NATIVE MICHIGAN PLANTS STIMULATE SOIL MICROBIAL SPECIES CHANGES AND PAH REMEDIATION AT A LEGACY STEEL MILL

A 1.3-acre phytoremediation site was constructed to mitigate polyaromatic hydrocarbon (PAH) contamination from a former steel mill in Michigan. Soil was amended with 10% (v/v) compost and 5% (v/v) poultry litter. The site was divided into twelve 11.89 m X 27.13 m plots, planted with approximately 35,000 native Michigan perennials, and soils sampled for three seasons. Soil microbial density generally increased in subplots of Eupatorium perfoliatum (boneset), Aster novae-angliae (New England aster), Andropogon gerardii (big bluestem), and Scirpus atrovirens (green bulrush) versus unplanted subplots. Using enumeration assays with root exudates, PAH degrading bacteria were greatest in soils beneath plants. Initially predominant, Arthrobacter were found capable of degrading a PAH cocktail in vitro, especially upon the addition of root exudate. Growth of some Arthrobacter isolates was stimulated by root exudate. The frequency of Arthrobacter declined in planted subplots with a concurrent increase in other species, including secondary PAH degraders Bacillus and Nocardioides. In subplots supporting only weeds, an increase in Pseudomonas density and little PAH removal were observed. This study supports the notion that a dynamic interplay between the soil, bacteria, and native plant root secretions likely contributes to in situ PAH phytoremediation.     

Effects of plant biomass, plant diversity, and water content on bacterial communities in soil lysimeters: implications for the determinants of bacterial diversity

Soils may comprise tens of thousands to millions of bacterial species. It is still unclear whether this high level of diversity is governed by functional redundancy or by a multitude of ecological niches. In order to address this question, we analyzed the reproducibility of bacterial community composition after different experimental manipulations. Soil lysimeters were planted with four different types of plant communities, and the water content was adjusted. Group-specific phylogenetic fingerprinting by PCR-denaturing gradient gel electrophoresis revealed clear differences in the composition of Alphaproteobacteria, Betaproteobacteria, Bacteroidetes, Chloroflexi, Planctomycetes, and Verrucomicrobia populations in soils without plants compared to that of populations in planted soils, whereas no influence of plant species composition on bacterial diversity could be discerned. These results indicate that the presence of higher plant species affects the species composition of bacterial groups in a reproducible manner and even outside of the rhizosphere. In contrast, the environmental factors tested did not affect the composition of Acidobacteria, Actinobacteria, Archaea, and Firmicutes populations. One-third (52 out of 160) of the sequence types were found to be specifically and reproducibly associated with the absence or presence of plants. Unexpectedly, this was also true for numerous minor constituents of the soil bacterial assemblage. Subsequently, one of the low-abundance phylotypes (beta10) was selected for studying the interdependence under particular experimental conditions and the underlying causes in more detail. This so-far-uncultured phylotype of the Betaproteobacteria species represented up to 0.18% of all bacterial cells in planted lysimeters compared to 0.017% in unplanted systems. A cultured representative of this phylotype exhibited high physiological flexibility and was capable of utilizing major constituents of root exudates. Our results suggest that the bacterial species composition in soil is determined to a significant extent by abiotic and biotic factors, rather than by mere chance, thereby reflecting a multitude of distinct ecological niches.     

