b
€SaCRITICAL REVIEW
1
c
xamination of the published scientific literature reveals persuasive evidence that plant roots, in conjunction with their associated microbial commuI nities, offer a potentially important treatment strategy for in situ biological remediation of chemically contaminated soils. Under a variety of environmental conditions, vegetation has been shown to enhance microbial degradation rates of organic chemical residues in soils. These findings are important because vegetation may provide a low-cost alternative or supplement to expensive, capitalintensive technologies for soil cleanup. Moreover, unlike technologies that merely relocate contaminants, vegetation promises a means of halting legal liability for hazardous waste residues in soils because hazardous compounds in the rhizosphere [root zone) are totally destroyed (mineralized). Historically, the use of plant systems as a waste treatment technology bas focused primarily on wastewater ( 1 ) . This work dealt mostly with the manipulation of operational parameters [e+, lagoon size or flow rate) to optimize biological removal of unwanted chemicals. Whether microorganisms in the root zone of aquatic plants contribute to improved water quality through detoxication of hazardous organic substances is unknown and relatively unexamined. This review will critically examine reports on the interaction of microorganisms with hazardous organic chemicals in the terrestrial rhizosphere. Studies on microbial degradation of agricultural chemicals in the rhizosphere, and recent research on the fate of nonagricultural chemicals in rhizosphere soils are presented. Collectively these studies provide a strong scientific basis to support field demonstrations of in situ degradation of toxicants in the rhizosphere. Moreover, investigations of the fundamental 2630 Emiron. Si.Technol., Vol. 27.No. 13,191
mechanisms wh microbial degra would provide insi tions of the plant for in situ remediation
The rhizosphere The rhizosphere, first described by Lorenz Hiltner in 1904, has been the focus of agricultural research for manv vears because of its imvortanck i o crop productivity. ?he rhizosphere is a zone of increased microbial activity and biomass at the root-soil interface that is under the influence of the plant root (2). This zone is distinguished from bulk soil by this root influence. Excellent comprehensive reviews on the rhizosphere are available (2,3); therefore, only a brief description of important rhizosphere characteristics is presented below. The overall effect of the plantmicrobe interaction is an increase in microbial biomass by an order of magnitude or more compared with that of microbial populations in bulk soils. This “rhizosphere effect” is often expressed quantitatively as the ratio of the number of microorganisms in rhizosphere soil to the number of microorganisms in nonrhizosphere soil, the WS ratio (4). Although WS ratios commonly range from 5 to 20, they can run as high as 100 or greater (5).This increase in microbial growth and activity in the rhizosphere may be res p o n s i b l e for t h e i n c r e a s e d metabolic degradation rate of cer-
T O D D A A ND E R S O N I
Iowa Stote University Ames, IA 50011-3140
E 1I 2 A B E T H
A
I
G U T H R IE
University of North Carolina Chapel Hill, NC27599-7400
B A R B A R A T , W A L T 0N oakRidge NationalLaboratory OakRidge, TN 37831-6038
tain xenobiotic compounds in the rhizosphere. Consequently, an intriguing question is whether selection for plants with supernodulating roots, proliferation of root hairs, or other genetically determined properties of plant roots would positively affect microbial degradation rates of specific toxicants in the rhizosphere. The actual comnosition of the microbial commun’ity in the rhizosphere is dependent on root type, plant species, plant age, and soil type (3,5,61,as well as other factors such as exposure history of the plant roots to xenobiotics (7-10). Generally, the rhizosphere is colonized by a predominantly gramnegative microbial community ( 5 ) . Carbon dioxide concentrations in the rhizosphere are generally higher than in nonvegetated soil, and rhizosphere soil pH may differ by 1-2 units from that of comparable nonvegetated soil. Oxygen concentrations, osmotic and redox potentials, and moisture content are other parameters influenced by the presence of vegetation (2).These parameters are further dependent on the properties of specific plant species. The continual change at the rootsoil interface, both physical and chemical, produces constant alterations in the soil structure and microbial environment. The interaction between plants and microbial communities in the rhizosphere is complex and has evolved to the mutual benefit of both organisms. Plants sustain large microbial populations in the rhizosphere by secreting substances such as carbohydrates and amino acids through root cells and by sloughing root epidermal cells. The magnitude of rhizodeposition by plants can be quite large ( 1 1 ) . Root cap cells, which protect the root from abrasion, may be lost to the soil at a rate of 10,000 cells per plant per day (3). In addition, root cells secrete mucigel, a gelatinous substance that is a lubricant for root penetration through the soil during growth.
