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

Mechanisms of Adsorption of Phenoxyalkanoic Herbicides on Fulvic and Humic Acids

Our recent study demonstrated that fulvic and humic acids are the major contributors to the adsorption of phenoxyalkanoic acid herbicides in soils. At very low pH, the neutral forms of these herbicides are bound directly to fulvic and humic acids, whereas at higher pH, their anionic forms are adsorbed mainly via bridges created by Al3+ species. The number of active sorption sites associated with Al3+ species complexed with fulvic acids is pH-dependent, whereas the number of corresponding sites in humic acids is pH-independent. Based on the results of the FTIR analysis, research into adsorption thermodynamics, and molecular modeling, an attempt was made in the present study to explain the adsorption mechanisms of six phenoxyalkanoic herbicides used currently in the European Union on the surfaces of the above fractions of humic substances. The obtained values of standard enthalpy (ΔH0) for the adsorption of the anionic forms of phenoxyalkanoic herbicides on fulvic or humic acids complexed with Al3+ were in the range of physical adsorption, i.e., from -8.4 kJ/mol to -2.9 kJ/mol for the former, and from -5.3 kJ/mol to -2.4 kJ/mol for the latter. The study demonstrated that the neutral forms of phenoxyalkanoic herbicides were bound to humic substances mainly via H-bonds, π-π stacking interactions, and hydrophobic interactions. Al3+ species were complexed with fulvic and humic acids to form outer-sphere complexes. Ternary outer-sphere complexes were also created between the anionic forms of phenoxyalkanoic acid herbicides and positively charged Al3+ species complexed with fulvic acids. The mechanisms of adsorption on humic acids involved a ligand exchange between a loosely bound hydroxyl group of hydrolyzed Al3+ complexed with this adsorbent and the anionic form of the herbicide. However, in this case, adsorption took place only in the presence of sufficiently strong hydrophobic and π-π stacking interactions supported by H-bonds. These findings elucidate why phenoxyalkanoic herbicides are mobile in the soil profile and are often rapidly degraded in soils.

Can the pH-dependent adsorption of phenoxyalkanoic herbicides in soils be described with a single equation?

The six phenoxyalkanoic acid herbicides currently used in the European Union have similar molecular structures. Therefore, we assumed the soil components involved in the adsorption mechanisms of these herbicides to be identical. The values of the adsorption distribution coefficient Kd, obtained via batch experiments involving typical Polish Arenosol, Luvisol, and Chernozem profiles with a native pH of 4.2-7.7, were examined using Lasso regression, as well as adsorption on isolated fractions of humic substances, Al2O3, and goethite. The neutral forms of the herbicides were adsorbed on the surface of fulvic acids available to them, covering soil mesopores with a size of > 2.5 nm. The models revealed that fulvic acids had a lognormal-like distribution in soil pores. Herbicide anions were adsorbed on the pH-dependent sites of Al oxyhydroxides and on the sites created by the Al+3 species adsorbed on the surface of fulvic acids (both sites were active up to pH 7.5), the sites of humic acids associated with the adsorbed Al3+ species, sites of Fe oxyhydroxides (active at pH < 5), and, to a limited extent, sites of humins. Two models describing the adsorption of phenoxyalkanoic acid herbicides in soils were created. A simpler model was based on humic substance fractions and the variables related to the potential acidity of soils. In the more extensive model, humic substance fractions and Al and Fe oxyhydroxide contents were used as predictors, and, where necessary, the predictors were combined with the modified Henderson-Hasselbalch formula to estimate the activity ranges of pH-dependent sorption sites. The study findings revealed that fulvic and humic acids were the main adsorbents of phenoxyalkanoic herbicides in soils, indicating that transporting of the herbicides with dissolved organic matter is an important mechanism of groundwater and surface water contamination with these chemicals.

