Simultaneous Biodegradation and Detoxification of the Herbicides 2,4-Dichlorophenoxyacetic Acid and 4-Chloro-2-Methylphenoxyacetic Acid in a Continuous Biofilm Reactor

The herbicides 2,4-diclorophenoxiacetic and 4-chloro-2-methylphenoxyacetic acids (2,4-D and MCPA) are widely used in agricultural practices worldwide. Not only are these practices responsible of surface waters contamination, but also agrochemical industries through the discharge of their liquid effluents. In this investigation, the ability of a 2,4-D degrading Delftia sp. strain to degrade the related compound MCPA and a mixture of both herbicides was assessed in batch reactors. The strain was also employed to remove and detoxify both herbicides from a synthetic effluent in a continuous reactor. Batch experiments were conducted in a 2-L aerobic microfermentor, at 28 °C. Continuous experiments were carried out in an aerobic downflow fixed-bed reactor. Bacterial growth was evaluated by the plate count method. Degradation of the compounds was evaluated by UV spectrophotometry, gas chromatography (GC), and chemical oxygen demand (COD). Toxicity was assessed before and after the continuous process by using Lactuca sativa seeds as test organisms. Delftia sp. was able to degrade 100 mg L^?1 of MCPA in 52 h. When the biodegradation assay was carried out with a mixture of 100 mg L^?1 of each herbicide, the process was accomplished in 56 h. In the continuous reactor, the strain showed high efficiency in the simultaneous removal of 100 mg L^?1 of each herbicide. Removals of 99.7, 99.5, and 95.0% were achieved for 2,4-D, MCPA, and COD, respectively. Samples from the influent of the continuous reactor showed high toxicity levels for Lactuca sativa seeds, while toxicity was not detected after the continuous process.     

Laboratory incubation studies of chlorophenoxyacetic acids in chernozemic soils

The relative persistence of 2,4-D, MCPA and 2,4,5-T in some Saskatchewan soils was assessed under laboratory conditions. Under moist conditions, 2,4-D and MCPA showed half-life times of between 14 and 41 days but the MCPA half-life was usually 1 or 2 days longer. 2,4,5-T exhibited a half-life period over twice the length of the other chemicals. The half-life times were directly correlated to microbial plate counts, the larger numbers of soil microorganisms being associated with shorter residence times. Half-lives depended on soil moisture content and the best moisture levels for chemical loss appeared to be just less than field capacity. The use of ¹⁴C in 2,4-D incubation studies showed that the initial cleavage of the 2,4-D molecule was associated with the ether linkage and was not a decarboxylation.     

Simultaneous enumeration of denitrifying and nitrate reducing bacteria in soil by a microtiter most-probable-number (MPN) procedure

A rapid, precise and accurate microtiter most-probable-number (MPN) procedure has been developed and tested for simultaneous (two reagent addition) enumeration of denitrifier and nitrate reducer populations in an agricultural soil. Values from 0.12 × 10⁶ to 0.22 × 10⁶ denitrifiers g^?1 soil were determined using two concentrations of each terminal electron acceptor (3.5 and 7.2 mm NO₂^? or 3.5 and 9.9 mm NO₃^? in nutrient broth. Nitrate reducer populations ranged from 0.97 × 10⁶ to 1.1 × 10⁶ organisms g^?1 soil when two concentrations of NO₃^? (3.5 and 9.9 mm) were employed. The denitrifiers thus constituted 12–20% of the nitrate reducer population. Neither method (tube or microtiter) or medium significantly altered the values obtained for these two groups of microorganisms.     

The millimetre-scale distribution of 2,4-D and its degraders drives the fate of 2,4-D at the soil core scale

