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b a RITICAL RE VIEW 1
c
xamination of the pub-
lished scientific literature
reveals persuasive evi-
dence that plant roots, in
conjunction with their as-
sociated microbial commu-
ities, offer a potentially
important treatment strategy for in
situ biological remediation of chem-
ically contaminated soils. Under a
variety of environmen tal condi-
tions, vegetation has been shown to
enhance microbial degradation
rates of organic chemical residues
in soils. These findings are impor-
tant because vegetation may pro-
vide a low-cost alternative or sup-
plement to expensive, capital-
intensive technologies for soil
cleanup. Moreover, unlike technol-
ogies that merely relocate contami-
nants, vegetation promises a means
of halting legal liability for hazard-
ous
waste residues in soils because
hazardous compounds in the rhizo-
sphere [root zone) are totally de-
stroyed (mineralized).
Historically, the use of plant sys-
tems as a waste treatment technol-
ogy bas focused primarily on waste-
water ( 1 ) . This work dealt mostly
with the manipulation of opera-
tional 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 rela-
tively unexamined.
This review will critically exam-
ine reports on the interaction of mi-
croorganisms with hazardous
or-
ganic chemicals in the terrestrial
rhizosphere. Studies
on
microbial
degradation of agricultural chemi-
cals in the rhizosphere, and recent
research on the fate of nonagricul-
tural chemicals in rhizosphere soils
are presented. Collectively these
studies provide a strong scientific
basis to support field demonstra-
tions of in situ degradation of toxi-
cants 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 imvor-
tain xenobiotic compounds in the
rhizosphere. Consequently, an in-
triguing question is whether selec-
tion for plants with supernodulat-
ing roots, proliferation of root hairs,
or other genetically determined
properties of plant roots would pos-
itively affect microbial degradation
rates of specific toxicants in the
rhizosphere.
The actual comnosition of the mi-
tanck
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. Ex-
cellent comprehensive reviews on
the rhizosphere are available 2, );
therefore, only a brief description of
important rhizosphere characteris-
tics is presented below.
The overall effect of the plant-
microbe 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 ef-
fect is often expressed quantita-
tively 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 in-
crease in microbial growth and ac-
tivity in the rhizosphere may be re-
s p o n s i b l e f or t h e i n c r e a s e d
metabolic degradation rate of cer-
crobial community in the rhizo-
sphere 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 colo-
nized by a predominantly gram-
negative 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 concen-
trations, osmotic and redox poten-
tials, and moisture content are other
parameters influenced by the pres-
ence of vegetation (2).These param-
eters are further dependent
on
the
properties of specific plant species.
The continual change at the root-
soil interface, both physical and
chemical, produces constant alter-
ations in the soil structure and mi-
crobial 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 rhizo-
sphere by secreting substances such
as carbohydrates and amino acids
through root cells and by sloughing
root epidermal cells. The magni-
tude
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.
0013-936X/93/0927-2630 04.00/0 1993 American Chemi cal Society
T O D D
N
D
E R
S
O N
Iowa
Stote University
Ames,
IA 50011-3140
1 2
B E T H G
U
T
H
R I
E
University
ofNorth
Carolina
Chapel
Hill,
NC27599-7400
B R
B
R
T
,
W
L T
0
oakRidge
National
aboratory
OakRidge,
TN
37831-6038
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This mucigel, along with other cell
secretions, constitutes root exudate
(12).
Soluble exudate includes ali-
phatic, aromatic, and amino acids
and sugars. Root cap cells and exu-
dates 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 nonula-
tions,
it
is the actual struct$e' of
the plant root that provides micro-
organisms 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 rhizo-
sphere of plants, microbial popula-
tions may be passively nourished
by root exudation and decaying
plant matter;
or the
presence of the
microorganisms may actually in-
duce liberation of certain organic
nutrients by co-evolved biochemi-
cal signals.
Pseudomonas
putida
and Azospirillum spp. are exam-
ples of bacteria capable of inducing
nutrient release
from
the roots
13).
