Spatial variability in 14C-herbicide degradation in surface and subsurface soils

The spatial variability in mineralization of atrazine, isoproturon and metamitron in soil and subsoil samples taken from a 135-ha catchment in north France was studied. Fifty-one samples from the top layer were taken to represent exhaustively the 31 agricultural fields and 21 soil types of the catchment. Sixteen additional samples were collected between depths of 0.7 and 10 m to represent the major geological materials encountered in the vadose zone of the catchment. All these samples were incubated with 14C-labelled atrazine under laboratory conditions at 28 degrees C. Fourteen selected surface samples which exhibited distinctly different behaviour for atrazine dissipation (including sorption and mineralization) were incubated with 14C-isoproturon and 14C-metamitron. Overall soil microbial activity and specific herbicide degradation activities were monitored during the incubations through measurements of total carbon dioxide and 14C-carbon dioxide respectively. At the end of the incubations, extractable and non-extractable (bound) residues remaining in soils were measured. Variability of herbicide dissipation half-life in soil surface samples was lower for atrazine and metamitron (CV < 12%) than for isoproturon (CV = 46%). The main contributor to the isoproturon dissipation variability was the variability of the extractable residues. For the other herbicides, spatial variability was mainly related to the variability of their mineralization. In all cases, herbicide mineralization half-lives showed higher variability than those of dissipation. Sorption or physicochemical soil properties could not explain atrazine and isoproturon degradation, whose main factors were probably directly related to the dynamics of the specific microbial degradation activity. In contrast, variability of metamitron degradation was significantly correlated to sorption coefficient (K(d)) through correlation with the sorptive soil components, organic matter and clay. Herbicide degradation decreased with depth as did the overall microbial activity. Atrazine mineralization activity was found down to a depth of 2.5 m; beyond that, it was negligible.     

Mineralization of 2,4‐D,mecoprop, isoproturon and terbuthylazine in a chalk aquifer

The potential to mineralize 2,4‐dichlorophenoxyacetic acid (2,4‐D), mecoprop, isoproturon and terbuthylazine was studied in soil and aquifer chalk sampled at an agricultural field near Aalborg, Denmark. Laboratory microcosms were incubated for 258 days under aerobic conditions at 10 °C with soil and chalk from 0.15–4.45 m below the surface. The [ring‐U‐¹⁴C]‐labeled herbicides were added to obtain a concentration of 6 µg kg^?1 and mineralization was measured as evolved [¹⁴C]carbon dioxide. The herbicides were readily mineralized in soil from the plough layer, except for terbuthylazine, which was mineralized only to a limited extent. In the chalk, lag periods of at least 40 days were observed, and a maximum of 51%, 33% and 6% of the added 2,4‐D, mecoprop and isoproturon, respectively, were recovered as [¹⁴C]carbon dioxide. Large variations in both rate and extent of mineralization were observed within replicates in chalk. No mineralization of terbuthylazine in chalk was observed. As a measure of the general metabolic activity towards aromatic compounds, [ring‐U‐¹⁴C]‐benzoic acid was included. It was readily mineralized at all depths. © 2000 Society of Chemical Industry     

Degradation of atrazine and isoproturon in the unsaturated zone: a study from Southern England

The potential for degradation of atrazine or isoproturon in the unsaturated zone of two boreholes was studied under laboratory conditions. Intact and uncontaminated samples were obtained from regular depths of 0–16.45 m and 0–9 m using a percussion coring technique. The results showed that the deep unsaturated zone contained micro-organisms capable of degrading atrazine or isoproturon. The rate of degradation was much faster in surface soil than in most unsaturated materials of both boreholes. The amount of atrazine remaining six months after incubation also varied between the two boreholes. A relatively small amount of atrazine was lost from sterilised samples, suggesting a significant role for microbial degradation. The addition of nutrient and energy sources into materials of low degradation capacity did not enhance the degradation of atrazine. Degradation rate was more related to the presence of a competent microbial population rather than to the presence of indigenous organic matter. However, the competent micro-organisms are more likely to be present when the organic matter content is high. The type and activity of these micro-organisms and their physical environment may have considerable influence on atrazine degradation and are likely to be responsible for much of the variation in the rate of degradation observed at different depths. © 1999 Society of Chemical Industry     

Degradation of atrazine and isoproturon in surface and sub-surface soil materials undergoing different moisture and aeration conditions

