Salt marsh rhizosphere affects microbial biotransformation of the widespread halogenated contaminant tetrabromobisphenol-A (TBBPA)

Estuarine sediments are the repository for a wide range of contaminants. Anthropogenic impacts and variations in the belowground biomass of salt marsh plants potentially select for different sediment microbial communities with different functional capabilities, including the ability to biotransform anthropogenic contaminants. There are large differences in both root morphology and the amount of fine root biomass of Spartina alterniflora and Phragmites australis; Spartina is the species commonly used to replace Phragmites in northeastern US salt marsh restoration projects. Our study compared the effect of these two macrophyte species on sediment microbial communities responsible for the biotransformation of the halogenated flame retardant tetrabromobisphenol A (TBBPA). Sediments were obtained from contaminated and uncontaminated salt marsh field sites in New Jersey. Anaerobic methanogenic sediment microcosms were established and incubated for up to 130 days. TBBPA was reductively dehalogenated resulting in the transient formation of two intermediates, identified as tribromobisphenol A and dibromobisphenol A, and the formation and accumulation of bisphenol A (BPA) as the end product. Spartina sediments from both sites were found to dehalogenate TBBPA more rapidly than the Phragmites or unvegetated sediments, resulting in greater production of BPA. Microbial community diversity as measured by in situ sediment phospholipid fatty acid (PLFA) composition prior to TBBPA exposure, was found to be higher in the uncontaminated sediments; differences in microbial PLFA diversity were not seen in contaminated sediments associated with either the different plant species or unvegetated sediment. The results of this study demonstrate that these two plant species affected sediment microbial community function with respect to dehalogenation capabilities, even though the disturbed and undisturbed sediments varied in microbial community composition.     

Microbial community composition and rhizodeposit-carbon assimilation in differently managed temperate grassland soils

Rhizodeposit-carbon provides a major energy source for microbial growth in the rhizosphere of grassland soils. However, little is known about the microbial communities that mediate the rhizosphere carbon dynamics, especially how their activity is influenced by changes in soil management. We combined a ¹³CO₂ pulse-labeling experiment with phospholipid fatty acid (PLFA) analysis in differently managed Belgian grasslands to identify the active rhizodeposit-C assimilating microbial communities in these grasslands and to evaluate their response to management practices. Experimental treatments consisted of three mineral N fertilization levels (0, 225 and 450 kg N ha⁻¹ y⁻¹) and two mowing frequencies (3 and 5 times y⁻¹). Phospholipid fatty acids were extracted from surface (0-5 cm) bulk (BU) and root-adhering (RA) soil samples prior to and 24 h after pulse-labeling and were analyzed by gas chromatography-combustion-isotope ratio mass spectrometry (GC-c-IRMS). Soil habitats significantly differed in microbial community structure (as revealed by multivariate analysis of mol% biomarker PLFAs) as well as in gram-positive bacterial rhizodeposit-C uptake (as revealed by greater ¹³C-PLFA enrichment following pulse-labeling in RA compared to BU soil in the 450N/5M treatment). Mowing frequency did not significantly alter the relative abundance (mol%) or activity (¹³C enrichment) of microbial communities. In the non-fertilized treatment, the greatest ¹³C enrichment was seen in all fungal biomarker PLFAs (C16:1ω5, C18:1ω9, C18:2ω6,9 and C18:3ω3,6,9), which demonstrates a prominent contribution of fungi in the processing of new photosynthate-C in non-fertilized grassland soils. In all treatments, the lowest ¹³C enrichment was found in gram-positive bacterial and actinomycetes biomarker PLFAs. Fungal biomarker PLFAs had significantly lower ¹³C enrichment in the fertilized compared to non-fertilized treatments in BU soil (C16:1ω5, C18:1ω9) as well as RA soil (all fungal biomarkers). While these observations clearly indicated a negative effect of N fertilization on fungal assimilation of plant-derived C, the effect of N fertilization on fungal abundance could only be detected for the arbuscular mycorrhizal fungal (AMF) PLFA (C16:1ω5). On the other hand, increases in the relative abundance of gram-positive bacterial PLFAs with N fertilization were found without concomitant increases in ¹³C enrichment following pulse-labeling. We conclude that in situ¹³C pulse-labeling of PLFAs is an effective tool to detect functional changes of those microbial communities that are dominantly involved in the immediate processing of new rhizosphere-C.     

