Mechanistic changes during phytoremediation of perchlorate under different root-zone conditions

Two types of hydroponic bioreactors were used to investigate the mechanisnistic changes during phytoremediation of perchlorate under different root-zone conditions. The bioreactors included: (1) an aerobic ebb-and-flow system planted with six willow trees, and (2) individual willow trees grown in sealed root-zone bioreactors. Rhizosphere probes were used to monitor for the first time during phytoremediation of perchlorate, diurnal swings in oxidation-reduction potential (E(H)), dissolved oxygen (DO), and pH. Radiolabeled (36Cl-labeled) perchlorate was used as a tracer in a subset of the sealed bioreactor experiments to quantify the contribution of phytodegradation and rhizodegradation mechanisms. Rhizodegradation accounted for the removal of 96.1 +/- 4.5% (+/-95% CI) of the initial perchlorate dose in experiments conducted in sealed hydroponic bioreactors with low DO and little or no nitrate N. Meanwhile, the contribution of rhizodegradation decreased to 76 +/- 14% (+/-95% CI) when nitrate (a competing terminal electron acceptor) was provided as the nitrogen source. Slower rates of phytoremediation by uptake and phytodegradation were observed under high nitrate concentrations and aerobic conditions, which allowed perchlorate to persist in solution and resulted in a higher fraction uptake by the plant. Specifically, the rate of removal of perchlorate from bulk solution ranged from 5.4 +/- 0.54 to 37.1 +/- 2.25 mg/L/d (+/-SE) in the absence of nitrate to 1.78 +/- 0.27 to 0.46 +/- 0.02 mg/L/d (+/-SE) at high nitrate concentration. The results of this study indicate that the root-zone environment of plants can be manipulated to optimize rhizodegradation and to minimize undesirable processes such as uptake, temporal phytoaccumulation, and slow phytodegradation during phytoremediation of perchlorate. Rhizodegradation is desired because contaminants resident in plant tissue may remain an ecological risk until completely phytodegraded.     

浮床植物系统对富营养化水体中氮、磷净化特征的初步研究

以浮床空心菜(Ipomoea aquatica)、水芹(Oenanthe javanica)和无植物系统为对象,研究了其在富营养化水体中对N、P的去除及其N₂O的排放情况.结果表明,浮床植物系统对水体中N、P具有良好的净化效果,植物组织所累积的N、P量分别占各自系统去除量的40.32%～63.87%,说明植物的同化吸收作用是N、P去除的主要途径.换水周期内浮床植物系统中硝化反应进行充分,而反硝化反应相对缓慢,导致系统具有较高的NH₄⁺-N去除率,而产生NO₃--N累积.植物的存在降低了系统中N₂O的排放通量.生长较好的空心菜系统在换水前后平均N₂O排放量最低,为17.14μgN·m^-2h^-1,空白高达8.08μgN·m^-2h^-1,水芹为37.38μg N·m^-2·h^-1.     

Sorption of cadmium and zinc from aqueous solutions by water hyacinth (Eichchornia crassipes)

The water hyacinth (Eichchornia crassipes) has been successfully utilized for the removal of Zn(II) and Cd(II) as well as their admixture from samples of aqueous solutions. The growth of the plant after 16 days of exposure to the metal ions showed an increasing trend up to 2.5 ppm of Cd(II) and 6.0 ppm of Zn(II) concentrations, however, the growth became nondetectable or inhibited above these concentrations. The overall metal uptake by the plant was dependent upon the concentration of the metal and the duration of the exposure time. The metal uptake from a mixture of Cd(II) and Zn(II) was reflected by a rate constant quite different from those solutions containing only one metal ion. An analysis of metal in roots and tops of the plants showed that more Zn(II) was accumulated in the root when compared to Cd(II). However, the accumulation factor for the tops and the roots for Cd(II) and Zn(II) was higher than those obtained admixture of Zn(II) and Cd(II). The rate of metal mobility in the root was slower than that in the top of the plant for Zn(II) and Cd(II). A water hyacinth based system can be used to remove Cd(II) and Zn(II) from water/wastewater.     

