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TECHNICAL REPORTS

Te ecacy o vegetative bufer strips (VBS) in removingherbicides deposited rom surace runof is related to theability o plant species to promote rapid herbicide degradation.

A growth chamber study was conducted to compare 14Catrazine (AR) degradation proles in soil rhizospheres romdiferent orage grasses and correlate AR degradation ratesand proles with microbial activity using three soil enzymes.Te plant treatments included: (i) orchardgrass (Dactylis

glomerataL.), (ii) smooth bromegrass (Bromus inermisLeyss.),(iii) tall escue (Festuca arundinaceaSchreb.), (iv) Illinoisbundle ower (Desmanthus illinoensis), (v) perennial ryegrass(Lolium perenneL.), (vi) switchgrass (Panicum virgatumL.), and (vii) eastern gamagrass (Tripsacum dactyloides). Soil

without plants was used as the control. Te results suggestedthat all plant species signicantly enhanced AR degradationby 84 to 260% compared with the control, but easterngamagrass showed the highest capability or promotingbiodegradation o AR in the rhizosphere. More than 90%o AR was degraded in the eastern gamagrass rhizospherecompared with 24% in the control. Dealkylation o atrazinestrongly correlated with increased enzymatic activities ob-glucosidase (GLU) (r= 0.96), dehydrogenase (DHG)(r= 0.842), and uorescein diacetate (FDA) hydrolysis (r= 0.702). Te incorporation o orage species, particularlyeastern gamagrass, into VBS designs will signicantly promotethe degradation o AR transported into the VBS by suracerunof. Microbial parameters widely used or assessment osoil quality, e.g., DHG and GLU activities, are promisingtools or evaluating the overall degradation potential o variousvegetative bufer designs or AR remediation.

Stimulated Rhizodegradation of Atrazine by Selected Plant Species

Chung-Ho Lin,* Robert N. Lerch, Robert J. Kremer, and Harold E. Garrett

A (AR) is one o the most widely applied herbicidesin the United States with an estimated 36.3 million kg oAR applied to more than 66% o all United States corn (ZeamaysL.) acreage in 2006 (USDA, 2007). Contamination o sur-ace and groundwater by AR and its chlorinated metabolites hasbeen linked to public health risks and ecological impacts (Hayeset al., 2002; Stanko et al., 2010; Swan et al., 2003).

Multispecies vegetative riparian bufer strips (VBS) have beenrecommended as a potentially cost-efective conservation prac-tice to reduce nonpoint-source pollution o adjacent waterways(Krutz et al., 2003; Schultz et al., 2000). Previous research hasshown that VBS used in agroecosystems reduce the transport o

and encourage the deposition o both dissolved and sediment-bound AR in surace runof (Arora et al., 1996; Cole et al.,1997; Hofman et al., 1995; Lin et al., 2007; Lowrance et al.,1997; Schultz et al., 1995; Schultz et al., 1991). Following thedeposition o AR into the VBS, only small ractions o depositedAR in VBS were taken up by plants or leached into the leachates.Rhizodegradation is considered the primary biotic mechanism orAR dissipation in VBS (Henderson et al., 2007; Lin et al., 2008;Schroll and Sabine, 2004; Singh et al., 2004). More than 15 ARdegradation products have been identied through either phys-iochemical or biochemical processes (Cook, 1987; Jensen, 1982;Sadowsky et al., 1998) (Fig. 1). Te major AR degradation prod-ucts include N-dealkylated metabolites, deethylatrazine (DEA),and deisopropylatrazine (DIA), and the hydroxylated metabolites,hydroxyatrazine (HA), deethylhydroxyatrazine (DEHA), anddeisopropylhydroxyatrazine (DIHA) (Fig. 1).