001 3-936X/93/0927-2630$04.00/0@ 1993 American Chemical Society
This mucigel, along with other cell secretions, constitutes root exudate (12). Soluble exudate includes aliphatic, aromatic, and amino acids and sugars. Root cap cells and exudates provide important sources of nutrients for microorganisms in the rhizosphere. Although modification of the soil in the rhimsphere by plant root exudates is an important process that influences microbial nonulations, it is the actual struct$e' of the plant root that provides microorganisms with surface area for colonization (2. 5, 6). The fibrous roots of grasses provide a larger surface area for colonization than do taproot systems (5). Once established in the rhizosphere of plants, microbial populations may be passively nourished by root exudation and decaying plant matter; or the presence of the microorganisms may actually induce liberation of certain organic nutrients by co-evolved biochemical signals. Pseudomonas putida and Azospirillum spp. are examples of bacteria capable of inducing nutrient release from the roots (13). In the absence of bacteria and fungi, plant exudate production decreases (21, subsequently providing fewer organic substrates to sustain microbial growth. The interaction of leguminous plants with nitrogen-fixing bacteria results in increased microbial biomass, plant growth, and root exudation, perhaps because of the increased availability of soil nitrogen in the presence of nitrogen-fixing bacteria. This in turn may lead to enhanced microbial degradation of organic compounds such as herbicides in the rhizosphere by these bacteria (141.
Sandmann and Loos (7)found an increase in the number of 2.4-D (2,4-dichlorophenoxyacetate)-degrading bacteria in rhizosphere soils of previously untreated sugar cane but not African clover. This research illustrates a potentially interesting theme common to the literature on microbial degradation of herbicides in the rhizosphere-the nossibilitv that rhizosuhere micro-
(10)showed elevated microbial counts in the rhizospheres of pesticide-treated plants. Corn, beans, and cotton plants treated with temik [Z-methyl-2(methy1thio)propionaldehyde O(methylcarbamoyl]oximel ( 8 ) bad higher microbial counts in some instances than those in untreated rhizospheres. Although the authors did not monitor degradation of temik, they hypothesized that the increased microbial numbers supported the idea that temik and other pesticides were metabolized by several types of rhizospheric microbiota. Gavrilova and co-workers (10) found >lOO-fold higher microbial counts in the rhizosphere of wheat, corn, and peas treated with diazinon [O,O-diethyl O(2isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate] than in treated soils without vegetation. Although no clear correlation could be established between microbial counts and the rate of diazinon degradation, the authors ' did isolate bacteria, fungi, and actinomycetes from wheat rhizosphere capable of degrading diazinon in pure cultures. Recently, Sat0 (16)found an eight-fold increase in heterotrophic and nitrifying bacteria in rice rhizosphere / after the addition of benthiocarb [S-p-chlorobenzyl diethylthiocarc bamate). These findings implicate the increase in microbial biomass as a cause of the decreased persistence of certain toxicants in the rhizosphere and also suggest that rhizosphere microorganisms can protect the plants from chemical injury (17, 28). Seibert and co-workers (19)observed an increase in atrazine (2chloro-4-ethylamino-6-isopropylamino-s-triazine) degradation at bial communities are involved in 5 ppm (mg/kg soil) by microorganprotecting the plant from chemical isms in the rhizosphere of corn folinjury. The phenoxy acid herbi- lowing the harvesting of corn shoots. cides, such as 2,4-D, control broad- This enhanced degradation was corleaf weeds. Sandmann and Loos (7) related with an increase in microbial suggested that the increase in 2,4-D- biomass in the presence of decomdegrading microorganisms in the posing corn roots. The authors also rhizosphere of sugar cane (a grass) attributed the increased degradation was a possible mechanism by to higher dehydrogenase activity obwhich the plant was protected from served i n the rhizosphere soil the herbicidal effects of 2,4-D and throughout the experimental period. that phenolic analogues in the exu- In the rhizosphere soil, the concendate contributed to selection of a tration of unchanged atrazine was microbial community responsible found to be lower than atrazine confor degrading 2,4-D. Conversely, centrations in nonrhizosphere soils. 2,4-D-degrading microbial popula- Concentrations of hydroxyatrazine tions in African clover, a plant sen- and two other hydroxylated metabolites were three times higher in the sitive to 2,4-D, were not increased. Earlier work by Abdel-Nasser and rhizosphere than were concentraco-workers (8) ,and Gavrilova et al. tions in nearby soils.