Temperature and substrate controls on microbial phospholipid fatty acid composition during incubation of grassland soils contrasting in organic matter quality

Soil incubations are often used to investigate soil organic matter (SOM) decomposition and its response to increased temperature, but changes in the activity and community composition of the decomposers have rarely been included. As part of an integrated investigation into the responses of SOM components in laboratory incubations at elevated temperatures, fungal and bacterial phospholipid fatty acids (PLFAs) were measured in two grassland soils contrasting in SOM quality (i.e. SOM composition), and changes in the microbial biomass and community composition were monitored. Whilst easily-degradable SOM and necromass released from soil preparation may have fuelled microbial activity at the start of the incubation, the overall activity and biomass of soil microorganisms were relatively constant during the subsequent one-year soil incubation, as indicated by the abundance of soil PLFAs, microbial respiration rate (r), and metabolic quotient (qCO₂). PLFAs relating to fungi and Gram-negative bacteria declined relative to Gram-positive bacteria in soils incubated at higher temperatures, presumably due to their vulnerability to disturbance and substrate constraints induced by faster exhaustion of available nutrient sources at higher temperatures. A linear correlation was found between incubation temperatures and the microbial stress ratios of cyclopropane PLFA-to-monoenoic precursor (cy17:0/16:1ω7c and cy19:0/18:1ω7c) and monoenoic-to-saturated PLFAs (mono/sat), as a combined effect of temperature and temperature-induced substrate constraints. The microbial PLFA decay patterns and ratios suggest that SOM quality intimately controls microbial responses to global warming.     

Microbial community composition on native and drastically disturbed serpentine soils

During construction of roads, entire hillsides can be cut away, dramatically disturbing the ecosystem. Microbial communities play important, but poorly understood roles in revegetating roadcuts because of the many functions they perform in nutrient cycling, plant symbioses, decomposition, and other ecosystem processes. Our objective was to determine relationships among microbial community composition, soil chemistry, and disturbance on a serpentine soil disturbed by a roadcut and then partially revegetated. We hypothesized that the adjacent undisturbed serpentine soil would have a different microbial community composition from barren and revegetated sections of the roadcut and that undisturbed soils would have the greatest microbial biomass and diversity. We measured phospholipid fatty acids (PLFA) and soil nutrient concentrations on barren and revegetated sections of the roadcut and on adjacent undisturbed serpentine and nonserpentine soils. Most roadcut samples had soil chemistry similar to the serpentine reference soil. The microbial biomass and diversity of barren sites was lower than that of revegetated or the serpentine reference site. The nonserpentine reference site had significantly (P≤0.05) greater microbial biomass than serpentine or disturbed sites but significantly lower relative proportions of actinomycetes, and slow growth biomarkers. The Barren site had the lowest microbial biomass and a significantly (P≤0.05) greater proportion of that biomass was fungi. Barren, revegetated, and serpentine sites all had dissimilar microbial community composition. The composition of the revegetated communities, however, was intermediate between the serpentine reference and barren soils, suggesting that community composition of revegetated soils is approaching that of an undisturbed site with similar soil chemistry.     

The microbial PLFA composition as affected by pH in an arable soil

The influence of soil pH on the phospholipid fatty acid (PLFA) composition of the microbial community was investigated along the Hoosfield acid strip, Rothamsted Research, UK - a uniform pH gradient between pH 8.3 and 4.5. The influence of soil pH on the total concentration of PLFAs was not significant, while biomass estimated using substrate induced respiration decreased by about 25%. However, the PLFA composition clearly changed along the soil pH gradient. About 40% of the variation in PLFA composition along the gradient was explained by a first principal component, and the sample scores were highly correlated to pH (R² = 0.97). Many PLFAs responded to pH similarly in the Hoosfield arable soil compared with previous assessments in forest soils, including, e.g. monounsaturated PLFAs 16:1ω5, 16:1ω7c and 18:1ω7, which increased in relative concentrations with pH, and i16:0 and cy19:0, both of which decreased with pH. Some PLFAs responded differently to pH between the soil types, e.g. br18:0. We conclude that soil pH has a profound influence on the microbial PLFA composition, which must be considered in all applications of this method to detect changes in the microbial community.     