The early effects of afforestation on biodiversity of grasslands in Ireland

The target rate of afforestation in Ireland over the next 30 years is 20,000 ha per year, which would result in an increase of the forest cover from the current 10% to 17%. In order to promote sustainable forest management practices, it is essential to know the composition and conservation value of habitats where afforestation is planned and the effects of subsequent planting upon biodiversity. The objectives of this study were to investigate changes in vegetation composition and diversity of grasslands 5 years after afforestation with Sitka spruce (Picea sitchensis) and determine the primary ecological and management factors responsible for these changes. Species cover, environmental and management data were collected from 16 afforested and unplanted improved and wet grassland site pairs in Ireland. Our results indicate that 5 years after tree planting, there were significant changes in richness, composition, and abundance of species. Competitive and vigorous grasses were more abundant in planted than in unplanted sites, as were generalist species found in both open and wooded habitats, while small-stature shade-sensitive species were less abundant. Vascular plant species richness and Shannon’s diversity index were higher in unplanted wet grassland, than in the planted sites. Bryophyte species richness was higher in planted improved grassland than in unplanted sites. The differences were primarily the result of the exclusion of grazing, ground preparation, changes in nutrient management and drainage for afforestation. Drainage ditches provided a temporary habitat for less competitive species, but the overall effect of drainage was to reduce the diversity of species dependent on wet conditions. Variance partitioning showed differences in the relative influences of environmental and management variables on biodiversity in the two habitats, probably due to the greater pre-afforestation grazing pressure and fertilisation levels in improved grasslands. The differences in biodiversity between planted and unplanted grasslands indicate that afforestation represents a threat to semi-natural habitats where distinctive and highly localised plant communities could potentially occur.     

Isolation and characterization of pyrene metabolizing microbial consortia from the plant rhizoplane

Most research on the ecology of PAH degrading bacteria in the rhizosphere has focused on individual strains that grow on specific PAHs. Thus, there are fundamental questions as to importance of microbial consortia for PAH degradation in the plant rhizosphere. The study reported here characterized cultivable pyrene degrading rhizoplane microbial communities from two different plant species using a root printing technique on agar plates. Colonies were revealed by formation of clearing zones on medium containing a thin film of pyrene on the surface of a mineral nutrient agar. Prints of the rhizoplane colonies were obtained from roots of Melilotus officinalis (sweet yellow clover) and Andropogon gerardii (big bluestem) plants. Phylogenetic characterizations of selected pyrene degrading colonies were assessed by PCR-DGGE and DNA sequencing. Results showed that different populations of cultivable pyrene degraders were obtained from representative consortia that were examined. Many of the PAH degrading consortia consisted of mixtures of bacterial species that were unable to degrade pyrene by themselves. While this study focused on culturable PAH degraders, the results suggest that pyrene degradation in the rhizosphere commonly involves the activity of bacterial consortia in which various species of bacteria interact to achieve PAH degradation.     

Increasing ibuprofen degradation in constructed wetlands by bioaugmentation with gravel containing biofilms of an ibuprofen‐degrading Sphingobium yanoikuyae

The aim of this study was to investigate the removal of ibuprofen in laboratory scale constructed wetlands. Four (planted and unplanted) laboratory‐scale horizontal subsurface flow constructed wetlands were supplemented with ibuprofen in order to elucidate (i) the role of plants on ibuprofen removal and (ii) to evaluate the removal performance of a bioaugmented lab scale wetland. The planted systems showed higher ibuprofen removal efficiency than an unplanted one. The system planted with Juncus effusus was found to have a higher removal rate than the system planted with Phalaris arundinacea. The highest removal rate of ibuprofen was found after inoculation of gravel previously loaded with a newly isolated ibuprofen‐degrading bacterium identified as Sphingobium yanoikuyae. This experiment showed that more than 80 days of CW community adaptation for ibuprofen treatment could be superseded by bioaugmentation with this bacterial isolate.     

Greenhouse and field assessment of phytoremediation for petroleum contaminants in a riparian zone

Greenhouse and field studies were conducted to evaluate the feasibility of phytoremediation for clean-up of highly contaminated sediments from Indiana Harbor. In the greenhouse study, plant species evaluated were willow (Salix exigua), poplar (Populus spp.), eastern gamagrass (Tripsacum dactyloides), arrowhead (Sagitaria latifolia), switchgrass (Panicum virgatum), and sedge (Carex stricta). Sediments with sedge, switchgrass, and gamagrass had significantly less residual total petroleum hydrocarbons (TPH) after one year of growth (approximately 70% reduction) than sediments containing willow, poplar, or no plants (approximately 20% reduction). Although not all polycyclic aromatic hydrocarbons (PAH) had concentration differences due to the presence of plants, residual pyrene concentrations in the unvegetated pots were significantly higher than in pots containing sedge, switchgrass, arrowhead, and gamagrass. As evaluated by TPH dissipation in the upper section of the pots, the sedge, switchgrass, and gamagrass treatments had higher TPH degradation than the unvegetated, willow and poplar treatments. These trends were similar for soil at the bottom of the pots, with the exception that in the switchgrass treatment, degradation was not significantly different than in the unvegetated soil. Two target contaminants, pyrene and benzo[b]fluoranthene, showed differences in degradation between planted and unvegetated treatments. In the field study, phytoremediation plant species were eastern gamagrass (T. dactyloides), switchgrass (P. virgatum), and sedge (C. stricta). In addition, rhizosphere characteristics of arrowhead (S. latifolia) and sedge were assessed. Arrowhead- and sedge-impacted soils were found to contain significantly more PAH-degrading bacteria than unvegetated soils. However, over the 12-month field study, no significant differences in contamination were found between the planted and unplanted soils for TPH and PAH concentrations. TPH concentrations near the canal were greater than concentrations further from the canal, indicating that the canal may have served as a continuous source of contamination during the study.     