Tebuconazole promotes spread of a multidrug-resistant plasmid into soil bacteria to form new resistant bacterial strains

The development of antibiotic resistance threatens human and environmental health. Non-antibiotic stressors, including fungicides, may contribute to the spread of antibiotic resistance genes (ARGs). We determined the promoting effects of tebuconazole on ARG dissemination using a donor, Escherichia coli MG1655, containing a multidrug-resistant fluorescent plasmid (RP4) and a recipient (E. coli HB101). The donor was then incorporated into the soil to test whether tebuconazole could accelerate the spread of RP4 into indigenous bacteria. Tebuconazole promoted the transfer of the RP4 plasmid from the donor into the recipient via overproduction of reactive oxygen species (ROS), enhancement of cell membrane permeability and regulation of related genes. The dissemination of the RP4 plasmid from the donor to soil bacteria was significantly enhanced by tebuconazole. RP4 plasmid could be propagated into more genera of bacteria in tebuconazole-contaminated soil as the exposure time increased. These findings demonstrate that the fungicide tebuconazole promotes the spread of the RP4 plasmid into indigenous soil bacteria, revealing the potential risk of tebuconazole residues enhancing the dissemination of ARGs in soil environments.

Improved Assessment of Soil Nonextractable Residues of the Pyrethroid Insecticide Cyphenothrin

The metabolic fate of pyrethroid insecticide cyphenothrin (1) [(RS)-α-cyano-3-phenoxybenzyl (1RS)-cis-trans-2,2-dimethyl-3-(2-methylprop-1-enyl)cyclopropanecarboxylate] in soils was investigated using ¹⁴C-labeled (1R)-cis/trans isomers at the cyclopropane ring. Both isomers degraded with half-lives of 19.0-47.4 days, and 48.9-56.0% and 27.5-38.7% of the applied radioactivity (AR) were mineralized to CO₂ and incorporated into nonextractable residues (NER), respectively, after 120 days at 20 °C. NER analyses revealed 37.5-42.2% (cis-1) and 44.9-54.1% (trans-1) of each residue at 30/120 days were comprised of ¹⁴C-amino acids (AAs) as microbial products. Assuming that 50% of microbial biomass is AAs, it was estimated that 11.3-22.9%AR (cis-1, 75.0-84.4% of NER) and 13.9-30.4%AR (trans-1, 89.8-108.2% of NER) were nonhazardous biogenic NER (bio-NER), while type I/II xenobiotic NER (xeno-NER) characterized by silylation was insignificant at 0.9-1.0%/2.8-3.3%AR (cis-1). Detailed ¹⁴C-AA quantitation indicated a high relevance of the tricarboxylic acid cycle and pyruvate pathway during bio-NER formation, offering new insights into the microbial assimilation of the chrysanthemic moiety.

Impact of historical legacy pesticides on achieving legislative goals in Europe

Pesticides are widely used in agriculture to optimise food production. However, the movement of pesticides into water bodies negatively impacts aquatic environments. The European Union (EU) aims to make food systems fair, healthy and environmentally friendly through its current Farm to Fork strategy. As part of this strategy, the EU plans to reduce the overall use and risk of chemical pesticides by 50 % by 2030. The attainment of this target may be compromised by the prevalence of legacy pesticides arising from historical applications to land, which can persist in the environment for several decades. The current EU Farm to Fork policy overlooks the potential challenges of legacy pesticides and requirements for their remediation. In this review, the current knowledge regarding pesticide use in Europe, as well as pathways of pesticide movement to waterways, are investigated. The issues of legacy pesticides, including exceedances, are examined, and existing and emerging methods of pesticide remediation, particularly of legacy pesticides, are discussed. The fact that some legacy pesticides can be detected in water samples, more than twenty-five years after they were prohibited, highlights the need for improved EU strategies and policies aimed at targeting legacy pesticides in order to meet future targets.

Improvement of Akanthomyces lecanii resistance to tebuconazole through UV-C radiation and selective pressure on microbial evolution and growth arenas