Microbial communities in forest floors under four tree species in coastal British Columbia

We compare forest floor microbial communities in pure plots of four tree species (Thuja plicata, Tsuga heterophylla, Pseudotsuga menziesii, and Picea sitchensis) replicated at three sites on Vancouver Island. Microbial communities were characterised through community level physiological profiles (CLPP), and profiling of phospholipid fatty acids (PLFA).Microbial communities from cedar forest floors had higher potential C utilisation than the other species. The F layer of the forest floor under cedar contained significantly higher bacterial biomass (PLFA) than the F layer under the other three tree species. There were differences in microbial communities among the three sites: Upper Klanawa had the highest bacterial biomass and potential C utilisation; this site also had the highest N availability in the forest floors. Forest floor H layers under hemlock and Douglas-fir contained greater biomass of Gram positive, Gram negative bacteria and actinomycetes than F layers based on PLFA, and H layers under spruce contained greater biomass of Gram negative bacteria than F layers. There were no significant differences in bacterial biomass between forest floor layers under cedar. Fungal biomass displayed opposite trends to bacteria and actinomycetes, being lowest in cedar forest floors, and highest in the F layer and at the site with lowest N availability. There were also differences in community composition among species and sites, with cedar forest floors having a much lower fungal:bacterial ratio than spruce, hemlock and Douglas-fir. The least fertile Sarita Lake site had a much greater fungal:bacterial ratio than the more fertile San Juan and Upper Klanawa sites. Forest floor layer had the greatest effect on microbial community structure and potential function, followed by site, and tree species. The similarity in trends among measures of N availability and microbial communities is further evidence that these techniques provide information on microbial communities that is relevant to N cycling processes in the forest floor.     

Composition of the microbial communities in the mineral soil under different types of natural forest

Phospholipid fatty acid (PLFA) patterns were used to describe the composition of the soil microbial communities under 12 natural forest stands including oak and beech, spruce-fir-beech, floodplain and pine forests. In addition to the quantification of total PLFAs, soil microbial biomass was measured by substrate-induced respiration and chloroform fumigation-extraction. The forest stands possess natural vegetation, representing an expression of the natural site factors, and we hypothesised that each forest type would support a specific soil microbial community. Principal component analysis (PCA) of PLFA patterns revealed that the microbial communities were compositionally distinct in the floodplain and pine forests, comprising azonal forest types, and were more similar in the oak, beech and spruce-fir-beech forests, which represent the zonal vegetation types of the region. In the nutrient-rich floodplain forests, the fatty acids 16:1ω5, 17:0cy, a15:0 and a17:0 were the most prevalent and soil pH seemed to be responsible for the discrimination of the soil microbial communities against those of the zonal forest types. The pine forest soils were set apart from the other forest soils by a higher abundance of PLFA 18:2ω6,9, which is typical of fungi and may also indicate ectomycorrhizal fungi associated with pine trees, and high amounts of PLFA 10Me18:0, which is common in actinomycetes. These findings suggest that the occurrence of azonal forest types at sites with specific soil conditions is accompanied by the development of specific soil microbial communities. The study provides information on the microbial communities in undisturbed forest soils which may facilitate interpretation of data derived from managed or even damaged or degraded forests.     