Microbial degradation o AR in rhizospheres usually requiresa group or consortium o microorganisms (Mandelbaum et al.,1993; Sadowsky et al., 1997). Te associated biochemical pro-cesses involve a wide spectrum o enzymatic reactions, e.g., deal-kylation, deesterication, hydroxylation, dehalogenation, andoxidization (Ambus, 1993; Bollag and Liu, 1990; Mandelbaumet al., 1993). Previous ndings have indicated that some soilenzymatic parameters widely used to assess soil quality in thepast might be useul as indicators to evaluate the overall potential

Abbreviations: ATR, atrazine; C3, cool-season; C

4, warm-season; DDA, didealkylatrazine;

DEA, deethylatrazine; DEHA, deethylhydroxyatrazine; DHG, dehydrogenase; DIA,

deisopropylatrazine; DIHA, deisopropylhydroxyatrazine; EG, eastern gamagrass;

FDA, uorescein diacetate; GLU, b-glucosidase; HA, hydroxyatrazine; HPLC, high-

performance liquid chromatography; IB, Illinois bundle ower; OG, orchardgrass;

RYE, perennial ryegrass; SM, smooth bromegrass; SW, switchgrass; TALL, tall fescue;

THAM, tris-hydroxymethyl-aminomethane; TOC, total organic carbon; TPF, triphenyl

formazan; TTC, triphenyltetrazolium chloride; VBS, vegetative buer strips.

C.-H. Lin and H. E. Garrett, Center for Agroforestry, Univ. of Missouri, Columbia,

MO 65211; R.N. Lerch and R.J. Kremer, USDAARS, Cropping Systems and Water

Quality Research Unit, Columbia, MO 65211. Assigned to Associate Editor Jose-

Julio Ortega-Calvo.

Copyright 2011 by the American Society of Agronomy, Crop Science

Society of America, and Soil Science Society of America. All rights

reserved. No part of this periodical may be reproduced or transmitted

in any form or by any means, electronic or mechanical, including pho-

tocopying, recording, or any information storage and retrieval system,

without permission in writing from the publisher.

J. Environ. Qual.

doi:10.2134/jeq2010.0440

Posted online 27 Apr. 2011.

Received 5 Oct. 2010.

*Corresponding author (Linchu@missouri.edu).

ASA, CSSA, SSSA

5585 Guilford Rd., Madison, WI 53711 USA

TECHNICAL REPORTS: BI OREEIATION AN BIOERAATION

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or herbicide biodegradation in VBS (Bergstrom et al., 1998;Zablotowicz et al., 2000). For instance, the uorescein diac-etate (FDA) hydrolytic enzyme activities, commonly used toestimate total soil microbial activity, were highly correlatedwith the deesterication o enoxapropethyl (Zablotowicz etal., 2000). Te dehydrogenase, which has been used as a validbiomarker o soil quality under diferent agronomic practices,is usually required to catalyze the oxidation and dehalogena-

tion o several herbicides and other organic compounds (Belleret al., 1996; Waarrde et al., 1993).Various VBS designs have been prescribed in the United

States to encourage deposition o dissolved and sediment-bound AR in surace runof (Lin et al., 2007; im and Jolly,1994; Udawatta et al., 2004). However, to date, ew studieshave compared the efectiveness o diferent plant rhizospheresin enhancing degradation o AR. Results rom a eld lysimeterstudy have shown stimulated dealkylation and hydroxylationrates o AR in rhizospheres o selected grass species, includingwarm-season and cool-season species (Lin et al., 2008). In the

same study, the increased degradation rateso AR in these grass treatments were oundstrongly correlated with increased microbialbiomass carbon. Te objectives o this studywere to investigate the efects o these plantspecies with additional species commonlyound near Midwest regions on the activi-ties o the soil microbial enzymes and theirassociation with the ate o isotope-labeledAR in rhizospheres. Te use o isotope-labeled AR allows us to quantiy themineralized AR and a wide spectrum oAR degradation products. Te goals o thestudy are (i) to compare AR degradationrates supported by soil rhizospheres romdiferent orage grasses and (ii) to correlateAR degradation proles with microbialactivities o soil enzymes, specically activi-ties o the b-glucosidase (GLU), dehydroge-nase (DHG), and FDA hydrolysis.

