| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK S7N 5A8, Canada
* Corresponding author (ken.vanrees{at}usask.ca)
Received for publication May 26, 2006.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Ectomycorrhizal (ECM) fungi are able to degrade a host of organic chemicals. Of 42 ECM species screened, 33 degraded at least one class of organic contaminant, and only 1 out of 21 species could not degrade at least one polycyclic aromatic hydrocarbon (PAH) (Gramss et al., 1999). Sixteen of these 21 species were able to degrade all five PAHs tested. Enzymes capable of degrading organic compounds are present in soils colonized by ECM fungi, and some lignin- and phenol-degrading enzymes have been observed to be associated with these fungi in sterile conditions (Gramss et al., 1999). Furthermore, interactions between ECM fungi and other microorganisms may be important in hydrocarbon degradation. When Paxillus involutus was grown in association with Pinus sylvestris in petroleum hydrocarbon-contaminated soil bacterial bio-films involved in hydrocarbon degradation were found on the surface of some hyphae (Sarand et al., 1998).
The ectomycorrhizal fungus P. tinctorius was selected for use in this study because it is known to associate with trees of the Populus genus, as well as many others (Navratil and Rochon, 1981; Cripps and Miller, 1995; Cairney and Chambers, 1999). Due to its wide host range, this fungus is commonly found in commercial inoculum. Given that this study was focused on utilizing an easily obtainable commercial inoculum that could be applied to a variety of phytoremediation applications, P. tinctorius was a good candidate. This fungus also is known to proliferate on marginal and industrially polluted soils (Marx and Artman, 1979; Agerer, 2002).
Mineral forms of N are often limited in hydrocarbon-contaminated soils due to net immobilization by microbes (Xu and Johnson, 1997). Ectomycorrhizal fungi are able to utilize organic forms of N and P in the soil and subsequently transfer these nutrients to the host. In fact, seedlings of several tree species colonized by Hebeloma crustuliniforme grew with protein as the sole N source because of external proteases produced by the fungi (Abuzinadah and Read, 1986). The ability of ECM fungi to utilize organic N sources could provide some benefit to trees growing in contaminated soil by giving access to forms of N not normally available for plant uptake.
The overall objective of this study was to assess the phytoremediation potential of the hybrid poplar/Pisolithus tinctorius association. Specific objectives were to quantify the effect of colonization by the ECM fungus Pisolithus tinctorius on aboveground biomass and fine root production, and nutrient acquisition by hybrid poplar grown in soil contaminated with diesel fuel.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
Pots were designed to enable minirhizotron observation of fine roots. Pots were constructed using schedule 40 polyvinyl chloride pipe with a 14.1-cm i.d., cut to a depth of 28 cm. Three rows of eight pots were assembled by drilling two 5-cm holes, 19 cm from the top of the pot, opposite each other in the sides of the pot and inserting a 400-cm-long, 5-cm-diam. acetate-butyrate minirhizotron tube horizontally through the pots. The minirhizotron tube was held in place by Mono Ultra caulk (Tremco, Toronto, ON, CA) around the outer walls of the pots. After insertion of the minirhizotron tube each pot had a volume of 4.1 L. The pots were then secured to a 5 by 56 by 400 cm piece of plywood. Any exposed tube surface was painted black to exclude light, then silver to reflect radiation. Tubes were capped with an aluminum can when not in use.
Hybrid Poplar Stock and Mycorrhizal Inoculum
Hybrid poplar (P. deltoides x (P. laurifolia x P. nigra) cv. Walker) rooted cuttings were purchased from Smoky Lake Forest Nursery, Smoky Lake, AB. Trees were approximately 6 mo old, shipped from cold storage in a dormant state and kept at –2°C. Thirty-six h before planting, seedlings were removed from cold storage and placed in the growth chamber to acclimate.
Mycorrhizal inoculum (Mycogrow Tree Tabs) was purchased from Fungi Perfecti LLC, Olympia, WA, USA. The inoculum was in tablet form containing spores of Pisolithus tinctorius and Rhizopogon spp. held together by an inert binder.