Effects of Indole-3-Acetic Acid (IAA), a Plant Hormone,on the Ryegrass Yield and the Removal of Fluoranthene from Soil

A soil culture experiment was conducted to determine whether a plant hormone, indole-3-acetic acid (IAA), could influence fluoranthene (Flu) removal from soil. Four treatments were utilized: (i) unplanted soil (CK), (ii) soil planted with ryegrass (P), (iii) soil planted with ryegrass and treated with 0.24 mg kg^?1 IAA (P+0.24), (iv) soil planted with ryegrass and treated with 2.4 mg kg^?1 IAA (P+2.4). The Flu initial concentration was 200 mg kg^?1. After 3 months, the percentage of Flu removal and plant root biomass were significantly increased under the P+2.4 and the removal rate was 35.68%. The total Flu content in plants was higher than that in the other treatments. The Flu concentration was significantly increased in the shoots, but not significantly altered in the roots. The highest translocation factor was observed in the P+2.4. Increase in number of bacteria, actinomycetes and fungi were observed in the planted treatments, and the amount of fungi was significantly increased in P+2.4. Flu removal was related to the Flu in ryegrass, and was insignificantly correlated with the stimulation of soil microflora, which suggesting that IAA may work mainly on improving plant growth, the Flu uptake, and eventually leading to enhanced remediation of Flu polluted soil.     

Re-sorption of organic compounds by roots of Zea mays L. and its consequences in the rhizosphere

The exudation of soluble carbon compounds from Zea mays roots was investigated over a 10 day growth period under sterile and non-sterile solution culture conditions. The results showed that plants grown in sterile static solution culture, where C was allowed to accumulate, released 8 times less C than plants grown under culture conditions in which the solutions were replaced daily. The increased C loss from plant cultures in which exudates were removed daily was attributable to, (a) the reduced potential for root re-sorption of previously lost C, and (b), increasing diffusion gradients between the root and the surrounding bathing solution increasing passive leakage of exudates from the roots. In treatments where C was removed daily from the root-bathing solution, 86% of the total C lost was of a soluble low molecular weight nature, whereas, in sterile and non-sterile static cultures, allowing the accumulation of C over 10 days, this was reduced to 67.5 and 48% respectively. The main C fluxes operating in a solution culture system (efflux and influx of C by both roots and microorganisms) were examined using a computer simulation model to describe movement of soluble sugar-C in both sterile and non-sterile conditions. In sterile static cultures where C was allowed to accumulate in solution over a 10 day growth period, 98% of the C exuded was re-absorbed by the plant. Where C was removed daily from the root-bathing solution this was reduced to 86%. The predicted patterns of C accumulation were similar to those found in the experiments. Simulations showed that the pattern of accumulation and final equilibrium concentrations were dependent on the rate of exudation, the spatial characteristics of exudation, solution volume, root growth rate and the presence of a microbial population. Simulations under non-sterile conditions showed that roots can compete with microorganisms for exudates in solution indicating the possible importance of re-sorption in a soil environment. The results clearly indicate that roots are capable of regulating the net amount of C released into a solution culture with the amount of C collected being highly dependent on the experimental conditions employed. The possible implications of soluble C influx on processes operating within the rhizosphere and in experimental systems is discussed.     

Effects of plant and influent C:N:P ratio on microbial diversity in pilot-scale constructed wetlands