The biodegradation of organic compounds in soil is a key process that has major implications for different ecosystem services such as soil fertility, air and water quality, and climate regulation. Due to the complexity of soil, the distributions of organic compounds and microorganisms are heterogeneous on sub-cm scales, and biodegradation is therefore partly controlled by the respective localizations of organic substrates and degraders. If they are not co-localized, transfer processes become crucial for the accessibility and availability of the substrate to degraders. This spatial interaction is still poorly understood, leading to poor predictions of organic compound dynamics in soils. The objectives of this work were to better understand how the mm-scale distribution of a model pesticide, 2,4-dichlorophenoxyacetic acid (2,4-D), and its degraders drives the fate of 2,4-D at the cm soil core scale. We constructed cm-scale soil cores combining sterilized and “natural” soil aggregates in which we controlled the initial distributions of 2,4-D and soil microorganisms with the following spatial distributions: i) a homogeneous distribution of microorganisms and 2,4-D at the core-scale, ii) a co-localized distribution of microorganisms and 2,4-D in a single spot (360 mm³) and iii) a disjoint localization of microorganisms and 2,4-D in 2 soil spots (360 mm³) separated by 2 cm. Two sets of experiments were performed: one used radiolabeled ¹⁴C-2,4-D to study the fate of 2,4-D, and the other used ¹²C-2,4-D to follow the dynamics of degraders. Microcosms were incubated at 20 °C and at field capacity (−31.6 kPa). At the core scale, we followed 2,4-D mineralization over time. On three dates, soil cores with microorganisms and 2,4-D localized in soil spots, were cut out in slices and then in 360 mm³ soil cubes. The individual soil cubes were then independently analysed for extractable and non-extractable ¹⁴C and for degraders (quantitative PCR of tfdA genes). Knowing the initial position of each soil cube allowed us to establish 3D maps of 2,4-D residues and degraders in soil. The results indicated that microorganisms and pesticide localizations in soil are major driving factors of i) pesticide biodegradation, by regulating the accessibility of 2,4-D to degrading microorganisms (by diffusion); and ii) the formation of non-extractable residues (NER). These results also emphasized the dominant role of microorganisms in the formation and localization of biogenic NER at a mm-scale. To conclude, these results demonstrate the importance of considering micro-scale processes to better understand the fate of pesticides and more generally of soil organic substrates at upper scales in soil and suggest that such spatial heterogeneity should not be neglected when predicting the fate of organic compounds in soils.     

Identification of in situ 2,4-dichlorophenoxyacetic acid-degrading soil microorganisms using DNA-stable isotope probing

Stable isotope probing (SIP) was used to investigate the microorganisms responsible for degradation of the herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D) in soil samples. Soils were unamended or amended with either unlabeled 2,4-D or UL(ring) ¹³C-2,4-D. Degradation of 2,4-D was complete after 17 days, whereas little removal (11±3%) was observed in the sterile controls. Terminal restriction fragment length polymorphism (TRFLP) on soil DNA after 17 days indicated a consistent increase in the relative abundance of one fragment (217 bp in Hae III digests) in soils spiked with 2,4-D (both unlabeled and labeled samples) compared to the unamended soils. DNA extracts from labeled and unlabeled 2,4-D amended soils were subject to ultracentrifugation, fractionation of centrifuged samples, followed by TRFLP on each fraction. TRFLP profiles from ultracentrifugation fractions illustrated that the same fragment experienced an increase in buoyant density (BD) in samples spiked with ¹³C-labeled 2,4-D. This increase in DNA BD indicates the organisms represented by this fragment were responsible for uptake and degradation of the herbicide. 16S rRNA sequencing of the heavy, ¹³C-enriched fraction suggests the organisms belong to the β subdivision of Proteobacteria. Herein, SIP facilitated the identification of unique organisms degrading 2,4-D in soil without the need for isolation and provided more direct evidence for a functional role of these organisms than would have been possible with the molecular-based methods alone.     

Denitrification in soil: Effects of herbicides

The effects of 20 herbicides on denitrification of nitrate in three soils were studied by determining the effects of 10 and 50μgg^?1 soil of each herbicide on the amounts of nitrate lost and the amounts of nitrite, N₂O and N₂ produced when soil samples were incubated anaerobically after treatment with nitrate. The herbicides used were butylate, EPTC, chlorpropham, propham, diuron, linuron, monuron, siduron, alachlor, trifluralin, 2,4-D amine, 2,4-D ester, atrazine, cyanazine, metribuzin, simazine, dalapon, chloramben, dicamba and dinoseb.None of the herbicides studied significantly affected denitrification of nitrate when applied at the rate of 10 μg g^?1 soil, but dinoseb increased the ratio of N₂ to N₂O in the gaseous products of denitrification when applied at this rate. Butylate, EPTC, diuron, simazine and dalapon had no significant effect on denitrification when applied at the rate of 50μgg^?1 soil, whereas metribuzin and dinoseb enhanced denitrification when applied at this rate. The influence of the other herbicides on denitrification when applied at the rate of 50μgg^?1soil depended on the soil, but all enhanced or inhibited denitrification in at least one soil.     