In situ assessment of phytotechnologies for multicontaminated soil management

Due to human activities, large volumes of soils are contaminated with organic pollutants such as polycyclic aromatic hydrocarbons, and very often by metallic pollutants as well. Multipolluted soils are therefore a key concern for remediation. This work presents a long-term evaluation of the fate and environmental impact of the organic and metallic contaminants of an industrially polluted soil under natural and plant-assisted conditions. A field trial was followed for four years according to six treatments in four replicates: unplanted, planted with alfalfa with or without mycorrhizal inoculation, planted with Noccaea caerulescens, naturally colonized by indigenous plants, and thermally treated soil planted with alfalfa. Leaching water volumes and composition, PAH concentrations in soil and solutions, soil fauna and microbial diversity, soil and solution toxicity using standardized bioassays, plant biomass, mycorrhizal colonization, were monitored. Results showed that plant cover alone did not affect total contaminant concentrations in soil. However, it was most efficient in improving the contamination impact on the environment and in increasing the biological diversity. Leaching water quality remained an issue because of its high toxicity shown by micro-algae testing. In this matter, prior treatment of the soil by thermal desorption proved to be the only effective treatment.     

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.     

Effects of Plant Biomass, Plant Diversity, and Water Content on Bacterial Communities in Soil Lysimeters: Implications for the Determinants of Bacterial Diversity

Soils may comprise tens of thousands to millions of bacterial species. It is still unclear whether this high level of diversity is governed by functional redundancy or by a multitude of ecological niches. In order to address this question, we analyzed the reproducibility of bacterial community composition after different experimental manipulations. Soil lysimeters were planted with four different types of plant communities, and the water content was adjusted. Group-specific phylogenetic fingerprinting by PCR-denaturing gradient gel electrophoresis revealed clear differences in the composition of Alphaproteobacteria, Betaproteobacteria, Bacteroidetes, Chloroflexi, Planctomycetes, and Verrucomicrobia populations in soils without plants compared to that of populations in planted soils, whereas no influence of plant species composition on bacterial diversity could be discerned. These results indicate that the presence of higher plant species affects the species composition of bacterial groups in a reproducible manner and even outside of the rhizosphere. In contrast, the environmental factors tested did not affect the composition of Acidobacteria, Actinobacteria, Archaea, and Firmicutes populations. One-third (52 out of 160) of the sequence types were found to be specifically and reproducibly associated with the absence or presence of plants. Unexpectedly, this was also true for numerous minor constituents of the soil bacterial assemblage. Subsequently, one of the low-abundance phylotypes (beta10) was selected for studying the interdependence under particular experimental conditions and the underlying causes in more detail. This so-far-uncultured phylotype of the Betaproteobacteria species represented up to 0.18% of all bacterial cells in planted lysimeters compared to 0.017% in unplanted systems. A cultured representative of this phylotype exhibited high physiological flexibility and was capable of utilizing major constituents of root exudates. Our results suggest that the bacterial species composition in soil is determined to a significant extent by abiotic and biotic factors, rather than by mere chance, thereby reflecting a multitude of distinct ecological niches.     