Materials and Methods

Experimental DesignTe experiment was conducted in anenvironment-controlled growth chamber(Environmental Growth Chambers GC72walk-in unit, Chagrin Falls, OH) with trip-licate replications o seven orage species: (i)orchardgrass (OR), (ii) smooth bromegrass(SM), (iii) tall escue (ALL), (iv) Illinoisbundle ower (IB), (v) perennial ryegrass(RYE), (vi) switchgrass (SW), and (vii)eastern gamagrass (EG). A control treat-ment with no plants was also included andmaintained with the same irrigation andertilization regime as other treatments. Teorage treatments included ve cool-season(C

3) species (OR, SM, ALL, IB, and RYE)

and two warm-season (C4) species (SW and

EG). Environmental conditions were: lightintensity o 1400 mmol photons m2 s1, light/dark period o15/9 h, humidity o 50%, and temperature at 25C (light)/20C(dark). wo-month-old seedlings were transplanted into 1-L,wide-mouth, polypropylene jars containing 1000 g o soil toallow growing or 100 d in a mixture o 60% sand and 40%Mexico silt loam (ne, smectitic, mesic Vertic Epiaquals) col-lected rom the A horizon o the soil prole. Te soils were

collected rom a site without previous history o AR appli-cation (were not previously exposed to AR). Te soils weresterilized in an autoclave or 60 min and exposed to ambientair in a greenhouse or 2 wk beore the experiment. Soil char-acterization, nitrogen content, and total organic carbon (OC)analyses were completed by the University o Missouri SoilCharacterization Laboratory using standard methods o analy-sis (Burt, 2010). Te soil particle size consisted o 108 g kg1clay, 338 g kg1 silt, and 557 g kg1 sand (sandy loam texture).Cation exchange capacity was 8.1 cmol kg1, soil pH 7.2, andinitial organic carbon content 6 g kg1. Te plants and control

Fig. 1. The major atrazine degradation pathway (Cook, 1987; Jensen, 1982; Sadowsky et al., 1998).

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treatments were irrigated with 100 mL o water once every otherday with a micro-dripping irrigation system. Te ertilizer solu-tion containing N, P, K, and micronutrients (Peters 20:20:20;Micromax Micronutrients, Te Scotts Company, Marysville,OH) was applied to each jar every week only during the rst3-wk establishment phase. Ater 3 mo (100 d) o plant growth,rhizosphere soil was separated rom each plant and soil mois-ture content was adjusted to 20%. Atrazine solution preparedwith 0.1 mCi o14CAR with a specic activity o 18.1 mCi/mmol (atrazine-ring-UL-14C, purity >95%; Sigma-Aldrich, St.Louis, MO) and nonradioactive AR was then added to 50 g(dry weight equivalent) o soil to achieve a nal concentrationo 100 mg kg1. Beore 14CAR application, a subsample othe rhizosphere soil was collected to determine b-glucosidase,dehydrogenase, and uorescein diacetate hydrolytic enzymeactivities. Measurement o GLU, DHG, and FDA activitieswere chosen as these enzymes have been commonly used inthe assessment o microbial activity and overallsoil quality. Te herbicide-treated soil was thenincubated in sealed 500-mL Mason jars or 115d at 25C in the dark. Atrazine-mineralized14CO

2was trapped using alkali traps (10 mL

o 2 M NaOH) placed in Mason jars and thetraps were periodically replaced on the rst,second, ourth, sixth, tenth, and sixteenthweeks throughout the incubation period. Aterend o experiment, roots and shoots were col-lected. Te biomass dry weight was recorded.

Chemical Analysis

Ater 115 days incubation, 14CAR and itsdegradation products were extracted with 250mL o 90% MeOH ollowed by 1 h o sonica-tion. Te extract was centriuged or approxi-mately 5 min at approximately 3000 rpm and

the supernatant was collected. Te extractionwas repeated and supernatants were combined.Te extract was concentrated to 10 mL usinga Savant concentrator (Holbrook, NY). Te14C-labeled 2,4-dichloro-phenoxyacetic acidwas added to each sample as an internal stan-dard or nal volume correction. Te nalextracts were concentrated to 200 L underN

2ow and injected into an SCL10Avp

high-perormance liquid chromatographysystem (HPLC) (Shimadzu, Columbia, MD).Te 14CAR and its degradation productswere separated using a silica-based Columbus

C8 column (4.6 mm by 250 mm, 5 m;Phenomenex, orrance, CA). Te radioactiv-ity was detected by an inline IN/US ScinFlowb-Ram Model 3 (ampa, FL) ow scintilla-tion analyzer (HPLCFSA). Injection volumewas 10 mL and mobile phase ow rate was 1mL min1. Te 14CAR and its metaboliteswere eluted with a two-part mobile phasegradient. Mobile phase A consisted o 0.1%H