lsis
1 .dem@%strations
Research on microbial transformations of organic compounds in the rhizosphere focuses mainly on agricultural chemicals such as insecticides and herbicides. A number of researchers have described an increase in pesticide degradation in the rhizospheres of a variety of plant species (Table 1). Occasionally, this increased degrading capacity correlates with increased numbers of pesticide-degrading microorganisms. The wide range of chemical structures and plant types reported in these studies suggests the involvement of multiple species of microorganisms ( 1 5 ) within a community, that is, microbial consortia, rather than a single member of that community.
L
2632 Envimn. Sci. Technol.. Vol. 27, No. 13, 1993
of in situ
Gramineae
Mecoprop" 2,4-Db MCPA'
Mixed microbial culture was capable of using compound source. None of the pure cultures was capable of using the compounds. Wheat is tolerant to this class 01 herbicides (phenoxy acids).
2,4-Db
High population of 2,4-D-degrading microorganisms in the rhizosphere of sugarcane, a plant tolerant to 2,4-D, compared with African clover, a plant sensitive to the herbicidal eflects of 2,4-D
7
Stimulated ammonium oxidalion (nitrification) in rhizosphere soil Increased mineralization in the rhizosphere, especially under flooded conditions '
Eightfold increase in heterotrophic bacteria in the rhizosphere of treated rice plants
16
Baci//ussp. isolated from rice rhizosphere could grow on oil residues but 31 only in the presence of root exudates. Rhizosphere microbial counts increased by 2 orders of magnitude
Vegetated microbial filters increased the removal of both aromatic and aliphatic compounds. Increased disappearance of PAHs in vegetated vs. nonvegetated soil columns Increase In production of atrazine degradation metabolites by rhizosphere microorganisms in the presence of decomposing roots Higher counts of microorganisms in treated vs. untreated rhizospheres
Rhizosphere trealments s gnificanlly ncreased mtial raies of mnera zaionoyafaclorot 1.1-1.9 Increase0 minemzatlon ot botn compodnds i n Ihe rhzosphere Descmes me rnponance 01 egunnous p anis tn rec a m ng petroleLrn.contammatea sites
Mh caJseo ennanceo nitrif callon ana m nera zation of organic SLQstanCes n tne (h zosphere Linaceae
Ammon lying. n tr lying and ce . J ose-oecomposing bacler a n tne m zospnere ncreaseo by 1 to 2 orders of magnitLoe
deza ly p ne grass nrod Soybean
Faoaceae P naceae Gramineae Compos.tae Faoaceae
.ncreased oegradal on 01 TCE n fhizosp contai
Canads
Typhaceae
Sddanants
Mineralizat.on01 sdtactants was more rapid in the rhizosphere th root. free sediments
2-(2-Methyl-4-chlomphenoxy)propianic acid. 42.4-Dichlorophen~xyaceti~ acid. E2-Melhyl-4-~hloraphen~xyacetic acid. d2,3-Oihydro-2,2-dimelhyl-
-benzofuranyl methylcarbamate. * O,Odiethyl-O-~nilrohenyl phosphorothioate. 'S-pchiorobenzyl diethylthiocarbamate.
