The application of the herbicide bromoxynil to a model soil-derived bacterial community: impact on degradation and community structure

AIMS: Bromoxynil degradation by soil micro-organisms has been shown to be co-oxidative in character. In this study, we investigate both the impact of the application of increasing bromoxynil concentrations on soil-derived bacterial communities and how these changes are reflected in the degradation of the compound. Our aim was to test the hypothesis that the addition of bromoxynil to a soil-derived bacterial community, and the availability of a readily utilizable carbon source would have an impact on bromoxynil degradation, and that would be reflected in the bacteria present in the soil community. METHODS AND RESULTS: Degradation of bromoxynil was observed in soil-derived communities containing 15 mg l(-1), but not 50 mg l(-1) of the compound, unless glucose was added. This suggests that the addition of carbon stimulates co-oxidative bromoxynil degradation by the members of the bacterial community. Measurable changes in the bacterial community indicated that the addition of bromoxynil led to deterministic selection on the bacterial population, i.e. the communities observed arise through the selection of specific micro-organisms that are best adapted to the conditions in the soil. The addition of bromoxynil was also shown to have a negative impact on the presence of alpha and gamma-proteobacteria in the soil community. CONCLUSION: Bromoxynil degradation is significantly inhibited in bacterial soil communities in the absence of readily accessible carbon. The application of bromoxynil appears to exert deterministic selection on the bacterial community. SIGNIFICANCE AND IMPACT OF THE STUDY: This study highlights the effects of increasing bromoxynil concentrations on a model bacterial population derived from soil. Soil communities show qualitative and quantitative differences to bromoxynil application depending on the availability of organic carbon. These findings might have implications for the persistence of bromoxynil in agricultural soils.     

Phenanthrene stimulates the degradation of pyrene and fluoranthene by <Emphasis Type="Italic">Burkholderia</Emphasis> sp. VUN10013

Pyrene and fluoranthene, when supplied as the sole carbon source, were not degraded by Burkholderia sp. VUN10013. However, when added in a mixture with phenanthrene, both pyrene and fluoranthene were degraded in liquid broth and soil. The amounts of pyrene and fluoranthene in liquid media (initial concentrations of 50 mg l⁻¹ each) decreased to 42.1% and 41.1%, respectively, after 21 days. The amounts of pyrene and fluoranthene in soil (initial concentrations of 75 mg kg⁻¹ dry soil each) decreased to 25.8% and 12.1%, respectively, after 60 days. None of the high molecular weight (HMW) polycylic aromatic hydrocarbons (PAHs) tested adversely affected phenanthrene degradation by this bacterial strain and the amount of phenanthrene decreased rapidly within 3 and 15 days of incubation in liquid broth and soil, respectively. Anthracene also stimulated the degradation of pyrene or fluoranthene by Burkholderia sp. VUN10013, but to a lesser extent than phenanthrene. The extent of anthracene degradation decreased in the presence of these HMW PAHs.     

Biodegradation kinetics of the benzimidazole fungicide thiophanate-methyl by bacteria isolated from loamy sand soil

Degradation of the fungicide thiophanate-methyl (TM) by Enterobacter sp. TDS-1 and Bacillus sp. TDS-2 isolated from sandy soil previously treated with TM was studied in mineral salt medium (MSM) and soil. Both strains were able to grow in MSM supplemented with TM (50 mg l⁻¹) as the sole carbon source. Over a 16 days incubation period, 60 and 77% of the initial dose of TM were degraded by strains TDS-1 and TDS-2, respectively, and disappearance of TM was described by first-order kinetics. Medium supplementation with glucose markedly stimulated bacterial growth; while the final rate of TM degradation was reduced by 21 and 27% for strains TDS-1 and TDS-2, respectively as compared to medium with TM only. Moreover, this additional carbon source changed the TM degradation kinetics, which proceeded according to a zero-order model. This effect was linked to substrate competition and/or a strong decrease of medium pH. Isolates degraded TM (100 mg kg⁻¹) in soil with rate constants of 0.186 and 0.210 day⁻¹, following first-order rate kinetics, and the time in which the initial TM concentration was reduced by 50% (DT50) in soils inoculated with strains TDS-1 and TDS-2 were 6.3 and 5.1 days, respectively. Analysis of TM degradation products in soil showed that the tested strains may have the potential to transform carbendazim (MBC) to 2-aminobenzimidazole (2-AB), and may be useful for a bioremediation of MBC-polluted soils.     

