Effects of sorption on biological degradation rates of (2,4-dichlorophenoxy) acetic acid in soils

2-Methyl-4-chlorophenoxyacetic acid (MCPA) sorption and desorption as a function of biochar properties and pyrolysis temperature

2-Methyl-4-chlorophenoxyacetic acid (MCPA) is a highly mobile herbicide that is frequently detected in global potable water sources. One potential mitigation strategy is the sorption on biochar to limit harm to unidentified targets. However, irreversible sorption could restrict bioefficacy thereby compromising its usefulness as a vital crop herbicide. This research evaluated the effect of pyrolysis temperatures (350, 500 and 800°C) on three feedstocks; poultry manure, rice hulls and wood pellets, particularly to examine effects on the magnitude and reversibility of MCPA sorption. Sorption increased with pyrolysis temperature from 350 to 800°C. Sorption and desorption coefficients were strongly corelated with each other (R2 = 0.99; P < .05). Poultry manure and rice hulls pyrolyzed at 800°C exhibited irreversible sorption while for wood pellets at 800°C desorption was concentration dependent. At higher concentrations some desorption was observed (36% at 50 ppm) but was reduced at lower concentrations (1-3% at < 5 ppm). Desorption decreased with increasing pyrolysis temperature. Sorption data were analyzed with Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherm models. Freundlich isotherms were better predictors of MCPA sorption (R2 ranging from 0.78 to 0.99). Poultry manure and rice hulls when pyrolyzed at higher temperatures (500 and 800°C) could be used for remediation efforts (such as spills or water filtration), due to the lack of desorption observed. On the other hand, un-pyrolyzed feedstocks or biochars created at 350°C could perform superior for direct field applications to limit indirect losses including runoff and leaching, since these materials also possess the ability to release MCPA subsequently to potentially allow herbicidal action.

Empirical model to assess leaching of pesticides in soil under a steady-state flow and tropical conditions

Abstract

An empirical model of leaching of pesticides was developed to simulate the concentration of fungicides throughout unsaturated soil. The model was based on chemical reactions and the travel time of a conservative tracer to represent the travel time required for water to flow between soil layers. The model's performance was then tested using experimental data from dimethomorph and pyrimethanil applied to the soil under field and laboratory conditions. The empirical model simulated fungicide concentration on soil solids and in soil solution at different depths over time (mean square error between 2.9 mg2 kg-2 and 61mg2 kg-2) using sorption percentages and degradation rates under laboratory conditions. The sorption process was affected by the organic carbon, clay, and the effective cation exchange capacity of the soil. The degradation rate values of dimethomorph (0.039 d-1-0.009 d-1) and pyrimethanil (0.053 d-1-0.004 d-1) decreased from 0 to 40 cm and then remained constant in deeper soil layers (60-80 cm). Fungicide degradation was a critical input in the model at subsurface layers. The model was determined to be a reliable mathematical tool to estimate the leachability of pesticides in tropical soil under a steady-state flow. It may be extended to other substances and soils for environmental risk assessment projects.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13762-023-05038-w.

Effects of soil factors on dimethyl disulfide desorption and the risk of phytotoxicity to newly-planted seedlings

Dimethyl disulfide (DMDS) is a relatively new soil fumigant used in agro-industrial crop production to control soil-borne pests that damage crops and reduce yield. The emissions of DMDS after fumigation reduce soil concentrations thus reducing the risk of phytotoxicity to newly planted crops. However, the factors affecting the desorption of DMDS from soil are unclear. In our study, the desorption characteristics of DMDS from soil were measured in response to continuous ventilation. The degradation of DMDS in soil was examined by thermal incubation. The phytotoxic response of newly-planted cucumber (Cucumis sativus) seedlings to DMDS residues was measured by a sand culture experiment. The results showed DMDS desorption and degradation rates fit a first-order model; that 92% of the DMDS desorption occurred in the first hour after fumigant application; and that residue concentrations in the soil at the end of the ventilation period were unlikely to be phytotoxic to newly-planted cucumber seedlings. By the third day of ventilation, the average desorption rate (ADR) of DMDS in Wenshan soil was 4.0 and 3.6 times, respectively, faster than that in Shunyi and Suihua soils and the ADR of DMDS in soil decreased by 40.0% when the soil moisture content increased from 3% to 12% (wt/wt). Moreover, within one hour of ventilation, the ADR of DMDS in soil decreased by 20.1% when the soil bulk density increased from 1.1 to 1.3 g cm-3. The degradation of DMDS in soil, however, was mostly influenced by soil type and moisture content. A slow degradation rate resulted in a high initial desorption concentration of DMDS in soil. Our results indicated that DMDS desorption from soil in response to continuous ventilation was affected by the soil type, moisture content and bulk density. Rapid degradation of DMDS in soil will lower the risk of phytotoxic residues remaining in the soil and reduce emissions during the waiting period. Acceleration of emissions early in the waiting period by managing soil moisture content or increasing soil porosity may shorten the duration of emissions. Alternatively, soil extraction technology could be developed to recover and reduce fumigant emissions.

Synthesizing Laboratory and Field Experiments to Quantify Dominant Transformation Mechanisms of 2,4-Dichlorophenoxyacetic Acid (2,4-D) in Aquatic Environments

Laboratory studies used to assess the environmental fate of organic chemicals such as pesticides fail to replicate environmental conditions, resulting in large errors in predicted transformation rates. We combine laboratory and field data to identify the dominant loss processes of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in lakes for the first time. Microbial and photochemical degradation are individually assessed using laboratory-based microcosms and irradiation studies, respectively. Field campaigns are conducted in six lakes to quantify 2,4-D loss following large-scale herbicide treatments. Irradiation studies show that 2,4-D undergoes direct photodegradation, but modeling efforts demonstrated that this process is negligible under environmental conditions. Microcosms constructed using field inocula show that sediment microbial communities are responsible for degradation of 2,4-D in lakes. Attempts to quantify transformation products are unsuccessful in both laboratory and field studies, suggesting that their persistence is not a major concern. The synthesis of laboratory and field experiments is used to demonstrate best practices in designing laboratory persistence studies and in using those results to mechanistically predict contaminant fate in complex aquatic environments.

Removal of stormwater dissolved organic nitrogen through biotransformation using activated carbon

Conventional bioretention systems are not effectively designed to remove stormwater dissolved organic nitrogen (DON). Biotransformation study on five organic nitrogenous compounds with different values for adsorption on coal activated carbon (AC) and bioavailability revealed that adsorption is a greater controlling factor for ammonification than bioavailability. This study also showed three apparent benefits: enhancement of the ammonification rate, ammonification of the bio-recalcitrant organic nitrogenous compounds, for example, pyrrole, and bio-regeneration of the adsorbent (coal AC). Low temperature (4°C) did not impact ammonification of leucine at a velocity of 34 cm/h, but negatively affected it at 61 cm/h. It was also observed that bed media height > 30 cm would not appreciably increase ammonification. Under intermittent wetting/draining conditions, the DON removal efficiency was more than 90%, indicating that DON was successfully removed through concurrent adsorption/ammonification, although generated ammonium in the effluent must be properly addressed. PRACTITIONER POINTS: Coal activated carbon appears a better material for DON ammonification compared with charcoal and quartz sand. A temperature as low as 4°C may not adversely impact DON ammonification at a velocity of 34 cm/h or less. A bed media depth of 30 cm is considered as adequate to promote DON ammonification. A larger depth may not be expected to improve ammonification. Ammonification of the bio-recalcitrant organic nitrogenous compounds, for example, pyrrole, and bio-regeneration of the adsorbent, for example, coal activated carbon, may be achieved.