Microbial processes within the rhizosphere of constructed wetlands are crucial to wastewater treatment, but the relation between microbial community diversity in rhizosphere, plant growth and water quality are unclear at present. The effects of plant growth, water C:N:P ratio and their interaction on microbial diversity in the rhizosphere were studied in synthetic wastewater in planted and unplanted wetlands during three different seasons. The physiological profile of microbial community-level in each wetland was assessed using substrate utilization patterns gathered via BIOLOG? ECO plates. Plant had a significant effect on AWCD parameter, since the planted wetlands usually had a higher the total microbial activity than the unplanted over the study period. The Shannon, Simpson and McIntosh indices in the planted wetlands were apparently higher than those in the unplanted wetlands under any C:N:P ratio influent condition especially in summer. It was also shown that the unplanted wetlands have a greater shift of the interstitial microbial community than the planted at different seasons, since plant rhizospheres produce a more ecologically stable system in order to resist against shifts in microbial community composition in response to C:N:P ratio change in wastewater. Principal component analysis and clustering analysis indicated that influent C:N:P ratio would induce similar microbial species in the planted wetlands and detach them from the unplanted wetlands.     

Sulphate uptake and metabolism in water hyacinth and salvinia during cadmium stress

Water hyacinth (Eichhornia crassipes (Mart.) Solms) and salvinia (Salvinia auriculata Aubl.) were exposed to toxic levels of Cd with the objective of evaluating its effect on sulphate uptake and metabolism. Plants were treated with 0 and 5 μmol L⁻¹ Cd for 3 days and, then sulphate uptake, ATP sulfurylase activity, soluble thiol content and Cd-binding complexes were determined. Water hyacinth showed a lower sulphate uptake, but its kinetic parameters were not affected by Cd. In salvinia, however, both V_max and affinity to sulphate (1/K_m) decreased with Cd treatment. The ATP sulfurylase activity increased in Cd-treated plant of both species, except in the roots of salvinia. In the presence of Cd water hyacinth always exhibited higher activity of this enzyme. The total soluble thiol content was always higher in water hyacinth. In Cd treated plants it increased in the leaves of water hyacinth, but decreased in salvinia. Cysteine content increased only in water hyacinth leaves, while γ-glutamylcysteine content increased in the two parts of the plants of both species after Cd treatment, especially in water hyacinth. Glutathione contents, on the contrary, after Cd treatment, reduced in both parts of the plants of water hyacinth but only in the leaves of salvinia. The unidentified thiol fraction content increased with Cd treatment in both species, especially in water hyacinth. Root and leaf extracts of both species showed peaks with maxima at A₂₆₅/A₂₈₀. In treated plants these peaks coincided with Cd content peaks indicating the formation of Cd-binding peptides. It was estimated that in the presence of Cd about 97% of Cd was associated with these complexes and water hyacinth had 28% more Cd-binding peptides than salvinia. Despite its lower sulphate uptake, water hyacinth showed higher rates of sulfur reduction and assimilation into soluble thiols. Possibly, glutathione is used in water hyacinth roots to synthesize hitherto unidentified Cd-binding peptides.     

Extraction and retrieval of potassium from water hyacinth (Eichhornia crassipes)

Studies were carried out on extraction and retrieval of potassium from water hyacinth (Eichhornia crassipes). The stem and leaf were subjected to 13 treatments. The highest rate of K removal following HCl treatment was 69.7% K. Most effective removal of suspended organic substances, Ca2+ and Mg2+ were achieved at pH approximately 13, when 88.0% of K remained in filtrate. Maximum K in precipitate following this step was achieved with tartaric acid additions at n(C4H6O6)/n(K+) of 1.72 when precipitating at 4 degrees C for 3h, which resulted in 72.3% of K removal from the solution. Over the entire process, 44.3% of K in the dried stem-leaf sample of water hyacinth was retrieved in the form of KC4H5O6. This process demonstrated the potential for use of water hyacinth as a resource of potassium to produce potassium salts and provide a valuable end use for the plant, which could be highly invasive in aquatic ecosystems.     

