Comparative analysis of three next-generation sequencing techniques to measure nosZ gene abundance in Missouri claypan soils.

Quantitative next-generation sequencing techniques have been critical in gaining a better understanding of microbial ecosystems. In soils, denitrifying microorganisms are responsible for dinitrogen (N2) production. The nosZ gene codes for nitrous oxide reductase, the enzyme facilitating the reduction of nitrous oxide (N2O) to N2. The objectives of this research were to: 1) understand how soil depth influences RNA concentration and nosZ gene abundance; 2) assess the spatial dependence of nosZ gene abundance in two claypan soil fields; and 3) compare and evaluate multiple RNA-based sequencing methods for quantifying nosZ gene abundance in soils in relation to dinitrogen (N2) production. Research sites consisted of two intensively studied claypan soil fields in Central Missouri, USA. Soil cores were collected from two landscape transects across both fields and analyzed for extractable soil RNA at two depths (0-15 cm and 15-30 cm). Measurements of nosZ gene abundance were obtained using real-time quantitative polymerase chain reaction (RT-qPCR), droplet digital polymerase chain reaction (ddPCR), and nanostring sequencing (NS). In both fields, soil RNA concentrations were significantly greater at 0-15 cm depth compared to 15-30 cm. These data indicated low overall soil microbial activity below 15 cm. Due to low quantities of extractable soil RNA in the subsoil, nosZ gene abundance was only determined in the 0-15 cm depth. Sequencing method comparisons of average nosZ gene abundance showed that NS results were constrained to a narrow range and were 10-20-fold lower than ddPCR and RT-qPCR at each landscape position within each field. Droplet digital PCR appears to be the most promising method, as it reflected changes in N2 production across landscape position.

Degradation kinetics of veterinary antibiotics and estrogenic hormones in a claypan soil.

Veterinary antibiotics and estrogens are excreted in livestock waste before being applied to agricultural lands as fertilizer, resulting in contamination of soil and adjacent waterways. The objectives of this study were to 1) investigate the degradation kinetics of the VAs sulfamethazine and lincomycin and the estrogens estrone and 17ß-estradiol in soil mesocosms, and 2) assess the effect of the phytochemical DIBOA-Glu, secreted in eastern gamagrass (Tripsacum dactyloides) roots, on antibiotic degradation due to the ability of DIBOA-Glu to facilitate hydrolysis of atrazine in solution assays. Mesocosm soil was a silt loam representing a typical claypan soil in portions of Missouri and the Central United States. Mesocosms (n = 133) were treated with a single target compound (antibiotic concentrations at 125 ng g-1 dw, estrogen concentrations at 1250 ng g-1 dw); a subset of mesocosms treated with antibiotics were also treated with DIBOA-Glu (12,500 ng g-1 dw); all mesocosms were kept at 60% water-filled pore space and incubated at 25 °C in darkness. Randomly chosen mesocosms were destructively sampled in triplicate for up to 96 days. All targeted compounds followed pseudo first-order degradation kinetics in soil. The soil half-life (t0.5) of sulfamethazine ranged between 17.8 and 30.1 d and ranged between 9.37 and 9.90 d for lincomycin. The antibiotics results showed no significant differences in degradation kinetics between treatments with or without DIBOA-Glu. For estrogens, degradation rates of estrone (t0.5 = 4.71-6.08 d) and 17ß-estradiol (t0.5 = 5.59-6.03 d) were very similar; however, results showed that estrone was present as a metabolite in the 17ß-estradiol treated mesocosms and vice-versa within 24 h. The antibiotics results suggest that sulfamethazine has a greater potential to persist in soil than lincomycin. The interconversion of 17ß-estradiol and estrone in soil increased their overall persistence and sustained soil estrogenicity. This study demonstrates the persistence of these compounds in a typical claypan soil representing portions of the Central United States.

Vegetative buffer strips show limited effectiveness for reducing antibiotic transport in surface runoff.

