Effect of the herbicides oxadiazon and oxyfluorfen on phosphates solubilizing microorganisms and their persistence in rice fields

Environmental fate and aquatic risk assessment of oxyfluorfen in California rice fields

The herbicide oxyfluorfen [OXY; 2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] recently emerged as a potential solution to combat herbicide resistance in California rice. Proposed as a preemergent applied preflood to soil, products are in development for use with OXY-tolerant rice strains. Currently, OXY is not registered for use with rice and its use in or near aquatic resources is restricted due to its high aquatic toxicity. Before OXY may be registered for use in California rice fields, its potential fate and aquatic risk must be evaluated. Particularly important is the identification of the minimum period water must be held on the field (water holding period) necessary for OXY to dissipate below levels of concern. In this assessment, the environmental fate of OXY and its risk to aquatic organisms under simulated California rice field conditions are characterized. The Pesticides in Flooded Applications Model (PFAM) was used to estimate environmental concentrations based on anticipated use patterns and water management practices in California (e.g., winter flooding, turnover, water holding, etc.). Two California rice field soil conditions were simulated in addition to standard soil conditions used in ecological risk assessment for rice. Results suggest OXY is likely to concentrate in sediment, dissipate slowly, and persist. Water holding period had little effect on paddy and release water concentrations. Risks from water column exposure were generally below levels of concern (LOC) for aquatic animals, whereas risks to aquatic plants, algae, and benthic invertebrates exceeded LOCs under all conditions evaluated. California rice field soil conditions were also associated with less risk compared with standard conditions. Reduced application rates were sufficient to reduce risk to acceptable levels in some situations. However, holding times up to 30 days had no effect on risk outcomes, suggesting water management needs of growers should be strongly considered when stipulating water holding periods for OXY.

Nanocomposite based slow release atrazine effectively controlled Striga asiatica incidence, and enhanced sugarcane yield

Sugarcane is a major industrial crop highly susceptible to parasitic weed (Striga spp.), causing a 38% reduction in cane yield due to a longer lag phase of 20-40 days, and wider spacing. Herbicides with a longer retention and slow-release nature could allow Striga seeds to germinate and be killed before attaching to the host. Therefore, a graphene oxide based nanoformulation loaded with atrazine was synthesized and evaluated under controlled and field conditions for its release kinetics, Striga control efficiency (SCE), and cane yield for two years (2018-2019) at two locations. In-vitro assays on release kinetics showed that the release rate of active ingredient (a.i.) from the nanocomposite loaded with atrazine (NCA) was slower (64.5%) than conventional atrazine (82.1%) on the 30th day in water. Similarly, cumulative release percentage of a.i. with NCA was 4.4% compared to atrazine (16.2%) at the initial 0-3 days in soil. Further, field evaluation (deep application in 12 cm furrows) of NCA at 1.25 kg a.i./ha at 95 days after planting (DAP) found superior in delaying Striga emergence by 18-20 days over atrazine. Furthermore, NCA recorded the highest efficacy (∼ 21%) across two locations owing to reduced Striga density (66.7-68.2%) and dry weight (39.3-48.9%). Consequently, NCA at 95 DAP produced higher cane (30.6-31.0%) and sugar (30.7-36.7%) yields. Therefore, carbon-based graphene oxide with a greater surface area and low production cost would offer an environmentally benign and alternative option in controlling Striga before its haustorium attaches to sugarcane roots. This formulation represents a novel direction for developing herbicides with enhanced performance and reduced environmental impact.

Acetyltransferase OsACE2 acts as a regulator to reduce the environmental risk of oxyfluorfen to rice production

The constant use of the pesticide oxyfluorfen (OFF) in farmland contaminates the soil, posing threats to crop growth and human health. To avoid the contamination of food crops with OFF, it is critically important to understand its absorption and degradation mechanisms. In this study, we characterized a new functional locus encoding an acetyltransferase (OsACE2) that can facilitate OFF degradation in rice. OsACE2 was drastically induced by OFF at 0.04-0.2 mg L^-1 for 6 days and the rice growth was significantly inhibited. To demonstrate the regulatory role of OsACE2 in resistance to OFF toxicity, we generated OsACE2 overexpression (OE) and knockout mutant using genetic transformation and gene-editing technologies (CRISPR/Cas9). The OE plants grown in the hydroponic medium showed improved growth (plant elongation and biomass), increased chlorophyll content, and reduced OFF-induced oxidative stress. The OsACE2-improved growth phenotypes of rice were attributed to the significantly lower OFF accumulation in OE plants. Conversely, knocking out OsACE2 resulted in compromised growth phenotypes compared to the wild-type (WT). Using LC-LTQ-HRMS/MS, five mono-metabolites and eleven conjugates of OFF were characterized through various canonical pathways, such as hydrolysis, oxidation, reduction, glycosylation, acetylation, malonylation, and interaction with amino acids. These metabolites increased in the OE plants, and five acetylated conjugates were reported for the first time. Collectively, OsACE2 plays a primary role in catabolizing OFF residues in rice through multiple degradation pathways and reducing OFF in its growth environment.