Impact of the nematophagous fungus Pochonia chlamydosporia on nematode and microbial populations

The microbial and nematode populations associated with two plants (tomato and cabbage) inoculated with the nematophagous fungus, Pochonia chlamydosporia var. chlamydosporia or root knot nematode (Meloidogyne incognita), or both, were compared with those in unplanted controls. The dominant factor affecting culturable microbial populations was found to be the presence or absence of tomato plants. Generally microbial colony counts were lowest in unplanted soil, small increases were associated with cabbage and significantly greater numbers with tomato plants. Differences in microbial diversity (estimated from community profiles of carbon substrate utlisation, using Biolog) were observed between planted and unplanted soils, however, there were few differences between soils with either of the two plants. The presence of P. chlamydosporia was associated with a reduction in the numbers of plant parasitic nematodes (51%-78%) including the migratory ectoparasites, whereas free-living nematodes, culturable bacteria and bacterial populations assessed by Biolog were unaffected by the application of fungus.     

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.     

Plant Species Composition Effects on Belowground Properties and the Resistance and Resilience of the Soil Microflora to a Drying Disturbance

We hypothesised that plant species composition and richness would affect soil chemical and microbial community properties, and that these in turn would affect soil microbial resistance and resilience to an experimentally imposed drying disturbance. We performed a container experiment that manipulated the composition and species richness of common pasture plant species (Trifolium repens, Lolium perenne, and Plantago lanceolata) by growing them in monoculture, and in all the possible two and three-way combinations, along with an unplanted control soil. Experimental units were harvested at four different times over a 16-month period to determine the effect of plant community development and seasonal changes in temperature and moisture on belowground properties. Results showed that plant species composition influenced soil chemistry, soil microbial community properties and soil microbial resistance and resilience. Soil from planted treatments generally showed reduced soil microbial resistance to drying compared to unplanted control soils. Soils from under T. repens showed a higher resistance and resilience than the soils from under P. lanceolata, and a higher resistance than soils from under L. perenne. We suggest that differences across soils in either resource limitation or soil microbial community structure may be responsible for these results. Plant species richness rarely affected soil microbial community properties or soil microbial resistance and resilience, despite having some significant effects on plant community biomass and soil nitrogen contents in some harvests. The effect that treatments had for most variables differed between harvests, suggesting that results can be altered by the stage of plant community development or by extrinsic environmental factors that varied with harvest timing. These results in combination show that soil microbial resistance and resilience was affected by plant community composition, and the time of measurement, but was largely unrelated to plant species richness.     

人工与天然云杉林土壤真菌群落多样性及菌群网络关系特征

土壤真菌群落多样性和菌群关系是维持生态系统的多样性及稳定性的关键。本文以粗枝云杉人工林和天然林为研究对象,利用高通量测序技术和生物信息学分析方法,研究了云杉根际和非根际土壤真菌群落组成、多样性及菌群网络关系。结果表明: 从群落组成上看,人工林中相对丰度最高的科是丝盖伞科,而天然林中是蜡壳耳科,两处林型下占比最高的属均为丝盖伞属。群落的β多样性在两处林型的根际、非根际下存在显著差异。环境变量与真菌类群的相对丰度和α多样性相关关系不显著,而草本覆盖度、土壤含水率、总有机碳和植被丰富度是群落β多样性的主要影响因素。网络分析显示,天然林土壤真菌菌群之间以负相关关系为主,表明天然林土壤中菌群之间主要存在竞争作用。比较两处林型下的根际、非根际土壤真菌菌群关系发现,非根际区域菌群之间负相关性均较高,表明非根际土壤中菌群的种间竞争作用可能要强于根际土壤。结合差异丰度分析,两处林型下根际和非根际之间存在显著差异的物种中仅有蜡壳耳科为真菌网络中共有的关键菌群,表明人工林和天然林土壤真菌群落结构中差异种群的变化可能对其群落稳定性影响较小。     

Comparison of Bacterial and Fungal Communities Between Natural and Planted Pine Forests in Subtropical China