Vegetative buffer strips (VBS) have been demonstrated to effectively reduce loads of sediment, nutrients, and herbicides in surface runoff, but their effectiveness for reducing veterinary antibiotic (VA) loads in runoff has not been well documented. The objective of this study was to determine the effectiveness of VBS vegetation and width on surface runoff loads of the VAs sulfamethazine (SMZ) and lincomycin (LIN). Experimental design of the plots (1.5 × 25 m) was a two-way factorial with four vegetation treatments (tall fescue [Festuca aruninacea Schreb.]; tall fescue with switchgrass [Panicum virgatum L.] hedge; warm-season native grass mix; and continuous fallow control), and four buffer widths (0, 2, 5, and 9 m). Turkey litter spiked with SMZ and LIN was applied to the source area (upper 7 m) of each plot, and runoff was collected at each width. Runoff was generated with a rotating boom simulator. Results showed VA loads in runoff at the 0-m sampler ranged from 3.8 to 5.9% of applied, and overall VA transport in runoff was predominately in the dissolved phase (90% for SMZ and 99% for LIN). Among vegetation treatments, only tall fescue significantly reduced loads of SMZ and LIN compared with the control, with load reductions of ∼30% for both VAs. Estimated field-scale reductions in VA loads showed that source-to-buffer area ratios (SBARs) of 10:1 to 20:1 reduced VA loads by only 7 to 16%. Overall, the grass VBS tested here were less effective at reducing SMZ and LIN loads in surface runoff than has been previously demonstrated for sediment, nutrients, and herbicides.

Estimating simazine-treated area in watersheds based on annual stream loads.

Existing data in the United States are insufficient for estimating pesticide-treated crop areas at the watershed scale. The objective of this research was to evaluate an approach for estimating simazine usage on corn (Zea mays L.) based on its transport to streams of the Salt River Basin (SRB) of Missouri, USA. Annual loads of total simazine and atrazine (parent + metabolites) were quantified for seven SRB watersheds from 2005 to 2017. Simazine-treated corn area was computed as the total simazine load (g) divided by total atrazine load (g ha-1 ) on a treated area basis; atrazine was used as surrogate in the absence of treated area simazine load data. From 2005 to 2010, an estimated 3.8-31% of the corn area within SRB watersheds was treated with simazine, and four of six watersheds had <10% of corn treated. In contrast, Long Branch Creek (2005-2017) and its sub-watersheds (2012-2017) had ≥20% of corn area treated with simazine. Key sources of variation in treated area estimates included extremely dry years with little simazine transport and disparities between spring-applied atrazine and fall-applied simazine transport. However, compared with national estimates for the SRB, these results estimated simazine usage that was generally one to two orders of magnitude greater and showed far more spatial and temporal variation among watersheds. These results demonstrated that this broadly applicable output-based method is a significant improvement over existing input-based national data for estimating pesticide usage in watersheds.

Clothianidin decomposition in Missouri wetland soils.

Neonicotinoid pesticides can persist in soils for extended time periods; however, they also have a high potential to contaminate ground and surface waters. Studies have reported negative effects associated with neonicotinoids and nontarget taxa, including aquatic invertebrates, pollinating insect species, and insectivorous birds. This study evaluated factors associated with clothianidin (CTN) degradation and sorption in Missouri wetland soils to assess the potential for wetland soils to mitigate potential environmental risks associated with neonicotinoids. Solid-to-solution partition coefficients (Kd ) for CTN sorption to eight wetland soils were determined via single-point sorption experiments, and sorption isotherm experiments were conducted using the two most contrasting soils. Clothianidin degradation was determined under oxic and anoxic conditions over 60 d. Degradation data were fit to zero- and first-order kinetic decay models to determine CTN half-life (t0.5 ). Sorption results indicated CTN sorption to wetland soil was relatively weak (average Kd , 3.58 L kg-1 ); thus, CTN has the potential to be mobile and bioavailable within wetland soils. However, incubation results showed anoxic conditions significantly increased CTN degradation rates in wetland soils (anoxic average t0.5 , 27.2 d; oxic average t0.5 , 149.1 d). A significant negative correlation was observed between anoxic half-life values and soil organic C content (r2  = .782; p = .046). Greater CTN degradation rates in wetland soils under anoxic conditions suggest that managing wetlands to facilitate anoxic conditions could mitigate CTN presence in the environment and reduce exposure to nontarget organisms.

