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1.
The long-term impact of tillage and residue management on soil microorganisms was studied over the growing season in a sandy loam to loamy sand soil of southwestern Quebec, growing maize (Zea mays L.) monoculture. Tillage and residue treatments were first imposed on plots in fall 1991. Treatments consisted of no till, reduced tillage, and conventional tillage with crop residues either removed from (−R) or retained on (+R) experimental plots, laid out in a randomized complete block design. Soil microbial biomass carbon (SMB-C), soil microbial biomass nitrogen (SMB-N) and phospholipid fatty acid (PLFA) contents were measured four times, at two depths (0-10 and 10-20 cm), over the 2001 growing season. Sample times were: May 7 (preplanting), June 25, July 16, and September 29 (prior to corn harvest). The effect of time was of a greater magnitude than those attributed to tillage or residue treatments. While SMB-C showed little seasonal change (160 μg C g−1 soil), SMB-N was responsive to post-emergence mineral nitrogen fertilization, and PLFA analysis showed an increase in fungi and total PLFA throughout the season. PLFA profiles showed better distinction between sampling time and depth, than between treatments. The effect of residue was more pronounced than that of tillage, with increased SMB-C and SMB-N (61 and 96%) in +R plots compared to −R plots. This study illustrated that measuring soil quality based on soil microbial components must take into account seasonal changes in soil physical and chemical conditions.  相似文献   

2.
There is growing interest in the potential of applying an electric field to soil to move and stimulate the degradation of contaminants, but we know little of the impact of this approach on exposed microbial communities. The effect of electrokinetics (3.14 A m−2) on soil bacterial and fungal communities was studied using soil cartridge microcosms (13 cm×5.4 cm×5.9 cm). After 27 days of electrokinetics, a zone of low pH (<4) was detected close to the anode. Soil exposed to electrokinetics and immediately adjacent to the anode demonstrated an increase in carbon substrate utilisation potential (≤290%) and microbial respiration rates. The diversity and structure of the bacterial community showed little response to electrokinetics, with the exception of soil close to the anode. Here, an increase in the percentage of Gram-positive species isolated was identified, most notably of Bacillus megaterium. Overall, the only detectable response of the microbial community was observed in soil immediately adjacent to the anode. The results of this study provide evidence that the application of electrokinetics has no serious negative effect on ‘soil microbial health’, thus endorsing its validity as a viable soil remediation technology.  相似文献   

3.
The microcalorimetric technique was used to analyse the influence of successive reforestations with Eucalyptus globulus Labill, a fast growing species, on the microbial activity in soil. With this aim, samples of similar origin soils, humic-eutrophic Cambisol, were collected from two adjacent land plots. One of the plots, to be taken as the reference, was not subjected to man activities for the last 100 years. The other plot was subjected to two reforestations in the last 20 years. For collection of the two samples, 100 m2 homogeneous and perfectly defined zones were chosen in each of the adjacent land plots to obtain final representative bulk samples of 400 g each.The study was carried out using the microbial population as a bioindicator of the productive potential in soil. The experiments were carried out using 1 g soil samples that were treated with 1.25 mg glucose and stored at 4 °C for 3 months. The analysis was complemented by the determination of physical, chemical, and biological properties and environmental parameters.The study was performed over 1 year and samples were seasonally collected (spring, summer, autumn and winter) in the same zone situated in Viveiro (Galicia, NW Spain) with the objective of checking the influence of environmental conditions on the microbial activity in the two soils studied.From the measured results, it can be observed that soils subjected to reforestations suffer not only important changes in physical structure, such as an increase in bulk density, from 600 to 660 kg m−3, a reduction in hydraulic conductivity, from 8.85×10−3 to 4.4×10−3 m s−1, or a decrease in the C-to-N ratio, from 13 to 8, but also they have lower microbial activity, for example 2.84 J g−1 in the reforested soil versus 6.14 J g−1 in the reference soil, in spring, thus presenting a lower productive potential that suggests the use of both amendments and adequate management techniques to ensure a sustainable exploitation, thus avoiding future degradation of soil.  相似文献   