Exploring soil microbial communities and soil organic matter: Variability and interactions in arable soils under minimum tillage practice

This study describes an integrated approach (1) to monitor the quantity and quality of water extractable organic matter (WEOM) and size, structure and function of microbial communities in space (depth) and time, and (2) to explore the relationships among the measured properties. The study site was an arable field in Southern Germany under integrated farming management including reduced tillage. Samples of this Eutric Cambisol soil were taken in July 2001, October 2001, April 2002 and July 2002 and separated into three depths according to the soil profile (0–10 cm, 10–28 cm and 28–40 cm). For each sample, the quantity and quality (humification index, HIX) of water extractable organic matter (WEOM) were measured concomitantly with soil enzyme activities (alkaline phosphatase, β-glucosidase, protease) and microbial community size (C_mic). Furthermore, microbial community structure was characterised based on the fingerprints of nucleic acids (DNA) as well as phospholipid fatty acids (PLFA). We observed strong influences of sampling date and depth on the measured parameters, with depth accounting for more of the observed variability than date. Increasing depth resulted in decreases in all parameters, while seasonal effects differed among variants. Principal component (PC) analysis revealed that both DNA and PLFA fingerprints differentiated among microbial communities from different depths, and to a smaller extent, sampling dates. The majority of the 10 PLFAs contributing most to PC 1 were specific for anaerobes. Enzyme activities were strongly related to C_mic, which was depending on water extractable organic carbon and nitrogen (WEOC and WEON) but not to HIX. HIX and WEOM interact with the microbial community, illustrated by (1) the correlation with the number of PLFA peaks (community richness), and (2) the correlations with community PC analysis scores.     

Substrate inputs and pH as factors controlling microbial biomass, activity and community structure in an arable soil

Our aim was to determine whether the smaller biomasses generally found in low pH compared to high pH arable soils under similar management are due principally to the decreased inputs of substrate or whether some factor(s) associated with pH are also important. This was tested in a soil incubation experiment using wheat straw as substrate and soils of different pHs (8.09, 6.61, 4.65 and 4.17). Microbial biomass ninhydrin-N, and microbial community structure evaluated by phospholipid fatty acids (PLFAs), were measured at 0 (control soil only), 5, 25 and 50 days and CO₂ evolution up to 100 days. Straw addition increased biomass ninhydrin-N, CO₂ evolution and total PLFA concentrations at all soil pH values. The positive effect of straw addition on biomass ninhydrin-N was less in soils of pH 4.17 and 4.65. Similarly total PLFA concentrations were smallest at the lowest pH. This indicated that there is a direct pH effect as well as effects related to different substrate availabilities on microbial biomass and community structure. In the control soils, the fatty acids 16:1ω5, 16:1ω7c, 18:1ω7c&9t and i17:0 had significant and positive linear relationships with soil pH. In contrast, the fatty acids i15:0, a15:0, i16:0 and br17:0, 16:02OH, 18:2ω6,9, 17:0, 19:0, 17:0c9,10 and 19:0c9,10 were greatest in control soils at the lowest pHs. In soils given straw, the fatty acids 16:1ω5, 16:1ω7c, 15:0 and 18:0 had significant and positive linear relationships with pH, but the concentration of the monounsaturated 18:1ω9 PLFA decreased at the highest pHs. The PLFA profiles indicative of Gram-positive bacteria were more abundant than Gram-negative ones at the lowest pH in control soils, but in soils given straw these trends were reversed. In contrast, straw addition changed the microbial community structures least at pH 6.61. The ratio: [fungal PLFA 18:2w6,9]/[total PLFAs indicative of bacteria] indicated that fungal PLFAs were more dominant in the microbial communities of the lowest pH soil. In summary, this work shows that soil pH has marked effects on microbial biomass, community structure, and response to substrate addition.     