CO2 reduction to acetic acid on the greigite Fe3S4{111} surface

Acetic acid (CH3-COOH) is an important commodity chemical widely used in a myriad of industrial processes, whose production still largely depends on homogeneous catalysts based on expensive rare metals. Here, we report a computational study on the formation of CH3-COOH from carbon dioxide (CO2) as an alternative chemical feedstock on the {111} surface of the low-cost greigite Fe3S4 catalyst. We have used density functional theory calculations with a Hubbard Hamiltonian approach and long-range dispersion corrections (DFT+U-D2) to simulate the various stages of the direct combination of C1 species of different composition to produce glyoxylic acid (CHO-COOH) as a key intermediate in the formation of CH3-COOH. Three reaction mechanisms are considered: (i) the main pathway where the direct formation of the C-C bond takes place spontaneously, followed by a step-wise reduction of CHO-CHOO to CH3-COOH; and the competitive pathways for the non-promoted and H-promoted elimination of hydroxy groups (OH) and water (H2O), respectively from (ii) the carboxyl; and (iii) the carbonyl end of the glyoxylate intermediates. The thermodynamic and kinetic profiles show that the energies for the intermediates on the main pathway are very similar for the two catalytic sites considered, although the activation energies are somewhat larger for the exposed tetrahedral iron (FeA) ion. In most cases, the intermediates for the deoxygenation of the carboxylic acid are less stable than the intermediates on the main pathway, which suggests that the molecule prefers to lose the carbonylic oxygen. The suitable surface properties of the Fe3S4{111} surface show that this material could be a promising sustainable catalyst in future technologies for the conversion of CO2 into organic acid molecules of commercial interest.

The degradation dynamics and rapid detection of thiacloprid and its degradation products in water and soil by UHPLC-QTOF-MS

Thiacloprid is a neonicotinoid insecticide used to control sucking and chewing insects of fruits and vegetables. Hydrolysis, photolysis of thiacloprid in aqueous solutions, and soil degradation of three typical types of soil in China were studied. UHPLC-QTOF/MS was used to acquire high-resolution mass spectrometry information of thiacloprid's degradation products in water and soil samples, and the UNIFI platform with integrated data processing function was used to find and identify degradation products. The degradation kinetics of thiacloprid was determined. Six transformation products (M271, M287, M269, M295, M279, M267) were found after the data processing workflow in the UNIFI platform by using the raw MS^E data. The structure of putative transformation products can be inferred based on the accurate mass of fragment ions and the automated spectral interpretation tools in the UNIFI platform. The structure of M271 was validated to be thiacloprid amide by comparing the ESI-MS² fragment ions in soil samples and thiacloprid amide standard. The TrendPlot function of UNIFI was used to demonstrate the kinetics of the transformation products. Reduction, hydrolysis, oxidation are the main reactions of thiacloprid in three tested soil in China and buffer solutions. This study provided a reference for the rapid identification of the transformation products of other pesticides in specific environmental conditions.

Impact of dichlorprop on soil microbial community structure and diversity during its enantioselective biodegradation in agricultural soils

Enantioselective biodegradation of racemic dichlorprop in two soils was investigated in the laboratory. Chiral separation of racemic dichlorprop was achieved by using HPLC with Phenomenex Lux Amylose-2. The first-order kinetic model fitted well the dissipation data of racemic dichlorprop and its pure R- and S-enantiomers. S-dichlorprop was preferentially degraded in both soils and enantioselectivity was affected by soil pH. The half-lives (DT₅₀) of S-dichlorprop were 8.22 days in soil A and 8.06 days in soil D, while R-dichlorprop was more persistent with DT₅₀ of 12.93 days in soil A and 12.38 days in soil D, respectively. Dichlorprop dissipated faster in soil D with lower organic matter content. In sterilized soils, neglected dissipation was observed and enantiomer fraction values remained constant, indicating that the enantioselective degradation was mainly controlled by soil microorganisms. Soil microbial community structure and diversity was assessed by Illumina MiSeq sequencing of 16S rRNA genes from dichlorprop and no dichlorprop contaminated microcosms. Compared with controls, dichlorprop application had no significant effect on microbial community structures at phylum level, but increased bacterial diversity and dichlorprop degradation related taxa in both soils. S-dichlorprop preferential degradation might be attributed to the S-enantiomer preferred degraders in the family of Sphingomonadaceae.

Soil properties, presence of microorganisms, application dose, soil moisture and temperature influence the degradation rate of Allyl isothiocyanate in soil

Allyl isothiocyanate (AITC) is a soil fumigant derived from plants that can effectively control soil-borne diseases. Fully understanding the impact of various factors on its degradation can contribute to its effectiveness against pests and diseases. First, orthogonal design determined the extraction method of AITC in soil, that is using ethyl acetate as the extraction reagent, vortexing for 1 min as the extraction method and holding for 30 min as the method time. Then we studied the effects of soil texture and environmental factors on the rate and extent of AITC degradation in soil. The half-lives of nine origins soils varied from 12.2 to 71.8 h that were affected by the soil's electrical conductivity, available nitrogen, pH and organic matter content. Biotic degradation of AITC contributed significantly (68%-90%) of the total AITC degradation in six soil types. The degradation rate of AITC decreased as the initial dose of AITC increased. The degradation rate of AITC in Suihua soil generally increased with increasing temperature and soil moisture. The effect of temperature on AITC degradation was more pronounced when the soil was moist, which has practical implications for the control of soil pests and diseases. In agricultural soil, the soil's characteristics and environmental factors should be considered when determining the appropriate AITC dose suitable for soil borne disease while at the same time minimizing emissions and impact on the environment.

Factors affecting coupled degradation and time-dependent sorption processes of tebuconazole in mineral soil profiles

This laboratory degradation and adsorption study aimed to determine the tebuconazole degradation parameters for 6 profiles of Polish mineral soils and to find links between the tebuconazole degradation rate, its adsorption, soil microbial activity and other significant soil properties. The values of the adsorption distribution coefficient K_d, obtained in batch experiments after 96 h of shaking were in the range of 6.2-34.6 mL g^-1. In both batch experiments and incubation experiments at 20 °C, the typical course of adsorption processes was observed, an initial rapid stage followed by a slow stage. In 3 of the 18 soils examined, adsorption was not reached within 51 days. The range of the half-life values was 201-433 days for the Ap horizon and up to 3904 days for subsoils, which were estimated using the two-site nonequilibrium adsorption model coupled with first-order degradation for dissolved and adsorbed pesticide. It was found that modeling the degradation of tebuconazole on the basis of the coefficients of microbial biomass activity for topsoil and two subsoils explained almost 96% of the variance of the estimated pore water degradation rate coefficients in examined soils. The degradation rate was also negatively correlated with the amount adsorbed in the time dependent adsorption sites. This fraction was the least available for soil microorganisms because it was strongly adsorbed in soil pores with a radius <2.5 nm, determined from the H₂O desorption isotherm. The degradation rate was also affected by the ratio of the water content in soil during degradation experiments to the water content at field capacity. The results indicated that degradation occurred in the soil liquid phase only.