Assessment of oxyfluorfen-tolerant rice systems and implications for rice-weed management in California

BACKGROUND

Weeds are a significant barrier to rice production in California, exacerbated by lack of chemical control options and herbicide-resistance in persistent aquaphilic species. Oxyfluorfen-tolerant rice, created at the Rice Experiment Station (RES) in Biggs, California, was developed to provide an agronomic program for managing problematic grass and sedge rice-weeds including Oryza sativa f. spontanea Roshev. (weedy 'red' rice). Hand-pulling is the most common removal method for O. sativa spontanea because there are no herbicides registered for this pest in California. Oxyfluorfen was used in combination with oxyfluorfen-tolerant rice in 2019 and 2021 field studies to evaluate rice injury and weed control efficacy on prevalent rice-weed species. Additional studies were conducted in 2021 on University of California Davis campus to determine pre-emergent oxyfluorfen efficacy on four California O. sativa spontanea accessions.

RESULTS

Fields studies indicated minimal crop injury in the first 28 days after seeding (DAS), but no observable injury at 60 DAS in both years. Weed control with oxyfluorfen alone was 87% or greater for all weeds rated with the exception of Schoenoplectus mucronatus (L.) Palla (ricefield bulrush), and Leptochloa fascicularis (Lam.) A. Gray (bearded sprangletop) in both years. All O. sativa spontanea exposed to soil-applied oxyfluorfen successfully emerged through the soil surface, but became completely necrotic 28 days after flooding.

CONCLUSION

Oxyfluorfen-tolerant rice system was demonstrated to be a viable management strategy for California rice growers who struggle with grass- and sedge-weed control as well as provide a novel herbicide option for California O. sativa spontanea management. © 2022 University of California, Davis. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Qualitative and Quantitative Analysis of the New Sulfone Bactericide 2-(4-Fluorophenyl)-5-(Methylsulfonyl)-1,3,4-Oxadiazole and Identification of Its Degradation Pathways in Paddy Water

Rapid and simple methods for the determination of Jiahuangxianjunzuo (JHXJZ) in paddy water, brown rice, soil and rice straw was developed and validated. This method involved the use of ultrahigh-performance liquid chromatography equipped with photodiode array detector. The most important factor was chromatographic conditions, as identified through an orthogonal experimental design. This method showed good recoveries and precisions, thereby indicating its suitability for monitoring of JHXJZ residues in paddy water, brown rice, soil and rice straw. Furthermore, hydrolysis experiment was conducted in the laboratory under pH = 7 buffer solutions, and its degradation product was identified as 2-(4-fluorophenyl)-5-methoxy-1,3,4-oxadiazole by high-resolution mass spectrometry. JHXJZ has a major degradation pathway in the water which the OH- nucleophilic attack the C5 of 1,3,4-oxadiazole ring. Then it leaves mesyl to form intermediate 5-(4-fluorophenyl)-1,3,4-oxadiazol-2-ol and the intermediate combined with methanol formed the degradation product 2-(4-fluorophenyl)-5-methoxy-1,3,4-oxadiazole by the loss of one H2O.The degradation pathways of JHXJZ under the present indoor simulation conditions were proposed.

Electrochemically assisted dewatering for the removal of oxyfluorfen from a coagulation/flocculation sludge

This work focuses on the evaluation of the electrochemical dewatering of sludge obtained in the coagulation of wastes polluted with oxyfluorfen. To do this, sludge samples were treated, aiming not only to reduce the sludge volume, but also to facilitate the degradation of oxyfluorfen contained in the cake via electrolysis with a boron-doped diamond anode. Results show that water can be effectively recovered through three sequential stages. First, a gravity-driven stage, that can recover around 60% of initial volume and where no oxyfluorfen is dragged. Then, a second stage that involves the application of pressure and which accounts for the recuperation of an additional 25% of the total volume of the water removed and in which oxyfluorfen also remained in the cake. Finally, an electrochemical stage, which involves the application of electricity with increasing electric fields (1.0, 2.0, 4.0, and 16.0 V cm^-1), accounting for the recovery of the rest of water released and where an electrolytic degradation of oxyfluorfen is obtained, whose extension depends on the electrode configuration used in the electro-dewatering cell. This electrode configuration also influences the retention or loss of oxyfluorfen from the cake, being the optimum choice the placement of the cathode downstream, next to the outlet of the dewatering cell.