In the absence of bacteria and
fungi, plant exudate production
decreases (21, subsequently pro-
Sandmann and Loos 7)ound
an
increase in the number of 2.4-D
(2,4-dichlorophenoxyacetate)-de-
grading bacteria i n rhizosphere
soils of previously untreated sugar
cane but not African clover. This re-
search illustrates
a
potentially inter-
esting theme common to the litera-
ture
on
microbial degradation of
herbicides in the rhizosphere-the
nossibilitv that rhizosuhere micro-
viding fewer organic substrates to
sustain microbial
growth.
The
in-
teraction of leguminous plants
with nitrogen-fixing bacteria re-
sults in increased microbial bio-
mass, plant growth, and root
exu-
dation, perhaps because of the
increased availability of soil nitro-
gen in the presence of nitrogen-fix-
ing
bacteria. This in turn may lead
to enhanced microbial degradation
of organic compounds such asher-
bicides in the rhizosphere by these
bacteria (141.
L
lsis
1
.
dem@ trati
on
s
/
of in
situ c
Research on microbial transfor-
mations of organic compounds in
the
rhizosphere focuses mainly on
agricultural chemicals such as in-
secticides and herbicides. A num-
ber of researchers have described an
increase in pesticide degradation in
the rhizospheres of a variety of
plant species (Table 1).Occasion-
ally, this increased degrading ca-
pacity correlates with increased
numbers of pesticide-degrading mi-
croorganisms. The wide range of
chemical structures and plant types
reported in these studies suggests
the involvement of multiple species
of microorganisms ( 15 ) within a
community, that is, microbial con-
sortia, rather than a single member
of
that community.
bial communities are involved in
protecting the plant from chemical
injury. The phenoxy acid herbi-
cides, such as 2,4-D, control broad-
leaf weeds. Sandmann and
Loos
7)
suggested that the increase in 2,4-D-
degrading microorganisms in the
rhizosphere of sugar cane (a grass)
was a possible mechanism by
which the plant was protected from
the herbicidal effects of 2,4-D and
that phenolic analogues in the exu-
date contributed to selection of a
microbial community responsible
for degrading 2,4-D. Conversely,
2,4-D-degrading microbial popula-
tions in African clover, a plant sen-
sitive to 2,4-D, were not increased.
Earlier work by Abdel-Nasser and
co-workers 8) and Gavrilova et al.
10) howed elevated microbial
counts in
the rhizospheres
of
pesti-
cide-treated plants. Corn, beans, and
cotton plants treated with temik
[Z-methyl-2 methy1thio)propional-
dehyde O(methylcarbamoyl]oximel
8)bad higher microbial counts in
some instances than those in un-
treated rhizospheres. Although the
authors did not monitor degradation
of temik, they hypothesized that the
increased microbial numbers sup-
ported 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(2-
isopropyl-4-methyl-6-pyrimid-
inyl) phosphorothioate] than in
treated soils without vegetation.
Although
no
clear correlation
could be established between mi-
crobial counts and the rate of di-
azinon degradation, the authors
did isolate bacteria, fungi, and
actinomycetes from wheat rhizo-
sphere capable of degrading diaz-
inon in pure cultures. Recently,
Sat0 16) ound an eight-fold in-
crease in heterotrophic and nitri-
fying bacteria in rice rhizosphere
after the addition of benthiocarb
[S-p-chlorobenzyl diethylthiocar-
bamate). These findings implicate
the increase
in
microbial biomass
as a cause of the decreased persis-
tence 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) b-
served an increase in atrazine ( 2 -
chloro-4-ethylamino-6-isopropyl-
amino-s-triazine) degradation at
5
ppm (mg/kg soil) by microorgan-
isms in the rhizosphere of corn fol-
lowing the harvesting of corn shoots.