供磷水平和根际效应协同影响含碱性磷酸酶基因细菌群落的网络复杂性和稳定性北大核心

【目的】通过研究长期不同供磷水平下根际、土体土壤中编码碱性磷酸酶基因(alkaline phosphatase gene,phoD)细菌群落特征、网络复杂性、群落的稳定性及其与磷酸酶活性之间的关系,揭示供磷水平和根际效应在调控土壤有机磷矿化中的微生物学机制。【方法】选取华北平原长期施磷的小麦-玉米轮作体系石灰性土壤为基质土壤,开展根箱试验。选取的试验处理包括3个供磷水平,分别是0、50.0、200.0 kg P/hm^(2)(分别表示为P0、P50、P200)。玉米种子播种30 d后,采集玉米的根际土和土体土。采用高通量测序技术分析根际和土体土壤中编码碱性磷酸酶基因(phoD)细菌群落,探究施肥及根际效应对含phoD基因细菌的群落特征、网络特征的影响及其与磷酸酶活性的关系。【结果】随着施磷量的增加,速效磷(available P,AP)和碱性磷酸酶(alkaline phosphatase,ALP)活性在根际、土体土壤中均显著提高,且两者呈显著正相关。phoD基因丰度在P0、P200处理的根际土壤中显著高于土体土壤。含phoD基因细菌群落的α多样性在P50处理下的根际土壤显著高于土体土壤。冗余分析(redundancy analysis,RDA)表明,土壤中AP、有机磷(organic P,Po)和全磷(total P,Pt)是影响微生物群落的主要因素。与不施磷处理(P0)相比,施磷处理(P50、P200)下根际土壤中网络节点数和连接数降低,而土体土壤中网络节点数和连接数增加;同时,施磷处理含phoD基因细菌群落的鲁棒性(robustness)在根际土壤中显著提高,而在土体土壤中显著降低。Mantel检验表明,含phoD基因微生物群落中的优势物种在根际土壤与AP、酸性磷酸酶(acid phosphatase,ACP)、内聚力(cohesion)和网络的鲁棒性显著相关,在土体土壤中无显著性。【结论】供磷水平及根际效应协同影响phoD基因丰度、含phoD基因细菌群落的α多样性、群落结构、优势物种、网络的复杂性及群落的稳定性,进而影响磷酸酶活性,调控了土壤中有机磷的矿化。     

尕海湿地草甸土退化过程土壤氮矿化演变特征

氮矿化是生态系统循环的重要环节之一,影响着生态系统功能和氮素生物地球化学循环,因此研究高寒湿地退化过程中土壤氮矿化演变特征,对揭示气候变化和人为活动干扰背景下的湿地土壤氮素循环过程具有重要意义。以尕海湿地4种不同退化梯度(未退化、轻度退化、中度退化、重度退化)土壤为研究对象,采用野外树脂芯原位培养方法,通过对植物生长季不同生长阶段(生长初期、生长盛期、枯萎期)土壤氮素矿化作用研究,分析湿地退化演替过程中土壤氮矿化时空变化特征及其与土壤环境因子和酶活性之间的关系。结果表明：尕海湿地退化对土壤氮矿化过程有显著抑制作用,与未退化(0.143 mg kg^-1 d^-1)相比,轻度退化、中度退化、重度退化的土壤净氮矿化速率分别减小了0.018 mg kg^-1 d^-1、0.025 mg kg^-1 d^-1、0.020 mg kg^-1 d^-1;随着退化程度加剧,土壤净氨化速率逐渐减小或者不变,而净硝化速率却增大。随时间推移,各退化...     