Mechanisms of Bond Cleavage during Manganese Oxide and UV Degradation of Glyphosate: Results from Phosphate Oxygen Isotopes and Molecular Simulations.

Degradation of glyphosate in the presence of manganese oxide and UV light was analyzed using phosphate oxygen isotope ratios and density function theory (DFT). The preference of C-P or C-N bond cleavage was found to vary with changing glyphosate/manganese oxide ratios, indicating the potential role of sorption-induced conformational changes on the composition of intermediate degradation products. Isotope data confirmed that one oxygen atom derived solely from water was incorporated into the released phosphate during glyphosate degradation, and this might suggest similar nucleophilic substitution at P centers and C-P bond cleavage both in manganese oxide- and UV light-mediated degradation. The DFT results reveal that the C-P bond could be cleaved by water, OH- or •OH, with the energy barrier opposing bond dissociation being lowest in the presence of the radical species, and that C-N bond cleavage is favored by the formation of both nitrogen- and carbon-centered radicals. Overall, these results highlight the factors controlling the dominance of C-P or C-N bond cleavage that determines the composition of intermediate/final products and ultimately the degradation pathway.

Veterinary Antibiotic Effects on Atrazine Degradation and Soil Microorganisms.

Veterinary antibiotics (VAs) in manure applied to agricultural lands may change agrichemical degradation by altering soil microbial community structure or function. The objectives of this study were to investigate the influence of two VAs, sulfamethazine (SMZ) and oxytetracycline (OTC), on atrazine (ATZ) degradation, soil microbial enzymatic activity, and phospholipid fatty acid (PLFA) markers. Sandy loam soil with and without 5% swine manure (w/w) was amended with 0 or 500 µg kgC radiolabeled ATZ and with 0, 100, or 1000 µg kg SMZ or OTC and incubated at 25°C in the dark for 96 d. The half-life of ATZ was not significantly affected by VA treatment in the presence or absence of manure; however, the VAs significantly ( < 0.05) inhibited ATZ mineralization in soil without manure (25-50% reduction). Manure amendment decreased ATZ degradation by 22%, reduced ATZ mineralization by 50%, and increased the half-life of ATZ by >10 d. The VAs had limited adverse effects on the microbial enzymes ß-glucosidase and dehydrogenase in soils with and without manure. In contrast, manure application stimulated dehydrogenase activity and altered chlorinated ATZ metabolite profiles. Concentrations of PLFA markers were reduced by additions of ATZ, manure, OTC, and SMZ; adverse additive effects of combined treatments were noted for arbuscular mycorrhizal fungi and actinobacteria. In this work, the VAs did not influence persistence of the ATZ parent compound or chlorinated ATZ metabolite formation and degradation. However, reduced CO evolved from VA-treated soil suggests an inhibition to the degradation of other ATZ metabolites due to an altered soil microbial community structure.

Long-term agroecosystem research in the central Mississippi river basin: introduction, establishment, and overview.