The fungicide "fluopyram" promotes pepper growth by increasing the abundance of P-solubilizing and N-fixing bacteria

Fluopyram, as a reasonably good fungicide and nematicide, is widely used to control agricultural pests worldwide. However, its effects on soil microbial communities and plant growth remain controversial. Therefore, in this study, we investigated the effects of three concentrations (0.5, 1.5, and 5.0 mg/kg) of the fluopyram (Lufuda 41.7% a.i., suspension concentrate, SC) on the pepper rhizosphere microorganisms and pepper seedlings growth in a plant growth room. Moreover, we also investigated the dissipation of fluopyram in the soil, pepper roots, and leaves across a time interval of 45 days. The results showed that fluopyram application increased the number of pepper rhizosphere phosphate (P)-solubilizing bacteria, the abundance of nitrogen (N)-fixing nifH genes, and the pepper seedling growth. The results of terminal restriction fragment length polymorphism (T-RFLP) analysis demonstrated that fluopyram did not alter rhizosphere bacterial community structure and diversity. However, fluopyram did increase the relative abundances of 138 bp and 400 bp T-RFs closely representing Bacillus and Rhizobium genera that were known as efficient plant growth promoting bacteria with P-solubilization and N-fixation properties. Corresponding to the increase of plant growth and beneficial microbes, the half-lives of fluopyram in soil and plant tissues also decreased that nevertheless suggested the role of plant-microbe interactions in the faster removal of fluopyram after application. Our results suggest that short-lived and easily degradable pesticides may have less toxicological effects on soil health while their judicious use may reshape plant-microbe interactions in favor of the plant growth.

Impact of Agrochemicals on Soil Microbiota and Management: A Review

The World Health Organization (WHO) states that in developing nations, there are three million cases of agrochemical poisoning. The prolonged intensive and indiscriminate use of agrochemicals adversely affected the soil biodiversity, agricultural sustainability, and food safety, bringing in long-term harmful effects on nutritional security, human and animal health. Most of the agrochemicals negatively affect soil microbial functions and biochemical processes. The alteration in diversity and composition of the beneficial microbial community can be unfavorable to plant growth and development either by reducing nutrient availability or by increasing disease incidence. Currently, there is a need for qualitative, innovative, and demand-driven research in soil science, especially in developing countries for facilitating of high-quality eco-friendly research by creating a conducive and trustworthy work atmosphere, thereby rewarding productivity and merits. Hence, we reviewed (1) the impact of various agrochemicals on the soil microbial diversity and environment; (2) the importance of smallholder farmers for sustainable crop protection and enhancement solutions, and (3) management strategies that serve the scientific community, policymakers, and land managers in integrating soil enhancement and sustainability practices in smallholder farming households. The current review provides an improved understanding of agricultural soil management for food and nutritional security.

Application of rice (Oryza sativa L.) root endophytic diazotrophic Azotobacter sp. strain Avi2 (MCC 3432) can increase rice yield under green house and field condition

Use of plant-associated beneficial microbes, especially endophytes are getting popular day by day as they occupy a relatively privileged niche inside different plant tissues with lesser competition for food and shelter than rhizosphere. The effects of different physical factors like temperature, rainfall, and seasonal variation and UV radiation on plant growth promoting endophytic communities are less pronounced than those on the rhizospheric and phylloplane microbes. This present work has been compromised with further utilization of an indigenous rice (Oryza sativa L.) root endophytic Azotobacter sp. strain Avi2 (MCC 3432) (AzA) as a bio-formulation for sustainable rice production based on several physiological parameters (plant height, root length/weight, leaf area, yield, chlorophyll contain), in-vitro comparative plant growth promoting assays, greenhouse and field experiments (dry and wet season). Treatments with AzA exhibited higher yield as well as maximal chlorophyll fluorescence (Fm) of flag leaves in flowering and grain filling stages indicating higher photosynthetic rates. Scanning electron microscopic image of rice roots demonstrated accumulation of bacterial biofilm at the junction of primary and lateral root confirming the root-colonizing ability of the bacterium. The results of the study were quite encouraging as AzA exhibited better vegetative and reproductive growth of rice in pot and field experiment compared to formulated rhizospheric Azotobacter sp. (commercial product). Apart from that plants treated with AzA (supplemented 50% nitrogenous fertilizer of recommended dose) exhibited similar yield parameters when it was compared with the recommended dose of fertilizer (RDF; 120:60:60 mg N:P:K kg^-1 soil/ without any bacterial). Therefore, it can be concluded that application of this plant growth promoting endophyte can reduce a substantial amount of N-fertilizer for field application.

