Vadose Zone Microbial Biobarriers Remove Nitrate from Percolating Groundwater

Microbial biobarriers are an established technique for cleansing contaminants from aquifers. This study evaluated their use under well-drained conditions within the vadose or unsaturated zone. Three sets of sand filled columns, the positive control, field-capacity, and sub-field-capacity groups, contained biobarriers formed by mixing sand with sawdust and soybean oil. The biobarriers were positioned 1 m from the top of the 145 cm columns. A fourth set of column, the negative control, contained no biobarrier. The positive control group’s biobarriers were saturated while biobarriers in the other groups were allowed to drain. At intervals water containing 20 mg l⁻¹ NO₃⁻–N was applied to the columns, the water was allowed to percolate through the columns, and the effluents were collected and analyzed. The biobarriers were highly effective at removing NO₃⁻. NO₃⁻–N in the effluents from the field-capacity, sub-field-capacity, and positive control groups averaged 0.4 ± 0.1, 0.6 ± 0.1, and 0.8 ± 0.1 mg l⁻¹, respectively, during the final weeks of the study while effluents from the negative control group averaged 17.9 ± 0.4 mg l⁻¹. The barriers removed NO₃⁻ even when the water content was in the 20–40% pore filled space range. During the 12-week study the field-capacity barriers lost 5.6% of their organic content while those in the sub-field-capacity group lost no detectable organic matter indicating that the barriers contained sufficient substrate to last for several years. Vadose zone biobarriers could provide a useful means of protecting surface waters and aquifers from NO₃⁻.     

Environmental Factors That Control Microbial Perchlorate Reduction

As part of a study to elucidate the environmental parameters that control microbial perchlorate respiration, we investigated the reduction of perchlorate by the dissimilatory perchlorate reducer Dechlorosoma suillum under a diverse set of environmental conditions. Our results demonstrated that perchlorate reduction by D. suillum only occurred under anaerobic conditions in the presence of perchlorate and was dependent on the presence of molybdenum. Perchlorate reduction was dependent on the presence of the enzyme chlorite dismutase, which was induced during metabolism of perchlorate. Anaerobic conditions alone were not enough to induce expression of this enzyme. Dissolved oxygen concentrations less than 2 mg liter⁻¹ were enough to inhibit perchlorate reduction by D. suillum. Similarly to oxygen, nitrate also regulated chlorite dismutase expression and repressed perchlorate reduction by D. suillum. Perchlorate-grown cultures of D. suillum preferentially reduced nitrate in media with equimolar amounts of perchlorate and nitrate. In contrast, an extended (40 h) lag phase was observed if a similar nitrate-perchlorate medium was inoculated with a nitrate-grown culture. Perchlorate reduction commenced only when nitrate was completely removed in either of these experiments. In contrast to D. suillum, nitrate had no inhibitory effects on perchlorate reduction by the perchlorate reducer Dechloromonas agitata strain CKB. Nitrate was reduced to nitrite concomitant with perchlorate reduction to chloride. These studies demonstrate that microbial respiration of perchlorate is significantly affected by environmental conditions and perchlorate reduction is directly dependent on bioavailable molybdenum and the presence or absence of competing electron acceptors. A microbial treatment strategy can achieve and maintain perchlorate concentrations below the recommended regulatory level, but only in environments in which the variables described above can be controlled.     

Microbial Communities in High and Low Recharge Environments: Implications for Microbial Transport in the Vadose Zone