Many challenges currently facing agriculture require long-term data on landscape-scale hydrologic responses to weather, such as from the Goodwater Creek Experimental Watershed (GCEW), located in northeastern Missouri, USA. This watershed is prone to surface runoff despite shallow slopes, as a result of a significant smectitic clay layer 30 to 50 cm deep that restricts downward flow of water and gives rise to a periodic perched water table. This paper is the first in a series that documents the database developed from GCEW. The objectives of this paper are to (i) establish the context of long-term data and the federal infrastructure that provides it, (ii) describe the GCEW/ Central Mississippi River Basin (CMRB) establishment and the geophysical and anthropogenic context, (iii) summarize in brief the collected research results published using data from within GCEW, (iv) describe the series of papers this work introduces, and (v) identify knowledge gaps and research needs. The rationale for the collection derives from converging trends in data from long-term research, integration of multiple disciplines, and increasing public awareness of increasingly larger problems. The outcome of those trends includes being selected as the CMRB site in the USDA-ARS Long-Term Agro-Ecosystem Research (LTAR) network. Research needs include quantifying watershed scale fluxes of N, P, K, sediment, and energy, accounting for fluxes involving forest, livestock, and anthropogenic sources, scaling from near-term point-scale results to increasingly long and broad scales, and considering whole-system interactions. This special section informs the scientific community about this database and provides support for its future use in research to solve natural resource problems important to US agricultural, environmental, and science policy.

Long-term agroecosystem research in the central Mississippi river basin: hydrogeologic controls and crop management influence on nitrates in loess and fractured glacial till.

Nitrogen from agriculture is known to be a primary source of groundwater NO-N. Research was conducted in a northeastern Missouri watershed to assess the impact of cropping systems on NO-N for a loess and fractured glacial till aquifer underlying claypan soils. Three cropped fields with 10 yr of similar management were each instrumented with 20 to 25 monitoring wells, 3 to 15 m in depth, in 1991 to 1992. Wells were sampled and analyzed for NO-N at least annually from 1991 to 2004. Initial NO-N concentrations were variable, ranging from undetectable to >24 mg L but averaged 7.0 mg L. Groundwater NO-N was significantly higher in Field 3, probably the result of concurrent applications of manure and N fertilizer before 1980. Overall changes in NO-N levels in Fields 1 and 2 were generally small; however, NO-N levels for Field 3 have decreased an average of 0.28 mg L yr. Excessive loading of N into the matrix of the glacial till may have had a long-term impact on NO-N for this field. Despite the presence of dissolved O in the aquifer, evidence of denitrification in some upper-landscape groundwater wells was found. The greatest decreases in NO-N concentration occurred as groundwater moved through an in-field tree line or through a riparian zone. While overall conclusions were complicated by the long-term impact of past management, the capacity of the till to buffer changes, hydrogeologic variability found among wells, and the activity of biological processes, we conclude that cropping practices during this study did not increase glacial till NO-N.

Long-term agroecosystem research in the central Mississippi river basin: hyperspectral remote sensing of reservoir water quality.

In situ methods for estimating water quality parameters would facilitate efforts in spatial and temporal monitoring, and optical reflectance sensing has shown potential in this regard, particularly for chlorophyll, suspended sediment, and turbidity. The objective of this research was to develop and evaluate relationships between hyperspectral remote sensing and lake water quality parameters-chlorophyll, turbidity, and N and P species. Proximal hyperspectral water reflectance data were obtained on seven sampling dates for multiple arms of Mark Twain Lake, a large man-made reservoir in northeastern Missouri. Aerial hyperspectral data were also obtained on two dates. Water samples were collected and analyzed in the laboratory for chlorophyll, nutrients, and turbidity. Previously reported reflectance indices and full-spectrum (i.e., partial least squares regression) methods were used to develop relationships between spectral and water quality data. With the exception of dissolved NH, all measured water quality parameters were strongly related ( ≥ 0.7) to proximal reflectance across all measurement dates. Aerial hyperspectral sensing was somewhat less accurate than proximal sensing for the two measurement dates where both were obtained. Although full-spectrum calibrations were more accurate for chlorophyll and turbidity than results from previously reported models, those previous models performed better for an independent test set. Because extrapolation of estimation models to dates other than those used to calibrate the model greatly increased estimation error for some parameters, collection of calibration samples at each sensing date would be required for the most accurate remote sensing estimates of water quality.

A simple method for isolation and purification of DIBOA-Glc from Tripsacum dactyloides.