CO2 and inorganic N supply modify competition for N between co-occurring grass plants, tree seedlings and soil microorganisms

Plant-plant and plant-soil interactions play a key role in determining plant community structure and ecosystem function. However, the effects of global change on the interplay between co-occurring plants and soil microbes in successional communities are poorly understood. In this study, we investigated competition for nitrogen (N) between soil microorganisms, grass plants and establishing tree seedlings under factorial carbon dioxide (CO₂) and N treatments. Fraxinus excelsior seedlings were germinated in the presence or absence of grass competition (Dactylis glomerata) at low (380 μmol mol⁻¹) or high (645 μmol mol⁻¹) CO₂ and at two levels of N nutrition in a mesocosm experiment. Pulse ¹⁵N labelling was used to examine N partitioning among plant and soil compartments. Dactylis exerted a strong negative effect on Fraxinus biomass, N capture and ¹⁵N recovery irrespective of N and CO₂ treatment. In contrast, the presence of Dactylis had a positive effect on the microbial N pool. Plant and soil responses to N treatment were of a greater magnitude compared with responses to elevated CO₂, but the pattern of Fraxinus- and microbial-N pool response to N and CO₂ varied depending on grass competition treatment. Within the Dactylis competition treatment, decreases in Fraxinus biomass in response to N were not mirrored by decreases in tree seedling N content, suggesting a shift from below- to above-ground competition. In the Dactylis-sown pots, ¹⁵N recovery could be ranked Dactylis > microbial pool > Fraxinus in all N and CO₂ treatment combinations. Inequalities between Fraxinus and soil microorganisms in terms of ¹⁵N recovery were exacerbated by N addition. Contrary to expectations, elevated CO₂ did not increase plant-microbe competition. Nevertheless, microbial ¹⁵N recovery showed a small positive increase in the high CO₂ treatment. Overall, elevated CO₂ and N supply did not interact on plant/soil N partitioning. Our data suggest that the competitive balance between establishing tree seedlings and grass plants in an undisturbed sward is relatively insensitive to CO₂ or N-induced modifications in N competition between plant and soil compartments.     

Long-term exclusion of plant-inputs to soil reduces the functional capacity of microbial communities to mineralise recalcitrant root-derived carbon sources

Microbial communities in soil are highly species-rich, recognition of which has led to the view that functional redundancy within communities may buffer many impacts of altered community structure on soil functions. In this study we investigated the impact of long-term (>50 years) exclusion of plant-inputs (bare-fallow treatment) on soil microbial community structure and on the ability of the microbial biomass to mineralise tracer additions of ¹³C-labelled plant-derived C-substrates. Exclusion of plant-inputs resulted in depletion of soil organic matter (SOM) and a reduction in microbial biomass size. The microbial community structure was also strongly affected, as indicated by the distinct phospholipid fatty acid (PLFA) profiles in bare-fallow and grassland soils. Mineralisation of labile plant-derived substrates was not perturbed by the bare-fallow treatment. The incorporation of labile plant-derived C into PLFA biomarkers was found to differ between soils, reflecting the distinct community structures of the soils and indicating that these substrates were utilised by a broad range of microbial groups. In contrast, the mineralisation of recalcitrant plant-derived substrates was reduced in bare-fallow soil and the fate of substrate-derived C within PLFA biomarkers was, initially, similar between the soils. These results indicate that utilisation of these recalcitrant substrates was a function restricted to specific groups, and that exclusion of plant-derived inputs to soil had reduced the capacity of bare-fallow microbial communities to utilise this substrate type. Therefore, the study suggests that long-term selective pressure on microbial communities, resulting in altered community structure, may also result in altered functional attributes. This structure-function relationship was apparent for utilisation of recalcitrant plant-derived substrates, but not for the more widely distributed attribute of labile C-substrate utilisation.     

Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands

Several recent studies have indicated that an enriched atmosphere of carbon dioxide (CO₂) could exacerbate the intensity of plant invasions within natural ecosystems, but little is known of how rising CO₂ impacts the belowground characteristics of these invaded systems. In this study, we examined the effects of elevated CO₂ and nitrogen (N) inputs on plant and soil microbial community characteristics of plant communities invaded by reed canary grass, Phalaris arundinacea L. We grew the invasive grass under two levels of invasion: the invader was either dominant (high invasion) at >90% plant cover or sub-dominant (low invasion) at <50% plant cover. Experimental wetland communities were grown for four months in greenhouses that received either 600 or 365 μl l⁻¹ (ambient) CO₂. Within each of three replicate rooms per CO₂ treatment, the plant communities were grown under high (30 mg l⁻¹) or low (5 mg l⁻¹) N. In contrast to what is often predicted under N limitation, we found that elevated CO₂ increased native graminoid biomass at low N, but not at high N. The aboveground biomass of reed canary grass did not respond to elevated CO₂, despite it being a fast-growing C3 species. Although elevated CO₂ had no impact on the plant biomass of heavily invaded communities, the relative abundance of several soil microbial indicators increased. In contrast, the moderately invaded plant communities displayed increased total root biomass under elevated CO_2, while little impact occurred on the relative abundance of soil microbial indicators. Principal components analysis indicated that overall soil microbial community structure was distinct by CO₂ level for the varying N and invasion treatments. This study demonstrates that even when elevated CO₂ does not have visible effects on aboveground plant biomass, it can have large impacts belowground.     

Plant species influence microbial diversity and carbon allocation in the rhizosphere

Plant species effects on microbial communities are attributed to changes in microbial community composition and biomass, and may depend on plant species specific differences in the quality of resources (carbon) inputs. We examined the idea that plant-soil feedbacks can be explained by a chance effect, which is the probability of a highly productive or keystone plant species is present in the community and will influence the functions more than the number of species per se. A ¹³C pulse labelling technique was applied to three plant species and a species mixture in a greenhouse experiment to examine the carbon flow from plants to soil microbial communities. The ¹³C label was given as CO₂ to shoots of a legume (Lotus corniculatus), a forb (Plantago lanceolata), a grass (Holcus lanatus) and a mixture of the three species. Microbial phospholipid fatty acids (PLFA) was analysed in order to determine the biomass and composition of the soil microbial community. The incorporation of the stable isotope into soil microorganisms was determined through GC-IRMS analyses of the microbial PLFAs. Plant species identity did not influence the microbial biomass when determined as total carbon of microbial phospholipid fatty acids. However, the labelled carbon showed that the grass monoculture (H. lanatus) and the plant mixture allocated more ¹³C into bacteria and actinomycete biomass than the other plant species. H. lanatus monocultures had also the highest amounts of ¹³C allocated to AM-fungi and saprophytic fungi. The carbon allocation from plants to soil microorganisms in a plant species mixture can thus be explained by the presence of a highly productive species that influence soil functions.     

Soil Microbial Community Composition and Diversity in the Rhizosphere of a Chinese Medicinal Plant

Rehmannia glutinosa is an important medicinal plant, but there is a serious problem of decreasing productivity with its continuous cropping on the same land. We hypothesize some relationships between this problem and the disturbed soil ecosystem. In this work, two community‐based microbiological measurements, community‐level physiological profiling (CLPP) using Biolog sole carbon (C) source utilization tests and phospholipid ester–linked fatty acid (PLFA) profiles, were used to evaluate soil microbial community function and composition of different R. glutinosa cropping soils. Field investigation showed that the problems with continuous cropping occurred not only in 2‐year continuous fields but also in 5‐year rotation fields. Soil basal respiration and metabolic quotient were significantly greater in R. glutinosa cropping soils than in the noncropping controls. In contrast, the Shannon index from the Biolog data set was lower in R. glutinosa cropping soils. Both CLPP‐ and PLFA‐based principal component analyses (PCA) showed distinct groupings of soil microbial communities in R. glutinosa rhizosphere, and 11 PLFAs representing different microbes were identified from the principal component scores of PLFAs. Among these, an abundance of PLFA 18:2ω6,9, which is a biomarker of soil fungi, was significantly higher in R. glutinosa cropping soils than control soils. These results suggest an alteration of soil microbial community following R. glutinosa cropping, and this might be an important reason for the constraints associated with continuous cropping.     