This enhanced degradation was cor-
related with
an
increase in microbial
biomass in the presence of decom-
posing corn roots. The authors also
attributed the increased degradation
to higher dehydrogenase activity ob-
served in the rhizosphere soil
throughout the experimental period.
In the rhizosphere soil, the concen-
tration of unchanged atrazine was
found to be lower th natrazine con-
centrations
in
nonrhizosphere soils.
Concentrations of hydroxyatrazine
and two other hydroxylated metabo-
lites were three times
higher
in the
rhizosphere than were concentra-
tions
in nearby soils.
2632
Envimn.
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Gramineae Mecoprop Mixed microbial culture was capable of using compound
2,4-Db source. None of the pure cultures was capable of using the compounds.
MCPA' Wheat is tolerant to this class
01
herbicides (phenoxy acids).
2,4-Db High population of 2,4-D-degrading microorganisms in the rhizosphere 7
of sugarcane, a plant tolerant to 2 ,4-D, compared with African clover, a
plant sensitive to the herbicidal eflects of 2,4-D
Stimulated ammonium oxidalion (nitri fication) n rhizosphere soil
Increased mineralization n the rhizosphere, especially under flooded
conditions
Eightfold increase in heterotrophic bacteria in the rhizosphere of treated 16
rice plants
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 sgnificanlly ncreased mtia l raies of
mnera zaio noya faclo rot 1.1-1.9
Increase0 minemzatlon ot botn compodnds n Ihe rhzosphere
Descmes me rnponance 1 egunn ous 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
Ammon lying.
n
tr lying and ce . J ose-oecomposing bacler a
n
tne
m
zospnere ncreaseo by 1 to
2
orders of magnitLoe
Linaceae
deza Faoaceae .ncreased oegradalon 1 TCE
n
fhizosp
ly
p ne P naceae contai
grass Gramineae
nrod Compos.tae
Soybean Faoaceae
Canads Typhaceae Sddanants Mineralizat.on
1
sdtactants was more rapid
in
the rhizosphere th
root. free sediments
2-(2-Methyl-4-chlomphenoxy)propianic acid. 42.4-Dichlorophen~xyacet i~cid. E2-Melhy l -4-~hloraphen~xyacet iccid. 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, oluene pxylene, bromoform, chloroform, 1.2-dohloroelhane. elrachtroethyiene.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. 2-
hXethyl-2(methyilhio)propionaldehyde
~(methylca*amoyi~oxime. dodecyl linear alkyibenzenssulfonate. dadecyl linear aIcol?oI ethoxylats, dodecykrime-
lhylammonium 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 re-
ported (20) for atrazine studied at
high concentrations characteristic
of waste sites. Rhizosphere soils
from grasses collected near the
boundaries of a pesticide-contami-
nated site mineralized
60
of atra-
zine added at 100 ppm 0.46
mM
after eight days, although a
lag
pe-
riod of three days was observed.
However, nonrhizosphere soil col-
lected within the site mineralized
280
of the atrazine within two to
three days, in some cases without a
lag period. Nonetheless,
60 miner-
alization of atrazine at
100
ppm in
rhizosphere soils of grasses collected
near the site boundary
has
mportant
implications for sites contaminated
with pesticide wastes.
served a rhizosphere effect on deg-
radation 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 rhizo-
spheres under nonflooded condi-
tions. Converselv, 22.6% of the ra-
Reddy and Sethunathan (21)con-
ducted fate studies of 14C-parathion
0,
diethyl-0-p-nitrophenylphos-
phorothioate) using rice and ob-
diocarbon evolved as %O, under
flooded conditions, which favor
rice growth. Reddy and Sethu-
nathan
21)
rgued that the proxim-
Environ SCLTechno1,VoI.