Tebuconazole (TEB) is a fungicide widely used in agriculture; however, its constant application has increased the emergence of resistant plant pathogenic fungal strains and reduced the effectiveness of fungi as biological control agents; for instance, the entomopathogenic and hyperparasitic fungus Akanthomyces lecanii, suitable for simultaneous biological control of insect pest and plant pathogenic fungi, is highly sensitive to fungicides. We carried out the induction of resistance to TEB in two wild type strains of A. lecanii by UV radiation and selective pressure in increasing fungicide gradients using a modified Microbial Evolution and Growth Arena (MEGA), to produce A. lecanii strains that can be used as biological control agent in the presence of tebuconazole. Nine UV-induced and three naturally adapted A. lecanii strains were resistant to TEB at the agriculturally recommended dose, and three irradiated strains were resistant to TEB concentration ten times higher; moreover, growth, sporulation rates, production of hydrolytic enzymes, and virulence against the hemipteran Coccus viridis, a major pest of coffee crops, were not affected in the TEB-resistant strains. These A. lecanii TEB-resistant strains would have a greater opportunity to develop and to establish themselves in fields where the fungicide is present and can be used in a combined biological-chemical strategy to improve insect and plant pathogenic fungal control in agriculture. Also, the selective pressure through modified MEGA plate methodology can be used for the adaptation of entomopathogenic filamentous fungi to withstand other chemical or abiotic stresses that limits its effectiveness for pest control.

An assessment of potential pesticide transmission, considering the combined impact of soil texture and pesticide properties: A meta-analysis

Pesticides are widely employed as a cost-effective means of reducing the impacts of undesirable plants and animals. The aim of this paper is to develop a risk ranking of transmission of key pesticides through soil to waterways, taking into account physico-chemical properties of the pesticides (soil half-life and water solubility), soil permeability, and the relationship between adsorption of pesticides and soil texture. This may be used as a screening tool for land managers, as it allows assessment of the potential transmission risks associated with the use of specified pesticides across a spectrum of soil textures. The twenty-eight pesticides examined were differentiated into three groups: herbicides, fungicides and insecticides. The highest risk of pesticide transmission through soils to waterways is associated with soils containing <20% clay or >45% sand. In a small number of cases, the resulting transmission risk is not influenced by soil texture alone. For example, for Phenmedipham, the transmission risk is higher for clay soils than for silt loam. The data generated in this paper may also be used in the identification of critical area sources, which have a high likelihood of pesticide transmission to waterways. Furthermore, they have the potential to be applied to GIS mapping, where the potential transmission risk values of the pesticides can be layered directly onto various soil textures.

Effects of biochar on the fate of conazole fungicides in soils and their bioavailability to earthworms and plants

The study showed novel findings about changes in the fate and bioavailability of conazole fungicides (CFs) after biochar (BC) addition to soil. Two contrasting soils (low- and high-sorbing of CF; L soils, H soils) were amended by three BCs (low-, moderate-, and high-sorbing of CF; L-BC, M-BC, H-BC) at 0.2% and 2% doses. Epoxiconazole (EPC) and tebuconazole (TBC) were then added to the soil-BC mixtures, and their degradation, bioaccumulation in earthworms (Eisenia andrei), and bioconcentration in lettuce (Lactuca sativa) were studied for three months. Also, stir bar sorptive extraction (SBSE) was performed to determine CF (bio)accessibility. The EPC and TBC degradation in the soil-BC mixtures followed usually the first-order decay kinetics. The BC addition prevalently decreased the pesticides degradation in the L soil mixtures but often increased it in the H soil mixtures. In general, EPC degraded less than TBC. BC type and dose roles in the pesticides degradation were unclear. The BC addition significantly reduced pesticide uptake to the earthworms in the L soil mixtures (by 37-96%) and in the H soil mixtures (by 6-89%) with 2% BC. The BC addition reduced pesticide uptake to the lettuce roots and leaves significantly-up to two orders of magnitude, and this reduction was strong in H soil mixtures at 2% of BC. The BC addition reduced the CF (bio)accessibility measured by SBSE in all L soil mixtures and some H soil mixtures with 2% BC. Although not significant, it also seems that the pesticide bioaccumulation, bioconcentration, and (bio)accessibility were decreasing according to the BC type (L-BC > M-BC > H-BC). The pesticide concentrations in the earthworms and lettuce correlated significantly to the SBSE results, which indicates this technique as a possible predictor of biotic uptake. Our results showed that the interactions were hard to predict in the complex soil-BC-pesticide system.