Experimental Design and Maintenance
Six replicates of four treatments were used in the experiment. The four treatments were a diesel-contaminated soil (–ECM/+Diesel), a diesel-contaminated soil with ECM inoculum (+ECM/+Diesel), uncontaminated soil with an ECM inoculum (+ECM/–Diesel), and uncontaminated soil with no addition of inoculum (–ECM/–Diesel). Two replicates of each treatment were randomly assigned within each row. At the time of planting two Mycogrow Tree Tabs were dissolved in 1 L of water for each treatment receiving inoculant. One tree was placed in a pot and the pot filled with 16 cm of soil. Five hundred milliliters of the inoculant solution was applied in the requisite treatments around tree roots at this depth. Pots were then filled to capacity, lightly packed as evenly as possible, and the remaining 500 mL of solution was added to the soil surface. Uninoculated treatments received 1 L of water with no inoculum in the same manner. To ensure that an equivalent amount of soil was present in each pot the weight of soil necessary to fill the first pot was recorded and all other pots were filled with the same amount of soil by weight.
Trees were grown in a growth chamber for 12 wk under a 16-h light/8-h dark cycle. Temperatures were maintained at 22 and 16°C during light and dark periods, respectively. A HydroSense volumetric soil probe (Campbell Scientific Ltd., Townsville, QLD, AU) was used to measure water content every 2 d. Field capacity was determined to correspond with a volumetric water content of 0.23 cm3 cm–3 (Tan, 1996). Pots were initially watered to a volumetric water content of 0.20 cm3 cm–3, which corresponded to 85% of field capacity. During the course of the study, pots with a volumetric water content of <0.10 cm3 cm–3 at the time of measurement were watered to volumetric water content of 0.20 cm3 cm–3. Six replicates of unplanted contaminated soil were placed in plastic pots of the same volume as the planted pots to serve as controls. The unplanted control soil was maintained at the same water contents as the planted treatments.
Data Collection and Processing
Minirhizotron Image Capture and Analysis
Image collection with the minirhizotron camera was accomplished by inserting the camera, with a 1.1-cm2 lens, into the tubes and capturing an image at 1.1-cm increments along the wall of the tube. A small hole drilled approximately 5 cm from the open end of the tube allowed the camera to be locked in place, and the handle of the camera was notched to facilitate incremental movement up the tube. Images were stored on a Sony Vaio (Sony Inc., Tokyo, JAP) laptop computer using I-CAP image capture software (Bartz Technology Co., Santa Barbara, CA, USA). Images were captured every 2 wk during the study.
Each observed root in every 1.1-cm2 image from the first sampling date was traced for length and diameter using RooTracker software (Version 2.0, Duke University, NC, USA). When images from subsequent sampling dates were uploaded into RooTracker, the length and diameter measurements traced on the previous sampling date are laid over the new image, allowing for new tracings to be made to reflect changes in the measured variables. When a root traced in a previous session was later observed to be missing, obscured by debris or soil, or when coloration became black (indicating death), the root was labeled "dead/missing" in RooTracker and traced in red (all other roots were traced in yellow). If, when tracing images for the subsequent 2 wk, these roots were still missing, or in the case of black or obscured roots became missing, they were deleted. Only missing roots were deleted, black and obscured roots continued to be traced as accurately as possible until such time as they may have become missing. Data from RooTracker was saved in Microsoft Excel (Microsoft Corp., Redmond, WA, USA) spreadsheets and organized by tube, frame, root, date, and root diameter. The plane intersect method script for SAS 9.1 (SAS Inst. Inc., Cary, NC, USA) was provided by the authors (Bernier and Robitaille, 2004) and used to acquire fine root production and biomass estimates.
Fine root mass density and soil coarse fraction are necessary secondary variables used by the plane intersect method to estimate fine root biomass and production (Bernier and Robitaille, 2004). Fine root mass density is calculated as the mass per unit volume of the root, and was measured by randomly selecting 20 roots (<2 mm diam.) from each treatment at the end of the study (Bernier et al., 2005). Values were 0.41, 0.37, 0.39, and 0.35 g cm–3 for the –ECM/+Diesel, +ECM/+Diesel, +ECM/–Diesel and –ECM/–Diesel treatments, respectively. Soil coarse fraction was estimated as 3% (w/w) by passing a 5-kg sample of soil through a 2-mm sieve and weighing the particles which were retained.