4.
The impact of secondary succession of grassland communities towards a Norway spruce forest on soil microbial community was studied on a belt transect established in the Pol’ana Mts., Central Europe. Data on understory vegetation, light availability, soil properties and microbial activity were collected on 147 plots distributed over regular grid. Moreover, distributions of functional groups of microorganisms were assessed using BIOLOG analysis on a subset of 27 plots. Mantel partial correlations between microbial community indicators and environmental variables showed that microbial activity generally decreased with increasing tree density and size, whereas it increased with increasing radiation at the soil surface, soil temperature, and cover and diversity of understory vegetation. Functional richness and diversity of microorganisms were positively correlated with solar radiation, but also with plant species richness and diversity. Abundance of several functional groups correlated closely with succession-related variables. Redundance analysis of microbial data provided slightly different outcomes. Forward selection yielded only two environmental variables significantly influencing the composition of the microbial community: tree influence potential and organic carbon content. Abundances of several functional microbial groups correlated with tree influence, documenting that microbial community changes are at least partially driven by the colonization of grassland by trees. Nevertheless, the relative importance of abiotic environment change and plant community succession on microbial community dynamics remains unresolved.  相似文献   

5.
The relationship between biodiversity and ecosystem functioning is of major scientific concern today. Few studies though have measured the interactions between soil microorganisms and plant diversity, the purpose of this study was to examine the link between plant diversity and microbial communities in fertilized versus unfertilized grasslands. Experiments were carried out on a permanent grassland in north-eastern France where agricultural practices had remained unchanged for the last 13 years. The experimental design included two plots of 300 m2 (fertilized at 120 kg N ha−1 or non-fertilized). Plots were replicated into three equal sub-plots (100 m2). From each sub-plot, six samples of soil and vegetation were taken at three dates during floristic development. At sampling, ground cover of each species was estimated, and total amount of C and N was determined in aboveground and root biomass. Soil samples were analyzed in order to measure the metabolic fingerprints of microorganisms using Biolog® GN2 microplates. Floristic composition and carbon substrate utilization patterns of rhizobacterial communities were more diversified in unfertilized than fertilized plots. In unfertilized plots, the development of Convolvulus arvensis and two legumes (Trifolium pratense and Trifolium repens) may help maintain observed floristic diversity. Moreover, an inversion of C and N distribution between aboveground and root biomass during the vegetation cycle probably induced a variation of rhizodeposition. This phenomenon could explain the differences of rhizobacterial metabolic fingerprints observed between experimental plots.  相似文献   

6.
The use of composts in agricultural soils is a widespread practice and the positive effects on soil and plants are known from numerous studies. However, there have been few attempts to compare the effects of different kinds of composts in one single study. The aim of this paper is to investigate to what extent and to which soil depth four major types of composts would affect the soil and its microbiota.In a crop-rotation field experiment, composts produced from (i) urban organic wastes, (ii) green wastes, (iii) manure and (iv) sewage sludge were applied at a rate equivalent to 175 kg N ha−1 yr−1 for 12 years. General (total organic C (Corg), total N (Nt), microbial biomass C (Cmic), and basal respiration), specific (enzyme activities related to C, N and P cycles), biochemical properties and bacterial genetic diversity (based on DGGE analysis of 16S rDNA) were analyzed at different depths (0-10, 10-20 and 20-30 cm).Compost treatment increased Corg at all depths from 11 g kg−1 for control soil to 16.7 g kg−1 for the case of sewage sludge compost. Total N increased with compost treatment at 0-10 cm and 10-20 cm depths, but not at 20-30 cm. Basal respiration and Cmic declined with depth, and the composts resulted in an increase of Cmic and basal respiration. Enzyme activities were different depend on the enzyme and among compost treatments, but in general, the enzyme activities were higher in the upper layers (0-10 and 10-20 cm) than in the 20-30 cm layer. Diversity of ammonia oxidizers and bacteria was lower in the control than in the compost soils. The type of compost had less influence on the composition of the microbial communities than did soil depth.Some of the properties were sensitive enough to distinguish between different compost, while others were not. This stresses the need of multi-parameter approaches when investigating treatment effects on the soil microbial community. In general, with respect to measures of activity, biomass and community diversity, differences down the soil profile were more pronounced than those due to the compost treatments.  相似文献   