Naturally occurring benzoxazinones (Bx) are a highly reactive class of compounds that have received particular attention in the past several decades. Recently, we identified 2-ß-D-glucopyranosyloxy-4-hydroxy-1,4-benzoxazin-3-one (DIBOA-Glc) as the compound present in the roots of Eastern gamagrass {Tripsacum dactyloides (L.)} responsible for atrazine degradation. However, characterization of the DIBOA-Glc/atrazine degradation reaction has been limited due to difficulties in attaining sufficient quantities of purified DIBOA-Glc. The objective of the study was to develop a simple purification and isolation method for obtaining bulk quantities of highly purified DIBOA-Glc. T. dactyloides roots were extracted with 90% aqueous methanol, and the crude extract was fractionated using an HPLC equipped with a C8 semi-prep column and fraction collector. UHPLC-DAD-MS/MS was used to confirm the identity of DIBOA-Glc in the fractions collected. Analysis by 13C and 1H NMR and DAD indicated that 542 mg of DIBOA-Glc with a purity of > 99% was obtained. The reactivity of the DIBOA-Glc was confirmed in a 16 hour assay with atrazine, which resulted in 48.5% ± 1.2% (SD) atrazine degradation. The method described here offers several advantages over existing extraction and synthesis methods, which are more cumbersome, use hazardous chemicals, and yield only small quantities of purified compound. The newly developed method will facilitate future research characterizing the chemical behavior of DIBOA-Glc and determine its potential as an atrazine mitigation and remediation tool.

Identification of an atrazine-degrading benzoxazinoid in Eastern gamagrass (Tripsacum dactyloides).

This study was part of a broader effort to identify and characterize promising atrazine-degrading phytochemicals in Eastern gamagrass (Tripsacum dactyloides ; EG) roots for the purpose of mitigating atrazine transport from agroecosystems. The objective of this study was to isolate and identify atrazine-degrading compounds in EG root extracts. Eastern gamagrass roots were extracted with methanol, and extracts were subjected to a variety of separation techniques. Fractions from each level of separation were tested for atrazine-degrading activity by a simple assay. Compounds were identified using high-performance liquid chromatography-tandem mass spectrometry. Results from the experiments identified 2-ß-d-glucopyranosyloxy-4-hydroxy-1,4-benzoxazin-3-one (DIBOA-Glc) as the compound responsible for atrazine degradation in the root extract fractions collected. 2-ß-d-Glucopyranosyloxy-1,4-benzoxazin-3-one (HBOA-Glc) was also identified in the root extract fractions, but it did not demonstrate activity against atrazine. Estimated root tissue concentrations were 210 mg kg(-1) (wet wt basis) for DIBOA-Glc and 71 mg kg(-1) for HBOA-Glc (dry wt basis, 710 ± 96 and 240 ± 74 mg kg(-1), respectively). This research was the first to describe the occurrence and concentrations of an atrazine-degrading benzoxazinone compound isolated from EG tissue.

Sulfamethazine Sorption to Soil: Vegetative Management, pH, and Dissolved Organic Matter Effects.

Elucidating veterinary antibiotic interactions with soil is important for assessing and mitigating possible environmental hazards. The objectives of this study were to investigate the effects of vegetative management, soil properties, and >1000 Da dissolved organic matter (DOM) on sulfamethazine (SMZ) behavior in soil. Sorption experiments were performed over a range of SMZ concentrations (2.5-50 µmol L) using samples from three soils (Armstrong, Huntington, and Menfro), each planted to one of three vegetation treatments: agroforestry buffers strips (ABS), grass buffer strips (GBS), and row crops (RC). Our results show that SMZ sorption isotherms are well fitted by the Freundlich isotherm model (log = 0.44-0.93; Freundlich nonlinearity parameter = 0.59-0.79). Further investigation of solid-to-solution distribution coefficients () demonstrated that vegetative management significantly ( < 0.05) influences SMZ sorption (ABS > GBS > RC). Multiple linear regression analyses indicated that organic carbon (OC) content, pH, and initial SMZ concentration were important properties controlling SMZ sorption. Study of the two most contrasting soils in our sample set revealed that increasing solution pH (pH 6.0-7.5) reduced SMZ sorption to the Armstrong GBS soil, but little pH effect was observed for the Huntington GBS soil containing 50% kaolinite in the clay fraction. The presence of DOM (150 mg L OC) had little significant effect on the Freundlich nonlinearity parameter; however, DOM slightly reduced SMZ values overall. Our results support the use of vegetative buffers to mitigate veterinary antibiotic loss from agroecosystems, provide guidance for properly managing vegetative buffer strips to increase SMZ sorption, and enhance understanding of SMZ sorption to soil.