Effect of Soil Fumigation on Degradation of Pendimethalin and Oxyfluorfen in Laboratory and Ginger Field Studies

Herbicides are usually applied to agricultural fields following soil fumigation to provide effective weed control in high-value cash crops. However, phytotoxicity has been observed in ginger seedlings following the application of herbicides in fumigated fields. This study tested a mixture of herbicides (pendimethalin and oxyfluorfen) and several fumigant treatments in laboratory and field studies to determine their effect on the growth of ginger. The results showed that soil fumigation significantly (P < 0.05) extended the degradation period of these herbicides in the field and in laboratory studies. The half-life of pendimethalin was extended by an average of approximately 1.29 times in the field and 1.74 times in the laboratory. The half-life of oxyfluorfen was extended by an average of about 1.19 times in the field and 1.32 times in the laboratory. Moreover, the extended period of herbicide degradation in the fumigant and nonfumigant treatments significantly reduced ginger plant height, leaf number, stem diameter, and the chlorophyll content. The study concluded that applying a dose below the recommended rate of these herbicides in chloropicrin (CP) or CP + 1,3-dichloropropene fumigated ginger fields is appropriate, as application of the recommended herbicide dose in fumigated soil may be phytotoxic to ginger.

Removal of oxyfluorfen from spiked soils using electrokinetic soil flushing with the surrounding arrangements of electrodes

This work reports the results of a study in which the remediation of soil that undergoes an accidental discharge of oxyfluorfen is carried out by using electrokinetic soil flushing (EKSF). Two different electrode configurations were tested, consisting of several electrodes surrounding an electrode of different polarity (so-called 1A6C, one anode surrounded by six cathodes, and 1C6A, one cathode surrounded by six cathodes). A pilot plant scale was used (with a soil volume of 175dm(3)) to perform the studies. During the tests, different parameters were measured daily (flowrates, pH, electrical conductivity and herbicide concentration in different sampling positions). Furthermore, at the end of the test, a complete post-mortem analysis was carried out to obtain a 3-D map of the pollution, pH and electrical conductivity in the soil. The results demonstrate that electrode arrangement is a key factor for effective pollutant removal. In fact, the 1A6C configuration improves the removal rate by 41.3% versus the 27.0% obtained by the 1C6A configuration after a period of 35days. Finally, a bench mark comparison of this study of soil remediation polluted with 2,4-D allows for significant conclusions about the scale-up and full-scale application of this technology.

Removal of oxyfluorfen from ex-situ soil washing fluids using electrolysis with diamond anodes

In this research, firstly, the treatment of soil spiked with oxyfluorfen was studied using a surfactant-aided soil-washing (SASW) process. After that, the electrochemical treatment of the washing liquid using boron doped diamond (BDD) anodes was performed. Results clearly demonstrate that SASW is a very efficient approach in the treatment of soil, removing the pesticide completely by using dosages below 5 g of sodium dodecyl sulfate (SDS) per Kg of soil. After that, complete mineralization of organic matter (oxyflourfen, SDS and by-products) was attained (100% of total organic carbon and chemical oxygen demand removals) when the washing liquids were electrolyzed using BDD anodes, but the removal rate depends on the size of the particles in solution. Electrolysis of soil washing fluids occurs via the reduction in size of micelles until their complete depletion. Lower concentrations of intermediates are produced (sulfate, chlorine, 4-(trifluoromethyl)-phenol and ortho-nitrophenol) during BDD-electrolyzes. Finally, it is important to indicate that, sulfate (coming from SDS) and chlorine (coming from oxyfluorfen) ions play an important role during the electrochemical organic matter removal.