Parameters affecting azoxystrobin and imidacloprid degradation in biobed substrates in the North Indian tropical environment

This study reports degradation of azoxystrobin (AZOXY) and imidacloprid (IMIDA) in the rice straw (RS)/corn cob (CC) and peat (P)/compost (C)-based biomixtures. The effect of biomixture preconditioning (10 days incubation prior to pesticide application), pesticide concentration and moisture content was evaluated. Results suggested that conditioning of biomixture greatly affected IMIDA degradation where half-life (t_1/2) was reduced by 5-9 times. This was attributed to higher microbial biomass carbon content and dehydrogenase activity in the conditioned biomixtures. Pesticide application in the conditioned biomixture did not show any negative impact on soil microbial parameters. Both pesticides degraded at faster rate in the rice straw-based biomixtures than in the corn cob-based biomixtures. Degradation slowed down with increase in initial concentration of pesticides in biomixture and 1.6-3.0 (AZOXY) and 2.4-3.6 (IMIDA) times increase in t_1/2 values was observed. The moisture content of biomixture showed positive effect on degradation which increased when moisture content was increased from 60 to 80% water holding capacity. The effect was significant for IMIDA degradation in the corn cob-based biomixtures and AZOXY degradation in the peat biomixtures. The rice straw-based biomixtures were better in degrading AZOXY and IMIDA and can be used in biopurification systems.

Real-time forecasting of pesticide concentrations in soil

Forecasting pesticide residues in soils in real time is essential for agronomic purposes, to manage phytotoxic effects, and in catchments to manage surface and ground water quality. This has not been possible in the past due to both modelling and measurement constraints. Here, the analytical transient probability distribution (pdf) of pesticide concentrations is derived. The pdf results from the random ways in which rain events occur after pesticide application. First-order degradation kinetics and linear equilibrium sorption are assumed. The analytical pdfs allow understanding of the relative contributions that climate (mean storm depth and mean rainfall event frequency) and chemical (sorption and degradation) properties have on the variability of soil concentrations into the future. We demonstrated the two uncertain reaction parameters can be constrained using Bayesian methods. An approach to a Bayesian informed forecast is then presented. With the use of new rapid tests capable of providing quantitative measurements of soil concentrations in the field, real-time forecasting of future pesticide concentrations now looks possible for the first time. Such an approach offers new means to manage crops, soils and water quality, and may be extended to other classes of pesticides for ecological risk assessment purposes.

Modeling the effect of adsorption on the degradation rate of propiconazole in profiles of Polish Luvisols

Laboratory adsorption and degradation studies were carried out to determine the effect of time-dependent adsorption on propiconazole degradation rates in samples from three Polish Luvisols. Strong propiconazole adsorption (organic carbon normalized adsorption coefficients K_oc in the range of 1217-7777 mL/g) was observed in batch experiments, with a typical biphasic mechanism with a fast initial step followed by the time-dependent step, which finished within 48 h in the majority of soils. The time-dependent step observed in incubation experiments was longer (duration from 5 to 23 d), and its contribution to total adsorption was from 20% to 34%. The half-lives obtained at 25 °C and 40% maximum water holding capacity of soil, were in the range of 34.7-112.9 d in the Ap horizon and in the range of 42.3-448.8 d for subsoils. The very strong correlations, between degradation rates in pore water and soil organic carbon and soil microbial activity, indicated that microbial degradation of propiconazole was most likely the only significant process responsible for the decay of this compound under aerobic conditions for the whole of the examined soil profiles. Modeling of the processes showed that only models coupling adsorption and degradation were able to correctly describe the experimental data. The analysis of the bioavailability factor values showed that degradation was not limited by the rate of propiconazole desorption from soil, but sorption affected the degradation rate by decreasing its availability for microorganisms.

In-situ atrazine biodegradation dynamics in wheat (Triticum) crops under variable hydrologic regime

A comprehensive biodegradation reaction network of atrazine (ATZ) and its 18 byproducts was coupled to the nitrogen cycle and integrated in a computational solver to assess the in-situ biodegradation effectiveness and leaching along a 5m deep soil cultivated with wheat in West Wyalong, New South Wales, Australia. Biodegradation removed 97.7% of 2kg/ha ATZ yearly applications in the root zone, but removal substantially decreased at increasing depths; dechlorination removed 79% of ATZ in aerobic conditions and 18% in anaerobic conditions, whereas deethylation and oxidation removed only 0.11% and 0.15% of ATZ, respectively. The residual Cl mass fraction in ATZ and 4 byproducts was 2.4% of the applied mass. ATZ half-life ranged from 150 to 247days in the soil surface. ATZ reached 5m soil depth within 200years and its concentration increased from 1×10^-6 to 4×10^-6mg/kg_dry-soil over time. The correlation between ATZ specific biomass degradation affinity Φ₀ and half-life t_1/2, although relatively uncertain for both hydrolyzing and oxidizing bacteria, suggested that microorganisms with high Φ₀ led to low ATZ t_1/2. Greater ATZ applications were balanced by small nonlinear increments of ATZ biodegraded fraction within the root zone and therefore less ATZ leached into the shallow aquifer.

Degradation of cyflumetofen and formation of its main metabolites in soils and water/sediment systems

Cyflumetofen is a novel benzoyl acetonitrile acaricide without cross-resistance to existing acaricides. In the present study, for the first time, the environmental behaviors of cyflumetofen and the formation of its main metabolites, 2-(trifluoromethyl) benzoic acid (B-1) and 2-(trifluoromethyl) benzamide (B-3), in the four types of soil (black soil, sierozem, krasnozem, and fluvo-aquic soil) and three types of water/sediment systems (Northeast Lake, Hunan paddy field, and Beijng Shangzhuang reservoir) under aerobic and anaerobic conditions were investigated. The degradation dynamics of cyflumetofen followed first-order kinetics. Under aerobic environment, the half-lives of cyflumetofen in black soil, sierozem, krasnozem and fluvo-aquic soil were 11.2, 10.3, 12.4, and 11.4 days. Under water anaerobic conditions, the half-lives were 13.1, 10.8, 13.9, and 12.8 days. The effects of different conditions and soil types on the half-lives of cyflumetofen were studied using a one-way ANOVA test with post hoc comparison (Tukey's test). It was shown that the differences in black soil, krasnozem, and fluvo-aquic soil were extremely significant difference (p < 0.05) under aerobic and water anaerobic conditions. And there is a strong correlation between half-life and pH. Under aerobic environment, the half-lives of cyflumetofen in Northeast Lake, Hunan paddy field, and Beijng Shangzhuang reservoir were 15.4, 16.9, and 15.1 days. Under anaerobic conditions, they were 16.5, 17.3, and 16.1 days. Analyzing the differences of the half-lives under aerobic and anaerobic conditions, the difference only in Shangzhuang reservoir was extremely significant difference (p < 0.05). In soils, cyflumetofen degraded metabolites B-1 and B-3, from the first day 0.24 % B-1 was generated, while, only very low levels of B-3 generated at the same time. As time increased, B-3 gradually increased, cyflumetofen reduced gradually. Until 100 days, there were about 3.5 % B-1 and B-3 in the soils. In the water/sediment systems, from the first day, it degraded into B-1 in the sediment, and in the water mainly degraded into B-3.