9
O,Odielhyl-O(Z-
isopropyl-6-melhyl-4-pyrimidinyl)phosphorothioate. Volatik organic compounds (benzene. bi henyl. chiombenzene, dimethylphthalale. ethylbenzene, naphthalene, pnilmloluene, toluene, pxylene, bromoform, chloroform, 1.2-dohloroelhane.lelrachtroethyiene. 1.1.1 -tnchiomethans. ',Polycyciic aromatic h drccarbons (benz[a]amhracene, chrysene. benzolalp rene, and dibenzla,hlanthracene. '2-Chloro-4-ethylamino-6-iso ropylamino-s-lnzins. "2hXethyl-2(methyilhio)propionaldehyde ~(methylca*amoyi~oxime. 'dodecyl linear alkyibenzens sulfonate. dadecyl linear aIcol?oI ethoxylats, dodecykrimelhylammonium chloride. rn Maleic hydrzide ( 1 , 2 - d i h y d r o - 3 , 6 - ~ r i d a i " ~ , o ""~1,1.2-trichloroethyiene. ).
Source: Reference 15.
Similar results were recently reported (20) for atrazine studied at high concentrations characteristic of waste sites. Rhizosphere soils from grasses collected near the boundaries of a pesticide-contaminated site mineralized 60% of atrazine added at 100 ppm (0.46 mM) after eight days, although a lag period of three days was observed. However, nonrhizosphere soil collected within the site mineralized
280% of the atrazine within two to three days, in some cases without a lag period. Nonetheless, 60% mineralization of atrazine at 100 ppm in rhizosphere soils of grasses collected near the site boundary has important implications for sites contaminated with pesticide wastes. Reddy and Sethunathan (21) conducted fate studies of 14C-parathion (0, @diethyl-0-p-nitrophenylphosphorothioate) using rice and ob-
served a rhizosphere effect on degradation similar to that observed by Seibert (19) using atrazine. Only 5.5% of the "C-parathion evolved as 14C0, from unplanted soils, whereas 9.2% evolved from rhizospheres under nonflooded conditions. Converselv, 22.6% of the radiocarbon evolved as %O, under flooded conditions, which favor rice growth. Reddy and Sethunathan (21)argued that the proxim-
Environ SCLTechno1 , VoI. 27, No. 13. 1993 2693
ity of the aerobic-anaerobic interface in the rice rhizosphere under flooded conditions favored the ring cleavage of parathion. Increased microbial deeradation rates in the rhizospherlmay also have been the result of greater 0, concentration provided by the rice roots (22,23). Thus, root exudates provide microorganisms with a wide range of organic substrates for use as carbon and energy sources. Because both parathion and the structurally related pesticide, diazinon, appear to be degraded initially by cometabolic attack (24,251,Hsu and Bartha (26) hypothesized that the rhizosphere would be especially favorable for cometabolic transformations of pesticides. Cometabolism is a process whereby microorganisms biochemically transform a substance while growing on an- I other substrate (5). The microorganism neither derives energy from the cometabolizedsubstance nor is incorporated into microbial biomass; however, the chemical structure of the cometabolized compound is changed. Using radiolabeled diazinon and parathion, Hsu and Bartha were able to show accelerated mineralization of these two insec- 1 ticides in the rhizosphere of the bush bean, Phaseolus vulgocis. Beans were chosen because of 1 their recognized inability to metabolize diazinon (25).Approximately 18% of the parathion and 13% of the diazinon were mineralized in the bean rhizospheres compared with 7.8% and 5.0% in the root-free soil for parathion and diazinon, respectively. Gunner and co-workers (25)previously found similar results w ith diazinon, although they did not observe an increase in microbial biomass in the rhizosphere after application of diazinon. Rather, a microbial isolate capable of diazinon metabolism proliferated. The rhizosphere may also provide a habitat in which microbial consortia capable of growth on organic contaminants may flourish. Lappin et al. (27)found that a microbial community isolated from wheat roots could grow on the herbicide mecoprop [2-(2-methyl-4-chlorophenoxy) propionic acid] as the sole carbon and energy source. The authors isolated five species of microorganisms, none of which could individually m w on mecoprop, not even when 6ultures included a readilv available carbon source for cometibolism. However, two or more species together could
degrade and grow on mecoprop as the sole carbon and energy source. This microbial community also degraded 2,4-D and MCPA (2-methyl+ ChloroDhenoxvaceticacidhMoreover. Anderson et h. (15)concluded that mimbial consortia, rather than individual microbial species, are likely to be involved in the degradation of numemus toxicants in the rhizosphere. Overall, these studies indicate the complexity of plant-microbetoxicant interactions and the complications that may hamper elucidation of the mechanisms by which toxicants are degraded in the rhizoni .