3PO

4bufer (pH = 2.1) and mobile phase

B was 100% acetonitrile. Te gradient started

at 10% and ramped linearly to 40% at 30 min, 75% at 40min, 10% at 45 min, and held at 10% or 14 min. Metaboliteswere identied by comparing the retention times o unlabeledstandards based on HPLCUV detection at 220nm (Fig. 2Aand B). Te standards including AR, DEA, DIA, HA, DIHA,DEHA, didealkylatrazine (DDA), ammeline, and ammelidewere purchased through ChemService (West Chester, PA). ourther conrm the presence o parent AR and its metabolites,the same set o standards and soil extracts were reinjected andeluted with the same stepwise gradient but with a basic buferconsisting o 5mM KH

2PO

4(pH = 7.7) as the mobile phase

A (Fig. 3A and B). Te 14CAR mineralization was deter-mined by measuring the radio activities o the evolved 14CO

2

using Beckman LS600 liquid scintillation counter (Beckman,Fullerton, CA). Te extraction eciencies o soil AR, DEA,and DIA were >95% and >75% o the applied 14C was recov-ered rom this study.

Fig. 2. Retention times of atrazine and its metabolites eluted with acidic mobile phase with0.1% H

3PO

4buer (pH 2.1) by HPLCUV (A: standards) and HPLCFSA (B: soils collected

from smooth bromegrass) analysis. Acronyms of the atrazine and its metabolites weredescribed as in Fig. 1.

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Journal of Environmental Quality Volume 40 JulyAugust 2011

Microbial Enzyme Activity

Te soil moisture content was determined beore the analysisand soil moisture content was adjusted to 20% beore analysis.Dehydrogenase activity was measured ollowing the proceduredescribed by Gerba and Brendecke (1995). Briey, this procedureuses 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-triphenyltetrazoliumchloride (C) as the electron acceptor, which is reduced to thered-colored, methanol-soluble, triphenylormazan (PF). Sixgrams o moist soil rom each sample was placed in a test tube.

Samples were incubated statically with 1 mL o 3% C and3 mL o 0.2 M CaCO

3bufer solution or 24 h at 37C. Tis

reaction was terminated with 10 mL o methanol and PF wasextracted with 30 mL o additional methanol. Te nal extract

was ltered with Whatman #42 lter paper and PF concentra-tion was determined spectrophotometrically at 485 nm.

o assess microbial carbon utilization eciency, GLUactivity was quantied according to procedures described byabatabai (1994). Te method is based on colorimetric mea-surement op-nitrophenol released by GLU when soil is incu-bated with bufered (pH 6.0) p-nitrophenyl-b-D-glucosidesolution. One gram o moist soil was sampled and incubatedwith solution containing 0.25 mL o toluene and 4 mL o 8mM tris-hydroxymethyl-aminomethane bufer (HAM; pH6) or 1 h at 37C. Ten, 1 mL o 0.5 M CaCl

2and 4 mL o

0.1 M HAM (pH 12) solution were added. Tis solution wasvortexed and ltered through Whatman #2 lter paper. Teabsorbance was measured at 410 nm.

Fluorescein diacetate hydrolytic activ-ity was determined by the enzymatic assayprocedures described by Bandick and Dick(1999). One gram (dry weight equivalent) omoist soil rom each sample was placed intoa 50-mL Erlenmeyer ask. wenty mL o 60mM sodium phosphate bufer (pH 7.6) and100 mL o 4.8 mM FDA were added to the

ask. Te ask was then capped with para-lm. Samples were incubated or 105 minon a reciprocal shaker at room temperature.Ater 105 min, 10 mL o acetone was addedto terminate the hydrolysis reaction. Sampleswere then ltered through Whatman #4 lterpaper and centriuged or 5 min. Fluorescein(the product o FDA hydrolysis) concentra-tion was determined spectrophotometricallyat 490 nm. Pairwise correlation analysis wasperormed to determine the relationshipsbetween the 14CAR degradation and activ-ities o soil enzymes.