Mineralisation and degradation of 2,4-dichlorophenoxyacetic acid dimethylamine salt in a biobed matrix and in topsoil

BACKGROUND

Biobeds are used for on-farm bioremediation of pesticides in sprayer rinsate and from spills during sprayer filling. Using locally sourced materials from Saskatchewan, Canada, a biobed matrix was evaluated for its effectiveness for mineralising and degrading 2,4-dichlorophenoxyacetic acid dimethylamine salt (2,4-D DMA) compared with the topsoil used in the biobed matrix.

RESULTS

Applying 2,4-D DMA to the biobed matrix caused a 2-3 day lag in CO2 production not observed when the herbicide was applied to topsoil. Despite the initial lag, less residual 2,4-D was measured in the biobed (0%) matrix than in the topsoil (57%) after a 28 day incubation. When the herbicide was applied 5 times to the biobed matrix, net CO2 increased immediately after each 2,4-D DMA application. Mineralisation of 2,4-D DMA was 61.9% and residual 2,4-D in the biobed matrix was 0.3% after 60 days, compared with corresponding values of 32.9 and 70.9% in topsoil.

CONCLUSION

The biobed matrix enhanced the mineralisation and degradation of 2,4-D DMA, indicating the potential for successful implementation of biobeds under Canadian conditions. The biobed matrix was more effective for mineralising and degrading the herbicide compared with the topsoil used in the biobed matrix. By correcting for biobed matrix and formulation blank, CO2 evolution was a reliable indicator of 2,4-D DMA mineralisation. © 2016 Society of Chemical Industry.

Using a Bayesian approach to improve and calibrate a dynamic model of polycyclic aromatic hydrocarbons degradation in an industrial contaminated soil

A novel kinetics model that describes the dynamics of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils is presented. The model includes two typical biodegradation pathways: the co-metabolic pathway using pseudo first order kinetics and the specific biodegradation pathway modeled using Monod kinetics. The sorption of PAHs to the solid soil occurs through bi-phasic fist order kinetics, and two types of non-extractible bounded residues are considered: the biogenic and the physically sequestrated into soil matrix. The PAH model was developed in Matlab, parameterized and tested successfully on batch experimental data using a Bayesian approach (DREAM). Preliminary results led to significant model simplifications. They also highlighted that the specific biodegradation pathway was the most efficient at explaining experimental data, as would be expected for an old industrial contaminated soil. Global analysis of sensitivity showed that the amount of PAHs ultimately degraded was mostly governed by physicochemical interactions rather than by biological activity.

Simulating Woodchip Bioreactor Performance Using a Dual-Porosity Model

There is a general understanding in the scientific community as to how denitrifying bioreactors operate, but we lack a quantitative understanding of the details of the denitrification process acting within them and comprehensive models for simulating their performance. We hypothesized that nitrate transport through woodchip bioreactors would be best described by a dual-porosity transport model where the bioreactor water is divided into a mobile domain (i.e., the water between the woodchips where it is free to flow and solute movement is by advection and dispersion) and an immobile domain of water (i.e., the water mostly within the woodchips that is stagnant and where solute movement is by diffusion alone). We calibrated the dual-porosity model contained in the HYDRUS model for a woodchip bioreactor using the results of a Br breakthrough experiment where we treated Br as a conservative nonadsorbing tracer. We then used the resulting model parameters to describe 2 yr of NO transport and denitrification within a bioreactor supplied by tile drainage. The only model parameters fitted to the NO data were either the zero- or first-order denitrification rate and its temperature dependence. The bioreactor denitrified 2.23 kg N (38%) of the NO entering it in 2013 and 3.73 kg N (49%) of the NO that entered it in 2014. The dual-porosity model fit the NO data very well, with fitted zero-order reaction rates of 8.7 and 6.8 mg N L d in 2013 and 2014, respectively, and corresponding first-order reaction rates of 0.99 and 1.02 d. For the 2-yr data set, both reaction rate models fit the data equally well. Consistent model parameters fitted for the 2 yr indicated that the model used was robust and a promising approach for modeling fate and transport of NO in woodchip bioreactors.

Adsorption and degradation of phenoxyalkanoic acid herbicides in soils: A review

The primary aim of the present review on phenoxyalkanoic acid herbicides-2-(2,4-dichlorophenoxy) acetic acid (2,4-D), 2-(4-chloro-2-methylphenoxy) acetic acid (MCPA), (2R)-2-(2,4-dichlorophenoxy) propanoic acid (dichlorprop-P), (2R)-2-(4-chloro-2-methylphenoxy) propanoic acid (mecoprop-P), 4-(2,4-dichlorophenoxy) butanoic acid (2,4-DB), and 4-(4-chloro-2-methylphenoxy) butanoic acid (MCPB)-was to compare the extent of their adsorption in soils and degradation rates to assess their potential for groundwater contamination. The authors found that adsorption decreased in the sequence of 2,4-DB > 2,4-D > MCPA > dichlorprop-P > mecoprop-P. Herbicides are predominantly adsorbed as anions-on organic matter and through a water-bridging mechanism with adsorbed Fe cations-and their neutral forms are adsorbed mainly on organic matter. Adsorption of anions of 2,4-D, MCPA, dichlorprop-P, and mecoprop-P is inversely correlated with their lipophilicity values, and modeling of adsorption of the compounds based on this relationship is possible. The predominant dissipation mechanism of herbicides in soils is bacterial degradation. The contribution of other mechanisms, such as degradation by fungi, photodegradation, or volatilization from soils, is much smaller. The rate of bacterial degradation decreased in the following order: 2,4-D > MCPA > mecoprop-P > dichlorprop-P. It was found that 2,4-D and MCPA have the lowest potential for leaching into groundwater and that mecoprop-P and dichlorprop-P have slightly higher potential. Because of limited data on adsorption and degradation of 2,4-DB and MCPB, estimation of their leaching potential was not possible.

Effect of formulation and repeated applications on the enantioselectivity of metalaxyl dissipation and leaching in soil

BACKGROUND

Soil incubation and column leaching experiments were conducted to address the question of whether the type of formulation (unsupported versus clay supported) and repeated applications of the chiral fungicide (RS)-metalaxyl affected the enantioselectivity of its dissipation and leaching in a slightly alkaline, loamy sand agricultural soil.