Effect of Pre-emergence Herbicides on Microbial Biomass and Biochemical Processes in a Typic Fluvaquent Soil Amended with Farm Yard Manure

Application of thiobencarb, pendimethalin and pretilachlor at rates of 7.5, 10.0 and 2.5 kg a.i. ha(-1), respectively, under laboratory conditions, significantly increased microbial biomass C, N and P, resulting in greater availability of C, N and P in soil amended with farm yard manure. Application of thiobencarb highly induced microbial biomass C (46.3 %) and N (40.6 %), while pretilachlor and thiobencarb augmented microbial biomass P to the extent of 14.9 % and 14.1 %, respectively. Application of pendimethalin retained the highest amount of total N (19.9 %), soluble NO3 (-) (56 %) and available P (69.5 %) in soil. A similar trend was recorded with thiobencarb for oxidizable organic C (18.1 %) and with pretilachlor for exchangeable NH4 (+) (65.8 %). At the end of the experiment, the highest stimulation of bacteria was recorded with thiobencarb (29.6 %), while pretilachlor harboured the maximum number of actinomycetes (37.2 %) and fungi (40 %) in soil compared to the untreated control.

Non-symbiotic N2-fixation and phosphate-solubility in Gangetic alluvial soil as influenced by pre-emergence herbicide residues

An experiment has been conducted under laboratory conditions to investigate the effect of two pre-emergence herbicides viz., thiobencarb (at 1.5 and 4.5 kg a.i. ha(-1)) and pretilachlor (at 0.5 and 1.5 kg a.i. ha(-1)), on the changes of growth and activities of aerobic non-symbiotic N2-fixing bacteria and phosphate-solubilizing microorganisms in relation to availability of mineral nitrogen and soluble phosphorus in the Gangetic alluvial soil (Typic Haplustept) of West Bengal, India. Application of herbicides, in general, significantly increased growth and activities of microorganisms, resulting in greater release of available nitrogen and soluble phosphorus in soil; and the stimulation was more pronounced when the herbicides were applied at their lower concentrations (recommended field application rates), more so with thiobencarb, as compared to pretilachlor. As compared to untreated control, application of thiobencarb at lower concentration increased the proliferation of aerobic non-symbiotic N2-fixing bacteria, phosphate-solubilizing microorganisms and non-symbiotic N2-fixing capacity of soil to the extent of 54.0, 44.6 and 31.7%, respectively; and accumulated the highest amount of available nitrogen (37.8%) and phosphorus (54.5%) in soil, while pretilachlor at field application rate highly induced (37.2%) phosphate-solubilizing capacity of soil. At higher concentration, pretilachlor was superior to thiobencarb in augmenting the growth and activities of phosphate-solubilizers. The results of the present study also indicated that gradual increase in concentration of the herbicides over their recommended field application rates was not much conducive for growth and activities of microorganisms, and subsequent release of nutrients in soil.

Residual effect of pre-emergence herbicides on microbial activities in relation to mineralization of C, N and P in the Gangetic alluvial soil of West Bengal, India

An experiment has been conducted under laboratory conditions to investigate the residual effect of three pre-emergence herbicides (thiobencarb, pendimethalin and pretilachlor) at fivefold field application rates (7.5, 10.0 and 2.5 kg a.i. ha(-1), respectively), on the changes of microbial activities and some biochemical processes in the Gangetic alluvial soil of West Bengal. Application of herbicides in general significantly increased microbial biomass resulting in greater mineralization of C, N and P in soil. The highest stimulation of microbial biomass C was recorded with thiobencarb (24.4%) followed by pendimethalin (23.4%). Microbial biomass N was highly induced under pretilachlor (54.5%) and thiobencarb (52.7%), while the stimulation of microbial biomass P was at par in the herbicide-treated soils. Compared to untreated control, the highest amount of organic C was retained with thiobencarb followed by pendimethalin. A similar trend was recorded with thiobencarb for total N, while pendimethalin induced exchangeable NH4 (+) and soluble NO3 (-) to the highest extent (42.2 and 34.5%, respectively). Regarding the availability of P in soil, pretilachlor manifested greater stimulation (33.1%) than thiobencarb (21.6%) and pendimethalin (11.4%). As compared to untreated control, thiobencarb harboured maximum number of bacteria (107.9%), while pretilachlor exerted the highest stimulations towards the proliferations of actinomycetes (132.6%) and fungi (149.5%) in soil.