Interaction between herbicides applied in mixtures alters the conception of its environmental impact

Herbicide mixtures have often been used to control weeds in crops worldwide, but the behavior of these mixtures in the environment is still poorly understood. Laboratory and greenhouse tests have been conducted to study the interaction of the herbicides diuron, hexazinone, and sulfometuron-methyl which have been applied alone and in binary and ternary mixtures in the processes of sorption, desorption, half-life, and leaching in the soil. A new index of the risk of leaching of these herbicides has also been proposed. The sorption and desorption study has been carried out by the batch equilibrium method. The dissipation of the herbicides has been evaluated for 180 days to determine the half-life (t_1/2). The leaching tests have been carried out on soil columns. The herbicides isolated and in mixtures have been quantified using ultra-high performance liquid chromatography coupled to the mass spectrometer. Diuron, hexazinone, and sulfometuron-methyl in binary and ternary mixtures have less sorption capacity and greater desorption when compared to these isolated herbicides. Dissipation of diuron alone is slower, with a half-life (t_1/2) = 101 days compared to mixtures (t_1/2 between 44 and 66 days). For hexazinone and sulfometuron-methyl, the dissipation rate is lower in mixtures (t_1/2 over 26 and 16 days), with a more pronounced effect in mixtures with the presence of diuron (t_1/2 = 47 and 56 and 17 and 22 days). The binary and ternary mixtures of diuron, hexazinone, and sulfometuron-methyl promoted more significant transport in depth (with the three herbicides quantified to depth P4, P7, and P7, respectively) compared to the application of these isolated herbicides (quantified to depth P2, P4, and P5). Considering the herbicides' desorption and solubility, the new index proposed to estimate the leaching potential allowed a more rigorous assessment concerning the risk of leaching these pesticides, with hexazinone and sulfometuron-methyl presenting a higher risk of contamination of groundwater.

Mechanisms of Tebuconazole Adsorption in Profiles of Mineral Soils

The study attempted to identify the soil components and the principal adsorption mechanisms that bind tebuconazole in mineral soils. The KF values of the Freundlich isotherm determined in 18 soils from six soil profiles in batch experiments after 96 h of shaking ranged from 1.11 to 16.85 μg1-1/n (mL)1/n g-1, and the exponent 1/n values from 0.74 to 1.04. The adsorption of tebuconazole was inversely correlated with the soil pH. Both neutral and protonated forms of this organic base were adsorbed mainly on the fraction of humins. The adsorption of the protonated form increased in the presence of hydrogen cations adsorbed in the soil sorption sites. Fourier transform infrared spectroscopy coupled with the molecular modeling studies and partial least squares regression analysis indicated that the tebuconazole molecule is bound in the organic matter through the formation of hydrogen bonds as well as hydrophobic and π-π interactions. Ion exchange was one of the adsorption mechanisms of the protonated form of this fungicide. The created mathematical model, assuming that both forms of tebuconazole are adsorbed on the organic matter and adsorption of the protonated form is affected by the potential acidity, described its adsorption in soils well.

Adsorption of bentazone in the profiles of mineral soils with low organic matter content

The current laboratory adsorption study aimed at determination of the values of adsorption distribution coefficient (K_d) of bentazone in the profiles of Arenosols, Luvisols, and Cambisols, which are the most common arable mineral soils in Poland. The study attempted to identify the soil components that bind bentazone and the principal adsorption mechanisms of this compound as well as create a model capable of predicting its adsorption in soils. The K_d values determined in batch experiments after 24 h of shaking were very low, and ranged from 0.05 to 0.30 mL/g for the Ap horizon and 0 to 0.07 mL/g for subsoils. The results indicated that the anionic form of bentazone was adsorbed on organic matter, while in acidic soils the neutral form of bentazone was adsorbed on organic matter and sand. The detailed analyses of mineralogical composition revealed that the principal mineral that was responsible for the adsorption of bentazone was quartz, which content was strongly positively correlated with the sand fraction. In soils with pH < 5 and an organic carbon content of < 0.35%, quartz exhibited much greater affinity for the neutral bentazone form than organic matter. Fourier transform infrared photoacoustic spectroscopy analyses supported by computational methods have shown the most probable mechanisms behind the adsorption of bentazone on quartz. The created model, assuming the adsorption of bentazone on organic matter and on sand and using the spectrophotometrically determined dissociation constant of bentazone, very well explained the K_d variance in the 81 examined soils, while correctly predicting the adsorption based on soil properties described in the published data.