RESULTS

Regardless of the type of formulation and the number of fungicide applications, the R-enantiomer of metalaxyl was degraded faster than the S-enantiomer, but the individual degradation rates of R- and S-metalaxyl were highly affected by the different application regimes assayed (t1/2 = 2-104 days). Repeated applications accelerated the degradation of the biologically active R-metalaxyl enantiomer, whereas they led to slower degradation of the non-active S-metalaxyl enantiomer. The type of formulation had less influence on the dissipation rates of the enantiomers. For all formulations tested, soil column leachates became increasingly enriched in S-enantiomer as the number of fungicide applications was increased, and application of metalaxyl to soil columns as clay-based formulations reduced the leaching of both enantiomers.

CONCLUSION

Pesticide application conditions can greatly influence the enantioselective dissipation of chiral pesticides in soil, and hence are expected to exert a great impact on both the biological efficacy and the environmental chiral signatures of pesticides applied as mixtures of enantiomers or racemates to agricultural soils.

Degradation of Fluxapyroxad in Soils and Water/Sediment Systems Under Aerobic or Anaerobic Conditions

The persistence and fate of fluxapyroxad were investigated including studies with four soils from Hunan (HN), Shanxi (SX), Jiangsu (JS), and Heilongjiang (HLJ) and two water/sediment systems (water/sediment systems 1 and 2) from Beijing, China. The results demonstrated that the biodegradation efficiency of fluxapyroxad in soils under aerobic conditions was higher than that observed under anaerobic conditions. The order of degradation capability was HLJ soil > JS soil > SX soil > HN soil, and fluxapyroxad dissipated faster in water/sediment system 2 than in system 1. The tested systems (four soils and two water/sediments systems) with rich organic matter content, high oxygen level and neutral pH had a high potential to degrade fluxapyroxad, possibly because rich organic matter and oxygen level stimulated microbial activity and the neutral pH was suitable for microbial growth. These results showed that fluxapyroxad exhibited high persistence in tested systems, with half-lives ≥157.6 day.

Sorption and degradation of triclosan in sediments and its effect on microbes

Sorption and degradation behavior of triclosan (TCS) and its effect on microbes were studied in three sediments spiked at different concentration levels (1, 10, and 100 μg g(-1)). TCS showed a strong affiliation to all the sediments with linear adsorption coefficients (Kd) that varied from 220 to 1092 L g(-1), and the adsorption capacity is related to the total organic carbon (TOC) contents of the sediments. The half-lives of TCS varied from 55 to 239 days, and were longer in sediment with higher Kd. TCS showed minor effect on the activities of fluorescein diacetate hydrolase, dehydrogenase, alkaline phosphatase, and urease in the 1 μg g(-1) treatment, but at higher levels, a short-term effect was observed followed by a rapid recovery except the urease activity in sediment with the lowest adsorption capacity. PCA plots of phospholipid fatty acid showed that the phenotypic community in sediments with low TOC were more sensitive to TCS. A positive relation between bacterial biomass and total microbial biomass suggests that changes of bacteria biomass were responsible for changes of total microbial biomass in treatments. Denaturing gradient gel electrophoresis analysis of the 16S rDNA showed that the bacterial community structure deviated further away from the control at higher TCS concentration levels, with similarity coefficients in Un-weighted Pair Group Mathematics Average clustering between control and 100 μg g(-1) treatment varied from 0.38 to 0.73. Both degradation rate and toxic effects of TCS decreased in sediment with higher sorption capacity, which can be attributed to a reduced bioavailablity.

The effects of woodchip- and straw-derived biochars on the persistence of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) in soils

Sorption and degradation are the primary processes controlling the efficacy and runoff contamination risk of agrochemicals. This study assessed the influence of two biochars, made from woodchips and straw at a pyrolysis temperature of 725°C and applied to a loamy sand and a sandy soil in the concentration of 5.3 g 100 g(-1) sandy soil and 4.1 g 100 g(-1) loamy sand soil, or 53 t ha(-1) for both soil types, on degradation of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA). Soils were spiked with 50 mg MCPA kg(-1) soil. In the sandy soil, significantly more MCPA remained after 100 days if amended with straw-derived biochar in comparison to wood-derived biochar. Both biochars types significantly increased urease activity (p<0.05) after 37 days in the loamy sand soil, but these differences disappeared after 100 days. A root and shoot elongation test demonstrated that the soils containing straw-derived biochar and spiked with MCPA, showed the highest phytotoxicity. Both biochars were found to retard MCPA degradation in loamy sand and sandy soils. This effect could not be explained only by sorption processes due to comparatively low developed micro/mesoporous structure of both biochars shown by BET surface analysis. However, an enhanced MCPA persistence and soil toxicity in sandy soil amended with straw biochar was observed and further studies are needed to reveal the responsible mechanisms.

Chemodynamics of chromium reduction in soils: implications to bioavailability

Chromium toxicity in soils can be mitigated by reduction of Cr(VI) to Cr(III) which is influenced by the presence of free Cr(VI) species in soil solution, and the supply of protons and electrons. In this study, the effects of Cr(VI) adsorption (i.e. availability of free Cr(VI) species in soil solution), soil pH (i.e. supply of protons) and three electron donor carbon sources [black carbon (BC), chicken manure biochar (CMB) and cow manure (CM)] on the reduction of Cr(VI) to Cr(III) in soils were investigated. The results indicated that the rate of Cr(VI) reduction decreased with an increase in Cr(VI) adsorption and soil pH, which is attributed to decreased supply of free Cr(VI) ions and protons, respectively. Among the three different amendments tested, BC showed the highest rate of Cr(VI) reduction followed by CM and CMB. Furthermore, addition of BC, CM and CMB decreased the bioavailability of Cr(VI) in contaminated soils. The high efficiency of BC on Cr(VI) reduction was due to the electron donor's functional groups such as phenolic, hydroxyl, carbonyl and amides. The study demonstrated that free form of Cr(VI) ions in soil solution and carbon amendments enriched with acidic functional groups favored the reduction of Cr(VI), thereby mitigating its bioavailability and toxicity in contaminated soils.

Partition, sorption and structure activity relation study of dialkoxybenzenes that modulate insect behavior

Some dialkoxybenzenes are promising new insect control agents. These compounds mimic naturally occurring odorants that modulate insect behavior. Before applying these compounds, however, their persistence and biodegradability at the application site and in the environment should be understood. The fate of organic compounds in the environment is a complex phenomenon which is influenced by many processes such as sorption to soil components, sedimentation, volatilization, and uptake by plants, as well as biotic and abiotic chemical degradation. In this study, the octanol-water partition coefficient, volatility and sorption on soil components (sand, clay and organic matter) of selected dialkoxybenzenes as well as structure activity relationships with regard to partition, volatility and sorption were investigated. Additionally, calculations of partition, molar volume and molecular surface areas were done, to understand structure-activity relationships of the physical properties.