Fate and distribution of arsenic in laboratory-scale subsurface horizontal-flow constructed wetlands treating an artificial wastewater

Knowledge regarding the fate, accumulation and distribution of arsenic inside constructed wetlands is still insufficient. Based on a complete mass balance analysis, the aim of this study was to investigate the fate and distribution of As in distinct wetland compartments and different segments along the wetland gradient. Experiments were carried out in laboratory-scale wetland systems, two planted with Juncus effusus and one unplanted, using an As-containing artificial wastewater. The obtained results revealed that the planted wetlands have a substantially higher As-mass retention capacity (59–61% of the total As inflow) than wetlands without plantation (only 44%). However, different loads of organic carbon within the inflowing artificial wastewater showed no remarkable influence on As-mass retention in the planted wetlands. Nearly 47–52% of the total inflowing As mass was found to be retained within the first half of the planted wetlands and this retention decreased step by step along the flow path. In contrast, only 28% of the total inflowing As mass was retained within the first half of the unplanted wetland. In general, a different fate and distribution of As was observed inside the planted and unplanted wetlands. Higher As concentrations were exhibited by the plant roots (51.5–161.5 mg As kg^?1 dry wt.) compared to the shoots (1.1–6.4 mg As kg^?1 dry wt.). Analysis of the total As-mass balance in the planted wetlands revealed that nearly 44–49% of the total inflowing As was recovered or concentrated within the plant roots, only 1% was sequestered within the plant shoots, 7–10% were entrapped or deposited within the gravel bed sediments, 2–3% were retained in the standing pore water, 39–41% were flushed out as outflow and the remaining 1–2% is still considered to be unaccountable. Total As accumulation in the plant shoots made a small contribution to the mass balance, and plant root biomass was found to be the most important compartment for As retention. In contrast, nearly 11% of the total inflowing As were found in the sediment, 2% in the standing pore water, 57% in the outflow and a substantially higher portion (nearly 30%) remained unaccountable in the unplanted bed, which might be released as volatile As compounds or lost from the system due to various unknown reasons. The results indicate that plants have a remarkable effect on As retention and stability of already retained As; hence planted wetlands might be a suitable option for treating As-contaminated wastewater.     

Characteristics of an endophytic pyrene-degrading bacterium of Enterobacter sp. 12J1 from Allium macrostemon Bunge

One pyrene-degrading endophytic bacterium was isolated from plants grown in polycyclic aromatic hydrocarbon-contaminated soils and identified as Enterobacter sp. 12J1 based on the 16S rDNA gene sequence analysis. Heavy metal and antibiotic resistance, degradation of pyrene, solubilization of inorganic phosphate and cell surface hydrophobicity characteristics of the isolate were further characterized. The isolate was also evaluated for promoting plant growth of wheat and maize and pyrene removal from pyrene-amended soil in pot experiments. High-performance liquid chromatograph (HPLC) analysis showed that the degradation rate of pyrene (5 mg l⁻¹) by the endophytic bacterial strain 12J1 was 83.8% under 28 °C for 7 days. The Enterobacter sp. 12J1 could produce indole acetic acid (IAA), siderophore and solubilize inorganic phosphate. The Enterobacter sp. 12J1 also has a cell surface hydrophobicity. In the live bacterial inoculation experiment, an increase in pyrene removal varying from 60% to 107% was observed in the planted soils treated with 100 mg kg⁻¹ of pyrene compared with the unplanted soils. The rate of pyrene removal increased by 43–65% in the live bacterium-inoculated planted soils compared with the dead bacterium-inoculated planted soils. Although there were no significant differences in the total culturable bacterial numbers between live and dead bacterial inoculation, the numbers of pyrene-degrading bacteria were significantly greater in the live bacterium-inoculated planted or unplanted soils. The isolate could colonize the tissue (root and stem) interiors and rhizosphere soils of wheat and maize after root inoculation.     

Cyanide degradation by water hyacinths. Eichornia crassipes (Mart.) Solms

Summary Cyanide degradation by water hyacinths, Eichornia crassipes (Mart.) Solms, in solutions containing 3–300 mg/l cyanide was investigated in batch tests. Water hyacinth was more efficient to remove free cyanide in the first 8 hours, compared to cyanide controls, free of plant. Gold mill synthetic effluents containing free cyanide (9 to 20 mg/l), thiocyanate (14 to 23 mg/l), and metallocyanides (iron, copper and zinc) was fed to a continuous lab. scale unit (6 l/h) to confirm the ability of water hyacinth to degrade free cyanide and that it can remove zinc and small amounts of iron. However, copper and thiocyanate remained untouched in the solution. According to the results, water hyacinth is only suitable to be used in conjunction with other cyanide wastewater treatments.     

