Atrazine mineralization in bulk soil and maize rhizosphere

An experiment was carried out in the greenhouse in order to compare atrazine mineralization in bulk soil and maize rhizosphere at different development stages. After 4, 8 and 12 weeks, we have (1) measured the soil microbial biomass C, (2) characterized the C substrate utilization profiles of the culturable microflora, and (3) analyzed atrazine mineralization. Microbial growth was stimulated in planted soil and different C substrate utilization patterns were obtained in bulk and rhizosphere soils during the first 2 months. During this period, laboratory tests for atrazine biodegradation revealed a lower mineralization potential in bulk than in planted soil. Atrazine mineralization was stimulated to a greater extent after atrazine application in the greenhouse but again the presence of plants had a favorable effect. After 12 weeks of cropping, the atrazine mineralization potential decreased in planted soil with or without prior atrazine application.     

Atrazine degradation in boreal nonagricultural subsoil and tropical agricultural soil

Purpose

The purpose of this study was to determine the natural atrazine degradation activity and the genetic potential in a soil profile spanning down to the groundwater zone, collected in Finland at a site where past use of atrazine has contaminated the groundwater, and in Indian agricultural topsoils having different histories of atrazine use.

Materials and methods

Atrazine degradation potential was assessed by quantifying the atrazine degradation genes atzA, trzN, and atzB by quantitative PCR reaction. Atrazine mineralization was studied by radiorespirometry in order to find out if these genes were expressed.

Results and discussion

Indian soils contained a large number up to 10⁴–10⁵ copies (g^?1 dry weight (dw) soil) of atrazine degradation genes after the first treatment with atrazine. These genes were also expressed, as up to 55 % of atrazine mineralized. Some unspecific binding of primers required thorough investigation and confirmation by sequencing of the qPCR products in the agricultural soil samples. The degradation capability of the nonagricultural boreal soil profile was much lower: atrazine degradation genes were present at detection limit (10² copies g^?1 soil), but mineralization studies indicated that these genes were not transcribed, since no or very little atrazine mineralization was observed.

Conclusions

Our results indicate that when atrazine was applied in agricultural practice, the soil atrazine degradation capacity was high. The organisms responsible for the degradation were effectively degrading atrazine already 3 months after the first treatment with atrazine. However, in boreal soil, decades after atrazine use had been discontinued, residual atrazine was not degraded even though a small number of degradation genes could still be detected in soil. There is a need for more specific primers for qPCR in tropical soils.     

土壤食细菌线虫对菲降解的影响

以菲作为多环芳烃污染物的代表,通过室内培养试验研究了在菲降解菌恶臭假单胞菌存在与否的条件下,接种食细菌线虫对未灭菌土壤中菲去除的影响。析因设计包括4个处理:单独接种恶臭假单胞菌(B),单独接种食细菌线虫(N),同时接种食细菌线虫和恶臭假单胞菌(BN)以及未接种线虫和细菌的对照(CK);分别在培养后第0、5、14、28天进行破坏性采样,测定土壤中菲的残留量,食细菌线虫和恶臭假单胞菌的数量,土壤FDA(荧光素二乙酸酯)水解酶、过氧化氢酶的活性。结果显示,相比CK,在培养前期(0~14天),处理N提高土壤中菲的去除率;而在培养后期(14~28天),BN处理降解菲的优势逐渐显现出来。在培养结束(28天)时,各处理菲的去除率依次为BN(48.2%)B(45.1%)N(44.4%)CK(43.5%)。试验结果还表明,接种食细菌线虫能显著(p0.05)促进细菌以及土壤酶活性。总之,食细菌线虫和细菌的交互作用可能促进土壤中菲的降解。     