The influence of different moisture and aeration conditions on the degradation of atrazine and isoproturon was investigated in environmental samples aseptically collected from surface and sub-surface zones of agricultural land. The materials were maintained at two moisture contents corresponding to just above field capacity or 90% of field capacity. Another two groups of samples were adjusted with water to above field capacity, and, at zero time, exposed to drying-rewetting cycles. Atrazine was more persistent (t(1/2) = 22-35 days) than isoproturon (t(1/2) = 5-17 days) in samples maintained at constant moisture conditions. The rate of degradation for both herbicides was higher in samples maintained at a moisture content of 90% of field capacity than in samples with higher moisture contents. The reduction in moisture content in samples undergoing desiccation from above field capacity to much lower than field capacity enhanced the degradation of isoproturon (t(1/2) = 9-12 days) but reduced the rate of atrazine degradation (t(1/2) = 23-35 days). This demonstrates the variability between different micro-organisms in their susceptibility to desiccation. Under anaerobic conditions generated in anaerobic jars, atrazine degraded much more rapidly than isoproturon in materials taken from three soil profiles (0-250 cm depth). It is suggested that some specific micro-organisms are able to survive and degrade herbicide under severe conditions of desiccation.     

Accelerated Degradation and Mineralization of Atrazine in Surface and Subsurface Soil Materials

In surface soils, atrazine is considered to be a moderately persistent herbicide, with half-lives ranging generally from one to two months. In subsoils, however, its degradation is generally slower. This paper reports the degradation of atrazine in soil and subsoil samples taken from six Belgian maize fields. Rapid degradation can take place in some samples taken from surface and in some from subsurface soils. Subsoil samples were found to degrade atrazine either very strongly or not at all. Experiments with [ring-U-¹⁴C] atrazine showed that the micro-organisms responsible for the rapid degradation cleave the triazine ring and extensively mineralize the molecule. © 1997 SCI.     

Rapid mineralisation of the herbicide isoproturon in soil from a previously treated Danish agricultural field

Mineralisation of the phenylurea herbicide isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea) and two of its known metabolites, 3-(4-isopropylphenyl)-1-methylurea (monodesmethyl-isoproturon) and 4-isopropylaniline, was studied in Danish agricultural soils with or without previous exposure to isoproturon. A potential for rapid mineralisation of isoproturon and the two metabolites was present in soils sampled from three plots within an agricultural field previously treated regularly with the herbicide, with 34-45%, 51-58% and 33-36% of the added [phenyl-U-14C]isoproturon, [phenyl-U-14C]monodesmethyl-isoproturon and [phenyl-U-14C]4-isopropylaniline metabolised to [14C]carbon dioxide within 30 days at 20 degrees C. In contrast, such extensive mineralisation of these three compounds was not observed within this period in soils sampled from two other agricultural fields without previous treatment with isoproturon. The mineralisation patterns indicated growth-linked metabolism of the three compounds in the previously exposed soils, and doubling times for [14C]carbon dioxide production ranged from 1.6 to 3.2, 1.0 to 2.1 and 1.3 to 1.7 days for isoproturon, monodesmethyl-isoproturon and 4-isopropylaniline, respectively. The ability to mineralise [phenyl-U-14C]isoproturon to [14C]carbon dioxide was successfully sub-cultured to a fresh mineral medium which provided isoproturon as sole source of carbon and nitrogen. One of the soils sampled from an agricultural field not previously treated with isoproturon showed accelerated mineralisation of [phenyl-U-14C]4-isopropylaniline toward the end of the experiment, with a doubling time for [14C]carbon dioxide production of 7.4days. This study indicates that the occurrence of rapid mineralisation of the phenyl ring of isoproturon to carbon dioxide is related to previous exposure to the herbicide, which suggests that microbial adaptation upon repeated isoproturon use may occur within agricultural fields.     