7.
Water availability strongly affects soil microbial activity and community composition. In a laboratory incubation we investigated the combined effect of soil moisture potential (−10 kPa, −135 kPa, and <−1500 kPa) and plant residue addition on soil enzyme activities (protease, β-glucosidase, β-glucosaminidase and exocellulase) and phospholipid fatty acid (PLFA) profiles. Soil respiration was positively correlated with soil moisture potential and significantly increased with the addition of residue. In the unamended soil, enzyme activities were little affected by soil moisture potential, nor did they change much over time. The addition of residue, however, significantly increased enzyme activity at each moisture level. Furthermore, all four enzyme activities were considerably higher in the amended dry soil than in amended samples with a higher moisture potential. In contrast, in the amended dry soil, respiration and microbial biomass were reduced compared to the amended samples with a higher moisture potential. The low microbial biomass in the amended dry soil was mainly due to a decrease in Gram-negative bacteria, while the fungal biomass reached similar levels at all water potentials. Therefore, shifts in microbial community composition alone cannot explain the increased enzyme activities in the dry soil. Other factors, such as increased fungal activity, stronger interactions between enzymes and soil particles due to thinner water films, may have contributed to the observed effects. Our results suggest that under dry conditions, potential enzyme activities may be decoupled from microbial biomass and respiration in the presence of substrates.  相似文献   

8.
We studied, under two different plant compositions, the short-term effects of glyphosate on rhizosphere soil microbial communities through the utilization of cultivation-dependent and -independent techniques. A short-term pot study was carried out using factorial treatments that included two different compositions of forage plant species (triticale versus a mixture of triticale and pea) and two concentrations of glyphosate (50 and 500 mg active ingredient kg−1 soil, as a commercial formulation, Roundup Plus) arranged in a completely randomized design experiment with four replicates. Control plants (no glyphosate added) were clipped in an attempt to compare two methods of weed control (manual = clipping; chemical = herbicide treatment). Rhizosphere soil was sampled 15 and 30 days after glyphosate treatment and the following soil components were determined: potentially mineralizable nitrogen, ammonium content, community-level physiological profiles using Biolog Ecoplates™, DNA microbial biomass and genotype diversity by means of PCR-DGGE. Fifteen days after herbicide treatment, a glyphosate-induced stimulation of the activity and functional diversity of the cultivable portion of the heterotrophic soil microbial community was observed, most likely due to glyphosate acting as an available source of C, N and P. On the other hand, 30 days after herbicide treatment, both the activity and diversity of the rhizosphere soil microbial communities showed an inconsistent response to glyphosate addition. Apart from its intended effect on plants, glyphosate had non-target effects on the rhizosphere soil microbial community which were, interestingly, more enhanced in triticale than in “triticale + pea” pots. Biolog™ was more sensitive than PCR-DGGE to detect changes in soil microbial communities induced by glyphosate and plant composition.  相似文献   

9.
Characterizing functional and phylogenetic microbial community structure in soil is important for understanding the fate of microbially-derived compounds during the decomposition and turn-over of soil organic matter. This study was conducted to test whether amino sugars and muramic acid are suitable biomarkers to trace bacterial, fungal, and actinomycetal residues in soil. For this aim, we investigated the pattern, amounts, and dynamics of three amino sugars (glucosamine, mannosamine and galactosamine) and muramic acid in the total microbial biomass and selectively cultivated bacteria, fungi, and actinomycetes of five different soils amended with and without glucose. Our results revealed that total amino sugar and muramic acid concentrations in microbial biomass, extracted from soil after chloroform fumigation varied between 1 and 27 mg kg−1 soil. In all soils investigated, glucose addition resulted in a 50-360% increase of these values. In reference to soil microbial biomass-C, the total amino sugar- and muramic acid-C concentrations ranged from 1-71 g C kg−1 biomass-C. After an initial lag phase, the cultivated microbes revealed similar amino sugar concentrations of about 35, 27 and 17 g glucosamine-C kg−1 TOC in bacteria, fungi, and actinomycetes, respectively. Mannosamine and galactosamine concentrations were lower than those for glucosamine. Mannosamine was not found in actinomycete cultures. The highest muramic acid concentrations were found in bacteria, but small amounts were also found in actinomycete cultures. The concentrations of the three amino sugars studied and muramic acid differed significantly between bacteria and the other phylogenetic microbial groups under investigation (fungi and actinomycetes). Comparison between the amino sugar and muramic acid concentrations in soil microbial biomass, extracted after chloroform fumigation, and total concentrations in the soil showed that living microbial biomass contributed negligible amounts to total amino sugar contents in the soil, being at least two orders of magnitude greater in the soils than in the soil inherent microbial biomass. Thus, amino sugars are significantly stabilized in soil.  相似文献   