Atrazine and metolachlor degradation in subsoils

Degradation of atrazine [2-chloro-4-etylamino-6-isopropylamino-1,3,5-triazine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)-acetamide] in sterile and non-sterile soil samples collected at two different soil depths (0-20 and 80-110 cm) and incubated under aerobic and anaerobic conditions was studied. Under aerobic conditions, the half-life of atrazine in non-sterile surface soil was 49 days. In non-sterile subsoil, the half-life of atrazine (119 days) was increased by 2.5 times compared in surface soils and was not statistically different from half-lives in sterile soils (115 and 110 days in surface soil and subsoil, respectively). Metolachlor degradation occurred only in non-sterile surface soil, with a half-life of 37 days. Under anaerobic conditions, atrazine degradation was markedly slower than under aerobic conditions, with a half-life of 124 and 407 days in non-sterile surface soil and non-sterile subsoil, respectively. No significant difference was found in atrazine degradation in both sterile surface soil (693 days) and subsoil (770 days). Under anaerobic conditions, degradation of metolachlor was observed only in non-sterile surface soil. Results suggest that atrazine degraded both chemically and biologically, while metolachlor degraded only biologically. In addition, observed Eh values of soil samples incubated under anaerobic conditions suggest a significant involvement of soil microorganisms in the overall degradation process of atrazine under anaerobic conditions.     

Selective enrichment of a pyrene degrader population and enhanced pyrene degradation in Bermuda grass rhizosphere

Rhizosphere soil has a more diverse and active microbial community compared to nonvegetated soil. Consequently, the rhizosphere pyrene degrader population (PDP) and pyrene degradation may be enhanced compared to nonvegetated bulk soil (NVB). The objectives of this growth chamber study were to compare (1) Bermuda grass (Cynodon dactylon cv. Guymon) growth in pyrene-contaminated and noncontaminated soils and (2) pyrene degradation and PDP among NVB, Bermuda grass bulk (BB), and Bermuda grass rhizosphere soil (BR). Soils were amended with pyrene at 0 and 500 mg kg^–1, seeded with Bermuda grass, and thinned to two plants per pot 14 days after planting (DAP). Pyrene degradation was evaluated over 63 days. The PDP was enumerated via a most probable number (MPN) procedure at 63 DAP. Bermuda grass root growth was more sensitive to pyrene contamination than shoot growth. Pyrene degradation followed first-order kinetics. Pyrene degradation was significantly greater in BR compared to BB and NVB with rate constants of 0.082, 0.050, and 0.052 day^–1, respectively. The PDPs were 8.01, 7.30, and 6.83 log₁₀ MPN g^–1 dry soil for BR, BB, and NVB, respectively. The largest PDP was in soil with the most rapid pyrene degradation. These results indicate that Bermuda grass can grow in pyrene-contaminated soil and enhance pyrene degradation through a rhizosphere effect.     

菲在黑麦草种植土壤中的降解及其对土壤酶的影响

研究了种植黑麦草对土壤中3环多环芳烃菲的动态降解作用。结果表明,黑麦草可以促进土壤中菲的降解,在75 d的盆栽试验里,种植黑麦草土壤中菲的可提取浓度明显低于不种植土壤（p<0.05）。在菲浓度为5 mg kg^-1、50 mg kg^-1、200 mg kg^-1的3种处理中,种植黑麦草壤中菲的降解率分别为81.07%、90.35%、84.94%,而不种植土壤中菲的降解率分别为75.34%、86.62%、67.60%。种植黑麦草增强了土壤中多酚氧化酶、脱氢酶和过氧化氢酶的活性以及增加土壤中微生物生物量碳的含量,即提高了土壤中生物活性,从而促进了土壤中菲的降解率。不同浓度菲处理,土壤中生物活性存在明显差异,高浓度菲(200 mg kg^-1)对土壤中生物活性产生较强的抑制作用,影响土壤中生物对菲的降解作用,从而揭示了植物促进菲降解的生物学及酶学机理。黑麦草对土壤中多环芳烃有较强的忍耐性,但过高的菲浓度对黑麦草的生长有影响。     

Influence of grass species and soil type on rhizosphere microbial community structure in grassland soils