The ability of indigenous micro-organisms to degrade isoproturon, atrazine and mecoprop within aerobic UK aquifer systems

The potential for the herbicides isoproturon, atrazine and mecoprop to degrade in the major UK aquifers of chalk, sandstone and limestone was studied using laboratory microcosms spiked at 100 microg litre(-1). Significant mecoprop degradation was only observed in sandstone groundwater samples. Atrazine transformation, based on the formation of metabolites, did occur in most groundwater samples, but only at a rate of 1-3% per year. A potential to degrade isoproturon was observed in groundwater samples from each of the aquifer types, with the most rapid and consistent degradation occurring at the sandstone field site. Biodegradation was confirmed by the formation of monodesmethyl- and didesmethyl-isoproturon. Isoproturon degradation potential rates obtained from the groundwater microcosms could not be correlated with either dissolved organic carbon or numbers of bacteria in the groundwater. It was noted that the ability of the groundwater at a field site to degrade a pesticide was not related to performance of the soil above.     

Atrazine behaviour in the different pedological horizons of two Argentinean non-till soil profiles

Atrazine behaviour was investigated in the different pedological horizons from profiles of two non-tilled soils, a Typic Argiustoll and an Entic Haplustoll from the Argentinean pampas. As atrazine use in field conditions was associated with maize cropping, only one type of soil received atrazine every other year. Atrazine behaviour was characterized through the balance of ¹⁴C-U-ring atrazine radioactivity among the mineralized fraction, the extractable fraction and the non-extractable bound residues. The composition of the extractable fraction was characterized. Atrazine mineralization was the main dissipation mechanism in the superficial horizon of the Argiustoll because of microbial adaptation after repeated atrazine applications. In contrast, little atrazine mineralization was found in the Haplustoll profile, and it decreased with depth. The capacity of the soil organic matter to form bound residues was characterized using soil-size fractionation. Atrazine-bound residues depended on the soil organic matter content and the size of the fraction. Organic matter in the largest size fractions had a higher capacity to form atrazine-bound residues. In the Argiustoll profile, the atrazine degradation capacity decreased in the subsurface horizons (Bt₁ and Bt₂), where a large part of bound residues were formed. The deepest horizon (BC) of this profile had a high capacity to degrade atrazine reaching this horizon after a lag period. In the Haplustoll profile, atrazine mineralization and bound residue formation followed the organic carbon mineralization pattern.     

Variation of pesticide sorption isotherm in soil at the catchment scale

The variation of the sorption isotherm of pesticides has seldom been explored at the catchment scale. Such a study was conducted at the scale of a 187-ha agricultural catchment for three herbicides: atrazine, isoproturon and metamitron. Partition coefficient (Kd) values were measured in batch experiments on 51 topsoil samples, and showed moderate variability at the catchment scale (coefficient of variation CV approximately 30%). Values of Kd ranged from 0.47 to 1.70 litre kg(-1) for atrazine, 0.47 to 1.81 for isoproturon, and 0.55 to 2.21 for metamitron. A clustering method was used to reduce the number of samples on which to measure sorption isotherms to 14. Sorption isotherms agreed with the Freundlich rather than the linear model. Kf parameters had CV values similar to those for Kd, with values ranging from 0.78 to 2.13 mg(1 - Nf) litre(Nf) kg(-1) for atrazine, 0.61 to 1.82 for isoproturon, and 0.69 and 2.58 for metamitron. Nf exponents showed little variation (CV < 5%). Nf values were between 0.86 and 0.98 for atrazine, 0.85 and 0.90 for isoproturon, and 0.82 and 0.87 for metamitron. More than 97% of the Kf catchment-scale variations could be explained by the variations of the soil organic carbon content.     

Removal of Atrazine Contamination in Soil and Liquid Systems Using Bioaugmentation

Reported levels of atrazine in soils at pesticide mix-load sites can vary between 7·9×10^-5 mM and 1·9 mM . We report on a mixed microbial culture, capable of degrading concentrations of atrazine in excess of 1·9 mM . At initial concentrations of 0·046 M and 0·23 M , the mixed population degraded 78% and 21% of atrazine in soil (100 days), respectively. At the same initial concentrations in liquid cultures, 90% and 56% of the atrazine was degraded (80 days), respectively. Decreased degradation in soil samples may have resulted from atrazine sorption to soil surfaces or decreased contact between the population and the herbicide. In the 0·23 M system, we attribute incomplete degradation to phosphorous depletion. Data for carbon dioxide evolution was fitted to a three-half-order regression model, but we feel that there are limitations of the application of this model to atrazine degradation. The population uses the herbicide as a nitrogen source and little carbon is incorporated into biomass, as the energy status of carbons in the ring leads to their direct evolution as [¹⁴C]carbon dioxide. This situation contributes to an evolution pattern that, when fitted to the three-half-order model, results in underestimation of the biomass produced. Data from our study suggest that our mixed culture could be used for bioremediation of atrazine at concentrations up to and exceeding those currently reported for agrochemical mixing-loading facilities. © 1997 SCI.     