Bacterial Communities Predominant in the Degradation of 13C-Labeled Pyrene in Red Soil

The bacterial community composition using a consortium (CON) with or without methyl-β-cyclodextrin (MCD), used to bioremediate the pyrene-contaminated soil was evaluated through stable-isotope probing (SIP). Microcosms were artificially contaminated with ¹³C pyrene at 15 ppm, and bacterial community composition was determined over a 14-day period. After 14 days, only 29.1% of pyrene was degraded in control, while 90.6% of pyrene was degraded by the bacterial community with MCD, showing the best bioremediation rate of all treatments. After 14 days, PAH degraders became dominated by Proteobacteria. Sphingomonas and Pseudomonas were the predominant genera in all treatments, and Pseudomonas was the main degrader from the bacterial community. The bacterial community was little affected by MCD. The results indicated that the combination of the bacterial community with MCD can be used to bioremediate PAHs polluted soil.     

Degradation of ioxynil and bromoxynil as measured by a modified spectrophotometric method.

A modified spectrophotometric method was developed to estimate ioxynil and bromoxynil residues. The method when compared with a 14C-tracer method was less sensitive but allowed rapid and accurate estimation of the herbicides. A clay loam soil with high organic matter content, which degraded ioxynil completely to CO2, also degraded bromoxynil completely. Bromoxynil degradation proceeded at a faster rate than that of ioxynil. The half-life of degradation was estimated to be 7 days for bromoxynil and 9-10 days for ioxynil. However, soil microorganisms which degraded ioxynil either completely to CO2 or partially did not seem to completely degrade bromoxynil. Degradation products from bromoxynil were detected on thin-layer chromatograms of extracts from pure cultures containing an exogenous carbon source. Complete degradation of bromoxynil and ioxynil in soil could be due to the action of different microorganisms.     

Bioreclamation of chlorophenol-contaminated soil by composting

Summary Microbiological decontamination of technical chlorophenol-containing soil by composting was studied. In two 50 m³ windrows the concentration of chlorophenols went down from 212 mg kg^-1 to 30 mg kg^-1 in 4 summer months and after the second summer of composting it was only 15 mg kg^-1. All chlorophenol congeners present in the technical chlorophenol were degraded, but the main dimeric impurities, polychlorinated phenoxyphenols were recalcitrant. The contaminated soil was found to contain chlorophenol-degrading microbes, 5x10⁶ cfu g^-1 of dry windrow soil. Laboratory experiments with samples from the windrow compost showed that chlorophenols were truly degraded and that chlorophenol loss by evaporation was less than 1.5% under the circumstances studied. Laboratory experiments also showed that degradation of chlorophenols (120 mg kg^-1) was accelerated when sterilized contaminated soil was inoculated with Rhodococcus chlorophenolicus (mineralizer of several chlorophenols) or naturally occurring microbes of the field composts. Biomethylation of chlorophenols in the composts was insignificant compared to biodegradation.     

Effect of soil moisture on bioremediation of chlorophenol-contaminated soil

A chlorophenol-contaminated soil was tested for the biodegradability in a semi-pilot scale microcosm using indigenous microorganisms. More than 90% of 4-chlorophenol and 2,4,6-trichlorophenol, initially at 30 mg kg^–1, were removed within 60 days and 30 mg pentachlorophenol kg^–1 was completely degraded within 140 days. The chlorophenols were degraded more effectively under aerobic condition than under anaerobic condition. Soil moisture had a significant effect with the slowest degradation rate of chlorophenols at 25% in the range of 10–40% moisture content. At 25–40%, the rate of chlorophenol degradation was directly related to the soil moisture content, whereas at 10–25%, it was inversely related. Limited oxygen availability through soil agglomeration at 25% moisture content might decrease the degradation rate of chlorophenols.     