Determination of Acidic Pesticides in the Drinking Water of Greece Using Capillary Gas Chromatography-Mass Spectrometry

There is no data currently available on acidic pesticides in the drinking water of Greece, although considerable quantities of them are in use. In this study, the occurrence of the six most important acidic herbicides in the drinking water of Greece was investigated. The target compounds studied include four chlorophenoxy herbicides, namely mecoprop, dichlorprop, MCPA and 2,4-D, and two other acidic herbicides, i.e. bromoxynil and bentazone. Analysis was carried out at a concentration level of 100 ng L^?1 using capillary gas chromatography-mass spectrometry (GC-MS) with selected ion monitoring (SIM). The method involved a pre-concentration with solid phase extraction and derivatization with pentafluorobenzyl bromide. Thirty-eight samples of drinking water from nine regions in Greece were screened. No herbicides were detected although fortification experiments with parallel water samples resulted in recovery rates better than 70%. The detection limits of the recovered compounds were found to be between 10 and 50 ng L^?1.     

Substrate specificity of chlorophenoxyalkanoic acid-degrading bacteria is not dependent upon phylogenetically related tfdA gene types

The phenoxyalkanoic acid herbicides constitute a group of chemically related molecules that have been widely used for over 50 years. A range of bacteria have been selected from various locations for their ability to degrade these compounds. Previously reported strains able to utilise 2,4-dichlorophenoxyacetic acid (2,4-D) include, Ralstonia eutropha JMP134, Burkholderia sp. RASC and Variovorax paradoxus TV1 and Sphingomonas sp. AW5 able to utilise 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). In addition a novel set of mecoprop-degrading strains including Alcaligenes denitrificans, Alcaligenes sp. CS1 and Ralstonia sp. CS2 are here described. It has been reported recently that TfdA enzymes, initially reported to have a role in 2,4-D catabolism are also involved in the first-step cleavage of related phenoxyalkanoate herbicides. However, a diversity of tfdA gene sequences have been reported. We relate the tfdA gene type to the metabolic ability of these strains. The tfdA-like genes were investigated by polymerase chain reaction amplification using a set of specific tfdA primers. Degradation ability was observed via phenol production from a range of unsubstituted and substituted phenoxyalkanoics including, 2,4-D, 2-methyl 4-chlorophenoxyacetic acid (MCPA), racemic mecoprop, (R)-mecoprop, 2-(2,4-dichlorophenoxy) propionic acid (racemic 2,4-DP), 2,4,5-T, 2,4-dichlorophenoxybutyric acid (2,4-DB), 4-chloro-2-methylphenoxybutyric acid (MCPB) and phenoxyacetate. Mecoprop-degrading strains showed partial tfdA sequences identical to the one described for V. paradoxus TV1 (a strain isolated on 2,4-D). However, substrate specificity was not identical as V. paradoxus exhibited greatest activity towards 2,4-D and MCPA only, whereas the mecoprop-degrading strains showed intense activity towards 2,4-D, MCPA, racemic mecoprop and (R)-mecoprop as substrates. However, Sphingomonas sp. AW5 which has been shown to carry a very different tfdA-like gene was the only strain to utilise the phenoxybutyric acid MCPB as a sole carbon source. In this study, we thus demonstrate that sequence diversity is not related to substrate specificity within the tfdA-like gene family. However, phylogenetically unrelated sequences may govern substrate specific activity.     