10.
In forest ecosystems, the external nitrogen (N) inputs mainly involve wet and dry depositions that potentially alter inorganic N availability in the soil and carbon (C) turnover. This study assesses the effect of a slow increase of inorganic N availability on microbial community activity and functionality in a Mediterranean forest soil. A four-month incubation experiment was performed with soil collected from the organic layer of a forest site and fertilized with a solution of ammonium nitrate. The fertilizer was supplied at an equivalent of 0, 10, 25, 50 and 75 kg N ha−1 (0, 0.3, 0.7, 1.3 and 2 mg N g−1 for control N0 and treatments N1, N2, N3 and N4, respectively). The incubation was carried out under optimal conditions, with the addition of the nutritive solution in small aliquots once a week to mimic the phenomenon of N deposition. In order to isolate the effect of N, the pH of the NH4NO3 solutions was adjusted to soil pH, and phosphorus was added in order to prevent any nutrient limitation effect. Inorganic N, C-mineralization, the activity of one oxidative enzyme (o-diphenol oxidase) and 8 hydrolitic enzymes (α-glucosidase, β-glucosidase, N-acetyl-β-d-glucosaminidase, cellulase, leucine amino-peptidase, acid phosphatase, butyric esterase and β-xylosidase) and the community level physiological profile (CLPP) were measured and analyzed during the whole incubation and at the end of the experiment as a proxy for microbial decomposition activity. In the first month, the highest N availability (N4) repressed the microbial respiration activity but stimulated microbial enzymatic activity, suggesting a change of C-pathways from spilling to enzymes and biomass investment. The treatments N1, N2 and N3 had no effect in the same period. Throughout the incubation, a general stress condition affected all the treated soils. As a consequence, treated soils exhibited higher respiration rates than the control. This was accompanied by a loss of functional diversity and an end-detected decline in biomass C. Although at the end of incubation most of the soil features showed a clear correlation with the inorganic N pool, the organic C content was strongly affected by different patterns of microbial activity during the experiment: the highest N treatment (N4) showed a lower C loss than the N3 treatment. Overall, the experiment showed how inorganic N availability can potentially alter the C cycle in a Mediterranean forest soil. The effect is non linear, depending on microbial community dynamics, on the community’s ability to adapt given the time scale of the process, and on N supply amount. Our study also revealed a common pattern in the short-term response to N addition in other, similar ecosystems with different climatic conditions.  相似文献   

11.
Municipal solid waste (MSW) composts have been used to maintain the long-term productivity of agroecosystems and to protect the soil environment from overcropping, changes in climatic conditions and inadequate management; they also have the additional benefit of reducing waste disposal costs. Since MSW may contain heavy metals and other toxic compounds, amendments cannot only influence soil fertility, but may also affect the composition and activity of soil microorganisms. The effects of MSW compost and mineral N amendments in a 6-year field trial on some physical-chemical properties, enzyme activities and bacterial genetic diversity of cropped plots (Beta vulgaris-Triticum turgidum rotation) and uncropped plots were investigated. The compost was added at the recommended and twice the recommended dosage (12, 24 t ha−1). Amendments of cropped plots with MSW compost increased the contents of organic C from 13.3 to 15.0 g kg−1 soil and total N from 1.55 to 1.65 g kg−1 soil. There were significant increases in dehydrogenase (9.6%), β-glucosidase (13.5%), urease (15.4%), nitrate reductase (21.4%) and phosphatase (9.7%) activities. A significant reduction in protease activity (from 3.6 to 2.8 U g−1 soil) was measured when a double dose of compost was added to the cropped plots. No dosage effect was detected for the other enzymes. Changes in the microbial community, as a consequence of MSW amendment, were minimal as determined using denaturing gradient gel electrophoresis, rDNA internal spacer analysis and amplified ribosomal DNA restriction analysis of bacteria, archaea, actinomycetes, and ammonia oxidizers. This indicates that there was no significant variation in the overall bacterial communities nor in selected taxonomic groups deemed to be essential for soil fertility.  相似文献   