The plane intersect method was develop by Bernier and Robitaille (2004) and uses only date of first sighting and root diameter as variables for estimating root biomass and production. These easily observable variables do away with uncertainties resulting from last sighting estimates and the possibility of skewed volumetric occupation data which can result from root/tube contact. The authors argue that since the minirhizotron tube is not representative of the soil body, the use of length to estimate fine root biomass and production may result in inaccuracies due to the tendency of roots to run along the tube surface (Bernier and Robitaille, 2004).
To better understand the method by which the plane intersect method estimates biomass and production imagine, if you will, a root growing through the soil. If the soil (and the root it contains) was then sliced into an infinite amount of layers across the roots surface, the root would be observed as an infinite series of two-dimensional, elliptical cross-sections (Bernier and Robitaille, 2004). The diameter of the root would then govern the length of its short axis and its angle of approach would govern the length of its long axis. If we think of the point of root/tube contact as a two-dimensional plane containing an elliptical root cross-section, then it can be expected that the long axis of the root is 
/
times greater than its diameter (Bernier and Robitaille, 2004). In this manner the plane intersect method uses diameter to produce biomass and productivity estimates in g m–2.
Harvest and Root Excavation
At the end of the experiment aboveground biomass was harvested, weighed, and dried at 60°C for 4 d. Leaves and woody biomass were separated and ground to pass through a 1-mm sieve using a Thomas Wiley Laboratory Mill Model 4 (Thomas Scientific, Swedesboro, NJ, USA). Subsamples of three stems from each treatment, weighing approximately 2 g each, were stored without drying for total petroleum hydrocarbon (TPH) analysis.
Roots were excavated from the soil by gentle washing over a 0.5-mm plastic mesh screen to catch any fragments. Excavated roots were then separated from the bole, enclosed in the mesh screen, and hand washed thoroughly in warm water. Roots were blotted dry, weighed, and stored at 4°C in plastic bags until TPH analysis. Whole plant biomass and root/shoot ratios in this study are calculated from final weights of above- and belowground biomass.
Before root excavation one 2-cm-diam. soil core was taken from each replicate within the two contaminated treatments for the entire depth of each pot. These samples were homogenized and used to assess residual diesel fuel concentrations in the soil, and were stored at –2°C until analysis.
Mycorrhizal Colonization
Mycorrhizal colonization counts were performed by randomly selecting 15 roots from each replicate of all treatments. Presence of ectomycorrhizae was assessed by visual inspection under a dissection microscope at 30x magnification. Identification to species was done using dissection and compound light microscopy (up to 400x magnification) and was based on the morphological characteristics of ramification, mantle, and rhizomorph type as described by Agerer (1999, 2002).
Total Petroleum Hydrocarbon Extraction and Plant Nitrogen and Phosphorus Concentrations
Plant leaves were acid digested following the method of Thomas et al. (1967). Subsequent analysis using a Technicon Autoanalyzer II (Labtronics Inc., Tarrytown, NY, USA) yielded plant contents of N as NH4 and P as PO4.
Total petroleum hydrocarbons were extracted from the soil samples by accelerated solvent extraction (Richter, 2000) using a Soxtec 2050 Autoextraction unit (Rose Scientific Ltd., Edmonton, AB, CA) with 50:50 hexane/acetone (v/v) as a solvent. After extraction the samples were concentrated by evaporating under vacuum to approximately 2 mL in a 60°C water bath on a Rotary evaporator. Each extract was then transferred to a 2-mL vial and loaded into a Varian CP-3800 gas chromatograph (GC; Varian Inc., Palo Alto, CA), with flame ionization detector (GC-FID), and cold on-column injection. A 0.2-µL portion of the sample was injected and analyzed for TPH (C10–C50). A Varian CP-SIL 5CB column having the dimensions 15 m by 0.25 mm i.d. with a stationary phase thickness of 0.25 µm was used for analytical separation. The carrier gas was hydrogen held at an initial pressure of 11.73 kPa for 1 min, then ramped to 38.64 kPa at 2.07 kPa min–1 and held for 9.3 min. Initial injector temperature was 60°C ramped to 300°C at 100°C min–1, and held for 20.9 min. Detector temperature was 300°C. Initial oven temperature was 40°C, held for 1 min and ramped to 300°C at 20°C min–1 and held for 9.3 min. The detection limit of the GC-FID is 7.5 mg kg–1. The detection limit of the method, including hydrocarbon spiking, extraction, and GC-FID detection is approximately 11.5 mg kg–1. Hydrocarbon concentrations below 11.5 mg kg–1 could not be accurately measured.