Layered double hydroxides as supports for the slow release of acid herbicides

A Mg/Al layered double hydroxide (LDH) was intercalated with the anionic herbicides 2,4-D, MCPA, and picloram by using three different methodologies: (i) direct synthesis (DS), (ii) regeneration (RE), and (iii) ion exchange (IE). The resulting complexes were characterized and assayed by batch release and column leaching tests, aiming at the controlled release of these herbicides. All the tested LDH-herbicide complexes displayed similar slow herbicide release properties in water, although the IE method seemed to result in complexes with a greater fraction of herbicide in a readily available form. Apparently, the LDH-herbicide complexes released most of the active ingredient present in the complexes at the end of the batch release experiment. This was attributed to the replacement of the intercalated herbicide by carbonate and hydroxyl anions from the aqueous solution. Compared to the free herbicides, the application of the three LDH-herbicide complexes (RE) to soil columns resulted in reduction in the maximum herbicide concentration in leachates and led to the retardation of herbicide leaching through the soil. All LDH-herbicide complexes presented an herbicidal efficacy similar to that of the free (technical) herbicides. Our results indicated the potential applicability of LDHs as supports for the preparation of slow release formulations of acid herbicides such as 2,4-D, MCPA, or picloram.     

Effects of 2,4-d on nitrogen fixation and carbon dioxide evolution in a soil microcosm

Two concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) 1.7 kg ha^?1 and 3.4 kg ha^?1 were applied to oats (Avena sativa L. ‘Orbit’) grown in terrestrial microcosms in a sandy soil. Carbon dioxide evolution and non-symbiotic N₂ fixation (C₂H₂ reduction) were measured weekly. On day 70 of the study, 2,4-D was applied a second time at the same application rates and soil CO₂ evolution and N₂ fixation were measured more frequently. Soil CO₂ evolution over 24 h period was significantly decreased by 40 to 50% at both application rates of 2,4-D. This response lasted less than 7 days. Nitrogen fixation was unaffected by 2,4-D except for an unexplained decrease observed in the 1.7 kg ha^?1 treatment 35 days after 2,4-D application. This effect was not observed on the following sampling date. The second application of 2,4-D failed to produce any significant change in soil CO₂ evolution or N₂ fixation. From these studies we conclude soil microbial populations either degraded or became acclimated to 2,4-D as a result of the initial application and that 2,4-D has no significant adverse effect on N₂ fixation or soil CO₂ evolution.     

Mineralization of carbon atoms of 14C-2,4-D side chain and degradation ability of bacteria in soil

The time-course of ¹⁴CO₂ formation in chernozem soil samples enriched with 1- or 2-¹⁴C-2, 4-dichlorophenoxyacetic acid (50 μg g g^?1 air-dry soil) was determined during incubation at 28°C. Except for the initial phase of decomposition, when the conversion of carboxyl carbon to ¹⁴CO₂ predominated over that of carbon in position 2, the rates of mineralization of the two carbon atoms of the side chain of the herbicide molecule exhibited no significant difference. The exponential phase of ¹⁴CO₂ evolution lasted from the 3rd to the 21st day of incubation; a semilogarithmic plot of its time dependence was strictly linear. The mineralization activity doubling time in this phase was 89.1 ± 3.6 h with 1-¹⁴CO-2, 4-D and 85.4 ± 5. l h with 2-¹⁴CO-2,4-D. An exponential decrease in mineralization activity was observed after 21 days, probably due to substrate exhaustion. The total proportion of radioactive carbon introduced into the soil in the form of 1- or 2-¹⁴CO-2,4-D and converted into ¹⁴CO₂ during 31 days of incubation was about 33%. Plate counts of bacteria increased during 35 days of incubation from 2.14 × 10⁸ to 2.8 × 10⁸ g^?1. The proportion of bacteria capable of producing ¹⁴CO₂ from the labelled herbicide increased in this period from 4.1 to 86.1%. This increase is probably directly responsible for the immediate onset of mineralization of the herbicide in soil treated previously with it or in soil inoculated with a suspension prepared from a sample previously incubated with the herbicide.     