Environmental contextualisation of potential toxic elements and polycyclic aromatic hydrocarbons in biochar

Nine dissimilar biochars, produced from varying feedstock at different pyrolysis temperatures, are appraised with respect to concentrations of potentially toxic elements, specifically, metals, metalloids and polycyclic aromatic hydrocarbons (PAHs). Concentrations of the metals and metalloids varied with the following ranges (mg kg(-1)): 0.02-0.94, Cd; 0.12-6.48, Cr; 0.04-13.2, Cu; 0.1-1.37, Ni; 0.06-3.87, Pb; 0.94-207, Zn and 0.03-0.27, As. Σ(16)PAH concentrations (16 Environmental Protection Agency (EPA) PAHs) range between 0.08 mg kg(-1) to 8.7 mg kg(-1). Subsequent comparison with background soil concentrations, concentration applied to the regulation of composted materials (Publicly Available Specification (PAS 100)) and European Union (EU) regulations relating to the application of sewage sludge to agricultural land suggest low risk associated with the concentrations of PTEs observed in biochar. Collectively, results suggest that environmental impacts attributable to metals, metalloids and PAHs associated with biochar following its application to soil are likely to be minimal.

Protozoan predation in soil slurries compromises determination of contaminant mineralization potential

Soil suspensions (slurries) are commonly used to estimate the potential of soil microbial communities to mineralize organic contaminants. The preparation of soil slurries disrupts soil structure, however, potentially affecting both the bacterial populations and their protozoan predators. We studied the importance of this "slurry effect" on mineralization of the herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA, (14)C-labelled), focussing on the effects of protozoan predation. Mineralization of MCPA was studied in "intact" soil and soil slurries differing in soil:water ratio, both in the presence and absence of the protozoan activity inhibitor cycloheximide. Protozoan predation inhibited mineralization in dense slurry of subsoil (soil:water ratio 1:3), but only in the most dilute slurry of topsoil (soil:water ratio 1:100). Our results demonstrate that protozoan predation in soil slurries may compromise quantification of contaminant mineralization potential, especially when the initial density of degrader bacteria is low and their growth is controlled by predation during the incubation period.

Relationship between biodegradation and sorption of phthalate esters and their metabolites in natural sediments

Regulatory evaluations of commercial chemicals in Canada, the United States, the European Union, and other countries aim to identify biodegradation rates of chemicals in natural soils and sediments. However, commonly used biodegradation testing methods are limited in their capacity to determine biodegradation rates under natural environmental conditions. As a result, widely varying biodegradation rates have been reported for many very hydrophobic substances. This variability causes difficulties in regulatory evaluations, potentially leading to chemical misclassification. In the present study, the authors developed a model of the relationship between biodegradation, sorption, and hydrophobicity, and tested the model in experiments that measured the biodegradation rates of a range of di-phthalate esters (DPEs) and mono-phthalate esters (MPEs) in natural sediments. The results indicate that DPEs and MPEs have the inherent capacity to be quickly degraded by microbes in sediments at a common rate, but that DPE biodegradation rates in natural sediments decrease with increasing phthalate ester sorption to sediments. The results show that inherently biodegradable substances that are subject to a high degree of sorption can be expected to exhibit long half-lives in natural sediments. The model provides a potential methodology for assessing biodegradation rates in natural sediments from inherent biodegradation rates measured in screening tests by accounting for chemical sorption. The present study indicates that a reduced rate of biodegradation is due to a reduced fraction of freely dissolved chemical concentration in the interstitial water, and that the environmental significance of sorption-reduced biodegradation rates needs to be viewed in the context of risk in chemical evaluations.

Validating potential toxicity assays to assess petroleum hydrocarbon toxicity in polar soil

Potential microbial activities are commonly used to assess soil toxicity of petroleum hydrocarbons (PHC) and are assumed to be a surrogate for microbial activity within the soil ecosystem. However, this assumption needs to be evaluated for frozen soil, in which microbial activity is limited by liquid water (θ(liquid)). Influence of θ(liquid) on in situ toxicity was evaluated and compared to the toxicity endpoints of potential microbial activities using soil from an aged diesel fuel spill at Casey Station, East Antarctica. To determine in situ toxicity, gross mineralization and nitrification rates were determined by the stable isotope dilution technique. Petroleum hydrocarbon-contaminated soil (0-8,000 mg kg(-1)), packed at bulk densities of 1.4, 1.7, and 2.0 g cm(-3) to manipulate liquid water content, was incubated at -5°C for one, two, and three months. Although θ(liquid) did not have a significant effect on gross mineralization or nitrification, gross nitrification was sensitive to PHC contamination, with toxicity decreasing over time. In contrast, gross mineralization was not sensitive to PHC contamination. Toxic response of gross nitrification was comparable to potential nitrification activity (PNA) with similar EC25 (effective concentration causing a 25% effect in the test population) values determined by both measurement endpoints (400 mg kg(-1) for gross nitrification compared to 200 mg kg(-1) for PNA), indicating that potential microbial activity assays are good surrogates for in situ toxicity of PHC contamination in polar regions.

Adsorption and desorption of chlorpyrifos to soils and sediments

Chlorpyrifos, one of the most widely used insecticides, has been detected in air, rain, marine sediments, surface waters, drinking water wells, and solid and liquid dietary samples collected from urban and rural areas. Its metabolite, TCP, has also been widely detected in urinary samples collected from people of various age groups. With a goal of elucidating the factors that control the environmental contamination, impact, persistence, and ecotoxicity of chlorpyrifos, we examine, in this review, the peer-reviewed literature relating to chlorpyrifos adsorption and desorption behavior in various solid-phase matrices. Adsorption tends to reduce chlorpyrifos mobility, but adsorption to erodible particulates, dissolved organic matter, or mobile inorganic colloids enhances its mobility. Adsorption to suspended sediments and particulates constitutes a major off-site migration route for chlorpyrifos to surface waters, wherein it poses a potential danger to aquatic organisms. Adsorption increases the persistence of chlorpyrifos in the environment by reducing its avail- ability to a wide range of dissipative and degradative forces, whereas the effect of adsorption on its ecotoxicity is dependent upon the route of exposure. Chlorpyrifos adsorbs to soils, aquatic sediments, organic matter, and clay minerals to differing degrees. Its adsorption strongly correlates with organic carbon con- tent of the soils and sediments. A comprehensive review of studies that relied on the batch equilibrium technique yields mean and median Kd values for chlorpyrifos of 271 and 116 L/kg for soils, and 385 and 403 L/kg for aquatic sediments. Chlorpyrifos adsorption coefficients spanned two orders of magnitude in soils. Normalizing the partition coefficient to organic content failed to substantially reduce variability to commonly acceptable level of variation. Mean and median values for chlorpyrifos partition coefficients normalized to organic carbon, K, were 8,163 and 7,227 L/kg for soils and 13,439 and 15,500 L/kg for sediipents. This variation may result from several factors, including various experimental artifacts, variation in quality of soil organic matter, and inconsistencies in experimental methodologies. Based on this review, there appears to be no definitive quantification of chlorpyrifos adsorption or desorption characteristics. Thus, it is difficult to predict its adsorptive behavior with certainty, without resorting to experimental methods specific to the soil or sediment of interest. This limitation should be recognized in the context of current efforts to predict the risk, fate, and transport of chlorpyrifos based upon published partition coefficients. Based on a comprehensive review of the peer-reviewed literature related to adsorption and desorption of chlorpyrifos, we propose the following key areas for future research. From this review, it becomes increasingly evident that pesticide partitioning cannot be fully accounted for by the fraction of soil or solid-matrix organic matter or carbon content. Therefore, research that probes the variation in the nature and quality of soil organic matter on pesticide adsorption is highly desirable. Pesticide persistence and bioavailability depend on insights into desorption capacity. Therefore, understanding the fate and environmental impact of hydrophobic pesticides is incomplete without new research being performed to improve insights into pesticide desorption from soils and sediments. There is also a need for greater attention and consistency in developing experimental methods aimed at estimating partition coefficients. Moreover, in such testing, choosing initial concentrations and liquid-solid ratios that are more representative of environmental conditions could improve usefulness and interpretation of data that are obtained. Future monitoring efforts should include the sampling and analysis of suspended particulates to account for suspended solid-phase CPF, a commonly underestimated fraction in surface water quality monitoring programs. Finally, management practices related to the reduction of off-site migration of CPF should be further evaluated, including alternative agricultural practices leading to reduction in soil erosion and structural best management practices, such as sedimentation ponds, treatment wetlands, and vegetated edge-of-field strips.