A Previously Unexposed Forest Soil Microbial Community Degrades High Levels of the Pollutant 2,4,6-Trichlorophenol

2,4,6-Trichlorophenol (2,4,6-TCP) is a hazardous pollutant that is efficiently degraded by some aerobic soil bacterial isolates under laboratory conditions. The degradation of this pollutant in soils and its effect on the soil microbial community are poorly understood. We report here the ability of a previously unexposed forest soil microbiota to degrade high levels of 2,4,6-TCP and describe the changes in the soil microbial community found by terminal restriction fragment length polymorphism (T-RFLP) analysis. After 30 days of incubation, about 50% degradation of this pollutant was observed in soils amended with 50 to 5,000 ppm of 2,4,6-TCP. The T-RFLP analysis showed that the soil bacterial community was essentially unchanged after exposure to up to 500 ppm of 2,4,6-TCP. However, a significant decrease in richness was found with 2,000 and 5,000 ppm of 2,4,6-TCP, even though the removal of this pollutant remained high. The introduction of Ralstonia eutropha JMP134 or R. eutropha MS1, two efficient 2,4,6-TCP degraders, to this soil did not improve degradation of this pollutant, supporting the significant bioremediation potential of this previously unexposed, endogenous forest soil microbial community.     

Susceptibility of the eggs of the field slug Deroceras reticulatum to contact with pesticides and substances of biological origin on artificial soil

The toxicity of 14 substances, including a number of pesticides, to the eggs of the pest slug Deroceras reticulatum was determined in laboratory experiments. Eggs were kept in contact with a precisely defined artificial soil to which a range of concentrations of the test substances had been applied. Mortality of the eggs was assessed every 24 h and the median lethal doses (LD₅₀) were determined. The herbicides bromoxynil, ioxynil and pyridate + bromoxynil, the insecticides thiocyclam, diflubenzuron and azadirachtin, the molluscicides metaldehyde and methiocarb, and other compounds such as carvone, iron‐EDDHA, saponin, and an extract of Pongamia pinnata, killed the eggs after periods of exposure ranging from 2 to 14 days, depending on the compound and the dose. Only two compounds, the insecticides imidacloprid and teflubenzuron, failed to kill the eggs of D. reticulatum at any of the doses tested. Values of LD₅₀ below 0.01 mg a.i. cm^?2 were obtained for the herbicides bromoxynil, ioxynil and pyridate + bromoxynil, and for the biological pesticide azadirachtin. The feasibility of slug egg control in different contexts is discussed.     

Effect of decabromodiphenyl ether (BDE 209) on soil microbial activity and bacterial community composition

In many areas of the world, polybrominated diphenyl ethers are ubiquitous due to their use as fire retardants. It is known that the hydrophobic characteristics of PBDEs cause them to sink in soil and sediment, yet their effect on microbes within the soil is not well understood. In this study, soil was treated with 1, 10, and 100 mg kg⁻¹ BDE 209 for up to 45 days. Treatment effects on soil enzymatic activities for urease and catalase were evaluated. The impact on the microbe community structure was estimated using denaturing gradient gel electrophoresis after polymerase chain reaction amplification of total genomic DNA, using bacterial variable V3 region targeted primers. The effects on the soil microbial community size and major bacterial groups were evaluated using fluorescence in situ hybridization analysis. Forty-five days after the addition of BDE 209, urease activity was suppressed by BDE 209, even at a concentration of 1 mg kg⁻¹. Catalase activity increased in the samples containing lower concentrations of BDE 209, but was suppressed in samples containing higher concentrations. The bacterial community also varied in response to the addition of BDE 209, and the variation of community composition differed among treatments. In addition, α, β and γ subclass proteobacteria decreased in the group of 100 mg kg⁻¹ BDE 209 spiked soil after 45 days of treatment. Throughout the experiment, no BDE 209 degradation was detected under darkness. These observations demonstrated that BDE 209 in soil, although of low bioavailability, had an adverse impact on the structure and function of the soil microbial community and microbial processes.     

