Amicarbazone, atrazine, and metribuzin behavior was examined in a field setting in Tennessee and in a laboratory setting using soils collected from Illinois and Tennessee. Fields planted to corn were sampled from 0 to 8 cm depth, and the samples were analyzed using methanolic extraction followed by tandem mass spectrometry analysis to determine residual herbicide concentrations. Conditions were favorable for herbicide degradation, including warm temperatures and adequate rainfall. All herbicide half-lives were <10 d. Laboratory research using soils with known atrazine-use histories showed that amicarbazone did not exhibit enhanced microbial degradation due to previous atrazine use. Apparent amicarbazone and metribuzin persistence levels implied that early-season weed control would be expected, but carryover to injure sensitive rotational crops would not be anticipated under these environmental conditions. Dissipation under field conditions of amicarbazone and metribuzin was not affected by being applied to separate plots or by coapplication to the same plots.

Atrazine applied at planting is commonly used for weed control in corn. With global climate change causing an increase in river flooding in the United States over the past decade, producers need information to determine the best course of action in flooded fields treated with atrazine into which they wish to immediately plant soybean. Studies were designed to understand the effect of flooding on atrazine residual activity including atrazine concentration, soybean injury, and soybean yield. In 2012, soybean yield in flooded treatments was reduced by prior atrazine application. In 2014, soybean injury was <10% in all plots, and nonflooded, atrazine-treated soils had yields equal to the nontreated. Findings from this research indicated that it is possible for producers to consider replanting soybean after atrazine application, with appropriate changes to product labeling.

The triazines are one of the most widely used herbicide classes ever developed and are critical for managing weed populations that have developed herbicide resistance. These herbicides are traditionally valued for their residual weed control in more than 50 crops. Scientific literature suggests that atrazine, and perhaps other s-triazines, may no longer remain persistent in soils due to enhanced microbial degradation. Experiments examined the rate of degradation of atrazine and two other triazine herbicides, simazine and metribuzin, in both atrazine-adapted and non-history Corn Belt soils, with similar soils being used from each state as a comparison of potential triazine degradation. In three soils with no history of atrazine use, the t1/2 of atrazine was at least four times greater than in three soils with a history of atrazine use. Simazine degradation in the same three sets of soils was 2.4 to 15 times more rapid in history soils than non-history soils. Metribuzin in history soils degraded at 0.6, 0.9, and 1.9 times the rate seen in the same three non-history soils. These results indicate enhanced degradation of the symmetrical triazine simazine, but not of the asymmetrical triazine metribuzin.

Laboratory experiments were conducted to determine the fate of 14C-EPTC in a soil that had a history of vernolate application and exhibited accelerated degradation of carbamothioate herbicides compared to nonhistory soil. A very rapid mineralization of the herbicide to 14CO2 was evident in history soil, compared to nonhistory soil. The two soils did not differ in the amounts of the EPTC lost through volatilization or in the nonextractable radioactive fractions. Except for small quantities of EPTC-sulfoxide and sulfone, no other metabolites were detected. Degradation of 14C-EPTC, as determined by evolution of 14CO2 in history soil, was drastically inhibited following soil sterilization by means of autoclaving or gamma irradiation. Soil disinfestation by solarization, methyl bromide, or metham had a pronounced inhibitory effect during the first 6 days, but was less effective than sterilization. Treatment of a history soil with the fungicide 2-methoxyethylmercury chloride and dietholate strongly inhibited EPTC degradation, while thiram and fentin acetate had only short lasting effects. Cycloheximide, an antifungal antibiotic, had little effect on the degradation of EPTC while chloramphenicol, an antibacterial antibiotic, inhibited the herbicide degradation. These results indicate that accelerated degradation of EPTC is linked to the activity of soil microorganisms, e.g. bacteria, and can be controlled by sterilization and chemical treatments.

The degradation of 14C-labeled metolachlor, acetochlor, and pretilachlor in control and soils with 10-yr metolachlor- history or 3-yr butylate history was studied by monitoring the evolution of 14CO2 in soil biometer flasks. The degradation rate of 14C-phenyl-labeled metolachlor was similar in soils with 0- and 10-yr metolachlor history over 52 d. 14C was released from carbonyl-labeled metolachlor about 40% faster than from phenyl-labeled metolachlor in both soils. Soil sterilization by autoclaving reduced significantly the metolachlor degradation rate in both soils. Degradation of 14C-labeled EPTC in soils with 3-yr butylate history was much faster than in soils with no history. Soil sterilization reduced the EPTC degradation rate, confirming microbial degradation. The degradation rates of acetochlor, metolachlor, and pretilachlor were similar in soils with and without butylate history. Most of the 14C that remained in history and nonhistory soils was extractable with ethyl acetate or sodium hydroxide. Three major metabolites of metolachlor and EPTC were detected by thin-layer chromatography (TLC) of extracts from both soils. In contrast to the situation with carbamothioate herbicides, soils exposed repeatedly to metolachlor or other acetanilides are not prone to become adapted to these herbicides. Soils with carbamothioate history did not exhibit any apparent cross-adaptation toward acetanilide herbicides.