A number of studies have reported species specific selection of microbial communities in the rhizosphere by plants. It is hypothesised that plants influence microbial community structure in the rhizosphere through rhizodeposition. We examined to what extent the structure of bacterial and fungal communities in the rhizosphere of grasses is determined by the plant species and different soil types. Three grass species were planted in soil from one site, to identify plant-specific influences on rhizosphere microbial communities. To quantify the soil-specific effects on rhizosphere microbial community structure, we planted one grass species (Lolium perenne L.) into soils from three contrasting sites. Rhizosphere, non-rhizosphere (bulk) and control (non-planted) soil samples were collected at regular intervals, to examine the temporal changes in soil microbial communities. Rhizosphere soil samples were collected from both root bases and root tips, to investigate root associated spatial influences. Both fungal and bacterial communities were analysed by terminal restriction fragment length polymorphism (TRFLP). Both bacterial and fungal communities were influenced by the plant growth but there was no evidence for plant species selection of the soil microbial communities in the rhizosphere of the different grass species. For both fungal and bacterial communities, the major determinant of community structure in rhizospheres was soil type. This observation was confirmed by cloning and sequencing analysis of bacterial communities. In control soils, bacterial composition was dominated by Firmicutes and Actinobacteria but in the rhizosphere samples, the majority of bacteria belonged to Proteobacteria and Acidobacteria. Bacterial community compositions of rhizosphere soils from different plants were similar, indicating only a weak influence of plant species on rhizosphere microbial community structure.     

氟磺胺草醚对大豆根际土壤微生物和酶活性的影响及其在根际的降解

【目的】 本研究旨在探讨不同浓度氟磺胺草醚在大豆根际土壤中的微生态效应,及其在根际土壤中的降解动态,为进一步研究除草剂的残留污染提供科学依据。 【方法】 以中黄13号大豆为材料,采用根箱进行了模拟栽培试验。设施用氟磺胺草醚3.75 mg/kg (低)、7.5 mg/kg (中)、18.75 mg/kg (高) 3个水平,以不添加氟磺胺草醚处理为对照,调查了大豆根际土壤细菌、真菌、放线菌数量,分析了根际土壤中过氧化氢酶、磷酸酶、脲酶、蔗糖酶4种酶活性,以及氟磺胺草醚在大豆根际土壤中的降解规律。 【结果】 低浓度氟磺胺草醚处理的大豆根际土壤细菌数量显著低于对照根际土壤 (P < 0.05),高浓度氟磺胺草醚处理在28 d时显著高于对照 ( P < 0.05);中浓度氟磺胺草醚处理与对照没有显著性差异。不同浓度氟磺胺草醚处理的大豆根际土壤真菌和放线菌数量与对照差异不显著。氟磺胺草醚处理的大豆根际土壤过氧化氢酶活性与对照没有显著差异;磷酸酶活性在取样初期略有降低;低浓度氟磺胺草醚处理的土壤脲酶活性显著降低,中浓度和高浓度处理对脲酶活性表现为先刺激后抑制;高浓度氟磺胺草醚处理的蔗糖酶活性在42 d和56 d时显著低于对照。高浓度氟磺胺草醚降解速率明显高于低浓度和中浓度,并且在试验初期降解迅速;3种浓度氟磺胺草醚在大豆根际土壤中的降解均符合一级动力学方程,降解半衰期由低浓度到高浓度逐渐变短。 【结论】 3种浓度氟磺胺草醚总体上降低大豆根际土壤中细菌的数量,而对大豆根际土壤真菌和放线菌的数量均没有显著影响。氟磺胺草醚对大豆根际土壤过氧化氢酶活性没有显著影响,在短期内对磷酸酶活性有一定程度的抑制作用,低浓度氟磺胺草醚可以显著降低大豆根际土壤脲酶活性,而高浓度氟磺胺草醚在试验后期可以显著抑制大豆根际土壤蔗糖酶活性。大豆根际土壤中氟磺胺草醚初始浓度越高,降解速率越快,半衰期越短。      

Increased degradation of phenanthrene in soil by Pseudomonas sp. GF3 in the presence of wheat

A phenanthrene-degrading bacterial strain Pseudomonas sp. GF3 was examined for plant-growth promoting effects and phenanthrene removal in soil artificially contaminated with low and high levels of phenanthrene (0, 100 and 200 mg kg⁻¹) in pot experiments. Low and high phenanthrene treatments significantly decreased the growth of wheat. Inoculation with bacterial strain Pseudomonas sp. GF3 was found to increase root and shoot growth of wheat. Strain GF3 was able to degrade phenanthrene effectively in the unplanted and planted soils. Over a period of 80 days the concentration of phenanthrene in soil in which wheat was grown was significantly lower than in unplanted soil (p<0.05). At the end of the 80-d experiments, 62.2% and 42.3% of phenanthrene had disappeared from planted soils without Pseudomonas sp. GF3 when the phenanthrene was added at 100 and 200 mg kg⁻¹ soil, respectively, but 84.8% and 70.2% of phenanthrene had disappeared from planted soils with the bacterial inoculation. The presence of vegetation significantly enhances the dissipation of phenanthrene in the soil. There was no significant difference in soil polyphenol oxidase activities among the applications of 0, 100 and 200 mg kg⁻¹ of phenanthrene. However, the enzyme activities in planted and unplanted soils inoculated with the strain Pseudomonas sp. GF3 were significantly higher than those of non-inoculation controls. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of wheat after inoculation.     