Characterization of alkalotolerant bacterial community by 16S rDNA-based denaturing gradient gel electrophoresis method for degradation of dibenzofuran in soil microcosm

An alkalotolerant bacterial community was developed by continuous enrichment in the chemostat in presence of dibenzofuran (DF) as sole carbon source. Six different types of bacterial isolates were cultured on nutrient broth agar plates together with six operational taxonomic units (OTUs) at pH 7.0 and pH 8.0 by 16S rDNA-DGGE method. However, isolates of microbial community was declined from three OTUs (pH 9.0) to two at pH 10.0 after enrichment in alkaline condition. Among the six isolates tested for degradation of DF, Pseudomonas sp. and Bacillus sp. the members of alkalotolerant bacterial community had better potency to degrade dibenzofuran. Alkalotolerant bacterial community introduced in soil microcosm for evaluation of survival of most suitable isolates and degradation of dioxin-like compound indicated more than 90% degradation of dibenzofuran after 45 days by the bacterial community enriched for 180 days in the chemostat at pH 10, however, microbial community was not competent to utilize even 50% DF after day 30, not enriched in the chemostat. The survival of competent bacteria monitored by DGGE method in soil microcosm indicated presence of two major alkalotolerant isolates for utilization of dibenzofuran, substantiated the results and significance of alkalotolerant bacteria for in situ bioremediation of dioxin-like compounds in the environment.     

Organic Amendments to Enhance Atrazine and Metamitron Degradation in Two Contaminated Soils with Contrasting Textures

Among all types of xenobiotics, pesticides such as herbicides play a significant role in soil and water pollution due to their wide usage all over the world. This study addresses the ability of organic amendments to enhance atrazine and metamitron degradation in two herbicide-contaminated soils with contrasting textures under laboratory conditions. Soil samples were collected from surface soils with textures of sandy loam and silty clay, from northeastern Iran. Initial concentration of herbicides was 50 mg · kg^{? 1} soil. Contaminated soil samples were treated with manure, compost and vermicompost at rates of 0, 0.5, and 2% (w/w). Residual concentrations of atrazine and metamitron were determined by HPLC at the end of incubation periods of 20, 40, and 60 days. Residual concentrations of atrazine were 46.5, 38.9, and 36.2 mg · kg^{? 1} after 20, 40, and 60 days incubation, respectively. Residual metamitron concentrations were clearly lower than atrazine. After 20, 40, and 60 days, concentrations of metamitron were 2.9, 1.0, and 0.6 mg · kg^{? 1}, respectively. Organic amendments at the rates of 0.5 and 2% showed similar effects on the enhancement of herbicide degradation in soils. However, no statistically significant effect was observed among types of organic amendments (α = 0.05). Degradation was affected by soil textures. Residual concentrations of herbicides were higher in sandy loam than in silty clay soil.     

Metabolic responses of novel cellulolytic and saccharolytic agricultural soil Bacteria to oxygen

Cellulose is the most abundant biopolymer in terrestrial ecosystems and is degraded by microbial communities in soils. However, relatively little is known about the diversity and function of soil prokaryotes that might participate in the overall degradation of this biopolymer. The active cellulolytic and saccharolytic Bacteria in an agricultural soil were evaluated by 16S rRNA ¹³C‐based stable isotope probing. Cellulose, cellobiose and glucose were mineralized under oxic conditions in soil slurries to carbon dioxide. Under anoxic conditions, these substrates were converted primarily to acetate, butyrate, carbon dioxide, hydrogen and traces of propionate and iso‐butyrate; the production of these fermentation end‐products was concomitant with the apparent reduction of iron(III). [¹³C]‐cellulose was mainly degraded under oxic conditions by novel family‐level taxa of the Bacteroidetes and Chloroflexi, and a known family‐level taxon of Planctomycetes, whereas degradation under anoxic conditions was facilitated by the Kineosporiaceae (Actinobacteria) and cluster III Clostridiaceae and novel clusters within Bacteroidetes. Active aerobic sub‐communities in oxic [¹³C]‐cellobiose and [¹³C]‐glucose treatments were dominated by Intrasporangiaceae and Micrococcaceae (Actinobacteria) whereas active cluster I Clostridiaceae (Firmicutes) were prevalent in anoxic treatments. A very large number (i.e. 28) of the detected taxa did not closely affiliate with known families, and active Archaea were not detected in any of the treatments. These collective findings suggest that: (i) a large uncultured diversity of soil Bacteria was involved in the utilization of cellulose and products of its hydrolysis, (ii) the active saccharolytic community differed phylogenetically from the active cellulolytic community, (iii) oxygen availability impacted differentially on the activity of taxa and (iv) different redox guilds (e.g. fermenters and iron reducers) compete or interact during cellulose degradation in aerated soils.     