Abstract Microbial communities along vertical transects in the unsaturated zone were evaluated at five sites in the Pasco Basin, in southeastern Washington State. Sites with contrasting recharge rates were chosen to maximize or minimize the potential for microbial transport. Pore water ages along the vertical transects were established using natural chloride tracers, and ranged from modern to either ∼15,000 yBP (years before present) or ∼30,000 yBP at the two low-recharge sites. Unsaturated flow processes were short-circuited by preferential flow at two of the three high-recharge sites, resulting in rapid movement of water through the vertical transects. Microbial numbers and biomass, based on plate counts, and phospholipid fatty acid (PLFA) concentrations decreased with depth at all sites. The majority (55–90%) of the culturable chemoheterotrophs recovered from most samples were streptomycete bacteria. 16S rRNA gene sequence and MIDI analyses indicated that 75% of the remaining isolates were Gram-positive bacteria (most likely species of Arthrobacter and Bacillus) 25% were Gram-negative bacteria (probably members of several genera in the alpha- and gamma-Proteobacteria). Comparison of microbial communities at low-recharge sites vs. high-recharge sites, where preferential flow occurs, revealed several differences that might be attributed to vertical transport of microbial cells at the high-recharge sites. Plate counts and PLFA analyses indicated that the proportion of streptomycetes, which were abundant at the surface but present in the subsurface as spores, decreased, or remained constant, with depth at the low-recharge sites, but increased with depth at the high-recharge sites. PLFA analyses also indicated that Gram-negative bacteria displayed increased nutrient stress with depth at the high-recharge sites characterized by preferential flow, but not at the low recharge site. This may be a result of advective transport of microbes to depths where it was difficult for them to compete effectively with the established community. Moreover, PLFA community structure profiles fluctuated considerably with depth at the low-recharge sites, but not at the high-recharge sites. This might be expected if transport were distributing the microbial community along the vertical profile at the high-recharge sites. In contrast to the high-recharge sites at which preferential flow occurs, filtration likely prevented vertical transport of microorganisms at the high-recharge site that was characterized by unsaturated flow. Received: 6 November 1996; Accepted: 9 May 1997     

Soil Microbial Communities of Eastern Antarctica

Investigation of microbial communities of Antarctica soils is a very important field of research that expands our knowledge of microbial participation in primary soil formation and specific features of their communities in extreme habitats, and it is of considerable interest in directed search of for microorganisms as potential biotechnological objects. The results of long-term (2012–2017) complex studies on soil microbial communities of the Russian East Antarctica polar stations at Shirmakher oasis (Novolazarevskaya station), the Larsemann Hills (Progress station), and the Tala Hills (Molodezhnaya station) are presented in this review. The assessment of biomass of soil microorganisms by the methods of direct microscopy has been carried out for the first time for this region. The general amount of microbial biomass is small; the fungi dominate (77–99%). The unique features of Antarctic soils are the high content and morphological diversity of small forms of microorganisms: fungi are presented by mainly single-celled structures (small spores and yeasts), while bacteria by ultrafine (filtering) forms. At the same time, microorganisms can significantly contribute to such important ecological functions of soil as the emission of greenhouse gases, especially during the warm season with the stable positive temperatures of the soil. This should be considered during creation of models and forecasts of global warming. The use of various isolation techniques for the analysis of the soil microbial population, together with the succession approach, significantly expand the information about taxonomic diversity of cultivated fungi and bacteria in Antarctica soils.     

Mineral Type Structures Soil Microbial Communities

Soil microorganisms living in close contact with minerals play key roles in the biogeochemical cycling of elements, soil formation, and plant nutrition. Yet, the composition of microbial communities inhabiting the mineralosphere (i.e., the soil surrounding minerals) is poorly understood. Here, we explored the composition of soil microbial communities associated with different types of minerals in various soil horizons. To this effect, a field experiment was set up in which mineral specimens of apatite, biotite, and oligoclase were buried in the organic, eluvial, and upper illuvial horizons of a podzol soil. After an incubation period of two years, the soil attached to the mineral surfaces was collected, and microbial communities were analyzed by means of Illumina MiSeq sequencing of the 16S (prokaryotic) and 18S (eukaryotic) ribosomal RNA genes. We found that both composition and diversity of bacterial, archaeal, and fungal communities varied across the different mineral surfaces, and that mineral type had a greater influence on structuring microbial assemblages than soil horizon. Thus, our findings emphasize the importance of mineral surfaces as ecological niches in soils.     