12.
The concentrations of organic C, labile organic fractions and the size and activity of the microbial community were measured to a depth of 30 cm below the plant row and at distances of 30 and 60 cm into the inter-row area under sugarcane under pre-harvest burning or green cane harvesting with retention of a crop residue (trash) mulch. Total root mass was similar under burning and trashing but under trashing there was a redistribution of roots towards the surface 0-10 cm in the inter-row space as roots proliferated beneath the trash mulch. Soil organic C content decreased in response to both increasing distance from the plant row (to a depth of 20 cm) and burning rather than trashing (to a depth of 10 cm). Declines in K2SO4-extractable C, light fraction C, microbial biomass C, basal respiration and aggregate stability in response to distance and burning were much more marked than those for organic C and occurred to a depth of 30 cm. Bulk density was greater under burnt than trashed sugarcane and was greater in the inter-row than row, particularly under burning. Heterotrophic functional diversity (measured by analysis of catabolic response profiles to 36 substrates) was also investigated. Principal component analysis of response profiles demonstrated that soils below the row and those under trashing at 30 cm out from this row were separated from the other soils on PC1 and the sample from the inter-row centre (60 cm out) under burning was separated from the others on PC2. Catabolic evenness was least for the latter soil. It was concluded that soil in the inter-row of burnt sugarcane receives few inputs of organic matter and that conversion to green cane harvesting with retention of a trash mulch greatly improves the organic matter, microbial and physical status of the inter-row soil.  相似文献   

13.
Changes in plant community structure, including the loss of plant diversity may affect soil microbial communities. To test this hypothesis, plant diversity and composition were experimentally varied in grassland plots cultivated with monocultures or mixtures of 2, 3 or 4 species. We tested the effects of monocultures versus mixtures and of plant species composition on culturable soil bacterial activity, number of substrates used and catabolic diversity, microbial biomass N, microbial respiration, and root biomass. These properties were all measured 10 months after seeding the experiment. Soil bacterial activity, number of substrates used and catabolic diversity were measured in the different plant communities using BIOLOG GN and GP microplates, which are redox-based tests measuring capacity of soil culturable bacteria to use a variety of organic substrates. Microbial biomass N, microbial respiration, and root biomass were insensitive to plant diversity. Culturable soil microbial activity, substrates used and diversity declined with declining plant diversity. Their activity, number of substrates used and diversity were significantly higher in plots with 3 and 4 plant species than in monocultures and in plots with 2 species. There was also an effect of plant species composition. Culturable soil microbial activity and diversity was higher in the four-species plant community than in any of the plant monocultures suggesting that the effect of plant diversity could not be explained by the presence of a particular plant species. Our results showed that changes in plant diversity and composition in grassland ecosystems lead to a rapid response of bacterial activity and diversity.  相似文献   

14.
The impact of DNA extraction protocol on soil DNA yield and bacterial community composition was evaluated. Three different procedures to physically disrupt cells were compared: sonication, grinding-freezing-thawing, and bead beating. The three protocols were applied to three different topsoils. For all soils, we found that each DNA extraction method resulted in unique community patterns as measured by denaturing gradient gel electrophoresis. This indicates the importance of the DNA extraction protocol on data for evaluating soil bacterial diversity. Consistently, the bead-beating procedure gave rise to the highest number of DNA bands, indicating the highest number of bacterial species. Supplementing the bead-beating procedure with additional cell-rupture steps generally did not change the bacterial community profile. The same consistency was not observed when evaluating the efficiency of the different methods on soil DNA yield. This parameter depended on soil type. The DNA size was of highest molecular weight with the sonication and grinding-freezing-thawing procedures (approx. 20 kb). In contrast, the inclusion of bead beating resulted in more sheared DNA (approx. 6-20 kb), and the longer the bead-beating time, the higher the fraction of low-molecular weight DNA. Clearly, the choice of DNA extraction protocol depends on soil type. We found, however, that for the analysis of indigenous soil bacterial communities the bead-beating procedure was appropriate because it is fast, reproducible, and gives very pure DNA of relatively high molecular weight. And very importantly, with this protocol the highest soil bacterial diversity was obtained. We believe that the choice of DNA extraction protocol will influence not only the determined phylogenetic diversity of indigenous microbial communities, but also the obtained functional diversity. This means that the detected presence of a functional gene—and thus the indication of enzyme activity—may depend on the nature of the applied DNA extraction procedure.  相似文献   