Microbial community structure and characteristics of the organic matter in soils under Pinus sylvestris, Picea abies and Betula pendula at two forest sites

 Microbial biomass C (C_mic), C mineralization rate, phospholipid fatty acid (PLFA) profiles and community level physiological profiles (CLPPs) using Biolog were determined from the humus and mineral soil layers in adjacent stands of Scots pine (Pinus sylvestris L.), Norway spruce [Picea abies (L.) Karst.] and silver birch (Betula pendula Roth) at two forest sites of different fertility. In addition, the Fourier-transformed infrared (FTIR) spectra were run on the samples for characterization of the organic matter. C_mic and C mineralization rate tended to be lowest under spruce and highest under birch, at the fertile site in all soil layers and at the less fertile site in the humus layer. There were also differences in microbial community structure in soils under different tree species. In the humus layer the PLFAs separated all tree species and in the mineral soil spruce was distinct from pine and birch. CLPPs did not distinguish microbial communities from the different tree species. The FTIR spectra did not separate the tree species, but clearly separated the two sites. Received: 3 December 1999     

Changes in microbial community structure and function following Sphagnum peatland restoration

This study examines the recovery of the microbial compartment following active restoration of a North American ombrotrophic peatland extracted for horticultural peat-based substrates and restored by the Sphagnum moss transfer method. We used phospholipid fatty acids (PLFAs) to portrait the microbial community structure and Community Level Physiological Profiles (CLPP) to describe the functional diversity of the microbial communities. Our results indicate that the PLFA profiles were different between the beginning and the end of the growing season, but that it was impossible to distinguish five different vegetation classes found along the disturbance-recovery gradient on the basis of the microbial community structure. The pH, the cover of mosses, Ledum groenlandicum and Eriophorum vaginatum var. spissum were the best environmental predictors for the PLFA composition. The newly formed peat found in aerobic conditions beneath restored Sphagnum carpets had the highest decomposition capacity, whereas the lowest rates were found in the surface samples of non-restored conditions or in the deepest horizons of the natural samples. A large proportion of the variation in the physiological profiles was explained with variables related to the vegetation cover, the physicochemical environment and the microbial structure of the community, which is very promising for future monitoring studies. Overall, this study demonstrates that the recovery of particular plant groups, namely mosses and shrubs in restored peatlands might be the driver of changes occurring in the structure of the microbial communities in restored peatlands.     

Changes in microbial communities and carbohydrate profiles induced by the mycorrhizal fungus (Glomus intraradices) in rhizosphere of olive trees (Olea europaea L.)

The influence of inoculation of olive trees with arbuscular mycorrhizal (AM) fungi, Glomus (G) intraradices, on microbial communities and sugar concentrations, were examined in rhizosphere of olive trees (Olea europaea L.). Analyses of phospholipid and neutral lipid fatty acids (PLFA and NLFA, respectively) were then used to detect changes in microbial community structure in response to inoculation of plantlets with G. intraradices.Microscopic observations studies revealed that the extraradical mycelium of the fungus showed formation of branched absorbing structures (BAS) in rhizosphere of olive tree. Root colonization with the AM fungi G. intraradices induced significant changes in the bacterial community structure of olive tree rhizosphere compared to non-mycorrhizal plants. The largest proportional increase was found for the fatty acid 10Me18:0, which indicated an increase in the number of actinomycetes in mycorrhizal rhizosphere soil, whereas the PLFAs i15:0, a15:0, i16:0, 16:1ω7 and cy17:0 which were used as indicators of bacteria decreased in mycorrhizal treatment compared to non-mycorrhizal control treatment. A highest concentration of glucose and trehalose and a lowest concentration of fructose, galactose, sucrose, raffinose and mannitol were detected in mycorrhizal rhizosphere soil. This mycorrhizal effect on rhizosphere communities may be a consequence of changes in characteristics in the environment close to mycorrhizal roots.     