Degradation of diphenamid in soil with accelerated degradation and in nontreated control soil and the involvement of soil microorganisms in these processes were investigated. Soil with accelerated degradation and mixed bacterial cultures originating from the same soil degraded diphenamid and its monodemethylated metabolite (diphen M-1) much faster than the control. The bidemethylated derivative (diphen M-2) was degraded much more slowly than diphenamid or diphen M-1. The abundance of fungi capable of degrading diphenamid was similar in the soils with and without accelerated degradation. Degradation of diphenamid by mixed bacterial cultures from a soil with accelerated degradation was much faster than by a culture from nontreated soil and was suppressed by the fungicides thiram and fentin acetate. These two fungicides, as well as the bactericide chloramphenicol, suppressed diphenamid degradation in mixed bacterial cultures. The antifungal cycloheximide and the actinomycete suppressor PCNB did not affect the degradation of diphenamid. Metabolism of 14C-diphenamid in a soil with accelerated degradation and in mixed bacterial cultures originating from the same soil showed similarities with regard to evolution of 14CO2 and production of certain metabolites, while the metabolism of diphenamid by Fusarium, which rapidly degrades diphenamid in cultures, was different with regard to the above parameters. This study suggests that development of accelerated degradation in a soil involves a shift in population and/or in activity of microbial degraders in favor of bacteria. It appears that the accelerated degradation occurred by inducing oxidative reaction which involves demethylation of diphenamid.

Greenhouse and field experiments were conducted to investigate degradation rates and efficacy of EPTC (S-ethyl dipropyl carbamothioate), butylate [S-ethyl bis(2-methylpropyl)carbamothioate], vernolate (S-propyl dipropylcarbamothioate), and cycloate (S-ethyl cyclohexylethylcarbamothioate) in soils with and without prior history of thiocarbamate use. Herbicide persistence was measured by an oat (Avena sativa L.) bioassay, and giant foxtail (Setaria faberi Herrm. # SETFA) control was visually estimated in field studies. Both greenhouse and field studies indicated that enhanced thiocarbamate degradation occurred when the same herbicide (EPTC, butylate, or vernolate) was applied over successive years. Addition of dietholate (O,O-diethyl O-phenyl phosphorothioate) or fonofos (O-ethyl-S-phenylethylphosphonodithioate) increased thiocarbamate persistence and giant foxtail control in soils with prior thiocarbamate exposure. Use of dietholate or fonofos two or three consecutive years with EPTC or butylate again resulted in enhanced degradation. EPTC persistence and giant foxtail control were not affected by prior treatments or tank mixes with carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). Rotation away from successive applications of the same thiocarbamate herbicide (EPTC, butylate, or vernolate) improved herbicide efficacy.

The influence of previous herbicide applications on the degradation rate of butylate [S-ethyl bis (2-methylpropyl)carbamothioate], EPTC (S-ethyl dipropyl carbamothioate), alachlor [2-chloro-N-(2,6-diethyl-phenyl)-N-(methoxymethyl)acetamide], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] was measured. Degradation studies were conducted on soils with zero to eight previous applications of butylate, zero and six consecutive annual applications of alachlor, and zero to seven previous applications of metolachlor. Previous applications of alachlor or of metolachlor did not affect the rate of degradation when the same herbicide was reapplied. Soils with previous butylate-use history had more rapid degradation of butylate or EPTC than soils with no previous butylate applications. Because soil sterilization reduced 14CO2 evolution to a low level, soil microorganisms could be the primary mechanism of butylate degradation. There was no difference between butylate or EPTC degradation rate in soils treated previously with butylate. Butylate could not be detected 28 days after treatment in soils previously treated with butylate, whereas soils not previously treated with butylate still contained biologically active butylate. Dietholate (O,O-diethyl O-phenyl phosphorothioate) added to butylate generally reduced the rate of butylate degradation in soils previously treated with butylate, but the butylate concentration always was lower in soils treated previously with butylate than in soils without previous butylate applications. Weed control in the field showed rapid loss of butylate and EPTC in soils with previous butylate treatment and that dietholate reduced the rate of butylate and EPTC degradation.