15.
Long-term diversity-disturbance responses of soil bacterial communities to copper were determined from field-soils (Spalding; South Australia) exposed to Cu in doses ranging from 0 through to 4012 mg Cu kg−1 soil. Nearly 6 years after application of Cu, the structure of the total bacterial community showed change over the Cu gradient (PCR-DGGE profiling). 16S rRNA clone libraries, generated from unexposed and exposed (1003 mg Cu added kg−1 soil) treatments, had significantly different taxa composition. In particular, Acidobacteria were abundant in unexposed soil but were nearly absent from the Cu-exposed sample (P<0.05), which was dominated by Firmicute bacteria (P<0.05). Analysis of community profiles of Acidobacteria, Bacillus, Pseudomonas and Sphingomonas showed significant changes in structural composition with increasing soil Cu. The diversity (Simpsons index) of the Acidobacteria community was more sensitive to increasing concentrations of CaCl-extractable soil Cu (CuExt) than other groups, with decline in diversity occurring at 0.13 CuExt mg kg−1 soil. In contrast, diversity in the Bacillus community increased until 10.4 CuExt mg kg−1 soil, showing that this group was 2 orders of magnitude more resistant to Cu than Acidobacteria. Sphingomonas was the most resistant to Cu; however, this group along with Pseudomonas represented only a small percentage of total soil bacteria. Changes in bacterial community structure, but not diversity, were concomitant with a decrease in catabolic function (BioLog). Reduction in function followed a dose-response pattern with CuExt levels (R2=0.86). The EC50 for functional loss was 0.21 CuExt mg kg−1 soil, which coincided with loss of Acidobacteria diversity. The microbial responses were confirmed as being due to Cu and not shifts in soil pH (from use of CuSO4) as parallel Zn-based field plots (ZnSO4) were dissimilar. Changes in the diversity of most bacterial groups with soil Cu followed a unimodal response - i.e. diversity initially increased with Cu addition until a critical value was reached, whereupon it sharply decreased. These responses are indicative of the intermediate-disturbance-hypothesis, a macroecological theory that has not been widely tested in environmental microbial ecosystems.  相似文献   

16.
Carbon (C) and nitrogen (N) fluxes are largely controlled by the small but highly bio-reactive, labile pools of these elements in terrestrial soils, while long-term C and N storage is determined by the long-lived recalcitrant fractions. Changes in the size of these pools and redistribution among them in response to global warming may considerably affect the long-term terrestrial C and N storage. However, such changes have not been carefully examined in field warming experiments. This study used sulfuric acid hydrolysis to quantify changes in labile and recalcitrant C and N fractions of soil in a tallgrass prairie ecosystem that had been continuously warmed with or without clipping for about 2.5 years. Warming significantly increased labile C and N fractions in the unclipped plots, resulting in increments of 373 mg C kg−1 dry soil and 15 mg N kg−1 dry soil, over this period whilst clipping significantly decreased such concentrations in the warmed plots. Warming also significantly increased soil microbial biomass C and N in the unclipped plots, and increased ratios of soil microbial/labile C and N, indicating an increase in microbial C- and N-use efficiency. Recalcitrant and total C and N contents were not significantly affected by warming. For all measured pools, only labile and microbial biomass C fractions showed significant interactions between warming and clipping, indicating the dependence of the warming effects on clipping. Our results suggest that increased soil labile and microbial C and N fractions likely resulted indirectly from warming increases in plant biomass input, which may be larger than warming-enhanced decomposition of labile organic compounds.  相似文献   

