-life (tj). 13-28 d), solubility in water of 184 mg 1. ' (pH 7), and apparent Kd ranging from 0.19 to 4.89 L kg I (Wolt et al., 1996). Little published dat:a, with Iimited degradation studies, exist on the behavior of cloransulani-methyl, nakirg this research important for understanding the enviroilmental fate of this new xenobiotic compound. The study of herbicide degradation and fate is vital for both maintaining environmental quality anid optimDiziiig agricultural practices. IIerbicidal activity and environmental behavior are both affected by the influence of enviroimental con(iitions on soil processes. Herbicide fate processes (e.g., sorption and dlesorption, volatilization, chemical hydrolysis, biodegradation, and bound residue formation) exhibit variability that can be dependent on factors such as soil temperature, moisture, redox potential, and pH, Of general importance are data relating the effects of moisture and temperature conditions on fate of xenobiotics in soil. Temperature responses for cenvironmental fate processes are variable, and range from temperature independence observed flor some sorption phenomena (H assett and Banwart, 1989) to hyperbolic (Mervosh et al., 1995a) or exponential functions (Leemley et al., 1988). Temperature optima are often associated with biological systems, whereas chemical processes generally exhibit exponential increases with temperature. For example, an Arrhenius plot of mesophilic bacterial growth in pure culture exhibited a response maximum at approximately 400 C, with reduced rates at higher and 786 C(UPPIES ElI AL: DEGRADATION 01 lower temperatures (Ing2aham, 1958). Thus, pesticide degradation controlled by biological factors would be expected to display temperature optima. The relative importance of chemical and biological degradative processes can be investigated bv examininig activation energy values from degradation studies over a ranige of temperatures. The activation energy is obtained by regressing the natural logarithilm of the rate constant against lt according to the Arrhenius equation: In K = (L'IRT) -+ In A Where K is the degradation rate constant, A is an empirical constait, t is temperature (K), R is the universal gas constant (8.3145 J K; mol ') and EL (kJ mol') is the activation energy for the reaction. 'l'he temperature responsiveniess of various degradation mechanismns has been claracterized using the E. of the svstem (Burmside, 1965; Hance, 1967; L.emley et al., 1988). Activation energy values greater than 60 kJ nmol iare frequently reported for chemical and physical reactions of xenobiotics in soil (Burnside, 1965; Hance, 1967; Lemiey et al., 1988), whereas biological reactions typically result in Fa values less than 30 kJ mol i (Meikle et al., 1973; Thirunaravanan et al., 1985). Meikle et al. (1973) suggested that the catalytic nature of biological reactions results in a lower energy requirement and henice, a diminished response to temperature. An objective of this study is to determine temperature dependence of cloransulam-nmethyl dissipation mechanisms. Soil moisture content affects both biological and abiotic environmenital fiate processes. A general trend of reduced degradation rates at low moisture contents has been observed for many pesticides, suclh as picloramn (Meikle et al., 1973), imazaquin and imazethapyr (Flint and Witt, 1997), trifluralin (Zimdahl and Gwynn, 1977), clomazonie (Mervosh et al., 1995a), and chlorsulfuroil (Thirunarayanianl et al., 1985). Factors such as limited diffusion of tile substrate (xenobiotic), increased toxicity (higher solution concenitration), and greater sorption at low moisture contents may result in less dissipation. At low enough water contents, physioiogical constraints result in reduced or no microbial activity, leading to retarded degradation, whereas reduced availability of oxygen may occur at higher moisture contents (1-leiweg. 1987; Flint and Witt, 1997). Ther efore, aerobic microbial activity is often maximal at a water content where substrate diffusion and oxygen supply are optimized (Skopp et al., 1990). Abiotic processes, suchi as sorption, transport, and volatilization, may also be dependent UponI diffusion kinetics, and thus arc also expected to respond to water content. TIhis studv investigates the importance of soil moisture on cloransulam-niethyl fate processes, :ncluding dissipation of parent in sorbed and solution pools, bound residue formation, and mineralizationi, using a range of onoisture contents optimal for soil microbial activity. Sorption of hydrophiobic organic compounds is usu ally greater in soils with mlore organic carbon, ofien resulting in bioavailability limitations for pesticide degradation (Sims et al., 1991, 1992). Ihough hydrophobic 7 2 (.()ORANSIULAM ME LIYL. ( I /787 sorption may be intrinsically temperature-independent at equilibrium (Hlassett and Banwart, 1989), the extent of sorption and resulting bioavailability constraints may be strongly influenced by temperature in thc noioequilibrium conditions present in unsatuirated soils (Mervosh et al., 1995b). Owing to sorption noneqLuilibriuim, the solution phase is typically depleted, preferentially over the sorbed phase in unsaturated soils (Meivosh et al., 1 995b). 1lowever, movement of substrates into inaccessible pore space may be more important than sorption in limiting bioavailability in many soils (Pignatello, 1989). 'ihe relative importanice of physical inaccessibility in xenobiotic biodegradiation appears to be a function of water coitent (Johnson et al., 1998). 'Ihe role of bioavailability in soil degradation of cloransillam-mnethiyi was investigated as a function of temperatire and moisture. The degradative characteristics of cloranisulammethyl have not been extensively deteniined in relation to temperature and soil moisture. The goal of this research was to demonstrate the important dissipation mechanisms of the xenobiotic cloransulam-methvl throughi soil pools and the influence of soil temperature and moisture on these processes and pathways. Ihle study employs a range of moisture conditions throughout which bioavailabilitv constraints should be more important than physiological stress (Harris, 1981, Skopp et al., 1990) and temperature conditions spanning the range expected at or near the soil surface where the pesticide is applied. MATERIALS AND METHODS