Results and Discussion

Rhizodegradation of Atrazine

Compared with the plant-ree control, theextent o AR degradation in the plantrhizospheres was signicantly enhancedby 84 to 260% (p < 0.01), through bothN-dealkylation and hydroxylation reac-tions (Fig. 4A and B). Among the plantspecies tested, EG showed the highestcapability or promoting AR degrada-tion. More than 90% o AR in the rhi-

zosphere o EG was degraded to less toxicmetabolites (Stoker et al., 2002; Stratton,1984), as compared with 24% in the con-trol. Te N-dealkylation was the primarydetoxication pathway in the EG treat-ments, accounting or 58% o the extract-able AR degradation products (Fig. 4B).Te DDA and DEA were two o the mostpredominate degradation products oundin the EG rhizospheres, accounting or28 and 23% o the total extracted 14C

Fig. 3. Retention times of atrazine and its metabolites eluted with basic mobile phase with5mM KH

2PO

4buer (pH 7.7) in HPLCUV (A: standards) and HPLCFSA (B: soils collected from

eastern gamagrass) analysis. Acronyms of the atrazine and its metabolites were described asin Fig. 1.

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activities, respectively. Hydroxylated metabolites,including DEHA (12%), DIHA (2.4%), and HA(8.8%), accounted or >23% o AR degradation inEG rhizospheres. In the other grass treatments, therates o AR degradation ranged rom 46 to 65%o the applied AR. Atrazine degradation rate wassimilar among rhizospheres o OR, SM, and SW(p > 0.05), where >60% o the applied 14CARwas degraded. Te similar proportion o hydrox-ylated metabolites across all grass treatments (p >0.01) indicated a similar degree by which hydroxyl-ation was stimulated in the plant rhizospheres. TeN-dealkylation, on the other hand, showed a broadrange rom

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Journal of Environmental Quality Volume 40 JulyAugust 2011

et al., 1993). However, a ew degraders, such as Pseudomonassp. strain atrazine-degrading Pseudomonas, isolated rom sitesheavily contaminated with AR, have evolved to use AR andits metabolites as C and N sources (Mandelbaum et al., 1993).Tese degraders possess a series oatz(e.g., atzAF) or trz genes(e.g., trzN) encoded on a sel-transmissible plasmid responsibleor various processes o AR degradation and ring cleavage,

resulting in rapid mineralization o AR to carbon dioxide,ammonia, and water. Te introduction o such degraders intothe rhizospheres is expected to urther stimulate the AR min-eralization process (Lin et al., 2009).

Pairwise comparison between AR metabolites and soilenzyme activities revealed that the overall AR degradationand N-dealkylation were strongly (p < 0.05) correlated to theactivities o all three enzymes, but the activity o GLU consis-tently showed the highest correlation coecients, particularlywith respect to the ormation o N-dealkylated metabolites(Fig. 6). Te enhanced AR degradation rates associated withstimulated microbial enzymatic activities in the presence ograsses were consistent with previous studies investigating theate o AR in plant microcosms (Henderson et al., 2007; Linet al., 2008; Smith et al., 2008). Similar, positive correlationsbetween enhanced AR N-dealkylation rates and increasedmicrobial biomass carbon in the rhizospheres have beenobserved in a eld study by Lin et al. (2008). In contrast, theormation o hydroxylated metabolites was not signicantlyrelated to any o the enzyme activities (p > 0.05). Te strongcorrelation between the enzyme activities and N-dealkylationand overall AR degradation suggested that any o the enzymesthat have been widely used or assessment o soil quality, par-

ticularly GLU, are very useul indicators ooverall AR degradation potential in VBSsoils. In contrast, the weaker correlationbetween enzyme activities and AR hydrox-ylation ound in this work suggested thatmeasured enzymes like GLU could be goodindicators o N-dealkylation but poor indi-cators o hydroxylation o AR.