I
I
i
esehch provides substantial evidence for P-d the
4~
”
u Moremediation
I
If surface soils. b .;i
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sphere. The following critical areas need to be investigated further: the influence of the size and structure of plant roots on toxicant degradation, the dynamic aspects of root turnover, including the possible release of toxicants to the soil during decay processes, the potential for roots to release surfactant compounds that may solubilize xenobiotics, and the role of rhizosphere microbial communities in humification processes that may reduce bioavailability of toxicants through stabilization with soil omanic matter. -
hazardous waste The studies reviewed above provide evidence for the accelerated mi-
2634 Environ. Sci. Technol.. Vol. 27, No. 13, 1993
Of
i
m b i a l degradation of agriculturally related organic compounds in the rhizosphere of a variety of plant species. Recent studies also indicate that the dkDDt?aranCeof nonaericultural chemic& is accelerated i’l the root zone. Collectively, these studies show results comparable to the work with pesticides: specifically, the degradation of a variety of nonagricultural chemicals and the capability of rhizosphere microorganisms to degrade them. The following target toxicants were examined in this research: polycyclic aromatic hydmcarbons in prairie gas rhizospheres (281, the enhanced degradation of 1,1,2-kichloroethylene (TCE) in soils collected from the rhizosphere (291, and increased TCE mineralization in whole plant-soil systems (30).Also documented in these studies are increased degradation of oil residues by microorganisms isolated from oil-polluted rice rhizospheres (311, increased mineralization of surfactants by microorganisms associated with cattail roots (32),surfactant mineralization in intact rhizospheres (331, and removal of a variety of EPA priority pollutants in nonvegetated filters and filters planted with the common reed, Phgmites communis (34).Several of these studies are discussed below. Rasolomanana and Balandreau (31)observed enhanced microbial degradation of oil by rhizosphere microorganisms. These observations were serendipitously discovered during the study of the improved growth of rice in soil to which oil residues had been applied. Rasolomanana and Balandreau, hypothesizing that the increased growth was brought about by the initial “removal” of the oil residues from the rhizosphere by specific microorganisms, isolated a Bacillus sp. that could grow on the oil residues only in the presence of rice root exudates. April1 and Sims (28)evaluated the persistence of four PAHsbenz[alanthracene, chrysene, benzo[alpyrene, and dibenz[a,h]anthracene-in the root zone of a mixture of eight prairie grasses in soil column studies. Residue analysis of the columns revealed that PAH disappearance was consistently greater in the vegetated than in the unvegetated controls. Although sterile soil controls were not included in the experiments, the authors speculated that microbial degradation of the PAHs accounted for the increased disappearance of
PAHs from the vegetated soil columns. However, the rhizosphere effect may have been obfuscated by the addition of manure to all soil columns during PAH addition. In other words, the difference in disappearance rates of the PAHs between the vegetated and the nonvegetated columns may have been much greater had the manure not been added to both columns. In addition, April1 and Sims speculated that humification of the PAHs may have accounted for increased disappearance in the vegetated columns. Furthermore, the possibility for root uptake and metabolism of the PAHs was not considered in these experiments, although the metabolic capabilities of vegetation are well documented (35, 36, 37) and plant uptake and metabolism of organic compounds may contribute to enhanced degradation of these materials at waste sites (38).Nonetheless, this study does provide evidence for the accelerated disappearance of hazardous organic compounds in the rhizosphere even though the cause of the disappearance was not established experimentally. Walton and Anderson observed accelerated degradation of TCE in slurries of rhizos here soils and mineralization of ’! C-TCE in rhizosphere soil samples collected from four plant species at a former solvent disposal site (29). These studies were conducted to provide a foundation for more rigorous whole-plant studies in which increased