Structural differences between bulk and rhizosphere soil

The physical characteristics of the soil at the root–soil interface are crucial because they determine both physical aspects of root function such as water and nutrient uptake and the microbial activity that is most relevant to root growth. Because of this we have studied how root activity modifies the structure and water retention characteristic of soil adjacent to the root for maize, wheat and barley. These plants were grown in pots for a 6‐week growth period, then the soil adjacent to the root (rhizosphere soil) and bulk soil aggregates were harvested. These soil aggregates were then saturated and equilibrated at matric potentials between ?600 kPa and saturation, and the water retention characteristics were measured. From subsamples of these aggregates, thin sections were made and the porosity and pore‐size distributions were studied with image analysis. Both image analysis and estimates of aggregated density showed that the rhizosphere soil and bulk soil had similar porosities. Growing different plants had a small but significant effect on the porosity of the soil aggregates. Image analysis showed that for all the plant species the structure of the rhizosphere soil was different to that of the bulk soil. The rhizosphere soil contained more larger pores. For maize and barley, water retention characteristics indicated that the rhizosphere soil tended to be drier at a given matric potential than bulk soil. This effect was particularly marked at greater matric potentials. The difference between the water retention characteristics of the bulk and rhizosphere soil for wheat was small. We compare the water retention characteristics with the data on pore‐size distribution from image analysis. We suggest that differences in wetting angle and pore connectivity might partly explain the differences in water retention characteristic that we observed. The impact of differences between the water retention properties of the rhizosphere and bulk soil is discussed in terms of the likely impact on root growth.     

Mobilization and turnover of soil phosphorus in the rhizosphere

Recent progress in methods enables a better understanding of the turnover of P in the rhizosphere. Examples of this progress are the separation of soil layers differing in proximity to the roots, improved methods for extraction and fractionation of soil P, application of ³²P isotope dilution analysis to follow P fluxes between various fractions and direct determination of microbially bound P and of root phosphatases.

• These methods were combined to investigate the following aspects
• –labile P pools, the P fluxes between these pools and their contribution to the P supply to growing maize roots
• –the role of microbial biomass in these interactions and the partition of mobilized P between plants and microorganisms
• –modifications of sorption and transport of P in the rhizosphere
• –plant availability of native and added organic phosphates, and the relative significance of root and soil phosphatases.

There is a significant transformation of P in the rhizosphere with a corresponding redistribution among fractions of different plant availability. About 9% of the inorganic ³²P added to soil were incorporated within 2 weeks into microbial and organic fractions. The transfer of P from non-exchangeable forms exceeded the depletion of the exchangeable P by a factor of 5. About 53% of the mobilized P originated from inorganic, the remaining 47% from organic fractions. Of the mobilized P 80% was taken up by the plants and 20% was found in the microbial biomass. Up to 90% of the P in the rhizosphere soil solution was organic with a maximum just outside the root zone. Soluble inositol hexaphosphate modified the sorption of inorganic P, thus shifting its equilibrium solution concentration. The phosphatase activity of the roots is considerable. Both root phosphatase activity and the utilization of inositol hexaphosphate depend on the P supply and nutritional status of plants with regard to P. It is concluded that the rhizosphere is a key site of P transformation with a significant mobilization of P from the non-exchangeable inorganic and organic fractions. Organic P fractions not only play a significant role as a P source but also modify important soil parameters related to the sorption and transport of P in the rhizosphere.     