Behaviour of oxyfluorfen in soils amended with edaphic biostimulants/biofertilizers obtained from sewage sludge and chicken feathers. Effects on soil biological properties

We studied the behaviour of oxyfluorfen herbicide at a rate of 4 l ha(-1) on biological properties of a Calcaric Regosol amended with two edaphic biostimulants/biofertilizers (SS, derived from sewage sludge; and CF, derived from chicken feathers). Oxyfluorfen was surface broadcast on 11 March 2013. Two days after application of oxyfluorfen to soil, both biostimulants/biofertilizers (BS) were also applied to the soil. An unamended soil without oxyfluorfen was used as control. For 2, 4, 7, 9, 20, 30, 60, 90 and 120 days of the application of herbicide to the soil and for each treatment, the soil dehydrogenase, urease, β-glucosidase and phosphatase activities were measured. For 2, 7, 30 and 120 days of the application of herbicide to the soil and for each treatment, soil microbial community was determined. The application of both BS to soil without the herbicide increased the enzymatic activities and soil biodiversity, mainly at 7 days of beginning the experiment. However, this stimulation was higher in the soil amended with SS than for CF. The application of herbicide in organic-amended soils decreased the inhibition of soil enzymatic activities and soil biodiversity. Possibly, the low-molecular-weight protein content easily assimilated by soil microorganisms is responsible for less inhibition of these soil biological properties.

Persistence of oxyfluorfen in soil, runoff water, sediment and plants of a sunflower cultivation

A field dissipation and transport study of oxyfluorfen in a sunflower cultivation under Mediterranean conditions have been conducted in silty clay plots (cultivated and uncultivated) with two surface slopes (1% and 5%). The soil dissipation and transport of oxyfluorfen in runoff water and sediment, as well as the uptake by sunflower plants, were investigated over a period of 191 days. Among different kinetic models assayed, soil dissipation rate of oxyfluorfen was better described by first-order kinetics. The average half-life was 45 and 45.5 days in cultivated plots with soil slopes 5% and 1% respectively, and 50.9 and 52.9 days in uncultivated plots with soil slopes 5% and 1%. The herbicide was detected below the 10 cm soil layer 45 days after application (DAA). Limited amounts of oxyfluorfen were moved with runoff water and the cumulative losses from tilled and untilled plots with slope 5% were estimated at 0.007% and 0.005% of the initial applied active ingredient, while for the plots with slope of 1%, the respective values were 0.002% and 0.001%. The maximum concentration of oxyfluorfen in sediment ranged from 1.46 μg g(-1) in cultivated plot with soil slope 1% to 2.33 μg g(-1) in uncultivated plot with soil slope 5%. The cumulative losses from tilled and untilled plots with slope 5% were estimated at 0.217% and 0.170% while for the plots with slope of 1%, the respective values were 0.055% and 0.025%. Oxyfluorfen was detected in sunflower plants until the day of harvest; maximum concentrations in stems and leaves (0.042 μg g(-1)) were observed 33 DAA and in roots (0.025 μg g(-1)) 36 DAA. In conclusion, oxyfluorfen hardly moves into silty clay soil and exhibited low run-off potential so it represents a low risk herbicide for the contamination of ground and adjacent water resources.

Effect of combined application of systemic herbicides on microbial activities in north bengal alluvial soil

An experiment was conducted under laboratory conditions to investigate the effect of combined application of three systemic herbicides, viz., fenoxaprop, pendimethalin and paraquat at 50 g, 1.0 kg and 1.0 kg a.i. ha(-1), respectively, on the changes of microbial biomass C, N and P in relation to transformations and availability of some plant nutrients in an alluvial soil (Typic Orchraqualf) of West Bengal, India. Application of the herbicides, in general, significantly increased microbial biomass C, N and P, resulting in greater availability of C, N and P in soil. The microbial biomass C was highly induced (28.4 %) when fenoxaprop was applied with pendimethalin, while fenoxaprop along with paraquat exerted maximum stimulation towards microbial biomass N (19.9 %) and biomass P (16.2 %). Application of fenoxaprop along with pendimethalin retained the highest amount of organic C (17.4 %), exchangeable NH4(+) (29 %) and available P (19.6 %), while fenoxaprop with paraquat augmented total N and soluble NO3(-) by 21.4 % and 25.2 %, respectively in soil.