mineralization of ‘C-TCE was also demonstrated (30). The plants tested represented a variety of root types: fibrous, tap, leguminous, and mycorrhizal. Two legumes, Lespedezo cuneoto and Glycine mox (soybean), enhanced soil microbial mineralization of 14CTCE, although only L. cuneoto is indigenous to the contaminated site. In addition, enhanced mineralization was observed in soil containing loblolly pine (Pinus toedo) seedlings, which have root-ectomycorrhizal associations. This raises the question of whether mycorrhizae contribute to the degradation of TCE and other hazardous organic compounds in the rhizosphere. Recently Donnelly and Fletcher (39)described the ability of ectomycorrhizal fungi to degrade certain congeners of polychlorinated biphenyls (PCBs)in vitro. In addition, Katayama and Matsumura (40) have recently shown that a rhizospherecompetent fungus, Trichoderma horzionum, was able to degrade a
variety of organochlorine compounds, including pentachlorophenol, endosulfan, and DDT. Because elevated %O, production was observed in the soils containing soybean germinated from commercially available seeds, prolonged exposure of the plant or soil microorganisms to the toxicant of interest may not be a requirement for enhanced degradation. However, prolonged exposure of soil microorganisms to the toxicant may speed degradation through the selective enrichment of those species in the microbial community that can survive and degrade the xenobiotic substrate. The relative importance of several variables in biodegrading TCE and other organic compounds in the rhizosphere is not well understood. These variables include the morphology or surface area of the root system (e.g., tap vs. fibrous], the selective influence of the root exudates, and the nature of microbe and root associations present (e.g., nitrogen-fixing and mycorrhizal). Other beneficial effects An important consideration for the use of vegetation in remediating contaminated surface soils is the potential transport of the parent compound or hazardous metabol i t e ( ~ from ) soil into plant tissue. Root uptake of organic compounds from soil solution depends largely on physicochemical properties of the compounds, environmental conditions, and plant characteristics (411. Because movement of chemicals into plants presents another potential route of exposure for humans and wildlife at contaminated sites, plant residue analysis is critical in addressing plant uptake of hazardous organic contaminants. A number of recent reviews (42-46) summarize the uptake and accumulation of organic chemicals from soil by vegetation. Overall, plant uptake is usually favored for small and low molecular weight polar compounds, whereas large and high molecular weight lipophilic compounds tend to be excluded from the root. Some researchers have proposed the use of vegetation in terrestrial environments to accumulate inorganic contaminants such as nitrates (471 and metals (48) as well as for removal of organic compounds from soils (38).
Conclusions The research discussed in this review provides substantial evidence
I ”.”
r-.
I
Todd A. Anderson is o member of the gmduote faculty and o research ossociate in the Pesticide Toxicology Lobomtory ot Iowa State University. He received a B.S. degree i n biological science from Peru Stote College, NE, and M.S. degree and Ph.D. in environmental toxicologyfrom the Universityof Tennessee. His reseorch interests include the environmental fate and effects of Industrial chemicals and pesticides ond bioremediation of contominoted sites.
I -.
\
I
--*
I
Elizabeth A. Guihrie is o graduate student in the Deportment of Environmental Sciences and Engineering ot the University of North Carolina. She received her B.S. degree in biology from Emory University ond served as a Peoce Corps volunteer in Gabon, Centml Africa. Her research interests ore the movement and effectsof chemical Contaminants in terrestrial ecosystems and biologicol remediotion of contominoted sites.
Barbam T. Walton is o senior scientist in the Environmental Sciences Division of Oak Ridge Notional Laboratory, She is on adjunct professor in the Deportment of Environmental Sciences ond Engineering at the University of North Carolina and holds adjunct faculty oppointments in ecology ond environmento1 toxicology at the University of Tennessee. She is o former president of the Society of Environmental Toxicology and Chemistry and a post officer of the Americon Board of Toxicology.