Nitrogen transformations and balance in a constructed wetland for nutrient-polluted river water treatment using forage rice in Japan

A pilot-scale surface-flow wetland planted with a new rice variety (Oryza sativa ’Kusahonami’) developed for livestock feed was constructed for treating nutrient-polluted river water. To calculate the balance between nitrogen removal and rice plant uptake of nitrogen, nitrogen removal from river water and nitrogen interactions among plants, soil water, and soil were investigated for this constructed wetland over two growing seasons in 2004 and 2005. The constructed wetland removed 33% of the total nitrogen entering with the river water. Rice plants were found to constitute the major nitrogen storage, with plant uptake being the major removal mechanism. The total inorganic nitrogen concentration in the rhizosphere changed seasonally because of plant uptake. Most nitrogen taken up by rice plants was contained in the aboveground biomass, with the mean amount being 34.0 g N m⁻². However, the nitrogen balance calculation suggested that rice plants uptake some nitrogen from soil, decreasing the available nitrogen in the soil of the lined impermeable wetland.     

Effect of the tropical grass Brachiaria brizantha (Hochst. ex A. Rich.) Stapf on microbial population and activity in petroleum-contaminated soil

The effect of the tropical pasture grass Brachiaria brizantha on numbers of bacteria, fungi and degraders of alkanes, aromatics, cycloalkanes and crude oil in petroleum hydrocarbon contaminated and uncontaminated savannah soil was evaluated. Substrate induced soil respiration and soil pH were compared between planted and unplanted soil. B. brizantha had a mostly increasing effect on microbial numbers. As an exception, growth of bacteria was not or negatively affected. Microbial respiration and pH were always lower in planted than in unplanted soil. Low pH may result from enhanced oil degradation in planted soil leading to an accumulation of organic acids. A comparable stimulation of crude oil degraders and fungi in planted soil points to the importance of fungi. Since they tolerate lower pH values than bacteria, they are considered to play a central role in oil degradation. Given that the enhancement of crude oil degradation under the influence of B. brizantha could not clearly be correlated to microbial numbers and activity, other factors like oxygen availability, plant enzymes and synergistic degradation by microbial consortia have to be considered.     

Technical note: fate and transport of jet fuel (JP-8) in soils with selected plants

Remediation of soil and groundwater contaminated by leaking fuel storage tanks may be assisted by plants, although plant effects on abiotic and biotic removal processes remain unclear. The objectives of this study were to investigate abiotic and biotic removal of JP-8, a kerosene-based jet fuel, in soils with plants, and to determine the effects of plant-induced water movement. Loss of JP-8 in a dry-soil, control column was 25% after 5 months, primarily due to volatilization and gas-phase diffusion. By comparison, managed treatments with simulated surface spills averaged 86% mass reduction at 5 months, indicating an important contribution of biodegradation. Overall JP-8 mass reduction was similar in surface and subsurface-irrigated systems, indicating water content, not mode of water application, influences bioremediation in near-surface systems. The JP-8 concentration reductions in soil columns contaminated above a simulated watertable were 36% after 3 months and 50% after 12 months for vegetated columns compared to 26% and 34% in unplanted columns. Downward movement of JP-8 in unplanted columns was double that in planted columns. Near the groundwater table, JP-8 persists longer than near the soil surface. Plants promote upward movement of water and help draw spilled JP-8 to aerobic near-surface soil.     