Microbial aspects of atrazine biodegradation in relation to history of soil treatment

Among 15 soils with different cropping practices, seven which had an history of repeated atrazine applications showed accelerated degradation of this herbicide. By contrast, grassland or agricultural soils with no recorded atrazine application, at least for the last three years, had a low degradation potential. No direct relation was found between the rate of atrazine mineralisation and the size of the microbial biomass. In adapted soils, the amounts of extractable residues were lowered and the very high percentages of radioactivity from [ring-¹⁴C]atrazine recovered as [¹⁴C]carbon dioxide demonstrated that N-dealkylation and deamidation were the only processes for micro-organisms to derive carbon and energy for heterotrophic growth. Kinetics of microbial ¹⁴C accumulation revealed that atrazine ring carbon could be incorporated by direct oxidative condensation with structural components of the bacterial or fungal cell whereas side-chain carbon was preferentially used for biosynthesis of new protoplasmic cell material, as confirmed by the high turnover rate of radiolabelled microbial components. From the determination of the Michaelis–Menten parameters, V_m and K_m in the presence of different selective biocides, it was possible to conclude that fungi were probably less active in atrazine degradation than bacteria and that over years the microbial atrazine-degrading community showed an increased efficiency. © 1999 Society of Chemical Industry     

Degradation of [14C]Terbuthylazine and [14C]Atrazine in Laboratory Soil Microcosms

The degradation and formation of major chlorinated metabolites of terbuthylazine and atrazine in three soils (loamy clay, calcareous clay and high clay) were studied in laboratory experiments using molecules labelled with ¹⁴C on the s-triazine ring. Soil microcosms were treated with the equivalent of 1 kg ha^-1 of herbicide and incubated in the dark for 45 days at 20(±1)°C. The quantity of [¹⁴C]carbon dioxide evolved in the soils treated with atrazine was negligible and could not be attributed to mineralization of the parent molecule. The mineralization of terbuthylazine accounted for 0·9–1·2% of the initial radioactivity. In the soils studied, the extrapolated half-lives varied from 88 to 116 days for terbuthylazine and 66 to 105 days for atrazine, with no significant differences for the three soils and the two molecules. The deethyl metabolites of the two s-triazines and the deisopropyl-atrazine metabolite appeared during the incubation in the three soils. The completely dealkylated metabolite was not detected in any of the soils. After 45 days of incubation, the non-extractable soil residues for the high clay, loamy clay and calcareous clay soils represented for terbuthylazine, 33·5, 38·3 and 43·1% and for atrazine, 19·8, 20·8 and 22·3% of the initial radioactivity. © 1997 SCI.     

Evidence for cross-adaptation between s-triazine herbicides resulting in reduced efficacy under field conditions

BACKGROUND: Enhanced atrazine degradation has been observed in agricultural soils from around the globe. Soils exhibiting enhanced atrazine degradation may be cross-adapted with other s-triazine herbicides, thereby reducing their control of sensitive weed species. The aims of this study were (1) to determine the field persistence of simazine in atrazine-adapted and non-adapted soils, (2) to compare mineralization of ring-labeled (14)C-simazine and (14)C-atrazine between atrazine-adapted and non-adapted soils and (3) to evaluate prickly sida control with simazine in atrazine-adapted and non-adapted soils.RESULTS: Pooled over two pre-emergent (PRE) application dates, simazine field persistence was 1.4-fold lower in atrazine-adapted than in non-adapted soils. For both simazine and atrazine, the mineralization lag phase was 4.3-fold shorter and the mineralization rate constant was 3.5-fold higher in atrazine-adapted than in non-adapted soils. Collectively, the persistence and mineralization data confirm cross-adaptation between these s-triazine herbicides. In non-adapted soils, simazine PRE at the 15 March and 17 April planting dates reduced prickly sida density at least 5.4-fold compared with the no simazine PRE treatment. Conversely, in atrazine-adapted soils, prickly sida densities were not statistically different between simazine PRE and no simazine PRE at either planting date, thereby indicating reduced simazine efficacy in atrazine-adapted soils.CONCLUSIONS: Results demonstrate the potential for cross-adaptation among s-triazine herbicides and the subsequent reduction in the control of otherwise sensitive weed species. Copyright (c) 2008 Society of Chemical Industry.     