Leaching of 2,4-D and dicamba from home lawns

Leaching of the broadleaf herbicides 2,4-D and dicamba from home lawns was monitored with ceramic extraction plates placed at a 0.2 m depth beneath undisturbed sod. The site was located on a Merrimac sandy loam. Four treatments, consisting of two rates of herbicide applications coupled with two irrigation regimes. were evaluated on 12 plots. The low herbicide rate consisted of 1.1 and 0.1 kg ha^?1 yr^?1 of 2,4-D and dicamba, respectively. The high rate used was 3.3 and 0.33 kg ha^?1 yr^?1 of 2,4-D and dicamba applied in three equal applications. Irrigation treatments were (1) minimal irrigation to avoid drought stress and percolation from the root zone and (2) overwatering at 37.5 mm week^?1. Geometric mean concentra tions of 2,4-D ranged from 0.55 to 0.87 μg L^?1 compared to 0.26 to 0.55 μg L^?1 for dicamba. The low application-minimum irrigation treatment generated significantly higher concentrations than the other treatments for both herbicides. The low concentrations observed for both herbicides suggest that excellent degradation conditions exist in the root zone of turfgrass during the summer months when application occurs.     

Soil microbial communities response to herbicide 2,4-dichlorophenoxyacetic acid butyl ester

2,4-Dichlorophenoxyacetic acid butyl ester (2,4-D butyl ester) is extensively applied for weed control in cultivation fields in China, but its effect on soil microbial community remains obscure. This study investigated the microbial response to 2,4-D butyl ester application at different concentrations (CK, 10, 100 and 1000 μg g^?1) in the soils with two fertility levels, using soil dilution plate method and phospholipid fatty acid (PLFA) analysis. Culturable microorganisms were affected by the herbicide in both soils, particularly at the higher concentration. After treating soil with 100 μg g^?1 herbicide, culturable bacteria and actinomycetes were significantly higher, compared to other treatments. Treatment of soil with 1000 μg g^?1 2,4-D butyl ester caused a decline in culturable microbial counts, with the exception of fungal numbers, which increased over the incubation time. PLFA profiles showed that fatty acids for Gram-negative (GN) bacteria, Gram-positive (GP) bacteria, total bacteria and total fungi, as well as total PLFAs, varied with herbicide concentration for both soil samples. As herbicide concentration increased, the GN/GP ratio decreased dramatically in the two soils. The higher stress level was in the treatments with high concentrations of herbicide (1000 μg g^?1) for both soils. Principal component analysis of PLFAs showed that the addition of 2,4-D butyl ester significantly shifted the microbial community structure in the two soils. These results showed that the herbicide 2,4-D butyl ester might have substantial effects on microbial population and microbial community structure in agricultural soils. In particular, the effects of 2,4-D butyl ester were greater in soil with low organic matter and fertility level than in soil with high organic matter and fertility level.     

Microbial and non-biological decomposition of chlorophenols and phenol in soil

The aerobic and anaerobic degradation of phenol and selected chlorophenols was examined in a clay loam soil containing no added nutrients. A simple, efficient procedure based on the high solubility of these compounds in 95% ethanol was developed for extracting phenol and chlorophenol residues from soil. Analysis of soil extracts with UV spectrophotometry showed that phenol,o-chlorophenol,p-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol and 2,4,6-trichlorophenol were rapidly degraded, whilem-chlorophenol, 3,4-dichlorophenol, 2,4,5-trichlorophenol and pentachlorophenol were degraded very slowly by microorganisms in aerobically-incubated soil at 23°C. Both 3,4,5-trichlorophenol and 2,3,4,5-tetra chlorophenol appeared to be more resistant to degradation by aerobic soil microorganisms at 23°C. None of the compounds examined were degraded by microorganisms in anaerobically-incubated soil at 23°C. Direct microscopic observation revealed that phenol and selected chlorophenols stimulated aerobic and to a lesser extent, anaerobic microbial growth in soil, and aerobic soil bacteria were responsible for the degradation of 2,4-dichlorophenol in aerobically-incubated soil at 23°C. Phenol,o-chlorophenol,m-chlorophenol,p-chlorophenol and 2,4-dichlorophenol underwent rapid non-biological degradation in sterile silica sand. Non-biological decomposition contributed, perhaps substantially, to the removal of some chlorophenols from sterile aerobically-incubated soil and both sterile and non-sterile anaerobically-incubated soil.     