Phytoremediation of soil contaminated with PCBs using different plants and their associated microbial communities

In this work, we evaluate the abilities of the plants Brassica juncea, Avena sativa, Brachiaria decumbens, and Medicago sativa to uptake polychlorinated biphenyls (PCBs) and induce degradation of soil microorganisms from contaminated soil. Removal of PCBs 44, 66, 118, 153, 170, and 180 was evaluated in both rhizospheric and nonrhizospheric soils. Microbial and bphA1 gene quantifications were performed by real-time PCR. The PCB concentrations in plant tissues and soil were determined, and a fluorescein diacetate (FDA) hydrolysis assay was used to measure microbial activity in soil. The removal percentages for all PCB congeners in planted soil versus unplanted control soil were statistically significant and varied between 45% and 63%. PCBs 118, 153, 138, and 170 were detected in Brachiaria decumbens roots at different concentrations. In planted soil, an increase in the concentration of bacteria was observed compared to the initial concentration and the concentration in unplanted control soil; however, no significant differences were identified between plants. The number of copies of the bphA1 gene was higher in rhizospheric versus non- rhizospheric soil for all plants at the end of the experiment. However, alfalfa and oat rhizospheric soil showed significant differences in the copy number of the bphA1 gene. In general, the concentration of fluorescein in the rhizospheric soil was greater than that in the nonrhizospheric soil. Although the plants had a positive effect on PCB removal, this effect varied depending on the type of PCB, the plant, and the soil.     

Dynamics of Arsenic Species in Laboratory‐Scale Horizontal Subsurface‐Flow Constructed Wetlands Treating an Artificial Wastewater

Knowledge regarding the dynamics of arsenic species and their interactions under gradient redox conditions in treatment wetlands is still insufficient. The aim of this investigation was to gain more information on the biotransformation of As and the dynamics of As species in horizontal subsurface‐flow constructed wetlands. Experiments were carried out in laboratory‐scale wetland systems, two planted with Juncus effusus and one unplanted, using an As‐containing artificial wastewater under defined organic C‐ and SO₄^2–‐loading conditions. Immobilization of As was found in all systems under conditions of limited C, mainly due to adsorption and/or co‐precipitation. The removal efficiencies were substantially higher in the planted systems (60–70 %) as compared to the unplanted system (37 % on average). Immobilization under the conditions mentioned above appeared to decrease over time in all systems. At the beginning, the dosage of organic carbon immediately caused intensive microbial dissimilatory sulfate reduction in all systems (in the range of 85–95 %) and highly efficient removal of total arsenic (81–96 % on average). Later on, in this operation period, the intensity of sulfate reduction and simultaneous removal of As decreased, particularly in the planted wetlands (ranging from 30–46 %). One reason could be the re‐oxidation of reduced compounds due to oxygenation of the rhizosphere by the emergent wetland plants (helophytes). A significant amount of reduced As [As(III)] was found in the planted systems (> 75 % of total As) during the period of efficient microbial sulfate reduction, compared to the unplanted system (> 25 % of total As). The immobilization of arsenic was found to behave more stably in the planted beds than in the unplanted bed. Both systems (planted and unplanted) were suitable to treat wastewater containing As, particularly under sulfate reducing conditions. The unplanted system seemed to be more efficient regarding the immobilization of As, but the planted systems showed a better stability of immobilized As.     

Growth characteristics of aquatic macrophytes cultured in nutrient-enriched water: I. Water hyacinth,water lettuce,and pennywort

Seasonal growth characteristics and biomass yield potential of 3 floating aquatic macrophytes cultured in nutrient nonlimiting conditions were evaluated in central Florida’s climatic conditions. Growth cycle (growth curve) of the plants was found to be complete when maximum plant density was reached and no additional increase in growth was recorded. Biomass yield per unit area and time was found to be maximum in the linear phase of the growth curve; plant density in this phase was defined as “operational plant density,” a density range in which a biomass production system is operated to obtain the highest possible yields. Biomass yields were found to be 106, 72, and41 t(drywt)ha^-1yr^-1, respectively, for water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), and pennywort (Hydrocotyle umbellata). Operational plant density was found to be in the range of 500–2,000 g dry wt m^-2 for water hyacinth, 200–700 g dry wt m^-2 for water lettuce, and 250–650 g dry wt m^-2 for pennywort. Seasonality was observed in growth rates but not in operational plant density. Specific growth rate (% increase per day) was found to maximum at low plant densities and decreased as the plant density increased. Results show that water hyacinth and water lettuce can be successfully grown for a period of about 10 mo, while pennywort, a cool season plant, can be integrated into water hyacinth/water lettuce biomass production system to obtain high yields in the winter.     