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ity of the aerobic-anaerobic inter-
degrade and grow on mecoprop as mbial degradation of agriculturally
face in the rice rhizosphere under
the sole carbon and energy
source.
related organic compounds in the
flooded conditions favored the ring This microbial community also de- rhizosphere of a variety of plant spe-
cleavage of parathion. Increased mi- graded
2,4-D
and MCPA (2-methyl+ cies. Recent studies also indicate that
crobial deeradation rates in the ChloroDhenoxvaceticacidhMoreover.
the
dkDDt?aranCe of nonaericultural
rhizospherlmay also have been the
result
of
greater
0,
concentration
provided by the rice
roots 22,23).
Thus, root exudates provide mi-
croorganisms with a wide range of
organic substrates for use as carbon
and energy sources. Because both
parathion and the structurally
re-
lated pesticide, diazinon, appear to
be degraded initially by cometa-
bolic attack 24,251, su and Bartha
(26)
hypothesized that the rhizo-
sphere would be especially favor-
able for cometabolic transforma-
tions of pesticides. Cometabolism
is a process whereby microorgan-
isms biochemically transform a
substance while growing
on
an-
other substrate (5). The microor-
ganism neither derives energy
from the cometabolized substance
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-
ticides in the rhizosphere of the
bush bean, Phaseolus vulgocis.
Beans were chosen because of
their recognized inability to me-
tabolize diazinon
(25).
Approxi-
mately 18 of the parathion and
13% of the diazinon were miner-
alized in the bean rhizospheres
compared with
7.8
and
5.0
in
the root-free soil for parathion
and diazinon, respectively.
Gun-
ner
and co-workers
25)
revi-
I
I
Anderson et
h. 15)
oncluded that
mimbia l consortia, rather th n indi-
vidual microbial species,
are
likely to
be involved in
the
degradation of
nu-
memus toxicants in the rhizosphere.
Overall, these studies indicate the
complexity of plant-microbe-
toxicant interactions and the com-
plications that may hamper elucida-
tion of the mechanisms by which
toxicants are degraded in the rhizo-
n
i
esehch
ith
usly found similar results w
diazinon, although they did not
provides
substantial
evidence for the
~
P d
u
Moremediation
f surface soils. i
b
i
observe an increase in microbial bi-
omass in the rhizosphere after ap-
plication of diazinon. Rather, a mi-
crobial isolate capable
of
diazinon
metabolism proliferated.
The rhizosphere may also provide
a habitat in which microbial consor-
tia capable of growth
on
organic con-
taminants may flourish. Lappin et al.
27)ound that a microbial commu-
nity isolated from wheat roots could
grow
on
the herbicide mecoprop
[2-(2-methyl-4-chlorophenoxy)
ro-
pionic acid]
as
the sole carbon and
energy source
The authors isolated
five species of microorganisms, none
of which could individually m w
n
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 turn-
over, including the possible release
of toxicants to the soil during decay
processes,
the potential for roots to release
surfactant compounds that may sol-
ubilize xenobiotics, and
the role of rhizosphere microbial
communities in humification pro-
cesses that may reduce bioavailabil-
ity of toxicants through stabiliza-
tion with soil omanic matter.
mecoprop, not even when 6ultures
included a readilv available carbon
-
Of hazardous
waste
source for cometibolism. However,
two or more species together could
The studies reviewed above pro-
vide evidence for the accelerated mi-
chemic& is accelerated
l
the root
zone. Collectively, these studies
show results comparable to the work
with
pesticides: specifically, the deg-
radation of a variety of nonagricul-
tur l chemicals and the capability of
rhizosphere microorganisms to de-
grade them. The following target toxi-
cants were examined in this research:
polycyclic aromatic hydmcarbons in
prairie g s rhizospheres 281, the en-
hanced degradation of 1,1,2-kichlo-
roethylene (TCE)
in soils
collected
from the rhizosphere (291,and in-
creased TCE mineralization in
whole plant-soil systems
(30).Also
documented in these studies are in-
creased degradation of
oil
residues
by microorganisms isolated from
oil-polluted rice rhizospheres 311,
increased mineralization of surfac-
tants
by microorganismsassociated
with cattail roots
(32),
urfactant
mineralization in intact rhizo-
spheres
331,
and removal of a vari-
ety of EPA priority pollutants in
nonvegetated filters and filters
planted with the common reed,
Ph gm it es communis (34).Several
of these studies are discussed be-
low.