Extraction of TPH from excavated tree roots consisted of taking 10 g of fresh roots from each replicate of contaminated soil treatments and placing in a ceramic mortar. Only roots <5 mm in diameter were used to ensure enough roots were included to provide a representative sample. Liquid N was then added to the bowl and the roots or stems crushed with a pestle as finely as possible. This method was used to protect against any dissipation of TPH that may come about from drying and produced good results after some initial trial and error.
A 5-g subsample of the crushed roots was extracted by the accelerated solvent method and treated to remove polar compounds before GC analysis. Three replicate stem samples from each treatment were extracted in the same way as roots. Only three replicates from each treatment were analyzed because it was postulated that the amount of TPH present in the stem would be negligible, as was found to be the case. Gas chromatography for roots and stems was performed in the same manner as for the soil samples.
Statistical Analysis
Analysis of fine root production was performed using mixed design ANOVA (SPSS version 13, SPSS Inc., Chicago, IL). The six biweekly estimates of production represented the within-subjects variables. Given that these variables were related in time it was necessary to use repeated measures analysis to determine effects. The four treatments represented the between-subjects factors. A significant "week x treatment" effect indicated that patterns of root production differed between treatments. Therefore, orthogonal polynomial trend analyses were run separately for each treatment to determine the effect of time on fine root production.
Due to the heterogeneity of variances of fine root production datasets, the significance of treatment effects on overall fine root production were determined using the Games-Howell multiple comparison test, which does not require equal variances for accuracy. Differences between treatments in N and P contents, final whole plant biomass, and residual soil and root TPH levels were assessed using one-way ANOVA. Multiple comparisons were made using the Bonferroni adjusted t test.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Fine Root Production
Mixed design ANOVA showed a significant between-subjects effect (Table 2). A posteriori testing revealed significantly greater fine root production in the –ECM/–Diesel and +ECM/–Diesel treatments than the –ECM/+Diesel and +ECM/+Diesel treatments (Table 3). More fine roots were produced by poplar grown in the +ECM/+Diesel treatment than the –ECM/+Diesel treatment.
|
|
|
|
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Nicolotti and Egli (1998) found that increasing concentrations of crude oil (up to 50 g kg–1) did not affect ECM infection. With contamination of 5 g kg–1 (the same as used in the present study) infection rates were still approximately 45 to 65%. Although infection in the present study was not as high as those found by Nicolotti and Egli (1998), the effect of diesel fuel contamination on infection seemed to be negligible.
Fine Root Production
Studies have reported enhanced host growth along with ectomycorrhizal colonization resulting from photosynthetic stimulation (Smith and Read, 1997), increased access to and uptake of soil nutrients (Vogt et al., 1991), and the ability of ECM fungi to use organic forms of these nutrients (Abuzinadah and Read, 1986). In this study aboveground growth was enhanced by ECM colonization in both contaminated and uncontaminated soil, while total fine root production was enhanced by ECM colonization only in the contaminated soil between the colonized and uncolonized treatments in the uncontaminated soil (Table 3). The lack of stimulated fine root production in the uncontaminated soil could be due to the fact that the experimental period was relatively short, although Baum et al. (2002) also observed no significant difference between root weights of poplar inoculated with L. laccata and those not inoculated after 6 mo. On the other hand, Navratil and Rochon (1981) did observe a stimulatory effect of P. tinctorius inoculum on total root length on the hybrid poplar clone P. euroamericana after 48 d. The absence of a stimulatory effect on fine root production in the uncontaminated soil is not surprising considering that the C cost of the ECM association to the tree can be great (Smith and Read, 1997). This increased C cost may lead to reduced fine root production in favor of aboveground production, to maximize rates of photosynthesis (Rygiewicz and Anderson, 1994; Smith and Read, 1997).