E2-3S在水平流人工湿地中的降解与转化

研究采用同时检测15种自由态雌激素(Free estrogens, FEs)及结合态雌激素(Conjugated estrogens, CEs; 包括雌激素手性分子、异构体、单位点及多位点结合的CEs)的方法, 探讨目标污染物17β-estradiol-3-sulfate (E2-3S)与其他类型雌激素(包括自由态雌激素与其他结合态雌激素)间的转化关系, 研究E2-3S在有/无植物水平流人工湿地(HFCWs)中的降解规律。结果显示: 在停留时间为1.5d的工况下, E2-3S在HFCWs进水端基质水平距离0 cm深度15 cm中转化率已达98%, E2-3S可转化为其他雌激素, 产物以FEs为最丰富(均占70%以上), 植物可以显著提高湿地DO浓度, 使FEs残留浓度比无植物CW(U-CW)更低, 有植物CW(P-CW)和U-CW对总雌激素去除效率分别为86%和58%; E2-3S的主要转化路径为经硫酯键断裂形成E2再氧化生成E1, 其次路径为直接氧化为E1-3S再水解生成E1, 少量路径为羟基化形成E3-3S再水解生成E3, 此外, E2-3S还可以产生痕量双位取代D-CEs(<总雌激素的5%), 且植物系统中存留量更低。     

Biodegrader metabolic expansion during polyaromatic hydrocarbons rhizoremediation

Root-microbe interactions are considered to be the primary process of polyaromatic hydrocarbon (PAH) phytoremediation, since bacterial degradation has been shown to be the dominant pathway for environmental PAH dissipation. However, the precise mechanisms driving PAH rhizostimulation symbiosis remain largely unresolved. In this study, we assessed PAH degrading bacterial abundance in contaminated soils planted with 18 different native Michigan plant species. Phenanthrene metabolism assays suggested that each plant species differentially influenced the relative abundance of PAH biodegraders, though they generally were observed to increase heterotrophic and biodegradative cell numbers relative to unplanted soils. Further study of >1800 phenanthrene degrading isolates indicated that most of the tested plant species stimulated biodegradation of a broader range of PAH compounds relative to the unplanted soil bacterial consortia. These observations suggest that a principal contribution of planted systems for PAH bioremediation may be via expanded metabolic range of the rhizosphere bacterial community.     

Influence of fertilizer levels on phytoremediation of crude oil-contaminated soils with the tropical pasture grass Brachiaria brizantha (hochst. ex a. rich.) stapf

Determination of fertilizer levels in phytoremediation of petroleum hydrocarbons is a complex issue, since nutrient demands of the plant and of degrading microorganisms in the rhizosphere have to be considered In the present work, three fertilizer levels were tested in a greenhouse experiment with the aim of optimizing growth of the tropical pasture grass Brachiaria brizantha and enhance microbial degradation of heavy crude oil in soil Fertilizer was applied twice in a concentration of 200, 300, and 400 mg each of N, P, and K per kg soil before and after the first sampling (14 wk). The medium fertilizer concentration resulted in best root growth and highest absolute oil dissipation (18.4%) after 22 wk The highest concentration produced best shoot growth and highest relative oil dissipation after 14 wk (10.5% less than unplanted control). In general, degradation of total oil and grease was higher in planted than in unplanted soil, but differences diminished toward the end of the experiment. Next to fertiizer quantity, its composition is an important factor to be further studied, including the form of available nitrogen (N-NO3- vs. N-NH4+). Field trials are considered indispensable for further phytoremediation studies, since greenhouse experiments produce particular water and nutrient conditions.