Detection of soluble Rhizobium japonicum antigens in soil by immunodiffusion

Innovative techniques for quantifying rhizobia in soil are needed. Immunodiffusion was adapted for use in evaluating four strains of Rhizobium japonicum incubated in soil. Reproducible precipitin bands were obtained using uniform soil cores 5 mm dia × 4 mm length collected with a cork borer. A flocculent of Ca(OH)₂ and MgCO₃ added to the microcores enhanced band formation. Clear distinct bands developed from populations in soil of 1 × 10⁸ cells g^?1 or greater. The complete set of bands which characterized cells cultured on artificial media did not develop from cells inoculated into soil, but at least one detectable band was always observed. Parameters of intensity value and band positioning were related to the population in the soil. Detection by immunodiffusion of a series of rhizobial populations in soil decreased with time almost linearly as the initial population decreased. Direct diffusion of antigens from soil and the development of specific precipitin bands provides a new approach to studying soil microorganisms.     

Use of sawdust for soil ph amendments

Abstract

Changes in pH values during 12 weeks incubation in soils treated with acidified sawdust (ACD‐SD)‐treated soils ranged from 5.03 to 5.89, from 9.88 to 10.35 in soil treated with alkalized sawdust (ALK‐SD), and ranged from pH 6.88 to 7.35 in untreated sawdust‐amended soil. In unamended soil, pH values were 6.80 to 7.35. Bacterial populations over the 12 weeks in ACD‐SD‐treated soils increased from 5×10⁶ to 167×10⁶ colonies while bacterial populations in ALK‐SD‐treated soils increased from 2×10⁶ to 54×10⁶. Fungal populations increased from 6×10⁴ to 11,333×10⁴ colonies per gram soil in ACD‐SD treated soils over the 12 week incubation. Untreated sawdust and control soil did not result in any significant changes in the fungal populations.     

Adsorption and degradation of 2,4-dichlorophenoxyacetic acid in spiked soil with Fe0 nanoparticles supported by biochar

This work studies the adsorption and degradation of 2,4-dichlorophenoxyacetic (2,4-D) in spiked soil with nanoscale Fe⁰ particles (nFe⁰) and biochar derived from maize straw. When biochar concentration was high, the adsorption capacity of soil was enhanced. Furthermore, 2,4-D degraded completely at loading rates of 0.33 and 0.17 g/L nFe⁰ plus biochar (initial 2,4-D concentration of 10 mg/g) within 40 h, according to equilibrium data. Additionally, the theoretical concentration of chloridion was approximately 84%. Further analysis indicated that the effect of nFe⁰ on 2,4-D degradation was weaker in soil columns than that in soil slurry. By contrast, 2,4-D degradation is positively influenced by biochar application, which prevented the aggregation and corrosion of Fe nanoparticles. Although the enhanced capacity for 2,4-D adsorption on the soil decelerated dechlorination rate, long-term nFe⁰ activity was generated. After 72 h, the efficiency of 2,4-D degradation was approximately 53.2% in the soil columns with biochar support.     

Degradation of [carboxyl-14C] 2,4-D and [ring-U-14C] 2,4-D in 114 agricultural soils as affected by soil organic carbon content

This study compared the degradation of [carboxyl-¹⁴C] 2,4-dichlorophenoxyacetic acid (2,4-D) (C2,4-D) and [ring-U-¹⁴C] 2,4-D (R2,4-D) in 114 agricultural soils (0–15 cm) as affected by 2,4-D sorption and soil properties (organic carbon content, pH, clay content, carbonate content, cation exchange capacity, total microbial activity). The sample area was confined to Alberta, Canada, located 49–60° north longitude and 110–120° west latitude and soils were grouped by soil organic carbon content (SOC) (0–0.99%, 1–1.99%, 2–2.99%, 3–3.99% and >4% SOC). Degradation rates of C2,4-D and R2,4-D followed first-order kinetics in all soils. Although total microbial activity increased with increasing SOC, degradation rates and total degradation of C2,4-D and R2,4-D decreased with increasing SOC because of increased sorption of 2,4-D by soil and reduced bioavailability of 2,4-D and its metabolites. Rates of R2,4-D degradation were more limited by sorption than rates of C2,4-D degradation, possibly because of greater sorption and formation of bound residues of 2,4-D metabolites relative to the 2,4-D parent molecule. Based on the sorption and degradation parameters quantified, there were two distinct groups of soils, those with less than 1% SOC and those with greater than 1% SOC. Specifically, soils with less than 1% SOC had, on average, 2.4 times smaller soil organic carbon sorption coefficients and 1.4 times smaller 2,4-D half-lives than soils with more than 1% SOC. In regional scale model simulations of pesticide leaching to groundwater, covering many soils, input parameters for each pesticide include a single soil organic carbon sorption coefficient and single half-life value. Our results imply, however, that the approach to these regional scale assessments could be improved by adjusting the values of these two input parameters according to SOC. Specifically, this study indicates that for 2,4-D and Alberta soils containing less than 1% SOC, the 2,4-D pesticide parameters obtained from generic databases should be divided by 2.5 (soil organic carbon sorption coefficient) and 1.5 (half-life value).     