Degradation of polycyclic aromatic hydrocarbons in soil by a two-step sequential treatment

The objectives of this work were to isolate the microorganisms responsible for a previously observed degradation of polycyclic aromatic hydrocarbons (PAH) in soil and to test a method for cleaning a PAH-contaminated soil. An efficient PAH degrader was isolated from an agricultural soil and designated as Mycobacterium LP1. In liquid culture, it degraded phenanthrene (58%), pyrene (24%), anthracene (21%) and benzo(a)pyrene (10%) present in mixture (initial concentration 50 μg ml⁻¹ each) and phenanthrene (92%) and pyrene (94%) as sole carbon sources after 14 days of incubation at 30°C. In soil, Mycobacterium LP1 mineralised ¹⁴C-phenanthrene (45%) and ¹⁴C-pyrene (65%) after 10 days. The good ability of this Mycobacterium was combined with the benzo(a)pyrene oxidation effect obtained by 1% w/w rapeseed oil in a sequential treatment of a PAH-spiked soil (total PAH concentration 200 mg kg⁻¹). The first step was incubation with the bacterium for 12 days and the second step was the addition of the rapeseed oil after this time and a further incubation of 22 days. Phenanthrene (99%), pyrene (95%) and anthracene (99%) were mainly degraded in the first 12 days and a total of 85% of benzo(a)pyrene was transformed during the whole process. The feasibility of the method is discussed.     

Mercury alters the bacterial community structure and diversity in soil even at concentrations lower than the guideline values

This study evaluated the effect of inorganic mercury (Hg) on bacterial community and diversity in different soils. Three soils—neutral, alkaline and acidic—were spiked with six different concentrations of Hg ranging from 0 to 200 mg kg⁻¹ and aged for 90 days. At the end of the ageing period, 18 samples from three different soils were investigated for bacterial community structure and soil physicochemical properties. Illumina MiSeq-based 16s ribosomal RNA (rRNA) amplicon sequencing revealed the alteration in the bacterial community between un-spiked control soils and Hg-spiked soils. Among the bacterial groups, Actinobacteria (22.65%) were the most abundant phyla in all samples followed by Proteobacteria (21.95%), Bacteroidetes (4.15%), Firmicutes (2.9%) and Acidobacteria (2.04%). However, the largest group showing increased abundance with higher Hg doses was the unclassified group (45.86%), followed by Proteobacteria. Mercury had a considerable negative impact on key soil functional bacteria such as ammonium oxidizers and nitrifiers. Canonical correspondence analysis (CCA) indicated that among the measured soil properties, Hg had a major influence on bacterial community structure. Furthermore, nonlinear regression analysis confirmed that Hg significantly decreased soil bacterial alpha diversity in lower organic carbon containing neutral and alkaline soils, whereas in acidic soil with higher organic carbon there was no significant correlation. EC₂₀ values obtained by a nonlinear regression analysis indicated that Hg significantly decreased soil bacterial diversity in concentrations lower than several guideline values.