磷脂脂肪酸分析方法及其在土壤微生物多样性研究中的应用

磷脂脂肪酸（PLFA）是活体微生物细胞膜的重要组分，不同类群的微生物可通过不同的生化途径合成不同的PLFA。一些PLFA可作为分析微生物量和微生物群落结构变化的“生物标记”。在土壤微生物分析中，越来越多地采用了PLFA方法。本文介绍了表征微生物的一些PLFA、从土壤中提取PLFA的方法以及用GC-MS分析PLFA的原理。本文还将常用的研究微生物多样性的几种方法进行了比较；传统的分析土壤微生物群落的方法依赖于培养技术，只能培养和分离出一小部分微生物群落；Biolog方法可用于研究土壤微生物群落生理多样性，即可反映微生物群落如何利用各种碳源底物，但对快速生长和适合在Biolog实验条件下生长的小部分群落成员有强烈的选择性；核酸分析方法的主要缺点是不能对土壤微生物进行定量分析；而PLFA方法相对于上述几种方法有诸多优势。本文通过一些实例证明PLFA方法的特色或独到之处，也分析了其缺点。采用PLFA方法并结合其他方法有助于获取土壤微生物群落多样性的更多和更全面而完整的信息。     

Response of microbial community composition and activity in agricultural and grassland soils after a simulated rainfall

Rainfall in Mediterranean climates may affect soil microbial processes and communities differently in agricultural vs. grassland soils. We explored the hypothesis that land use intensification decreases the resistance of microbial community composition and activity to perturbation. Soil carbon (C) and nitrogen (N) dynamics and microbial responses to a simulated Spring rainfall were measured in grassland and agricultural ecosystems. The California ecosystems consisted of two paired sets: annual vegetable crops and annual grassland in Salinas Valley, and perennial grass agriculture and native perennial grassland in Carmel Valley. Soil types of the respective ecosystem pairs were derived from granitic parent material and had sandy loam textures. Intact cores (30 cm deep) were collected in March 1999. After equilibration, dry soil cores (approx. −1 to −2 MPa) were exposed to a simulated Spring rainfall of 2.4 cm, and then were measured at 0, 6, 24, and 120 h after rewetting. Microbial biomass C (MBC) and inorganic N did not respond to rewetting. N₂O and CO₂ efflux and respiration increased after rewetting in all soils, with larger responses in the grassland than in the agricultural soils. Phospholipid fatty acid (PLFA) profiles indicated that changes in microbial community composition after rewetting were most pronounced in intensive vegetable production, followed by the relict perennial grassland. Changes in specific PLFA markers were not consistent across all sites. There were more similarities among microbial groups associated with PLFA markers in agricultural ecosystems than grassland ecosystems. Differences in responses of microbial communities may be related to the different plant species composition of the grasslands. Agricultural intensification appeared to decrease microbial diversity, as estimated from numbers of individual PLFA identified for each ecosystem, and reduce resistance to change in microbial community composition after rewetting. In the agricultural systems, reductions in both the measures of microbial diversity and the resistance of the microbial community composition to change after a perturbation were associated with lower ecosystem function, i.e. lower microbial responses to increased moisture availability.     

Impact of coloniser plant species on the development of decomposer microbial communities following deglaciation

The effects of coloniser plant species on microbial community growth and composition were investigated on recently deglaciated terrain at Glacier Bay, south-east Alaska. Analysis of microbial communities using phospholipids fatty acid analysis (PLFA) revealed that Alnus and Rhacomitrium had the greatest impact on microbial growth, increasing total PLFA by some 6-7 fold relative to bare soil, whereas Equisetum led to a 5.5 fold increase in total PLFA relative to bare soil. These coloniser species also had significant effects on the composition of their associated microbial communities. Rhacomitrium, Alnus, and Equisetum increased bacterial PLFA, a measure of bacterial biomass, relative to bare soil. Rhacomitrium and Alnus also dramatically increased the concentration of the fungal fatty 18:2ω6 in soil relative to bare soil, by 12-fold and 8-fold, respectively. The net effect of the above changes was a significant increase in the ratio of fungal: bacterial fatty acids in soil associated with Alnus and Rhacomitrium, but not Equisetum. Possible reasons for these effects of particular plants on microbial communities are discussed, as is their significance in relation to the development of microbial communities in relatively sterile, recently deglaciated ground.