Although mean total fine root production in the +ECM/–Diesel and –ECM/–Diesel treatments was not significantly different, the patterns of production over time were dissimilar (Table 3 and Fig. 1). Fine root production in the –ECM/–Diesel treatment showed a negative linear trend, indicating that an initial flush of fine roots occurred in this treatment and that production decreased thereafter. A negative linear trend in fine root production in trees over the growing season is commonly reported in the literature (Hendrick and Pregitzer, 1996; Steele et al., 1997). Given that this study spanned 12 wk, or the equivalent of a short growing season, a negative linear trend probably reflects relatively normal growing conditions. This behavior may be due to the fact that trees coming out of a dormant phase produce a strong initial flush of root growth to access soil nutrients (Hendrick and Pregitzer, 1996).
The quartic trend observed in the +ECM/–Diesel treatment describes a fluctuating pattern of root production over the study period (Table 2 and Fig. 1). Trees associated with ECM fungi in a natural setting can evince a linear trend in fine root production; however, in the present case of the +ECM/–Diesel treatment the quartic trend could be a product of the fluctuating nutrient requirements of the fungi and the hybrid poplar.
Inoculating hybrid poplar with ECM increased fine root production in diesel-contaminated soil, but not to the same level of production measured in the uncontaminated treatments. Many ECM fungi are capable of degrading hydrocarbons and other organic xenobiotics (Gramss et al., 1999). The increased fine root production may be the result of reduced contaminant levels in the immediate vicinity of the root/hyphal association. The fungus may create a buffer around the root system as a result of metabolizing hydrocarbons, or by inducing bacterial degradation of these compounds. Although the ability of P. tinctorius to degrade hydrocarbons has not been assessed specifically, it was demonstrated to degrade 2,4,6-trinitrotoluene (TNT) (Meharg and Cairney, 2000). Furthermore, ECM fungi can stimulate bacterial activity in their vicinity; a phenomenon known as the mycorrhizosphere effect (Meharg and Cairney, 2000). Some researchers have attributed increased hydrocarbon degradation in mycorrhizal associations to this effect, rather than the fungi themselves (Meharg and Cairney, 2000). Another reason for increased fine root production in the +ECM/+Diesel treatment compared to the –ECM/+Diesel treatment may be better access to nutrients in the soil as a result of fungal scavenging of organic substrates and subsequent nutrient transfer to the host plant (Smith and Read, 1997).
Fine root production in the –ECM/+Diesel treatment remained quite low until 8 wk. Between 8 and 10 wk, fine root production increased approximately threefold, at which time fine root production was similar to that of the other treatments. This lag in fine root production was likely due to the inhibitory effects of the diesel on root growth. After 8 wk the effect was mitigated probably by the removal of the contaminants from the system by processes such as microbial degradation, plant uptake, root adsorption, volatilization, or reduced bioavailability. In the +ECM/+Diesel treatment, on the other hand, the inhibitory effects of the diesel on fine root production were alleviated more rapidly. Fine root production in the +ECM/+Diesel treatment increased markedly between 2 and 4 wk, and by 6 wk fine root production in the +ECM/+Diesel treatment had reached levels comparable to those in the +ECM/–Diesel and –ECM/–Diesel treatments. Again, this increased production was presumably due to the ECM association stimulating nutrient access and/or contaminant detoxification in the area immediately surrounding the roots. Sequestration of hydrocarbons in the fungal sheath may also have protected roots from direct hydrocarbon exposure.
Whole-Plant Biomass, Root/Shoot Ratios, and Nutrient Status
Trees in the uncontaminated soil that were colonized by ECM fungi resulted in the greatest whole-plant biomass production of all of the treatments (Table 3). This stimulatory effect of ECM colonization on host plant growth has often been documented in the literature (Smith and Read, 1997). Poplar grown in both diesel-contaminated treatments produced significantly less biomass than poplar in the uncontaminated treatments, but colonized plants in contaminated soils performed better than those that were not colonized. Reasons for this enhanced performance are likely to be the same as those discussed in relation to fine root production alone.
Root/shoot ratios in the present study are similar to other studies involving young hybrid poplar (Proe et al., 2002; Glynn et al., 2003). Diesel contamination caused root/shoot ratios to increase compared to uncontaminated soils. Typically, root/shoot ratios decrease with ECM colonization due to increased nutrient uptake capability (Smith and Read, 1997), although some researchers have found increased ratios because of the added weight of the fungal biomass (Alexander, 1981; Smith and Read, 1997). In this study, diesel contamination was the most important factor governing root/shoot ratios, given that higher ratios were induced in contaminated treatments even though overall fine root production was suppressed. Increased root/shoot ratios in plants grown in contaminated soils are most likely an indication of hormesis due to contact with xenobiotic compounds or a response to stress resulting from nutrient deficiencies. Hormesis refers to the stimulated growth of an organism in response to small amounts of toxic substances and has been documented in other plant species in hydrocarbon-contaminated soils (Salanitro et al., 1997; Kirk et al., 2002). Pregitzer et al. (1993) demonstrated that fine root production is stimulated in nutrient-limited conditions as a means to access a larger volume of soil.
