Field and laboratory experiments were conducted in 1983 and 1984 to compare the degradation of dichlormid (2,2-dichloro-N,N-di-2-prophenylacetamide), dietholate (O,O-diethyl O-phenylphosphorothioate), EPTC (S-ethyl dipropyl carbamothioate), and butylate [S-ethyl bis(2-methylpropyl) carbamothioate] in soils that had or had not been previously treated with each chemical. EPTC + dichlormid, butylate + dichlormid, EPTC + dichlormid + dietholate, or butylate + dichlormid + dietholate were applied annually for 4 yr to field plots to establish treatment histories. Butylate and EPTC degradation was enhanced in soils with previous histories of butylate and EPTC, respectively. Degradation rates of dietholate were equivalent in untreated soils or soils previously treated with EPTC + dietholate or butylate + dietholate.

Reduced effectiveness of carbamothioate (thiocarbamate) herbicides in certain soils has been attributed to rapid herbicide degradation by soil microorganisms. Studies were conducted to determine if greater populations of EPTC (S-ethyl dipropyl carbamothioate)-degrading microorganisms were responsible for increased rates of degradation observed following repeated applications of EPTC to a Grenada silt loam soil. EPTC-degrading microorganism populations, measured with a 14C-MPN (most-probable-number) technique, were not larger in soils with accelerated rates of EPTC degradation, and degrader populations did not increase after application of 6 mg EPTC/kg of soil. Degrader populations increased after application of 60 mg EPTC/kg of soil only in soil previously treated for 6 yr with EPTC. Increased rates of metabolism of EPTC were apparently responsible for the increased rates of degradation, rather than increased populations of degraders.

Soils did not remain enhanced for EPTC degradation when EPTC was applied in a biennial rotation with cyanazine, cycloate, or alachlor as indicated by laboratory analysis and early-season weed control in the year of EPTC use. Control of late-season weeds was less when EPTC was rotated with alachlor than with a first-time use of EPTC. EPTC degradation remained enhanced in an EPTC-butylate rotation. The reversion rate of soil from an enhanced condition to normality was gradual and varied with location, year, and number of prior EPTC applications. At Clay Center and Scottsbluff, soils reverted to a nonenhanced EPTC degradation rate 18 months after initial EPTC application. At Scottsbluff, soil was not enhanced for EPTC degradation 18 months after the second of two annual EPTC applications. Soil from Clay Center was partially enhanced for EPTC degradation 18 months after the second of two annual EPTC applications but was not enhanced after 30 months.

Field research was conducted in 1982 and 1983 to characterize the persistence of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)pyridinone} in five Georgia soils. Fluridone persisted less than 365 days in all soils, with shorter persistence upon reapplication in 1983 which indicated the potential for enhanced microbial degradation. A significantly higher rate of loss in 1983 compared to 1982 was recorded in the Greenville sandy clay and Dothan loamy sand soils. A higher rate of loss was recorded for the 1.7 kg ai/ha than the 0.6 kg/ha treatment in the Bradson clay loam and Rome gravelly clay loam soils. No grain sorghum [Sorghum bicolor (L.) Moench. ‘BR 64’) injury was observed in a field bio assay planted in the spring of 1984. Herbicide leaching did not appear to be an important method of loss.

Butylate [S-ethyl bis(2-methylpropyl)carbamothioate] persistence was compared in three soils that were treated annually with four thiocarbamate herbicides in 1981 and 1982. Butylate degraded faster in soils treated two consecutive years with butylate + dichlormid (2,2-dichloro-N,N-di-2-prophenylacetamide) than in soils treated for 2 yr with EPTC (S-ethyl dipropyl carbamothioate) + dichlormid, cycloate (S-ethyl cyclohexylethylcarbamothioate), vernolate (S-propyl dipropylcarbamothioate) + dichlormid, or from previously untreated soils. Butylate + dichlormid applications in 1981 and 1982 enhanced butylate degradation in 1983 at all locations and reduced weed control at two locations. Previously untreated soil or soil previously treated with two annual applications of butylate + dichlormid was treated with 6 ppmw butylate, incubated for 14 days, analyzed for residual butylate, and bioassayed for herbicide activity. Bioassay indicated higher herbicidal activity than was predicted by the residual butylate concentration. A herbicidally active metabolite may be produced during the soil degradation of butylate.