Our nding suggested that EG providedmore avorable rhizosphere conditions tosustain higher AR biodegradation activi-ties and support higher numbers o ARdegraders than C

3plants to promote vari-

ous biochemical degradation processes tooccur (Bollag and Liu, 1990; Mahmoodand Renuka, 1990; Qian et al., 1997). Teobserved diferences in enzymatic activitiesand AR degradation potential among theorage species may be a result o the difer-ences in their morphological, physiological,and biochemical characteristics. Our nd-ing has supported other eld and laboratory

studies suggesting equivalent or higher deg-radation rates o AR and other herbicidesin soils planted with warm-season speciescompared with cool-season species (Beldenand Coats, 2004; Lin et al., 2008; Smith etal., 2008). Warm-season species, includingEG and SW (both C4 species), partition

more carbohydrates to roots as compared with C3 cool-seasongrasses. (Dhawan and Goyal, 2004; Jiang et al., 2002). Greaterpartitioning o carbohydrates to the root system likely resultedin higher concentrations o root-derived, bioavailable organiccarbon and other nutrients in the rhizospheres o the C

4spe-

cies. It should be noted that both perennial C4

EG and SW

plants in our study were not allowed to achieve ull maturityduring the 100-d growing period. In this study, EG partitioned>87% o the carbohydrates to the root biomasssimilar toOR and SM. However, due to the possible root-derived N andhigher N utilization in C

4EG as compared with other treat-

ments, particularly the C3

grass treatments (Oaks, 1994), theconcentrations o total N in EG rhizosphere were ound to be42% higher than in the control and 50.1% higher than C

3

grass treatments. Te high nitrogen use eciency o C4

plantsmay be related to biological nitrication inhibition Subbaraoet al. (2009). Biological nitrication inhibition is the resulto plant-induced alterations o the rhizosphere chemical andmicrobial conditions through the release o root exudates.

Although the link between soil herbicide dissipation ratesand root biomass have been established by Smith et al. (2008),the same relationship between root biomass and AR degra-dation was not ound in this study (r= 0.45 p = 0.31). Forexample, EG has a signicantly lower root biomass comparedwith OR and SM (able 1), but the AR degradation rates inEG rhizosphere were signicantly higher (p < 0.005). We havealso careully examined the levels o soil OC among grassand other treatments, and they were not signicantly diferent(0.70.9%). Tis nding led us to believe that the diferencein organic carbon composition and carbohydrates constitutes

Fig. 5. Total mineralization of atrazine and microbial enzyme activities in soils with forage

treatments. Control treatment is bare ground. Error bar represents the standard error of themean (n = 3). Values in parentheses represent the Pearson coecient of correlation betweenmineralization of14Catrazine and enzymatic activity. *Dehydrogenase and b-glucosidaseactivities are expressed in molg-1h-1. The FDA hydrolytic activity is expressed in (nmolg-1h-1) 0.01. The total mineralization of atrazine was signicantly enhanced by easterngamagrass (p < 0.001). The dierences in mineralization among control and other speciestreatments were not signicant.

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in the rhizospheres may also inuence AR degradation pro-les. Future studies, including speciation o the carbohydrateconstituents (e.g., sugars speciation) in the EG root exudates,are needed to test this hypothesis.

In addition, many oxidative and hydrolytic secondarymetabolite compounds contained in root exudates o C

4spe-

cies,

such as hydroxamic acids, have been shown to directlyhydrolyze AR and its chlorinated metabolites, resulting inrapid ormation o the hydroxylated metabolites (Martin-Netoet al., 1994; Wenger et al., 2005). For instance, the AR-degrading benzoxazinones, DIMBOA (3,4-dihydro-2,4-hy-droxy-7-methoxy-2H-1,4-benzoxazin-3-one), and its analogs

Fig. 6. Pairwise comparison between atrazine metabolites and soil enzyme activities. The level of condence ellipse = 0.68.

Table 1. Average dry weight (g) of biomass of root and shoot biomass and the partition of carbohydrates (g). The same letters within the column werenot signicantly (P< 0.05) dierent (least signicant dierence test,n = 3)

Treatments Root Shoot Root/shoot ratios Partitioning of carbohydrates to the root

g %

OR 378.4a 37.3b 10.2a 91.0a

SM 400.1a 37.9b 10.6a 91.4a

TALL 295.0b 52.5a 5.6b 84.9ab

IB 97.1c 25.5c 3.8c 79.2c

RYE 130.3c 24.6c 5.3ab 84.1ab

SW 242.8b 58.5a 4.1c 80.6c

EG 272.3b 37.9b 7.2ab 87.8ab

OR = orchardgrass; SM = smooth bromegrass; TALL = tall fescue; IB = Illinois bundle ower; RYE = perennial ryegrass; SW = switchgrass; EG = east-

ern gamagrass.