Diversity of 16S-rRNA and nifH genes derived from rhizosphere soil and roots of an endemic drought tolerant grass,Lasiurus sindicus

Lasiurus sindicus is a highly nutritive, drought tolerant, perennial grass, endemic to the Thar Desert of Rajasthan, India. In order to characterize the diversity of bacteria associated with roots of this grass that had survived severe drought stress, 16S-rRNA gene clone libraries were established from RT-PCR amplified products of the total RNA extracted from the washed roots and rhizosphere soil samples. Eight major bacterial taxa were identified in a total of 121 16S-rRNA gene clones. The majority of sequences belonged to Gram-positive bacteria, Actinobacteria being the most predominant ones, closely followed by Firmicutes. Most of the sequences showed similarity with sequences from cultivated bacteria or uncultivated environmental clones associated with arid, semi-arid environments, cold deserts and contaminated soils. PCR amplification of nifH genes using total DNA as template produced a total of 48 nifH clones from the rhizosphere soil and root samples and revealed a predominance of nifH sequences closely affiliated to Pseudomonas pseudoalcaligenes, isolated in a previous study from root samples of Lasiurus sindicus. Some nifH sequences showed close similarity to cultivated diazotrophs like Azospirillum brasilense, Rhizobium sp., and a variety of uncultured nitrogen fixing bacteria. Thus, this study provides us with evidence that L. sindicus harbors a diversity of bacteria with potential for nitrogen fixation.     

木糖氧化无色杆菌反硝化亚种细菌的分离鉴定及其菲降解特性研究

利用选择性富集培养及升华法,从石油污染的土壤中分离到2株菲降解细菌,它们在以菲为唯一碳源的培养基上生长良好。应用BIOLOG细菌鉴定系统和分子生物学方法对两株细菌进行鉴定,两株菌分别为坚强芽孢杆菌(Bacillus.firmus)和木糖氧化无色杆菌反硝化亚种(Achromobacter.xylosoxidanssub.sp.denitrificans),两株菌均具有邻苯二酚氧化酶活性。两株细菌在液体培养条件下都表现较强降解菲的能力,液体培养60.h约90%的加入菲被降解。通过测定液体培养基中菲浓度和菌体密度变化发现,菌株降解菲的量与其生长密度相关;随着菌体浓度(吸光度)的增加,代谢底物菲的浓度明显降低,两株菌混合使用能够大幅度提高降解菲的能力。     

Persistence of hexachlorocyclohexane isomers in soil planted with rice and in rice rhizosphere soil suspensions

Summary The relative persistence of -, and -isomers of hexachlorocyclohexane (HCH) was studied in a flooded soil with and without rice seedlings under greenhouse conditions. -HCH was more stable than - and -HCH in both planted and unplanted systems. - and -HCH decreased to negligible levels (5.5% for the -isomer and 2.4% for the -isomer) after 30 days in planted and unplanted soils. During the same period, 30.9% of the added -HCH was recovered from planted soil and 50.6% from unplanted soil. Likewise, in anaerobically (H₂ + CO₂ atmosphere) incubated mineral salts solution inoculated with suspensions from rice rhizosphere and non-rhizosphere soils, -HCH decreased to low levels (< 15%) within 5 days. Most of the added -HCH was recovered from mineral solution inoculated with nonrhizosphere soil suspension even after 30 days while -HCH decreased to 53.6% of the original level in mineral solution inoculated with rice rhizosphere soil suspension. The data reveal that the degradation of anaerobically unstable HCH isomers is not retarded by the possible aeration of a flooded soil by rice roots.     

Determination of the extent of rhizosphere soil

Abstract

The extent of the rhizosphere was investigated by using root volume and root length in ten replications. The experiment was conducted using split cylindrical pots, 23 cm long and 7.5 cm in diameter. Sorghum (Sorghum bicolor) plants were grown in a calcareous soil of low phosphorus (P) status. Fertilized soil (750 g soil and 250 g sand) was placed in a closed‐bottom PVC tube. At harvest, plant roots were gently removed from the pots and the roots were shaken five times in order to reduce variation between samples. The soil that was easily shaken from the root surface was assumed to be non‐rhizosphere soil, and the soil adhering to the root segment after a gentle shake was considered to be rhizosphere soil. The rhizosphere thickness was found to have a range of 0.39 to 0.64 mm from the root surface (0.51 mm average thickness). Rhizosphere soil mass was also calculated and found to be on average 22% of the total soil mass.     

Impact of Al and Fe on the development of phenanthrene catabolism in soil

Purpose

Heavy metals often occur as co-contaminants with polycyclic aromatic hydrocarbons (PAHs) and reportedly have adverse effects on biodegradation. In this study, the development of ¹⁴C-phenanthrene mineralisation in soil co-contaminated with aged or freshly added Al or Fe amendment was assessed.