     

黄河三角洲刺槐白蜡混交对土壤细菌群落结构及多样性的影响

为探讨黄河三角洲刺槐白蜡混交对土壤细菌群落结构及多样性的影响,通过高通量测序技术分析比较了刺槐白蜡混交林及刺槐纯林、白蜡纯林土壤细菌群落结构及多样性。结果表明:(1)混交林与两种纯林土壤细菌群落共36门。酸杆菌门、变形菌门、放线菌门(相对丰度大于10%)为刺槐白蜡混交林与两种纯林土壤中共有的优势菌群;硝化螺旋菌门为刺槐纯林土壤中的优势菌群。不同人工林土壤中各门细菌相对丰度差异显著。(2)混交改变了土壤细菌群落结构,提高了细菌多样性。刺槐白蜡混交林土壤细菌物种数、Chao1指数、Shannon指数分别为1934.5、2629.1、9.1,显著高于两种纯林。(3)相关性分析表明,土壤含水量与放线菌门细菌呈显著正相关;pH与芽单胞菌门细菌呈极显著正相关,与酸杆菌门细菌呈显著负相关。细菌多样性与土壤含水量呈显著正相关,与速效钾、有机质含量呈显著负相关。研究表明,刺槐白蜡混交林土壤细菌群落结构与两种纯林之间有一定差异,多样性差异显著,刺槐白蜡混交改变细菌群落结构,提高细菌多样性。     

高寒地区不同退化草地植被特性和土壤固氮菌群特性及其相关性

选取东祁连山不同退化程度的高寒草地为研究对象,调查研究其植物种类、植被盖度、高度、地上生物量等植物指标以及土壤好气性自生固氮菌和嫌气性自生固氮菌数量,在此基础上,采用real-time PCR的方法扩增nifH基因,测定不同退化程度草地土壤中固氮菌相对于土壤总细菌的量,以探讨草地退化过程中植被及土壤固氮菌群的变化规律,结果发现:随着退化程度的加深,草地植物种类逐渐减少,并且优势植物发生变化,毒杂草逐渐增多,植被的高度、盖度、地上生物量都逐渐降低。对土壤固氮菌的研究则表明,土壤好气性自生固氮菌和嫌气性自生固氮菌的数量在不同退化草地随草地退化程度的加重而减少,在同一退化程度草地土壤则是随土层深度加深而下降。对土壤固氮菌nifH基因扩增的结果也表明随着退化加剧,土壤固氮菌相对于土壤总细菌的比例在降低,进一步说明草地退化过程中土壤固氮菌不仅是数量上的下降,更是群落结构层面的变化。对植被特性和土壤固氮菌含量的相关分析表明,植被特性和土壤中固氮菌含量呈显著相关。研究从土壤固氮菌群的角度研究了草地退化的过程,说明了二者具有协同性,研究和治理草地退化必须重视土壤功能菌群尤其是固氮菌群的作用。     

Comparison of DNA‐ and RNA‐based bacterial community structures in soil exposed to 2,4‐dichlorophenol

Aims: To examine the effect of the pollutant 2,4‐dichlorophenol on DNA‐ and RNA‐based bacterial communities in soil. Methods and Results: Soil was exposed to 100 mg kg^?1 of 2,4‐dichlorophenol (2,4‐DCP), and degradation was monitored over 35 days. DNA and RNA were coextracted, and terminal restriction fragment length polymorphism (T‐RFLP) was used to report changes in bacterial communities in response to the presence of the chlorophenol. The phylogenetic composition of the soil during degradation was determined by creating a clone library of amplified 16S rRNA sequences from both DNA and reverse‐transcribed RNA from exposed soil. Resulting clones were sequenced, and putative identities were assigned. Conclusions: A significant difference between active (RNA‐based) and total (DNA‐based) bacterial community structure was observed for both T‐RFLP and phylogenetic analyses in response to 2,4‐DCP, with more pronounced changes seen in RNA‐based communities. Phylogenetic analysis indicated the dominance of Proteobacteria in both profiles. Significance and Impact of the Study: This study describes the response of soil bacterial communities to the addition of the xenobiotic compound 2,4‐DCP, and highlights the importance of including RNA‐based 16S rRNA analysis to complement any molecular study in a perturbed soil.