Effect of systemic herbicides on microbial biomass in relation to availability of some plant nutrients in an alluvial soil of West Bengal

An experiment was conducted under laboratory conditions to investigate the effect of three herbicides, viz., fenoxaprop, pendimethalin and paraquat, at their recommended field application rates (50 g, 1.0 kg and 1.0 kg a.i. ha⁻¹, respectively), on the changes of microbial biomass C, N and P in relation to transformations and availability of plant nutrients in an alluvial soil (Typic Orchraqualf) from West Bengal, India. Application of herbicides, in general, significantly increased the microbial biomass C, N and P, resulting in greater availability of these plant nutrients in soil. The microbial biomass C was highly increased due to application of fenoxaprop (39.8%) followed by paraquat (28.2%). Application of pendimethalin led to the maximum stimulation of microbial biomass N (37.1%), while microbial biomass P was increased by 15.2% following the incorporation of paraquat into the soil. Compared to untreated control soil, the soil retained the highest amount of organic C (19.8%) when it was treated with pendimethalin. A similar trend was recorded with fenoxaprop for total N (19.6%) and exchangeable NH₄⁺ (21.3%) in soil. The stimulation of soluble NO₃⁻ was highest under fenoxaprop (22.3%) followed by paraquat (20.7%). Regarding the availability of P in soil, paraquat manifested greater stimulation (17.8%) than fenoxaprop (15.4%) or pendimethalin (13.1%). Application of paraquat also gave the highest amount of total P (17.8%) in the soil solution.

Field dissipation of oxyfluorfen in onion and its dynamics in soil under Indian tropical conditions

Oxyfluorfen, a diphenyl-ether herbicide is being used to control annual and perennial broad-leaved weeds and sedges in a variety of field crops including onion. The present study was aimed to investigate the dynamics and field persistence of oxyfluorfen in onion plant, bulb and soil under Indian tropical conditions. Application of four rates of oxyfluorfen viz., 200, 250, 300 and 400 g AI ha(-1) as pre-emergence gave good weed control in field experiment with onion. The oxyfluorfen residue dissipated faster in plant than in soil respectively, with a mean half-life of 6.1 and 11.2 days. Dissipation followed first-order kinetics. In laboratory column leaching experiments, 17 percent of the applied oxyfluorfen was recovered from the soil and indicates its solubility in water and mobility in sandy clay loam soil was low. A sorption study revealed that the adsorption of oxyfluorfen to the soil was highly influenced by the soil organic carbon with the Koc value of 5450. The study concludes that the dissipation of oxyfluorfen in soil and onion was dependent on the physico-chemical properties of the soil and environmental conditions.

Soil application of dinitroaniline and arylphenoxy propionic herbicides influences the activities of phosphate-solubilizing microorganisms in soil

An experiment was conducted under laboratory conditions to investigate the effect of two systemic herbicides, viz. pendimethalin (a dinitroaniline) and quizalofop (an arylphenoxy propionic acid) at their recommended field application rates (1.0 kg and 50 g active ingredient per hectare, respectively), either separately or in a combination, on growth and activities of phosphate-solubilizing microorganisms in relation to their effects on biochemical transformations and availability of organic carbon, total and available phosphorus in a Typic Haplustept soil of West Bengal, India. Application of herbicides, in general, significantly stimulated the growth and activities of phosphate-solubilizing microorganisms which increased microbial biomass resulting in higher accumulation of oxidizable organic carbon, total and available phosphorus in soil as compared to untreated control. The combined application of both the herbicides highly stimulated the proliferations of phosphate-solubilizing microorganisms, while pendimethalin alone significantly accentuated phosphate-solubilizing capacities 36.4% as compared to untreated control and retained highest amount of total phosphorus due to greater microbial activities in soil. The separate application of quizalofop also manifested an induced effect on the proliferations of phosphate-solubilizing microorganisms and accounted significant amounts of organic carbon and available phosphorus in the soil system. The results of the present study thus indicated that the cited herbicides at their field application rates can be safely used to eradicate weeds in the crop fields.

Effect of pendimethalin and quizalofop on N2-fixing bacteria in relation to availability of nitrogen in a Typic Haplustept soil of West Bengal, India

An experiment was conducted under laboratory conditions to investigate the effect of two systemic herbicides viz., pendimethalin and quizalofop, at their recommended field rates (1.0 kg and 50 g active ingredient ha(- 1), respectively) on the growth and activities of non-symbiotic N(2)-fixing bacteria in relation to mineralization and availability of nitrogen in a Typic Haplustept soil. Both the herbicides, either singly or in a combination, stimulated the growth and activities of N(2)-fixing bacteria resulting in higher mineralization and availability of nitrogen in soil. The single application of quizalofop increased the proliferation of aerobic non-symbiotic N(2)-fixing bacteria to the highest extent while that of pendimethalin exerted maximum stimulation to their N(2)-fixing capacity in soil. Both the herbicides, either alone or in a combination, did not have any significant difference in the stimulation of total nitrogen content and availability of exchangeable NH(4)(+) while the solubility of NO(3)(-) was highly manifested when the herbicides were applied separately in soil.