To improve our understanding of the changes in bacterial and fungal diversity in natural pine and planted forests in subtropical region of China, we examined bacterial and fungal communities from a native and a nearby planted pine forest of the Mt. Lushan by constructing clone libraries of 16S and 18S rRNA genes. For bacterial communities, Proteobacteria and Acidobacteria were dominant bacterial taxa in both two types of forest soils. The Shannon–Wiener diversity index, rarefaction curve analysis, and LibShuff analysis suggest that these two forests contained similar diversity of bacterial communities. Low soil acidity (pH ≈ 4) of our study forests might be one of the most important selection factors determining growth of acidophilic Acidobacteria and Proteobacteria. However, the natural forest harbored greater level of fungal diversity than the planted forest according to the Shannon–Wiener diversity index and rarefaction curve analysis. Basidiomycota and Ascomycota were dominant fungal taxa in the soils of natural and planted forests, respectively. Our results suggest that fungal community was more sensitive than the bacterial community in characterizing the differences in plant cover impacts on the microbial flora in the natural and planted forests. The natural and planted forests may function differently due to the differences in soil fungal diversity and relative abundance.     

Effectiveness of Vetiver Grass (Vetiveria Zizanioides L. Nash) for Phytoremediation of Endosulfan in Two Cotton Soils from Burkina Faso

The influence of vetiver grass (Vetiveria zizanioides) on the fate of endosulfan was studied using a vertisol and a lixisol soils from cotton-growing areas of Burkina Faso. Endosulfan adsorption isotherms were prepared for planted and unplanted soils. Pot experiments were then conducted for six months. For both soils, endosulfan adsorption was higher on planted soils (K_f= 6.53–9.73 mg^1–nLⁿkg^–1) than on unplanted soils (6.27–7.24 mg^1–nLⁿkg^–1). In unplanted soils, vertisol adsorbed more endosulfan than lixisol. From the pot experiments, the estimated half-lives of endosulfan in unplanted soils (40.6 to 43.1 days) were higher than in planted soils (34.5 to 40.6 days) containing a greater number of endosulfan-degrading microorganisms. Six months after treatment, endosulfan was not detected in soils. The effectiveness of vetiver in promoting adsorption and the disappearance of endosulfan in both studied soils should be validated on the cotton plot scale in Burkina Faso.     

The interacting effects of temperature and plant community type on nutrient removal in wetland microcosms

Treatment wetlands can remove nutrients from inflow sources through biogeochemical processes. Plant composition and temperature play important roles in the nutrient removal efficiency of these wetlands, but the interactions between these variables are not well understood. We investigated the seasonal efficiency of wetland macrophytes to reduce soil leachate concentrations of total nitrogen and total phosphorus in experimental microcosms. Each microcosm contained one of six vegetation treatments: unplanted, planted with one of four species (Carex lacustris, Scirpus validus, Phalaris arundinacea and Typha latifolid) in monoculture or planted with an equal abundance of all four species. Microcosms were also subjected to two temperature treatments: insulated microcosms and microcosms exposed to environmental conditions. A constant nutrient solution containing 56 mg/l N and 31 mg/l P was added to all microcosms three times a week. Water samples were analyzed monthly for total dissolved nitrogen and total dissolved phosphorous. Microcosms exhibited a typical pattern of seasonal nutrient removal with higher removal rates in the growing season and lower rates in the winter months. In general, planted microcosms outperformed unplanted microcosms. Among the plant treatments, Carex lacustris was the least efficient. The four remaining plant treatments removed an equivalent amount of nutrients. Insulated microcosms were more efficient in the winter and early spring months. Although a seasonal pattern of nutrient removal was observed, this variation can be minimized through planting and insulation of wetlands.     

PYRENE EFFECTS ON RHIZOPLANE BACTERIAL COMMUNITIES

Certain plant species promote biodegradation of polycyclic aromatic hydrocarbons (PAHs), but few studies have examined the microbial populations that are associated with the rhizoplane of these plants. In this study, the bacterial composition of the rhizoplane were characterized for four plant species during in soils with different histories of exposure to PAH and in the presence or absence of a pyrene spike at 100 mg kg^?1 pyrene. Three of the plant species including Andropogon gerrardii, Panicum coloratum and Melilotus officinalis were known to stimulate PAH degradation. Wheat (Triticum aestivum) was used as a reference species. Results showed that after 90 days, approximately 45% of the pyrene spike disappeared from soil without plants. In contrast, cultivation of plants resulted in 95% disappearance of pyrene. There were no significant differences in the extent of pyrene disappearance for different plants. In all cases, 16S rRNA gene profiles of the rhizoplane were less complex in the pyrene-spiked soils, suggesting that richness and evenness of the predominant bacteria were reduced. Our results show that pyrene contamination results in significant shifts in the composition of rhizosphere bacterial communities that are still further influenced by the plant species and prior exposure history to PAH contamination.     