Atrazine is the most widely used herbicide in the U.S. and has been detected in surface water and groundwater. Technologies are needed for onsite and in situ remediation of water and soil containing atrazine. We investigated the potential of using fine-grained, zero-valent iron (Fe0) to remove atrazine and promote its degradation in contaminated water and soil. Atrazine loss from aqueous solution increased with increasing Fe0 concentration (w/v). Agitating 20 μg 14C-ring-labeled atrazine L−1 with 10% Fe0 (w/v) removed 92% of the 14C from solution within 48 h. Only about 4% of the 14C lost from solution was extractable from the iron with 3 mM CaCl2 (readily available pool), 81% was extractable with CH3CN (potentially available pool), and 11% was unextractable residues. A companion experiment indicated that most of the 14C extracted from the iron with 3 mM CaCl2 after the 48-h Fe0 treatment was unaltered atrazine, while the CH3CN extract contained approximately 33% atrazine and 48% was unidentified atrazine transformation products. Treating a highly contaminated solution (20 mg atrazine L−1) with 20% Fe0 (w/v) removed 88% of the 14C (added as 14C-ring-labeled atrazine) from solution within 48 h. Deethylatrazine was the main atrazine transformation product detected in solution after treatment, but small amounts of deisopropylatrazine, didealkylatrazine, and hydroxyatrazine were also found. Treating Sharpsburg surface soil containing 1 mg atrazine kg−1 with Fe0 (2%, w/w) increased atrazine mineralization from 4.1 to 11.2% after 120 d. Pyrite (4% FeS2, w/w) also increased atrazine mineralization in surface soil, but was less effective in the presence of NO3 − or SO4 2− (100 mg kg−1 soil). Adding 2% Fe0 (w/w) and 100 mg NO3 − kg−1 to contaminated subsurface soil increased atrazine mineralization from 0.4 to 8.2% within 120 d, and unextractable residues increased from 4.6 to 9.8%. These results indicate iron can sorb atrazine and promote its transformation in water and soil.

Degradation of flurtamone in a Greenville sandy clay loam, a Cecil loam, and a Dothan loamy sand with 0, 1, or 2 yr of previous flurtamone field use was evaluated under controlled conditions. Soil sterilization by autoclaving significantly reduced flurtamone dissipation rate in all soils. Enhanced degradation of flurtamone was observed in a Greenville sandy clay loam after 1 yr of previous flurtamone field use and in a Cecil loam after 2 yr of previous flurtamone field use. No enhancement of flurtamone degradation was observed in a Dothan loamy sand. Flurtamone degradation kinetics in these studies was described as a first-order process. Microbial populations in each soil showed no major changes in total bacterial numbers due to preexposure to flurtamone in the field.

Accelerated degradation of the herbicide diphenamid (N,N-dimethyl-α-phenylbenzeneacetamide) was investigated in Israeli soils. Repeated application of this herbicide in the soil in the laboratory enhanced its degradation, which increased with an increasing number of applications. After the fourth application nearly 100% of the herbicide was degraded within 5 days of incubation, whereas only slight degradation was observed during the first 25 days in a previously untreated soil. Accelerated degradation was also observed in a soil collected from a field with previous diphenamid treatments. Fumigation with methyl bromide or treating the soil with the fungicide fentin acetate (triphenyltin acetate) was effective in decreasing the accelerated degradation of diphenamid, whereas the fungicides TMTD (tetramethylthiuram disulfide) and TBZ [2-(4-thiazolyl)benzimidazole] were only partially effective in inhibiting accelerated degradation. In the laboratory, fentin acetate prevented the degradation of diphenamid applied to previously untreated soil. Several fungi capable of degrading diphenamid were isolated from soils with or without accelerated degradation.

An indirect enzyme-linked immunosorbent assay (ELISA) for EPTC was developed that may be useful for predicting enhanced carbamothioate herbicide degradation in adapted soils. A modified radioimmunoassay (RIA) procedure was used to detect antibodies specific for EPTC and establish antibody dilutions for optimum ELISA sensitivity. Antibodies were obtained from a rabbit injected with a butylate-analog protein conjugate. An indirect ELISA subsequently was developed for EPTC with a detection range of 10 to 1280 ng ml−1 in a 1:3 aqueous dilution of a 90:10 (v/v) acetonitrile (ACN):water soil extract. EPTC recovery from soil was > 96%, as determined by conventional gas chromatography. Measurement of low concentrations of residual EPTC with the ELISA was less quantitative than by gas chromatography, but the immunoassay provided a rapid, potentially cost-effective method for screening soils to determine enhanced carbamothioate degradation.