Enhanced degradation of atrazine under field conditions correlates with a loss of weed control in the glasshouse

Enhanced degradation of atrazine has been reported in the literature, indicating the potential for reduced residual weed control with this herbicide. Experiments were conducted to determine the field dissipation of atrazine in three cropping systems: continuous Zea mays L. (CC) receiving atrazine applications each year, Gossypium hirsutum L.-Z. mays rotation (CCR) receiving applications of atrazine once every 2 years and a no atrazine history soil (NAH). Subsequent laboratory and greenhouse experiments were conducted with soil collected from these cropping systems to determine atrazine degradation, mineralization and residual weed control. Field dissipation of atrazine followed first-order kinetics, and calculated half-life values for atrazine combined over 2003 and 2005 increased in the order of CC (9 d) = CCR (10 d) < NAH (17 d). Greenhouse studies confirmed that the persistence of atrazine was approximately twofold greater in NAH soil than in CC or CCR soil. Biometer flask mineralization studies suggested that enhanced degradation of atrazine was due to rapid catabolism of the s-triazine ring. Glasshouse efficacy studies revealed a loss of residual weed control in CC and CCR soil compared with NAH soil. These data indicate that, under typical Mississippi Delta field conditions and agronomic practices, the persistence of atrazine may be reduced by at least 50% if the herbicide is applied more than once every 24 months. Glasshouse studies suggest that under these conditions a loss of residual weed control is possible.     

Evidence for the Accelerated Degradation of Isoproturon in Soils

The herbicide isoproturon was degraded rapidly in a sandy loam soil under laboratory conditions (incubation temperature, 15°C; soil moisture potential, -33 kPa). Degradation was inhibited following treatment of the soil with the antibiotic chloramphenicol, but unaffected by treatment with cycloheximide, thus indicating an involvement of soil bacteria. Rapid degradation was not observed with other phenylurea herbicides, such as diuron, linuron, monuron or metoxuron incubated in the same soil under the same experimental conditions. Three successive applications of isoproturon to ten soils differing in their physicochemical properties and previous cropping history induced rapid degradation of the herbicide in most of them under laboratory conditions. There were, however, no apparent differences in ease of induction of rapid degradation between soils which had been treated with isoproturon for the last five years in the field and those with no pre-treatment history. A mixed bacterial culture able to degrade isoproturon in liquid culture was isolated from a soil in which the herbicide degraded rapidly.     

Capacity of model biobeds to retain and degrade mecoprop and isoproturon

Biobeds are used to increase the adsorption and degradation of pesticide spillage on sites used for mixing and loading and for cleaning of sprayers. The adsorption and the rate of degradation of 14C-labelled isoproturon and mecoprop (MCPP) at concentrations from 0.0005 to 25 000 mgkg(-1) were determined in biobed soil. Further leaching of the two herbicides was determined in a model biobed with a surface area of 2 m2. The biobed material showed enhanced ability to adsorb the two herbicides. Kd was 5.2 litre kg(-1) for isoproturon and 1.6 litre kg(-1) for MCPP in biobed material, which is higher than in natural soil. In different experiments with natural soil, Kd ranges from 0.07 to 0.6 litrekg(-1) for MCPP and from 1.5 to 4.6 litre kg(-1) for isoproturon in soils with varying organic carbon content. Degradation of MCPP was rapid at concentrations from 0.0005 to 500 mg kg(-1), delayed at 5000 mg kg(-1), and very slow at 25 000 mg kg(-1). For isoproturon, the relative degradation was most rapid at the lowest concentration and decreasing with increasing concentrations. After 120 days, between 55% and 8% 14C was evolved as 14CO2 at concentrations between 0.0005 and 25 000 mg kg(-1). The rate of evolution of 14CO2 indicated that degradation rates at low concentrations were of first-order and at higher concentrations of zero-order. Leaching of MCPP and isoproturon was determined in a newly established model biobed during a 2-year period. About 13% of applied MCPP and 1.4% of applied isoproturon leached out during the winter following the first autumn application (worst-case scenario). Leaching was completely prevented when the biobed had a well-developed grass cover and was covered during the winter.     