Impact of Fumigants on Soil Microbial Communities

Agricultural soils are typically fumigated to provide effective control of nematodes, soilborne pathogens, and weeds in preparation for planting of high-value cash crops. The ability of soil microbial communities to recover after treatment with fumigants was examined using culture-dependent (Biolog) and culture-independent (phospholipid fatty acid [PLFA] analysis and denaturing gradient gel electrophoresis [DGGE] of 16S ribosomal DNA [rDNA] fragments amplified directly from soil DNA) approaches. Changes in soil microbial community structure were examined in a microcosm experiment following the application of methyl bromide (MeBr), methyl isothiocyanate, 1,3-dichloropropene (1,3-D), and chloropicrin. Variations among Biolog fingerprints showed that the effect of MeBr on heterotrophic microbial activities was most severe in the first week and that thereafter the effects of MeBr and the other fumigants were expressed at much lower levels. The results of PLFA analysis demonstrated a community shift in all treatments to a community dominated by gram-positive bacterial biomass. Different 16S rDNA profiles from fumigated soils were quantified by analyzing the DGGE band patterns. The Shannon-Weaver index of diversity, H, was calculated for each fumigated soil sample. High diversity indices were maintained between the control soil and the fumigant-treated soils, except for MeBr (H decreased from 1.14 to 0.13). After 12 weeks of incubation, H increased to 0.73 in the MeBr-treated samples. Sequence analysis of clones generated from unique bands showed the presence of taxonomically unique clones that had emerged from the MeBr-treated samples and were dominated by clones closely related to Bacillus spp. and Heliothrix oregonensis. Variations in the data were much higher in the Biolog assay than in the PLFA and DGGE assays, suggesting a high sensitivity of PLFA analysis and DGGE in monitoring the effects of fumigants on soil community composition and structure. Our results indicate that MeBr has the greatest impact on soil microbial communities and that 1,3-D has the least impact.     

Cross-Site Soil Microbial Communities under Tillage Regimes: Fungistasis and Microbial Biomarkers

The exploitation of soil ecosystem services by agricultural management strategies requires knowledge of microbial communities in different management regimes. Crop cover by no-till management protects the soil surface, reducing the risk of erosion and nutrient leaching, but might increase straw residue-borne and soilborne plant-pathogenic fungi. A cross-site study of soil microbial communities and Fusarium fungistasis was conducted on six long-term agricultural fields with no-till and moldboard-plowed treatments. Microbial communities were studied at the topsoil surface (0 to 5 cm) and bottom (10 to 20 cm) by general bacterial and actinobacterial terminal restriction fragment length polymorphism (T-RFLP) and phospholipid fatty acid (PLFA) analyses. Fusarium culmorum soil fungistasis describing soil receptivity to plant-pathogenic fungi was explored by using the surface layer method. Soil depth had a significant impact on general bacterial as well as actinobacterial communities and PLFA profiles in no-till treatment, with a clear spatial distinction of communities (P < 0.05), whereas the depth-related separation of microbial communities was not observed in plowed fields. The fungal biomass was higher in no-till surface soil than in plowed soil (P < 0.07). Soil total microbial biomass and fungal biomass correlated with fungistasis (P < 0.02 for the sum of PLFAs; P < 0.001 for PLFA 18:2ω6). Our cross-site study demonstrated that agricultural management strategies can have a major impact on soil microbial community structures, indicating that it is possible to influence the soil processes with management decisions. The interactions between plant-pathogenic fungi and soil microbial communities are multifaceted, and a high level of fungistasis could be linked to the high microbial biomass in soil but not to the specific management strategy.     

Population Changes of Soil Microbial Communities Induced by Hydrocarbon and Heavy Metal Contamination

Substrate utilization tests with Biolog® plates were used to obtain information on shifts in community composition and on changes in the metabolic diversity and activity of microorganisms in soil polluted with hydrocarbons. and/or heavy metals. Differences between the patterns of substrate utilization of endogenous microorganisms of pristine and contaminated soils were investigated by multivariate analysis. Population changes and shifts in metabolic diversity were observed both after hydrocarbon pollution or heavy metal contamination. The overall activity on the 95 Biolog® Gram-negative (GN) substrates correlated well with the respiration rate of the soil. Soils contaminated with hydrocarbons showed higher metabolic potentials than the corresponding controls. In contrast, heavy metal pollution caused both lower metabolic activity and a loss in diversity. The Biolog® assay was found to be suitable to describe changes in functional diversity of soils caused by hydrocarbon contamination or heavy metal stress.     

Nitrate Reduction by Cassava

Nitrate reductase (NR) was assayed in vivo in cassava (Manihotesculenta Crantz). Activity in the leaves ranged from 0 to 2.51µmole of NO₃^– reduced g^–1 h^–1, withno activity in the younger leaves (leaf 1 on top). NR activitywas localized in the sides and toward the tip of the lobes ofthe leaf. (Received December 10, 1985; Accepted April 8, 1986)     