Materials and methods

¹⁴C-phenanthrene mineralisation was assessed using respirometry; respirometers incorporated a Teflon-lined screw-capped CO₂ trap containing 1-M NaOH within a glass scintillation vial. The production of ¹⁴CO₂ was assessed by the addition of Ultima Gold liquid scintillation fluid to the CO₂ traps and subsequent liquid scintillation counting. Enumeration of phenanthrene-degrading bacteria was achieved by counting the colony forming unit count using the spread plate method.

Results and discussion

This investigation considered the effects of Al and Fe (50, 100, 250 and 500 mg/kg) on ¹⁴C-phenanthrene biodegradation in soil over 63-day contact time. Fresh Al amendments at lower concentrations (50 and 100 mg/kg) stimulated phenanthrene catabolism (p <0.05) at t?=?21 and 42 days which may reflect an ‘Arndt–Schulz’ effect, but phenanthrene catabolism was significantly reduced (p <0.05) in 500 mg/kg aged Al this could be due to Al toxicity to phenanthrene degraders. Phenanthrene mineralisation was stimulated in the highest Fe concentration (500 mg/kg) in aged and fresh Fe amendments at t?=?21 days. This could be because Fe is an essential requirement for microbial growth.

Conclusions

The impact of Al or Fe on the catabolism of ¹⁴C-phenanthrene was dependent on incubation time and Al was more toxic than Fe to soil PAH catabolic activity. This could be because Al is a non-essential microbial requirement. Bioremediation of soils co-contaminated with PAH and heavy metal is a complex problem; therefore, studies on the impact of metals on PAHs biodegradation highlight the risks and biodegradation potential in contaminated soil.     

Links between pseudometallophytes and rhizosphere microbial communities in a metalliferous soil

Short-term improvements in soil health derived from pseudometallophytes growth and metal phytoremediation were quantified based upon specific microbial properties of potential value as bioindicators of soil functioning. To this aim, plant consortia, consisting of 1–3 pseudometallophytes with different metal-tolerance strategies (hyperaccumulator: Noccaea caerulescens; accumulator: Rumex acetosa; excluder: Festuca rubra), were grown in a mine soil. At the end of the experiment, soil microbial biomass, activity, structural and functional community profiling, and stability were determined. Growing together with N. caerulescens stimulated the growth of the other two pseudometallophytes. The combination of R. acetosa and N. caerulescens extracted the highest amounts of Zn. Except for β-glucosidase, a negative correlation was found between enzyme activities and number of pseudometallophytes present in the study pots. Microbial biomass C was highest in the presence of all three pseudometallophytes. The combination of different pseudometallophyte species, which may allow for a greater exploitation of potential niche space, appears promising for phytoremediation. When quantifying soil health, the importance of measuring various types of soil microbial properties has been highlighted, as the response observed was different in each of them.     

Soil microbial community dynamics and phenanthrene degradation as affected by rape oil application

The effects of rape oil application on soil microbial communities and phenanthrene degradation were characterized by examining phenanthrene concentrations, changes in microbial composition and incorporation of [¹³C] phenanthrene-derived carbon into phospholipid fatty acids (PLFAs). A Haplic Chernozem was incubated with and without rape oil in combination with and without phenanthrene over 60 days. High-performance liquid chromatography (HPLC) analysis showed a net reduction in extractable phenanthrene in the soils treated with rape oil but no net reduction in the soils without rape oil. Rape oil application increased the total PLFA content and changed microbial community composition predominantly due to growth of fungal groups and Gram-positive bacterial groups. Under rape oil and phenanthrene amendment all detected microbial groups grew until day 24 of incubation. The ¹³C PLFA profiles showed ¹³C enrichment for the PLFAs i14:0, 15:0, 18:0, 18:1ω5 and the fungal biomarker 18:2ω6,9 under rape oil application. Fungal PLFA growth was highest among detected all PLFAs, but its ¹³C incorporation was lower compared to the Gram-positive and Gram-negative bacteria PLFAs. Our results demonstrate the effect of rape oil application on the abundance of microbial groups in soil treated with phenanthrene and its impact on phenanthrene degradation.