Stimulated rhizodegradation of atrazine by selected plant species.

The efficacy of vegetative buffer strips (VBS) in removing herbicides deposited from surface runoff is related to the ability of plant species to promote rapid herbicide degradation. A growth chamber study was conducted to compare C-atrazine (ATR) degradation profiles in soil rhizospheres from different forage grasses and correlate ATR degradation rates and profiles with microbial activity using three soil enzymes. The plant treatments included: (i) orchardgrass ( L.), (ii) smooth bromegrass ( Leyss.), (iii) tall fescue ( Schreb.), (iv) Illinois bundle flower (), (v) perennial ryegrass ( L.), (vi) switchgrass ( L.), and (vii) eastern gamagrass (). Soil without plants was used as the control. The results suggested that all plant species significantly enhanced ATR degradation by 84 to 260% compared with the control, but eastern gamagrass showed the highest capability for promoting biodegradation of ATR in the rhizosphere. More than 90% of ATR was degraded in the eastern gamagrass rhizosphere compared with 24% in the control. Dealkylation of atrazine strongly correlated with increased enzymatic activities of ß-glucosidase (GLU) ( = 0.96), dehydrogenase (DHG) ( = 0.842), and fluorescein diacetate (FDA) hydrolysis ( = 0.702). The incorporation of forage species, particularly eastern gamagrass, into VBS designs will significantly promote the degradation of ATR transported into the VBS by surface runoff. Microbial parameters widely used for assessment of soil quality, e.g., DHG and GLU activities, are promising tools for evaluating the overall degradation potential of various vegetative buffer designs for ATR remediation.

Reducing herbicides and veterinary antibiotics losses from agroecosystems using vegetative buffers.

Multiple species vegetative buffer strips (VBSs) have been recommended as a cost-effective approach to mitigate agrochemical transport in surface runoff derived from agronomic operations, while at the same time offering a broader range of long-term ecological and environmental benefits. However, the effect of VBS designs and species composition on reducing herbicide and veterinary antibiotic transport has not been well documented. An experiment consisting of three VBS designs and one continuous cultivated fallow control replicated in triplicate was conducted to assess effectiveness in reducing herbicide and antibiotic transport for claypan soils. The three VBS designs include (i) tall fescue, (ii) tall fescue with a switchgrass hedge barrier, and (iii) native vegetation (largely eastern gamagrass). Rainfall simulation was used to create uniform antecedent soil moisture content in the plots and to generate runoff. Our results suggested that all VBS significantly reduced the transport of dissolved and sediment-bound atrazine, metolachlor, and glyphosate in surface runoff by 58 to 72%. Four to 8 m of any tested VBS reduced dissolved sulfamethazine transport in the surface runoff by more than 70%. The tall fescue VBS was overall most effective at reducing dissolved tylosin and enrofloxacin transport in the runoff (>75%). The developed exponential regression models can be used to predict expected field-scale results and provide design criteria for effective field implementation of grass buffers. Our study has demonstrated that an optimized VBS design may achieve desired agrochemical reductions and minimize acreage removed from crop production.

Adsorption of isoxaflutole degradates to aluminum and iron hydrous oxides.