Mineralization of the herbicide MCPA in urban soils is linked to presence and growth of class III tfdA genes

Mineralization and sorption of ¹⁴C-ring labeled herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) were quantified along with the tfdA gene abundance in 7 different soils. The soils tested were five gravel soils from urban locations, one soil from the embankment of a railway track, and finally an agricultural soil as a control. The mineralization experiments were performed with a concentration of MCPA of 5 mg/kg and incubated at 10 °C for a period of 60 days.With K_d values ranging from 0.04 to 0.41 l kg⁻¹ the sorption experiments revealed that binding of MCPA to the six gravel soils was lower compared to the control soil which had a K_d value of 0.91 l kg⁻¹. The potential for MCPA mineralization varied from less than 5 to over 55% mineralized in 60 days. The most rapid MCPA mineralization was observed in the soil from the Danish railway tracks with 55% mineralized after only 18 days. The mineralization data was fitted to degradation kinetic models, which indicated that growth occurred as a response to MCPA degradation in most of the soils.Soil DNA was extracted and tfdA genes responsible for the first step in MCPA degradation were quantified by real-time PCR (qPCR) at appropriate time points throughout the mineralization experiments. Indicating growth of specific MCPA degraders, the abundance of class III tfdA genes showed an increase during MCPA mineralization in those soils able to mineralize MCPA.These findings emphasize the importance of the presence of microorganisms that are able to readily degrade MCPA, to avoid groundwater leaching following use on urban gravel areas that possess low binding ability of the compound.     

Immediate Effects of Time and Method of Alfalfa Termination on Soil Mineral Nitrogen,Moisture, Weed Control,and Seed Yield,Quality, and Nitrogen Uptake

Three field experiments were conducted on Gray Luvisol (Typic Cryoboralf) soils in northeastern Saskatchewan to compare the effects of alfalfa (Medicago sativa Leyss) stand termination with tillage and herbicides at different times on mineral nitrogen (N) (ammonium-N and nitrate-N) and moisture content of soil in spring (experiments 1 and 2), soil moisture, volunteer alfalfa and dandelion control, plant density, seed yield, protein concentration and N uptake for wheat (Triticum aestivum L.), barley (Hordeum vulgare L), canola (Brassica rapa L.), and pea (Pisum sativum L.) crops (experiment 3). Termination treatments included combinations of times (in mid-June after cut 1, in mid-August after cut 2 and in mid-May during spring) and methods [tillage alone, herbicides alone (glyphosate + 2,4-D amine and also clopyralid + 2,4-D ester in experiment 3) and these herbicides + tillage]. Tillage alone significantly increased spring soil nitrate-N levels over herbicides alone or herbicides + tillage. Termination after cut 1 had the highest levels of soil nitrate-N. There was little effect of time and method of termination on soil ammonium-N and moisture content in spring. Herbicides + tillage generally provided better control of both volunteer alfalfa and dandelion in the four crops than tillage or herbicides alone. In general, alfalfa termination with herbicides alone significantly reduced plant density, seed yield, and N uptake of all crops and protein concentration of cereals only due to effects on levels of soil nitrate-N, dandelion control, and crop injury by clopyralid or 2,4-D residues in soil. Plant density, seed yield, N uptake and protein concentration of crops tended to decline with delay in termination time. The results of this study support the use of some tillage in alfalfa stand termination in helping to control volunteer alfalfa and dandelion and optimize annual crop yields and quality.