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Journal of Environmental Quality Volume 40 JulyAugust 2011

have been ound to be abundant in the C4

AR-tolerant plants(Wenger et al., 2005). In a separate study, we also observed arapid chemical hydrolysis o AR when AR reacted with rootextract rom EG root tissues (data are not shown).

Although a satisactory relationship o soil quality withquantitative pesticide biodegradation does not exist (Lockeand Zablotowicz, 2004), some indirect relationships based onmicrobial activity, specically enzyme activity, can be madebased on assessments o soils and pesticide application. Forexample, FDA hydrolase may be a potential indicator or bio-degradative activity o complex organic molecules, includingsoil-applied pesticides (Zablotowicz et al., 2000). Staddonet al. (2001) concluded that increased levels o this enzymeobserved in VBS soils partly contributed to enhanced degrada-tion o the herbicide metolachlor. Tereore, agroecosystemsthat include continuous vegetation are associated with high soilquality because an optimal unctional soil system that developscan also enhance other benecial activities, including biodeg-radation o introduced pesticides (Karlen et al., 1999; Lockeand Zablotowicz, 2004). Similar to our nding, previous soilquality studies also demonstrated that microbial activity, basedon activities o enzymes including FDA hydrolase, DHG, and

GLU, increased considerably more under grassland vegetationthan soils under intensive cultivation (Acosta-Martinez et al.,2008; Kremer and Li, 2003). Tereore, soil managed underVBS not only improves its physical status (e.g., improvedaggregate stability and inltration rates), but it also improvesthe biological status o the soil via stimulated microbial activ-ity and enhanced organic matter accumulation, leading toan overall increase in soil quality and higher bioremediationpotential or degradation o organic herbicides.

Te work reported here identied EG as an outstanding can-didate to be incorporated into VBS or mitigating the of-sitetransport o AR via its ability to acilitate AR degradationin the rhizosphere. A eld rainall stimulation study conducted

on a claypan soil landscape also demonstrated the superior per-ormance o an EG bufer over other cool-season grass bufersin reducing both dissolved and sediment-bound AR trans-port in surace runof (Lin et al., 2007). Approximately 75 to80% o the dissolved and sediment-bound AR transportedin surace runof was removed by 4 m o EG bufer. Tus, theimplementation o bufers using EG could efectively improveboth deposition and subsequent degradation o AR with lessland taken out o production. Among the other treatments,SW, OR, and SM also showed greatly enhanced degradationo AR in this study and in a eld lysimeter study (discussedabove). While the warm-season grasses have superior ability toacilitate AR degradation in soils, their establishment is not

always practical, timely, or economical. Tus, it is importantto recognize that cool-season species, particularly OR and SM,also acilitate enhanced degradation and they are easier andmore economical to establish.

Conclusions

All plant species tested signicantly enhanced AR degradationby creating conditions that enhanced microbial enzyme activityin the rhizosphere. Te AR mineralization was signicantlycorrelated to GLU and DHG activities in the rhizosphere othe grass treatments, although mineralization accounted or

no more than 7% o the applied 14CAR. All three enzymessignicantly correlated to the ormation o N-dealkylatedmetabolites and overall AR degradation, but they were notsignicantly correlated to the ormation o hydroxylated metab-olites, suggesting that measured enzymes like GLU could begood indicators o overall AR degradation and N-dealkylationbut poor indicators o hydroxylation. Te incorporation o EGinto VBS designs is highly recommended to promote dissipa-tion o AR transported rom croplands and deposited in theVBS. However, cool-season grasses, particularly OR and SM,also signicantly enhanced AR degradation and they are morereadily established than the C

4species. Te results rom this

study also showed that the microbial enzyme activities widelyused or assessment o soil quality, particularly GLU activity, arepromising as indicators or evaluating the overall AR degrada-tion potential o various vegetative bufer designs.

AcknowledgmentsTis work was unded through the University o Missouri Centeror Agroorestry under cooperative agreement 5862275029

with the USDAARS. Any opinions, ndings, conclusions, orrecommendations expressed in this publication are those o theauthor(s) and do not necessarily reect the view o USDA or the

University o Missouri. Mention o specic companies, products, ortrade names is made only to provide inormation to the reader anddoes not constitute endorsement by USDAARS.

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