Dégradation de l'isoproturon et disponibilité de ses résidus dans le sol

Degradation if isoproturon and availability of residues in soil The availability and degradation of ¹⁴C-ring-labelled isoproturon in soil was investigated over 140 days under controlled laboratory conditions. Degradation of the active ingredient followed and 65 days later only a minor fraction (0.6%) of the parent molecule remained extractable. A demethylated-isoproturon metabolite was detectable in soil from day 15 (2.6%). The amount of ¹⁴CO₂ derived from the ¹⁴C benzene ring label and liberated over time indicated that a total of 13.6% isoproturon was mineralized during the incubation period. In parallel, the amount of ¹⁴C residue extracted from the soil by water followed by methanol or remaining within the soil—analysed by combustion—was also determined at intervals. After 140 days, 72% of the radiolabel added remained in the soil as non-extractable residue. The degradation half-life of extractable isoproturon was an estimated 14 days.     

莠去津在土壤中的残留动态和淋溶动态

利用HPLC法对土壤中莠去津的残留动态、淋溶动态进行了研究。结果显示,莠去津以有效成分2.25 kg/hm2和4.50 kg/hm2的剂量施用时,在土壤中的半衰期分别为19.1 d和18.1 d,即其半衰期与莠去津的施用浓度无关,属于典型的一级动力学反应。在120 d的玉米生长期中,土壤中莠去津在不断降解代谢的同时,逐渐向深层土壤中淋溶,多数莠去津持留在表层土壤中。施用莠去津27 d后,高浓度处理小区莠去津的淋溶深度超过30 cm,深度为10~15 cm处的土壤在施用后27 d莠去津的浓度最大。同一土壤深度,莠去津在高浓度处理小区的残留量要远高于低浓度处理小区。这些结果显示,减小莠去津的用量可以减少莠去津在土壤中的移动,表明低剂量施用莠去津是保护地下水免受污染的一种有效措施。影响莠去津的淋溶作用的主要因素包括使用量和土壤的理化特性。     

Enhanced degradation of some soil-applied herbicides

In a field experiment involving repeated herbicide application, persistence of simazine was not affected by up to three previous doses of the herbicide. With propyzamide, there was a trend to more rapid rates of degradation with increasing number of previous treatments. Persistence of linuron and alachlor was affected only slightly by prior applications. In a laboratory incubation with soil from the field that had received four doses of the appropriate herbicide over a 12–month period, there was again no effect from simazine pretreatments on rates of loss. However, propyzamide, linuron and alachlor all degraded more rapidly in the previously treated than in similar untreated soil samples. Propyzamide, linuron, alachlor and napropamide degradation rates were all enhanced by a single pretreatment of soil in laboratory incubations, whereas degradation rates of isoproturon, metazachlor, atrazine and simazine were the same in pretreated and control soil samples.     

Metribuzin degradation in soil: I—effects of soybean residue amendment,metribuzin level,and soil depth

Plant residue and soil depth effects on metribuzin degradation were investigated. Dundee silt loam soil collected at depth increments of 0–10 cm (SUR) and 10–35 cm (SUB) was treated with labeled [5^?14 C]metribuzin. Samples were assayed at several time points up to 140 days after treatment. Soybean residue was added to half of the SUR samples (RES), with remaining SUR unamended (NORES). None of the SUB samples were amended with soybean residue. Metribuzin mineralization to ¹⁴CO₂ proceeded more slowly in RES and SUB than in NORES and SUR, respectively. Extractable components in SUR samples included polar metabolites, plus deaminated metribuzin (DA) in the RES, and parent metribuzin in the NORES. Deaminated diketometribuzin (DADK) and metribuzin comprised major ¹⁴C components extracted from SUB, while in SUR, faster degradation of metabolites resulted in metrizubin as the primary identifiable compound. Unextractable ¹⁴C increased until day 35 for both RES and NORES, after which it remained constant for NORES. but declined for RES. A corresponding rise in RES polar ¹⁴C suggested that as soybean residue decomposed, ¹⁴C bound in the residue was released as extractable polar material. Soil with soybean residue accumulation may alter metabolite degradation patterns, but does not impede initial metribuzin degradation. Depth differences in metribuzin degradation were attributed to reductions in microbial activity with increasing soil depth.