Environ. Sci. Technol.. Vol. 27, No. 13, 1993 26%
for the potential role of vegetation in facilitating microbial degradation for in situ bioremediation of surface soils contaminated with hazardous organic compounds. Support for this concept comes from the fundamental microbial ecology of the rhizosphere, documented acceleration of microbial degradation of agricultural chemicals in the root zone, and recent research addressing degradation of nonagricultural hazardous organic compounds in the rhizosphere. Further understanding of the critical factors influencing the plant-microbe-toxicant interaction in soils will permit more rapid realization of this new approach to in situ bioremediation. Especially promising areas for further research are the following: the species-specific properties of the plant, such as root morphology and plant physiology; ecological and physiological characteristics of the microbial communities associated with plant roots; and the role of root exudates in selection of those communities. Microbially mediated humification processes in the rhizosphere may have an important influence on the persistence and bioavailability of toxicants in surface soils. Also important may be the role of nonbacterial plant associations in the rhizosphere, such as the presence of mycorrhizae or the influence of abiotic factors such as nutrient additions, aeration, and multiple chemical stresses. A better understanding of the mechanistic interactions between plant roots and their surrounding microbial communities will favor successful field demonstrations and permit effective selection and management of vegetation to achieve in situ bioremediation. Acknowledgments The authors thank A. M. Hoylman a n d C. W. Gehrs, OakRidge National Laboratory, Oak Ridge, TN; T. C. Hazen of the Savannah River Site, Aiken, SC; a n d F. K. Pfaender, University of N o r t h Carolina, Chapel Hill, for helpful contributions to this work. The Office of Technology Develo ment a n d the Office of Environmentaf Restoration and Waste Management, U.S. Department of Energy, supported this work. Oak Ridge National Laboratory is managed by Martin Marietta Energy Systems, Inc., under contract DE-AC05-840R21400 with the U.S. Department of Energy. Environmental Sciences Division Publication No. 4142.
References (1) Walton, B. T.; Anderson, T. A. Curr. Opin. Biotechnol. 1992,3,267-70. (2) Curl, E. A,; Truelove, B. The Rhizo-
sphere; Springer-Verlag: Berlin and Heidelberg, 1986. Campbell, R. Plant Microbiology; Edward Arnold: Baltimore, MD, 1985. Katznelson, H. Soil Sci. 1946, 62, 343-54.
Atlas, R. M.; Bsrtha, R. Microbial Ecology: Fundamentals and Applications; BenjaminICummings: Menlo Park, CA, 1992. Bolton, H.; Fredrickson, J. K.; Elliot, L. F. In Soil Microbial Ecology; Metting, F. B., Ed.; Marcel Dekker: New York, 1993;pp. 27-64. Sandmann, E.R.I.C.; Loos, M. A. Chemosphere 1984, 23, 1073-84. Abdel-Nasser, M.; Makawi, A. A.; Abdel-Moneim, A. A. Egypt. J. Microbiol. 1979,14,37-44.
Abuyeva, A. A,; Bagayev, V. B. Izvestiya Timiiryazevsk. Skh. Akad. 1975,2,127-30. (10)Gavrilova, E. A,; Kruglov, Y. V.; Ga-
rankina, N. G. Tr. Vses. NauchnoIssled. Instit. Skh. Mikrobiologii. 1983,52,67-70. (11)Paul E. A.; Clark, F. E. Soil Microbiol-
o g y and Biochemistry; Academic Press: New York, 1989. (12)Rovira, A. D.; Foster, R. C.; Martin, J. K. In T h e Soil-Root Interface; Harley, J. L.; Russell, R. S., Eds.; Academic Press: New York, 1979; pp. 1-4. (13)Prikryl, Z.; Vancura, V. Plant Soil 1980,57,69-83. (14)Liu, C.-M. et al. Appl. Environ. Microbiol. 1991,57, 1799-1804. (15)Anderson, T. A.; White, D. C.; Wal-
ton, B. T. In Biotransformations: Microbial Degradation of Health-Risk Compounds; Singh, V., Ed.; in press. (16)Sato, K. In Interrelationships between Microorganisms and Plants in Soil; Vancura, V.; Kunc, F., Eds.; Elsevier: New York, 1989;pp. 335-42. (17)Herring, R.; Bering, C. L. Bull. Environ. Contam. Toxicol. 1988, 40, 626-3 2. (18)Krueger, J. P.; Butz, R. G.; Cork, D. J. J. Agric. Food Chem. 1991,39,1000-03. (19) Seibert, K.; Fuehr, F.; Cheng, H. H. In
Theory and Practical Use of SoilApplied Herbicides Symposium; European Weed Resource Society: Paris, 1981;pp. 137-46. ( 2 0 ) Mandelbaum, R. T.; Wackett, L. P.; Allan, D. L. Appl. Environ. Microbiol. 1993,59,1695-1701. (21)Reddy, B. R.; Sethunathan, N. Appl. Environ. Microbiol. 1983,45,826-29. (22)van Raalte, M. H. Hortus Botanicus,
Bogorien sia,
Javu
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