17.
Soil profiles are often many meters deep, but with the majority of studies in soil microbiology focusing exclusively on the soil surface, we know very little about the nature of the microbial communities inhabiting the deeper soil horizons. We used phospholipid fatty acid (PLFA) analysis to examine the vertical distribution of specific microbial groups and to identify the patterns of microbial abundance and community-level diversity within the soil profile. Samples were collected from the soil surface down to 2 m in depth from two unsaturated Mollisol profiles located near Santa Barbara, CA, USA. While the densities of microorganisms were generally one to two orders of magnitude lower in the deeper horizons of both profiles than at the soil surface, approximately 35% of the total quantity of microbial biomass found in the top 2 m of soil is found below a depth of 25 cm. Principal components analysis of the PLFA signatures indicates that the composition of the soil microbial communities changes significantly with soil depth. The differentiation of microbial communities within the two profiles coincides with an overall decline in microbial diversity. The number of individual PLFAs detected in soil samples decreased by about a third from the soil surface down to 2 m. The ratios of cyclopropyl/monoenoic precursors and total saturated/total monounsaturated fatty acids increased with soil depth, suggesting that the microbes inhabiting the deeper soil horizons are more carbon limited than surface-dwelling microbes. Using PLFAs as biomarkers, we show that Gram-positive bacteria and actinomycetes tended to increase in proportional abundance with increasing soil depth, while the abundances of Gram-negative bacteria, fungi, and protozoa were highest at the soil surface and substantially lower in the subsurface. The vertical distribution of these specific microbial groups can largely be attributed to the decline in carbon availability with soil depth.  相似文献   

18.
Canola crops have been shown to inhibit soil-borne pathogens in following crops. This effect is mainly attributed to the release of low molecular S-containing compounds, such as isothiocyanates, during microbial degradation of the crop residues. We have assessed the effect of low concentrations of phenylethylisothiocyanate (PEITC) on soil microbial communities as well as its rate of degradation in soil and determined the concentration of PEITC and the microbial community structure in the rhizosphere of canola. PEITC was degraded within 96 h by soil microorganisms. PEITC added to the soil daily for 5 d affected both bacterial and eukaryotic community structure, determined by PCR-DGGE. Community structures of bacteria and eukaryotes changed at PEITC concentrations between 1300 and 3790 pmol g−1 soil fresh weight but was unaffected at lower concentrations. The PEITC concentration in the rhizosphere of living canola roots was greater in first order laterals than in second order laterals. The maximal PEITC concentration detected in the rhizosphere was 1827 pmol g−1. Redundancy analysis of the DGGE banding patterns indicated a significant correlation between the PEITC concentration in the rhizosphere and the community structure of the active fraction of eukaryotes and bacteria in the rhizosphere. Other important factors influencing the microbial community structure were soil moisture and plant dry matter. It is concluded that canola may affect the soil microbial community structure not only after incorporation of canola residues but also during active growth of the plants.  相似文献   

19.
Although elevation of CO2 has been reported to impact soil microbial functions, little information is available on the spatial and temporal variation of this effect. The objective of this study was to determine the microbial response in a northern Colorado shortgrass steppe to a 5-year elevation of atmospheric CO2 as well as the reversibility of the microbial response during a period of several months after shutting off the CO2 amendment. The experiment was comprised of nine experimental plots: three chambered plots maintained at ambient CO2 levels of 360 μmol mol−1 (ambient treatment), three chambered plots maintained at 720 μmol mol−1 CO2 (elevated treatment) and three unchambered plots of equal ground area used as controls to monitor the chamber effect.Elevated CO2 induced mainly an increase of enzyme activities (protease, xylanase, invertase, alkaline phosphatase, arylsulfatase) in the upper 5 cm of the soil and did not change microbial biomass in the soil profile. Since rhizodeposition and newly formed roots enlarged the pool of easily available substrates mainly in the upper soil layers, enzyme regulation (production and activity) rather than shifts in microbial abundance was the driving factor for higher enzyme activities in the upper soil. Repeated soil sampling during the third to fifth year of the experiment revealed an enhancement of enzyme activities which varied in the range of 20-80%. Discriminant analysis including all microbiological properties revealed that the enzyme pattern in 1999 and 2000 was dominated by the CO2 and chamber effect, while in 2001 the influence of elevated CO2 increased and the chamber effect decreased.Although microbial biomass did not show any response to elevated CO2 during the main experiment, a significant increase of soil microbial N was detected as a post-treatment effect probably due to lower nutrient (nitrogen) competition between microorganisms and plants in this N-limited ecosystem. Whereas most enzyme activities showed a significant post-CO2 effect in spring 2002 (following the conclusion of CO2 enrichment the previous autumn, 2001), selective depletion of substrates is speculated to be the cause for non-significant treatment effects of most enzyme activities later in summer and autumn, 2002. Therefore, additional belowground carbon input mainly entered the fast cycling carbon pool and contributed little to long-term carbon storage in the semi-arid grassland.  相似文献   

20.
Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO2 (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.  相似文献   

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