Rasolomanana and Balandreau
31)
observed enhanced microbial
degradation of oil by rhizosphere
microorganisms. These observa-
tions were serendipitously dis-
covered during the study of the
improved growth of rice in soil to
which oil residues had been ap-
plied. Rasolomanana and Balan-
dreau, hypothesizing that the in-
creased 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)
valuated
the persistence of four PAHs-
benz[alanthracene, chrysene, ben-
zo[alpyrene, and dibenz[a,h]an-
thracene-in the root zone of a
mixture of eight prairie grasses in
soil column studies. Residue analy-
sis of the columns revealed that
PAH disappearance was consis-
tently greater in the vegetated than
in the unvegetated controls. Al-
though sterile soil controls were not
included in the experiments, the
authors speculated that microbial
degradation of
the
PAHs accounted
for the increased disappearance of
2634
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PAHs from the vegetated soil col-
umns. However, the rhizosphere ef-
fect may have been obfuscated by
the addition of manure to all soil
columns during PAH addition. In
other words, the difference in dis-
appearance rates of the PAHs be-
tween the vegetated and the
non-
vegetated columns may have been
much greater had the manure not
been added to both columns.
In addition, April1 and Sims spec-
ulated that humification of the PAHs
may have accounted for increased
disappearance in the vegetated col-
umns. 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 en-
hanced degradation of these materi-
als at waste sites
(38).
Nonetheless,
this study does provide evidence for
the accelerated disappearance of haz-
ardous organic compounds in the
rhizosphere even though the cause of
the disappearance was not estab-
lished experimentally.
Walton and Anderson observed
accelerated degradation of TCE in
slurries of rhizos here soils and
mineralization of C-TCE in rhizo-
sphere soil samples collected from
four plant species at a former sol-
vent disposal site
(29).
These stud-
ies were conducted to provide a
foundation for more rigorous
whole-plant studies in which in-
creased mineralization of C-TCE
was also demonstrated 30).The
plants tested represented a variety
of root types: fibrous, tap, legumi-
nous, and mycorrhizal. Two le-
gumes, Lespedezo
cuneoto
and Gly-
cine mox (soybean), enhanced soil
microbial mineralization of 14C-
TCE, although only L. cuneoto is in-
digenous to the contaminated site.
In addition, enhanced mineraliza-
tion was observed in soil containing
loblolly pine
(Pinus
toedo
seed-
lings, which have root-ectomycor-
rhizal 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 ectomy-
corrhizal fungi to degrade certain
congeners of polychlorinated bi-
phenyls (PCBs) in vitro. In addition,
Katayama and Matsumura 40) have
recently shown that a rhizosphere-
competent fungus, Trichoderma
horzionum
was able to degrade a
variety of organochlorine com-
pounds, including pentachlorophe-
nol,
endosulfan, and DDT.
Because elevated %O, produc-
tion was observed in the soils con-
taining soybean germinated from
commercially available seeds, pro-
longed exposure of the plant or soil
microorganisms to the toxicant of
interest may not be a requirement
for enhanced degradation. How-
ever, prolonged exposure of soil mi-
croorganisms to the toxicant may
speed degradation through the se-
lective enrichment of those species
in
the microbial community that
can survive and degrade the xenobi-
otic substrate. The relative impor-
tance of several variables in biode-
grading TCE and other organic
compounds in the rhizosphere is
not well understood. These vari-
ables include the morphology or
surface area of the root system (e.g.,
tap vs. fibrous], the selective influ-
ence of the root exudates, and the
nature of microbe and root associa-
tions 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 metabo-
l i te(~)rom soil into plant tissue.