Role of soil sorption and microbial degradation on dissipation of mesotrione in plant-available soil water

Mesotrione is a carotenoid biosynthesis-inhibiting herbicide labeled for pre-emergence and postemergence weed control in corn production. Understanding the factors that influence the dissipation of mesotrione in soil and in the plant-available water (PAW) is important for the environmental fate assessment and optimal weed management practices. The present research investigated the role of soil properties and microbial activities on the interrelated sorption and degradation processes of mesotrione in four soils by direct measurements of PAW. We found that mesotrione bound to the soils time dependently, with approximately 14 d to reach equilibrium. The 24-h batch-slurry equilibrium experiments provided the sorption partition coefficient ranging from 0.26 to 3.53 L kg(-1), depending on soil organic carbon and pH. The dissipation of mesotrione in the soil-bound phase was primarily attributed to desorption to the PAW. Degradation in the PAW was rapid and primarily dependent on microbial actions, with half-degradation time (DT(50)) <3 d in all four soils tested. The rapid degradation in the PAW became rate limited by sorption as more available molecules were depleted in the soil pore water, resulting in a more slowed overall process for the total soil-water system (DT(50) <26 d). The dissipation of mesotrione in the PAW was due to microbial metabolism and time-dependent sorption to the soils. A coupled kinetics model calibrated with the data from the laboratory centrifugation technique provided an effective approach to investigate the interrelated processes of sorption and degradation in realistic soil moisture conditions.

Cultivation-independent analysis of archaeal and bacterial communities of the formation water in an Indian coal bed to enhance biotransformation of coal into methane

Biogenic origin of the significant proportion of coal bed methane has indicated the role of microbial communities in methanogenesis. By using cultivation-independent approach, we have analysed the archaeal and bacterial community present in the formation water of an Indian coal bed at 600-700 m depth to understand their role in methanogenesis. Presence of methanogens in the formation water was inferred by epifluorescence microscopy and PCR amplification of mcrA gene. Archaeal 16S rRNA gene clone library from the formation water metagenome was dominated by methanogens showing similarity to Methanobacterium, Methanothermobacter and Methanolinea whereas the clones of bacterial 16S rRNA gene library were closely related to Azonexus, Azospira, Dechloromonas and Thauera. Thus, microbial community of the formation water consisted of predominantly hydrogenotrophic methanogens and the proteobacteria capable of nitrogen fixation, nitrate reduction and polyaromatic compound degradation. Methanogenic potential of the microbial community present in the formation water was elucidated by the production of methane in the enrichment culture, which contained 16S rRNA gene sequences showing close relatedness to the genus Methanobacterium. Microcosm using formation water as medium as well as a source of inoculum and coal as carbon source produced significant amount of methane which increased considerably by the addition of nitrite. The dominance of Diaphorobacter sp. in nitrite amended microcosm indicated their important role in supporting methanogenesis in the coal bed. This is the first study indicating existence of methanogenic and bacterial community in an Indian coal bed that is capable of in situ biotransformation of coal into methane.

Modeling spatial variation in microbial degradation of pesticides in soil

Currently, no general guidance is available on suitable approaches for dealing with spatial variation in the first-order pesticide degradation rate constant k even though it is a very sensitive parameter and often highly variable at the field, catchment, and regional scales. Supported by some mechanistic reasoning, we propose a simple general modeling approach to predict k from the sorption constant, which reflects bioavailability, and easily measurable surrogate variables for microbial biomass/activity (organic carbon and clay contents). The soil depth was also explicitly included as an additional predictor variable. This approach was tested in a meta-analysis of available literature data using bootstrapped partial least-squares regression. It explained 73% of the variation in k for the 19 pesticide-study combinations (n = 212) in the database. When 4 of the 19 pesticide-study combinations were excluded (n = 169), the approach explained 80% of the variation in the degradation rate constant. We conclude that the approach shows promise as an effective way to account for the effects of bioavailability and microbial activity on microbial pesticide degradation in large-scale model applications.

Migration of Radionuclides in Geologic Media: Fundamental Research Needs

An assessment of the fundamental research needs in understanding and predicting the migration of radionuclides in the subsurface is provided. Emphasis is on the following three technical areas: (1) aqueous speciation of radionuclides, (2) the interaction of radionuclides with substrates, and (3) intermediate-scale interaction studies. This research relates to important issues associated with environmental restoration and remediation of DOE sites contaminated with mixed radionuclide-organic wastes.

Bioavailability of methyl parathion adsorbed on clay minerals and iron oxide

Adsorption, desorption and degradation by Pseudomonas putida of methyl parathion (O,O-dimethyl O-p-nitrophenyl phosphorothioate) on montmorillonite, kaolinite and goethite were studied. Metabolic activities of methyl parathion-degrading bacteria P. putida in the presence of minerals were also monitored by microcalorimetry to determine the degradation mechanism of methyl parathion. Montmorillonite presented higher adsorption capacity and affinity for methyl parathion than kaolinite and goethite. The percentage of degradation of methyl parathion adsorbed on minerals by P. putida was in the order of montmorillonite>kaolinite>goethite. The presence of minerals inhibited the exponential growth and the metabolic activity of P. putida. Among the examined minerals, goethite exhibited the greatest inhibitory effect on bacterial activity, while montmorillonite was the least depressing. The biodegradation of adsorbed methyl parathion by P. putida is apparently not controlled by the adsorption affinity of methyl parathion on minerals and may be mainly governed by the activity of the methyl parathion-degrading bacteria. The information obtained in this study is of fundamental significance for the understanding of the behavior of methyl parathion in soil environments.

Mineralization and degradation of glyphosate and atrazine applied in combination in a Brazilian Oxisol

The aim of this study was to investigate the behavior of the association between atrazine and glyphosate in the soil through mineralization and degradation tests. Soil treatments consisted of the combination of a field dose of glyphosate (2.88 kg ha⁻¹) with 0, ½, 1 and 2 times a field dose of atrazine (3.00 kg ha⁻¹) and a field dose of atrazine with 0, ½, 1 and 2 times a field dose of glyphosate. The herbicide mineralization rates were measured after 0, 3, 7, 14, 21, 28, 35, 42, 49, 56 and 63 days of soil application, and degradation rates after 0, 7, 28 and 63 days. Although glyphosate mineralization rate was higher in the presence of 1 (one) dose of atrazine when compared with glyphosate alone, no significant differences were found when half or twice the atrazine dose was applied, meaning that differences in glyphosate mineralization rates cannot be attributed to the presence of atrazine. On the other hand, the influence of glyphosate on atrazine mineralization was evident, since increasing doses of glyphosate increased the atrazine mineralization rate and the lowest dose of glyphosate accelerated atrazine degradation.