Isoxaflutole is a preemergence herbicide that has been marketed as a substitute for atrazine. It is rapidly transformed to a more stable and soluble diketonitrile degradate (DKN) after field application and can further degrade to a benzoic acid degradate (BA) within soil. However, no previous research has been conducted to investigate DKN and BA sorption to metal oxide minerals. The primary objective of this research was to elucidate the interactions of DKN and BA with synthetic hydrous aluminum and iron oxides (HAO and HFO, respectively) to understand how variably charged minerals may influence adsorption of these compounds in soil. The herbicide degradates did adsorb to HAO and HFO, and the data were well described by the Freundlich model (R2 > 0.91), with Nvalues ranging from 0.89 to 1.2. Adsorption isotherms and Kd values demonstrate that BA is adsorbed to HFO to a greater extent than other degradate-mineral combinations that were studied. The degree of hysteresis between adsorption/desorption isotherms was characterized as slight (hysteresis index values < 1.7), suggesting weak DKN and BA retention to HFO and HAO oxide surfaces. Degradate adsorption was observed to greatly diminish as suspension pH increased. Attenuated total reflectance-Fourier transform infrared spectra show no evidence that DKN or BA adsorb to mineral surfaces as inner-sphere complexes under hydrated conditions. Instead DKN and BA adsorb to positively charged metal oxide surfaces as outer-sphere or diffuse ion swarm complexes via electrostatic attraction. This research indicates that metal oxides may serve as important retardants for DKN and BA migration through acidic soils enriched with aluminum and iron oxides.

Dissipation of sulfamethazine and tetracycline in the root zone of grass and tree species.

The detection of veterinary antibiotics (VAs) in drinking water resources resulting from manure disposal operations has raised public health concerns. Previous studies have demonstrated the benefits of using multispecies vegetated buffer strips (VBS) to reduce agrichemical transport from agroecosystems. However, VA fate and subsequent effects of VAs on microbial activities in the root zone ofVBS have not been well documented. A growth chamber study was conducted to investigate dissipation of two commonly administered VAs, sulfamethazine (SMZ) and tetracycline (TC), and the relationship between VA dissipation and soil enzyme activities in the root zone of selected plant species. Switchgrass, eastern gammagrass, orchardgrass, and a hybrid poplar tree were grown in pots containing a Mexico silt loam/sand mixture for 3 mo, followed by plant biomass removal and collection of root zone soil. Radiolabeled (3H) SMZ or TC was applied to the soils and samples were incubated in the dark for 5 wk. Among the plant species studied, hybrid poplar showed enhanced capability for promoting SMZ dissipation. The half-lives of SMZ in soil planted to the poplar tree were significantly reduced by the enhanced enzymatic activity. Comparison of soil enzymatic activities between the antibiotic treatments revealed that fluorescein diacetate hydrolytic and glucosaminidase enzyme activities were significantly lower in TC-treated soils than in SMZ-treated soils. The beta-glucosidase activities were similar between the two VA treatments. Correlation analyses showed that the half-life of SMZ in the soil was negatively correlated with enzymatic activity. Enhanced SMZ dissipation in soil planted to hybrid poplar suggests that incorporation of this plant species in VBS may mitigate deleterious effects of SMZ in the environment.

Evaluation of PCR-based quantification techniques to estimate the abundance of atrazine chlorohydrolase gene atzA in rhizosphere soils.

There are many challenges in the accurate quantification of bacterial genes, such as the atrazine-degrading enzyme antA from Pseudomonas sp. strain ADP, from soil samples. We compared four quantitative methods for enumeration of atrazine-degrading bacteria in rhizosphere environments and utilized the optimal probe-based real-time polymerase chain reaction (PCR)-based method in an ongoing bioremediation experiment to monitor atzA copy number over time. We compared three quantitative PCR (qPCR) based methods--quantitative competitive PCR and two real-time qPCR methods--to traditional dilution-plate counting techniques. The optimal real-time qPCR assay was then used to monitor atzA copy number over time in the robust atrazine-degrading Pseudomonas sp. strain ADP-spiked rhizosphere environment. The use of sensitive and reliable probe-based real-time qPCRs for the enumeration of bacterial catabolic genes allows for their detection from soil samples and monitoring of potential degradative populations over time. The addition of arrazine-biodegrading bacteria into arrazine-contaminated sites to remove entrapped atrazine is a promising approach for mitigating atrazine pollution and its metabolites. The methodology contained herein will allow for optimal monitoring of atzA in rhizosphere soil with or without the addition of biodegradative Pseudomonas sp. strain ADP of bacteria.