Acute exposure of the aquatic macrophyte Callitriche obtusangula to the herbicide oxadiazon: the protective role of N-acetylcysteine

In this study we investigated the acute exposure of the aquatic macrophyte Callitriche obtusangula to the herbicide oxadiazon (Ronstar). The toxic effects on C. obtusangula were evaluated, 24h after exposure, by assessing visible necrotic leaf lesions and, 12 h after exposure, via analyses of dead cells and hydrogen peroxide (H2O2) deposits localized by histocytochemical analysis with Trypan blue and 3,3'-diaminobenzidine (DAB), respectively. As a result, we found that 0.1275 microg L(-1) a.i. (active ingredient) oxadiazon was the maximum concentration that produced no observable adverse effects (NOAEC) both at leaf and tissue levels, at any considered exposure time. Additionally, we assayed the protective effect of pre-treatment with 0.25 mM N-acetylcysteine (NAC), a cysteine donor, on the damage caused by the toxic herbicidal dose of 6.37 microg L(-1) a.i to C. obtusangula, correlating the NAC observed protection to the direct H2O2-scavenging and to the enhancement of glutathione parameters. NAC-treated plants showed a fourfold increase in the GSH (reduced glutathione)+GSSG (oxidised glutathione) content (149.2 nmol g(-1) FW) compared to controls (36.1 nmol g(-1) FW); in the NAC+oxadiazon treatments, the GSH+GSSG content was more than fivefold higher (202.1 nmol g(-1) FW). GSH showed a similar trend in NAC and NAC+oxadiazon treatments, being six- (130.0 nmol g(-1) FW) and eightfold (185.0 nmol g(-1) FW) higher, respectively, compared to controls (20.7 nmol g(-1) FW). Accordingly, the GSH/GSSG ratio in NAC- and NAC+oxadiazon-treated plants was significantly increased compared to controls, indicating alleviation of oxidative stress.

Effect of systemic herbicides on N2-fixing and phosphate solubilizing microorganisms in relation to availability of nitrogen and phosphorus in paddy soils of West Bengal

A field experiment has been conducted with four systemic herbicides viz., butachlor [N-(butoxymethyl)-2-chloro-2',6'-diethyl-acetanilide], fluchloralin [N-(2-chloroethyl)-(2,6-dinitro-N-propyl-4-trifluoromethyl) aniline], oxadiazon [5-terbutyl-3-(2,4-dichloro-5-isopro poxyphenyl)-1,3,4-oxadiazol-2-one] and oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenyl)-4-(trifluoromethyl) benzene] at their recommended field rates (2.0, 1.5, 0.4 and 0.12kga.i.ha(-1), respectively) to investigate their effects on growth and activities of aerobic non-symbiotic N(2)-fixing bacteria and phosphate solubilizing microorganisms in relation to availability of nitrogen and phosphorus in the rhizosphere soils as well as yield of the rice crop (Oryza sativa L cv. IR-36). Application of herbicides, in general, highly stimulated the population and activities of the target microorganisms, which resulted in a greater amount of atmospheric nitrogen fixation and phosphate solubilization in the rhizosphere soils of the test crop. The greater microbial activities subsequently augmented the mineralization and availability of nitrogen and phosphorus in the soil solution, which in turn increased the yield of the crop. Among the herbicides, oxyfluorfen was most stimulative followed by fluchloralin and oxadiazon in augmenting the microbial activities in soil. Butachlor also accentuated the mineralization and availability of nitrogen due to higher incitement of non-symbiotic N(2)-fixing bacteria in paddy soil. The grain and straw yields of the crop were also significantly increased due to the application of oxyfluorfen (20.2% and 21%) followed by fluchloralin (13.1% and 15.4%) and butachlor (9.1% and 10.2%), respectively.

Inductive effects of environmental concentration of atrazine on Escherichia coli and Enterococcus faecalis

Atrazine solutions (0.1, 1, 10 and 100 microg/L) inoculated with Escherichia coli and Enterococcus faecalis under natural conditions significantly increased (p < or = 0.05) the population levels of both test bacteria; it indicates the ability of bacterial cells to degrade atrazine and to use the original compound or its degradation products as nutrient(s). In some cases, alterations in the morphology of the colonies were also observed on selective solid media. Biochemical differentiation was also found and, on the other hand, a loss of culturability was recorded; this suggests that bacteria have entered in a viable but nonculturable state. A re-appearance of the colonies occurred after inoculation on tryptone-soy agar with atrazine.