Polynomial response of 2,4-D mineralization to temperature in soils at varying soil moisture contents, slope positions and depths

The herbicide 2,4-D [2,4-(dichlorophenoxy) acetic acid] is a widely used broadleaf control agent in cereal production systems. Although 2,4-D soil-residual activity (half-lives) are typically less than 10 days, this herbicide also has as a short-term leaching potential due to its relatively weak retention by soil constituents. Herbicide residual effects and leaching are influenced by environmental variables such as soil moisture and temperature. The objective of this study was to determine impacts of these environmental variables on the magnitude and extent of 2,4-D mineralization in a cultivated undulating Manitoba prairie landscape. Microcosm incubation experiments were utilized to assess 2,4-D half-lives and total mineralization using a 4 × 4 × 3 × 2 factorial design (with soil temperature at 4 levels: 5, 10, 20 and 40°C; soil moisture at 4 levels: 60, 85, 110, 135 % of field capacity; slope position at 3 levels: upper-, mid- and lower-slopes; and soil depth at 2 levels: 0-5 cm and 5-15 cm). Half-lives (t(½)) varied from 3 days to 51 days with the total 2,4-D mineralization (M(T)) ranging from 5.8 to 50.9 %. The four-way interaction (temperature × moisture × slope × depth) significantly (p < 0.001) influenced both t(½) and M(T). Second-order polynomial equations best described the relations of temperature with t(½) and M(T) as was expected from a biological system. However, the interaction and variability of t(½) and M(T) among different temperatures, soil moistures, slope positions, and soil depth combinations indicates that the complex nature of these interacting factors should be considered when applying 2,4-D in agricultural fields and in utilizing these parameters in pesticide fate models.

Bioavailability of organoclay formulations of atrazine in soil

Pesticide formulations based on organoclays have been proposed to prolong the efficacy and reduce the environmental impact of pesticides in soil. This research addressed the question of whether atrazine in organoclay-based formulations is irreversibly sorbed or is bioavailable for bacterial degradation in soil. Different cations of l-carnitine (CAR), tyramine (TYRAM), hexadimethrine (HEXADIM), phenyltrimethylammonium (PTMA), hexadecyltrimethylammonium (HDTMA), and Fe(III) were incorporated into Na-rich Wyoming montmorillonite (SWy-2) and Ca-rich Arizona montmorillonite (SAz-1) at 100% of the cation exchange capacity (CEC) of the clays as a strategy to enhance the affinity of the clay minerals for atrazine. A Buse loam soil from Becker, MN, was treated with three organoclay-based formulations of 14C-atrazine or free herbicide and incubated for 2 weeks. To determine the bioavailability of 14C-atrazine, the soil was inoculated with Pseudomonas sp. strain ADP, which rapidly mineralizes atrazine. At day 0, and after a 2 week incubation, mineralization and the amount of 14C-atrazine residues distributed between the aqueous-extractable, methanol-extractable, and bound fractions in the soil were determined to characterize the availability of nonaged and aged atrazine residues. By the end of the 2 week incubation, the microorganisms had mineralized >80% of the initial readily available (water-extractable) and >70% of the less readily available (methanol-extractable) 14C-atrazine in the soil. Bound residues increased from <4% at day 0 to ∼17% after the 2 week incubation for both the formulated and free forms of atrazine. The results of these incubation experiments show that the bioavailabilities of atrazine were similar in the case of the organoclay formulations and as free atrazine. This indicated that whereas more atrazine was sorbed and less likely to be transported in soil, when formulated as organoclay complexes, it was ultimately accessible to degrading bacteria, so that the herbicide is likely to be naturally attenuated by soil microorganisms.

Structure of a laccase-mediated product of coupling of 2,4-diamino-6-nitrotoluene to guaiacol, a model for coupling of 2,4,6-trinitrotoluene metabolites to a humic organic soil matrix

This work presents laccase-mediated model reactions for coupling of reduced 2,4,6-trinitrotoluene (TNT) metabolites to an organic soil matrix. The structure of an isolated coupling product of 2,4-diamino-6-nitrotoluene (2,4-DANT) to guaiacol as humic constituent was determined. Among several structures, the compound was identified conclusively to be the trinuclear coupling product 5-(2-amino-3-methyl-4-nitroanilino)-3,3(prm1)-dimethoxy-4,4(prm1)-diphenoqu inone. The compound has a weight of 409 g mol(sup-1) and may serve as a model reaction for the biogenic formation of bound residues in soil from TNT by coupling aminotoluenes (reduced TNT metabolites) to humic constituents. A linear correlation of the substrate consumption to the enzyme activity was detected. Based on this observation, the described reaction of 2,4-DANT coupling to guaiacol may be used for determination of laccase activity since the reaction was not inhibited by other compounds of culture supernatants. We propose a two-step mechanism for the coupling reaction because 2,4-DANT was not transformed by laccases in the absence of guaiacol and guaiacol oxidation was independent of the presence of 2,4-DANT. The first reaction step is a laccase-mediated dimerization of two guaiacol monomers with subsequent oxidation to a diphenoquinone. The second step is the nucleophilic addition of 2,4-DANT to the ortho position of the carbonyl group of the diphenoquinone structure.

Soil temperature and moisture effects on the persistence of synthetic androgen 17alpha-trenbolone, 17beta-trenbolone and trendione

Trenbolone acetate (TBA) is a synthetic androgenic steroid hormone administered as a subcutaneous implant for growth promotion in beef cattle. The primary metabolite excreted in manure from implanted cattle is 17alpha-trenbolone with lesser amounts of 17beta-trenbolone and trendione also present. At 22 degrees C and favorable moisture conditions in a controlled laboratory environment, trenbolone degrades to trendione in a few hours; however, these conditions are often not what exist in the field. Therefore, aerobic degradation rates of 17alpha-trenbolone, 17beta-trenbolone and trendione were determined in a sandy soil and silty clay loam under a range of temperature and water availability combinations that may be expected in the field. A first-order exponential decay model was used to estimate rates and generally resulted in good model fits to the data. Degradation rates decreased with decreasing water availability (i.e., more negative soil matric potential) and decreasing temperature. However, when water availability was substantially reduced (-1.0MPa), hotter temperatures (35 degrees C) significantly reduced trenbolone degradation rates. Once temperature was low enough to limit microbial activity, no further changes were observed with decreasing matric potential. Trendione also exhibited similar moisture and temperature dependent degradation, but persisted longer than the parent trenbolone. The latter was discussed in light of extracellular versus intracellular enzymatic degradation and sorption. Half lives at colder temperatures (5 degrees C) even under favorable moisture conditions were 2-3d for the trenbolone isomers and approached 10d for trendione.