Field and laboratory studies were conducted to examine the influence of previous pesticide use, number of prior EPTC (S-ethyl dipropyl carbamothioate) and butylate [S-ethyl bis(2-methylpropyl)carbamothioate] applications, and pretreatment of soil with EPTC and butylate metabolites on EPTC and butylate degradation. EPTC degradation was enhanced in Clay Center and Scottsbluff soils following three annual EPTC, butylate, or vernolate (S-propyl dipropylcarbamothioate) applications, but was not affected in soils following three annual atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile}, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], or cycloate (S-ethyl cyclohexylethylcarbamothioate) applications. EPTC degradation rate was not affected in a Scottsbluff soil following three annual carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) applications, but was partially enhanced in a Clay Center soil. Self- and cross-enhanced degradation of EPTC and butylate did not change regardless of the number of prior annual applications of each herbicide. The degradation rates of EPTC and butylate were not affected by soil pretreated with butylate sulfone, but the degradation rates-were fully enhanced by pretreatment with the respective sulfoxides of each herbicide. In cross-enhancement testing, EPTC and butylate degradation was partially enhanced in Clay Center soil pretreated with the sulfoxides of each.

Most well-drained Mississippi Delta soils have been used for cotton production, but corn has recently become a desirable alternative crop, and subsequently, atrazine use has increased. Between 2000 and 2001, 21 surface soils (0 to 5 cm depth) with known management histories were collected from various sites in Leflore, Sunflower, and Washington counties of Mississippi. Atrazine degradation was assessed in 30-d laboratory studies using 14C-ring–labeled herbicide. Mineralization was extensive in all soils with a history of one to three atrazine applications with cumulative mineralization over 30 d ranging from 45 to 72%. In contrast, cumulative mineralization of atrazine from three soils with no atrazine history was only 5 to 10%. However, one soil with no history of atrazine application mineralized 54 and 29% of the atrazine in soils collected in 2000 and 2001, respectively. Methanol extracted 15 to 23% of the 14C-atrazine 7 d after treatment in soils having two applications within the past 6 yr, whereas 65 to 70% was extracted from no-history soils. First-order kinetic models indicated soil with 2 yr of atrazine exposure exhibited a half-life of less than 6 d. Most probable number (MPN) estimates of atrazine-ring mineralizing-microorganisms ranged from 450 to 7,200 propagules g−1 in atrazine-exposed soils, and none were detected in soils with no history of atrazine use. Although most soils exhibited rapid atrazine mineralization, analysis of DNA isolated from these soils by direct or nested polymerase chain reaction (PCR) failed to amplify DNA sequences with primers for the atzA atrazine chlorohydrolase gene. These results indicate that microbial populations capable of accelerated atrazine degradation have developed in Mississippi Delta soils. This may reduce the weed control efficacy of atrazine but also reduce the potential for off-site movement. Studies are continuing to identify the genetic basis of atrazine degradation in these soils.

Atrazine is widely used to control broadleaf weeds and grasses in corn, sorghum, and sugarcane. Field persistence data published before 1995 showed that the average half-life of atrazine in soil was 66 d, and farmers expect to achieve weed control with a single application for the full season. However, reports of enhanced atrazine degradation in soil from fields that have a history of atrazine applications are increasing. A rapid laboratory assay was developed to screen soils for enhanced atrazine degradation. Soil (50 g) was placed in a 250 ml glass jar and treated with 7.5 ml of water containing atrazine (5 µg ai ml−1) and capped with a Teflon-lined lid. The assay was conducted at room temperature (25 C). Soil subsamples (1.5 to 3 g) were removed at 0, 1, 2, 4, 8, and 16 d after treatment and extracted with an equal weight of water (wt/vol). The atrazine in the water extract was assayed with high-pressure liquid chromatography (HPLC). The half-life of atrazine in soils with a history of use was ≤ 1.5 d, whereas the half-life in soils with no history of atrazine use was > 8 d. The advantages of this assay are (1) the ease of set up; (2) the rapidity of extraction, and (3) the simplicity of the quantification of the atrazine.

Field studies examined the interaction of soil pH with differing levels of atrazine exposure over 4 yr. Soil pH was 5.2 to 7.1 with atrazine exposures ranging from none (0) to eight applications over a 4-yr period (for each year, one application at planting and one early postemergence). The entire plot area was uniformly managed to reduce potential confounding effects due to cropping history, tillage, and other factors. Soil from all plots previously treated with atrazine displayed rapid atrazine dissipation, with half-lives under laboratory conditions being less than 4 d in plots of pH 5.5 or greater and less than 8 d in the field. Soil pH had a marked effect, with slower atrazine dissipation in those plots that had a pH 5.5 or less. This effect of pH and previous atrazine history was consistent in laboratory and field environments. Implications of these findings include probable reduction in weed control due to more rapid atrazine dissipation and potentially reduced loadings into surface water due to this phenomenon.