Effects of Rapeseed Oil on the Rhizodegradation of Polyaromatic Hydrocarbons in Contaminated Soil

Plants have the ability to promote degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soil by supporting PAH degrading microorganisms in the rhizosphere (rhizodegradation). The aim of this study was to evaluate if rapeseed oil increases rhizodegradation because various studies have shown that vegetable oils are able to act as extractants for PAHs in contaminated soils and therefore might increase bioavailability of PAHs for microbial degradation. In this study different leguminous and grass species were tested. The results suggested a significant impact of vegetable oil (1 and 3% w/w) on plant growth (decrease of plant height and biomass). The results of the pot experiment showed a decrease in the PAH content of the soil without amendment of rapeseed oil after six months. In soil amended with 1% and 3% of oil, there was no decrease in PAH content within this period. Although no enhancement of PAH degradation by plants could be measured in the bulk soil of the pot experiments, a rhizobox experiment showed a significant reduction of PAH content in the rhizosphere of alfalfa (Medicago sativa cv. Europe). Our investigations also showed significant differences in the degradation behaviour of the 16 individually analysed PAHs.     

Bacterial community evaluation during establishment of tall fescue (Festuca arundinacea) in soil contaminated with pyrene

A greenhouse study was conducted to monitor microbial community dynamics related to contaminant concentration and plant growth during the phytoremediation of pyrene. Soil microbial communities in both bulk and rhizosphere soils were assessed using a polymerase chain reaction--denaturing gradient gel electrophoresis (PCR-DGGE). However, no correlation between pyrene concentration and bacterial community shifts was observed. Even though plants significantly enhanced pyrene degradation (undetectable after 91 d for planted treatments compared with 82 mg/kg for unplanted treatments after 147 d with initial concentration of 758 mg/kg), biodegradation may have been accomplished by the pre-existing microbial community. It is also possible that the method was unable to pick up subtle community shifts, considering that the carbon source from pyrene was only 3.7% of the existing soil organic matter. This research suggests that plants significantly enhance degradation and mineralization of pyrene in soil, although there was no conclusive evidence supporting any specific bacterial activity responsible for this enhancement.     

Linkage between bacterial and fungal rhizosphere communities in hydrocarbon-contaminated soils is related to plant phylogeny

Phytoremediation is an attractive alternative to excavating and chemically treating contaminated soils. Certain plants can directly bioremediate by sequestering and/or transforming pollutants, but plants may also enhance bioremediation by promoting contaminant-degrading microorganisms in soils. In this study, we used high-throughput sequencing of bacterial 16S rRNA genes and the fungal internal transcribed spacer (ITS) region to compare the community composition of 66 soil samples from the rhizosphere of planted willows (Salix spp.) and six unplanted control samples at the site of a former petrochemical plant. The Bray–Curtis distance between bacterial communities across willow cultivars was significantly correlated with the distance between fungal communities in uncontaminated and moderately contaminated soils but not in highly contaminated (HC) soils (>2000 mg kg⁻¹ hydrocarbons). The mean dissimilarity between fungal, but not bacterial, communities from the rhizosphere of different cultivars increased substantially in the HC blocks. This divergence was partly related to high fungal sensitivity to hydrocarbon contaminants, as demonstrated by reduced Shannon diversity, but also to a stronger influence of willows on fungal communities. Abundance of the fungal class Pezizomycetes in HC soils was directly related to willow phylogeny, with Pezizomycetes dominating the rhizosphere of a monophyletic cluster of cultivars, while remaining in low relative abundance in other soils. This has implications for plant selection in phytoremediation, as fungal associations may affect the health of introduced plants and the success of co-inoculated microbial strains. An integrated understanding of the relationships between fungi, bacteria and plants will enable the design of treatments that specifically promote effective bioremediating communities.