Ectomycorrhizal colonization increased leaf N and P content of trees growing in both uncontaminated and contaminated soils compared to the respective uncolonized treatments. It is well known that ECM fungi are important in nutrient acquisition, due to their ability to utilize organic N sources and greatly increase plant P uptake (Harley and McCready, 1950; Abuzinadah and Read, 1986). The fact that leaf concentrations in the +ECM/+Diesel treatment were boosted to the same level as the +ECM/–Diesel treatment is noteworthy. These similar nutrient concentrations demonstrate an important potential benefit for phytoremediation applications in that ECM colonization provides the host tree with greater nutrient access over uncolonized trees in diesel-contaminated soils. The fact that no significant difference was observed in N and P concentrations between the –ECM/–Diesel and –ECM/+Diesel treatments demonstrates that uncolonized trees growing in an uncontaminated soil were no better at accessing N and P than uncolonized trees in a contaminated environment, where nutrient deficiencies are common.
Total Petroleum Hydrocarbon Concentration Remaining in the Soil
Even though ECM colonization increased fine root production and aboveground biomass in hybrid poplar grown in contaminated soils, actual phytoremediation capability was reduced. In this study, 336.5 mg kg–1 of TPH remained in the soil in the +ECM/+Diesel treatment after 12 wk. In the –ECM/+Diesel treatment only 253.3 mg kg–1 of TPH remained. Although research has shown that ECM fungi degrade hydrocarbons in pure culture and help to initiate cometabolism in the soil (Gramss et al., 1999), the reduced remediation capability seen here may be due to ECM fungi suppressing bacterial activity in the soil due to outcompetition for water and nutrients, as well as the incorporation of C that would enter the soil from root exudates into fungal biomass (Olsson et al., 1996). The production of antibacterial substances by ECM fungi also has been documented (Rasanayagam and Jeffries, 1992). Joner et al. (2006) observed lower PAH degradation rates in phytoremediation experiments with ECM fungi, which the researchers attributed to depletion of mineral N and P by the fungi.
Total petroleum hydrocarbon concentrations extracted from the roots of hybrid poplar were approximately three times greater in the +ECM/+Diesel treatment than in the –ECM/+Diesel treatment. In fact, 354.1 mg kg–1 of TPH were sequestered in the roots of the +ECM/–Diesel treatment, compared to only 102.2 mg kg–1 in the –ECM/+Diesel treatment. Hydrophobins exist within the mycelial cell walls of certain species of ECM fungi, including P. tinctorius (Bücking et al., 2002). These water-repellent complexes lower the ion permeability of the fungal sheath and also may serve as a sorption site for hydrocarbons in the soil. Root sequestration of contaminants by partitioning onto the fungal sheath is in itself a phytoremediation process, due to the fact that the contaminants have been removed from the soil. However, this phenomenon may only be temporary given that hydrocarbons not degraded at the root surface could reenter the soil on root senescence.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The present study suggests that while ECM fungi helped hybrid poplar to better tolerate hydrocarbon-contaminated soils, their usefulness in phytoremediation may be limited during the initial phase of establishment. This is evidenced by the smaller concentrations of TPH that remained in the soil after 12 wk in the –ECM/+Diesel treatment compared to the +ECM/+Diesel treatment, although compared to unplanted controls both of the planted treatments reduced contamination levels. A goal of many phytoremediation strategies is simply to effectively establish plants on a contaminated site. Therefore, over the long term the stimulation of hybrid poplar root and shoot growth achieved by ECM fungal colonization may improve tree establishment, long-term productivity, and lead to an increased ability to compete with weeds on these contaminated sites.
|
ACKNOWLEDGMENTS |
|---|
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Soil Science Society of America Journal | Journal of Plant Registrations | The Plant Genome | |||