The dissipation of flurtamone was determined in three Georgia soils in 1987 to 1989. Flurtamone dissipation in soil was initially rapid but gradually slowed throughout the sampling interval. Dissipation was not affected by rate (0.8 or 1.7 kg ai ha-1) or application method (PPI or PRE). Dissipation rates were slightly more rapid in the Greenville and Dothan soils than in the Cecil soil, with calculated initial half-lives for each respective soil being 6 to 7, 8 to 10, and 9 to 23 days. There was no effect of previous flurtamone exposure on the dissipation rate in soil at any location.

Sugarcane growers have observed reduced residual activity of atrazine on organic soils in the Everglades Agricultural Area (EAA) of south Florida. Field studies were conducted between 2011 and 2012 to determine the rate of dissipation of atrazine at 2.24, 4.48, and 8.96 kg ha−1 and metribuzin at 0.56, 1.12, and 2.24 kg ha−1 in the top 10 cm of soil in sugarcane fields in the EAA. The bioavailable fraction of atrazine dissipated more rapidly than the total amount of atrazine in the soil. Half-lives of the total and bioavailable fraction of atrazine ranged between 3.9 to 12.1 d and 1.0 to 7.5 d, respectively. Metribuzin dissipated much more slowly than atrazine on organic soils. Similarly, dissipation of the bioavailable fraction of metribuzin was more rapid than was the dissipation of the total amount of metribuzin in the soil. Half-lives of the total and bioavailable fraction of metribuzin ranged between 16.2 and 24.8 d and 6.0 and 14.3 d, respectively. These results indicate that enhanced atrazine degradation occurs on organic soils under field conditions in the EAA, resulting in shorter residual atrazine activity. This implies that metribuzin is a better option for weed control in sugarcane grown on organic soils of the EAA exhibiting enhanced atrazine degradation.

Simazine is an important management tool for weed control in vineyards because of its relatively low price, reliable control of several problem weeds, and long residual activity. After repeated and extensive use of simazine, several growers in the Central Valley of California expressed concerns about reduced, residual weed control with this herbicide. Experiments were conducted to evaluate the rate of simazine dissipation in soils with differing simazine-use histories and to determine whether residual weed control differed among sites. Two raisin vineyards were used in all studies, one with extensive simazine-use history (adapted) and one with no recent simazine-use history (nonadapted). Results indicated that simazine dissipation from biotic processes was fourfold greater in soil with a long simazine-use history relative to soil with no recent simazine applications. In the field, simazine persisted longer at the nonadapted site, and weed-control duration was affected by dissipation rate. Central Valley vineyard soils that have had repeated simazine applications can develop enhanced, microbial degradation, and reduced, residual weed control is possible; however, weed control is also affected by environmental conditions and other crop management practices.

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.

In order to select efficient and competitive glufosinate-degrading bacteria, two soils which had been treated with glufosinate annually for more than 5 yr were screened. Three strains tolerant to this herbicide were identified by 16S rDNA analysis as Burkholderia sacchari, Serratia marcescens, and Pseudomonas psychrotolerans. In addition, a moderately tolerant strain, P. citronellolis, was isolated from a soil which had received glufosinate treatment for only 6 mo. In culture medium containing high concentration of glufosinate, the former three strains showed significant ability to degrade this glutamine synthetase inhibitor, suggesting that glufosinate-degrading bacteria would be readily found in soils after a long-term induction or selection. A subsequent biodegradation experiment showed that 30 and 50% of glufosinate was degraded 7 and 21 d after treatment (DAT), respectively, in sterilized soils inoculated with the above-mentioned three tolerant strains. While more than 30% of the glufosinate in nonsterilized soils was degraded 7 DAT by the indigenous edaphic microbes, inoculation with the three selected strains enhanced glufosinate degradation to nearly 50%. A study on the competition from edaphic microorganisms in soils by polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) analysis revealed that within 21 d after inoculation (DAI), the propagation of B. sacchari and P. psychrotolerans was not affected, whereas that of the less tolerant P. citronellolis was inhibited. This observation suggests that a long-term herbicide exposure is a promotive factor in generating bacterial strains having high degradation efficiency and showing vigorous propagation under the competition pressure arising from indigenous microbes.