Influence of watershed system management on herbicide concentrations in Mississippi Delta oxbow lakes.

The Mississippi Delta Management Systems Evaluation Area (MD-MSEA) project was established in 1994 in three small watersheds (202 to 1,497 ha) that drain into oxbow lakes (Beasley, Deep Hollow, and Thighman). The primary research objective was to assess the implications of management practices on water quality. Monthly monitoring of herbicide concentrations in lake water was conducted from 2000 to 2003. Water samples were analyzed for atrazine, cyanazine, fluometuron, metolachlor, and atrazine metabolites. Herbicide concentrations observed in the lake water reflected cropping systems of the watershed, e.g., atrazine and metolachlor concentrations were associated with the level of corn and sorghum production, whereas cyanazine and fluometuron was associated with the level of glyphosate-sensitive cotton production. The dynamics of herbicide appearance and dissipation in lake samples were strongly influenced by herbicide use, lake hydrology, rainfall pattern, and land management practices. The highest maximum concentrations of atrazine (7.1 to 23.4 microg L(-1)) and metolachlor (0.7 to 14.9 microg L(-1)) were observed in Thighman Lake where significant quantities of corn were grown. Introduction of s-metolachlor and use of glyphosate-resistant cotton coincided with reduced concentration of metolachlor in lake water. Cyanazine was observed in two lakes with the highest levels (1.6 to 5.5 microg L(-1)) in 2000 and lower concentrations in 2001 and 2002 (<0.4 microg L(-1)). Reduced concentrations of fluometuron in Beasley Lake were associated with greater use of glyphosate-resistant cotton and correspondingly less need for soil-applied fluometuron herbicide. In contrast, increased levels of fluometuron were observed in lake water after Deep Hollow was converted from conservation tillage to conventional tillage, presumably due to greater runoff associated with conventional tillage. These studies indicate that herbicide concentrations observed in these three watersheds were related to crop and soil management practices.

Degradation of isoxaflutole (balance) herbicide by hypochlorite in tap water.

Chlorine has been widely employed for the disinfection of drinking water. Additionally, it has the capacity to oxidize many organic compounds in water. Isoxaflutole (Balance; IXF) belongs to a new class of isoxazole herbicides. Isoxaflutole has a very short soil half-life and rapidly degrades to a stable and phytotoxic metabolite, diketonitrile (DKN). Further degradation of DKN produces a nonbiologically active benzoic acid (BA) metabolite. In experiments using high-performance liquid chromatography-UV spectroscopy (HPLC-UV) and HPLC tandem mass spectrometry (HPLC-MS/MS), DKN was found to rapidly react with hypochlorite in tap water, yielding the BA metabolite as the major end product. One milligram per liter (19 microM) of hypochlorite residue in tap water was able to completely oxidize up to 1600 microg/L (4.45 micromol/L) of DKN. In tap water, the disappearance of IXF was much more rapid than in DI water. As soon as the IXF is hydrolyzed to DKN, the DKN quickly reacts with the OCl(-) to form nonphytotoxic BA. As a result, the herbicide solutions prepared with tap water at 500 microg/L will no longer possess any herbicidal activity after 48 h of storage. However, in agronomic settings, highly concentrated tank solutions (600-800 mg/L) may be prepared with tap water since the conversion of IXF to BA would represent <5% of the herbicide; therefore, any impact on the herbicide efficacy would be negligible. Results of this study show that current chlorination disinfection protocols in municipal water systems would completely eliminate the phytotoxic form of this new herbicide, DKN, from drinking water supplies; yet, farmers can use chlorinated tap water without significant loss of efficacy.