Root uptake of organic compounds
from soil solution depends largely
on physicochemical properties of
the compounds, environmental
conditions, and plant characteris-
tics 411. Because movement of
chemicals into plants presents an-
other potential route of exposure for
humans and wildlife at contami-
nated 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 accumu-
lation 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 com-
pounds tend to be excluded from
the root. Some researchers have
proposed the use of vegetation in
terrestrial environments to accumu-
late inorganic contaminants such as
nitrates 471and metals ( 4 8 )as well
as for removal of organic com-
pounds from soils 38).
Conclusions
The research discussed in this re-
view provides substantial evidence
r .
I
Todd
A.
Anderson is
o
member of the
gmduote faculty and
o
research ossoci-
ate in the Pesticide Toxicology Lobom-
tory
ot I owa
State University.
H e
re-
ceived a B.S. degree in biological
science from Peru Stote College NE
and
M.S.
degree and Ph.D. in environ-
mental toxicology rom the Universityof
Tennessee. His reseorch interests in-
clude the environmental fate and effects
of Industrial chemicals and pesticides
ond bioremediation
of
contominoted
sites.
. --*
- - I
ElizabethA. Guihrie is
o
graduatestu-
dent in the Deportment
of
Environmen-
tal Sciences and Engineeringot the Uni-
versity 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 interestsore the movement and
effectsof chemical Contaminants in ter-
restrial ecosystems and biologicol reme-
diotion
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 Deport-
ment of Environmental Sciences ond
Engineering at the Universityof North
Carolina and holds adjunct faculty op-
pointments in ecology ond environmen-
to1 toxicology
t
the Universityof Ten-
nessee. She is o former president of he
Society of Environmental Toxicology
and Chemistry and a post officerof the
Americon Board
of
Toxicology.
Environ. Sci. Technol..
Vol. 27,
No.
13, 1993
26
8/11/2019 Bioremediation in the Rhizosphere
7/7
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 funda-
mental microbial ecology of the
rhizosphere, documented accelera-
tion of microbial degradation of ag-
ricultural chemicals in the root
zone, and recent research address-
ing degradation of nonagricultural
hazardous organic compounds in
the rhizosphere. Further under-
standing of the critical factors influ-
encing the plant-microbe-toxicant
interaction in soils will permit more
rapid realization of this new ap-
proach to in situ bioremediation.
Especially promising areas for
fur-
ther 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 com-
munities. Microbially mediated hu-
mification processes in the rhizo-
sphere may have an important
influence on the persistence and bio-
availability 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 addi-
tions, aeration, and multiple chemi-
cal stresses. A better understanding
of the mechanistic interactions be-
tween plant roots and their
sur-
rounding microbial communities
will favor successful field demon-
strations and permit effective selec-
tion and management of vegetation
to achieve in situ bioremediation.
Acknowledgments
T h e a u t h o r s t h a n k A . M . H o y lm a n a n d
C . W . G ehr s , O akR i dge N a t i ona l L abor a-
t o r y , O ak R i dge , T N ;
T.
C. Hazen of the
S a v a n n a h R i v e r S i t e , A i k e n , S C; a n d
F .
K . P f a e n d e r , U n i v e r s i t y o f N o r t h
Carol ina , Chape l Hi l l , for helpful cont r i -
bu t i on s t o t h i s w or k . T he O f f ice o f T ech-
no l ogy D eve lo men t an d t he O f f ice o f
E n v i r o n m e n t a f R e s t o r at i o n
and
W ast e
M a n a g e m e n t , U . S . D e p a r t m e n t
of
E n-
e r g y, s u p p o r t e d t h i s w o r k . O a k R i d g e
Nat ional Laboratory
is
managed by Mar -
t i n Mar i e t ta E ne r gy S ys t ems , I nc . , under
con t r ac t D E - A C 05- 840R 21400 w i t h t he
U . S . D e p a r t m e n t o f E n e r g y . E n v i r o n -
mental S c i e n c e s D i v i s i o n P u b l i c a t i o n
N o. 4142.
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