Inhibition of Nitrate Reduction by Chromium(VI) in Anaerobic Soil Microcosms

Chromium is often found as a cocontaminant at sites polluted with organic compounds. For nitrate-respiring microbes, Cr(VI) may be not only directly toxic but may also specifically interfere with N reduction. In soil microcosms amended with organic electron donors, Cr(VI), and nitrate, bacteria oxidized added carbon, but relatively low doses of Cr(VI) caused a lag and then lower rates of CO₂ accumulation. Cr(VI) strongly inhibited nitrate reduction; it occurred only after soluble Cr(VI) could not be detected. However, Cr(VI) additions did not eliminate Cr-sensitive populations; after a second dose of Cr(VI), bacterial activity was strongly inhibited. Differences in microbial community composition (assayed by PCR-denaturing gradient gel electrophoresis) driven by different organic substrates (glucose and protein) were smaller than when other electron acceptors had been used. However, the selection of bacterial phylotypes was modified by Cr(VI). Nine isolated clades of facultatively anaerobic Cr(VI)-resistant bacteria were closely related to cultivated members of the phylum Actinobacteria or Firmicutes. In Bacillus cereus GNCR-4, the nature of the electron donor (fermentable or nonfermentable) affected Cr(VI) resistance level and anaerobic nitrate metabolism. Our results indicate that carbon utilization and nitrate reduction in these soils were contingent upon the reduction of added Cr(VI). The amount of Cr(VI) required to inhibit nitrate reduction was 10-fold less than for aerobic catabolism of the same organic substrate. We speculate that the resistance level of a microbial process is directly related to the diversity of microbes capable of conducting it.Chromium(VI) is a toxic metal that can negatively affect bioremediation of organic compounds in sites where chromium and organic pollutants cooccur (36). Under oxygen-limited conditions, chromium(VI) can be reduced (biologically or chemically) to insoluble and relatively nontoxic Cr(III) (22). Despite the potential interactions between biotic and chemical components, the responses of anaerobic microbial activities to Cr(VI) have not been well studied (6, 7, 42, 43).Under anaerobic conditions, an important factor in the catabolism of organic carbon is the availability of electron acceptors. Nitrate is of special interest because it is often found as a copollutant in contaminated soils (18). Nitrate-reducing bacteria are facultative anaerobes commonly found in environmental samples and can couple the reduction of nitrate to the oxidation of diverse organic substrates (10, 13). The effect of Cr(VI) on natural denitrifying communities or pure cultures of denitrifying bacteria is not well characterized (8, 29). The environmental effects of Cr(VI) on denitrification are of particular interest because in addition to acute toxicity to the cell, Cr(VI) may compete with nitrate as an electron acceptor (15, 30). However, in other denitrifying bacteria (for example, Staphylococcus spp.), no competitive interactions were reported (45).The purpose of this study was to extend our work on the effects of Cr(VI) upon microbes in soil that mediate discrete chemoheterotrophic processes such as the use of O₂ (30) or Fe⁺³ (26) as terminal electron acceptors. We examined denitrification to determine whether the putative direct impact of Cr(VI) on the biochemistry of nitrate reduction would alter community dynamics from what had been observed with other terminal electron acceptors. In addition, we can add this data set to previous work to analyze the range of sensitivities to Cr(VI) that were found across a broad array of chemoheterotrophic processes.     

Deadwood Reduces the Variation in Soil Microbial Communities Caused by Experimental Forest Gaps

Ecosystems - Downed woody debris (DWD) creates heterogeneous microsites that are beneficial to enhancing biodiversity and that represent persistent nutrient and carbon pools. Windthrow events...     

Functional Potential of Soil Microbial Communities in the Maize Rhizosphere

Microbial communities in the rhizosphere make significant contributions to crop health and nutrient cycling. However, their ability to perform important biogeochemical processes remains uncharacterized. Here, we identified important functional genes that characterize the rhizosphere microbial community to understand metabolic capabilities in the maize rhizosphere using the GeoChip-based functional gene array method. Significant differences in functional gene structure were apparent between rhizosphere and bulk soil microbial communities. Approximately half of the detected gene families were significantly (p<0.05) increased in the rhizosphere. Based on the detected gyrB genes, Gammaproteobacteria, Betaproteobacteria, Firmicutes, Bacteroidetes and Cyanobacteria were most enriched in the rhizosphere compared to those in the bulk soil. The rhizosphere niche also supported greater functional diversity in catabolic pathways. The maize rhizosphere had significantly enriched genes involved in carbon fixation and degradation (especially for hemicelluloses, aromatics and lignin), nitrogen fixation, ammonification, denitrification, polyphosphate biosynthesis and degradation, sulfur reduction and oxidation. This research demonstrates that the maize rhizosphere is a hotspot of genes, mostly originating from dominant soil microbial groups such as Proteobacteria, providing functional capacity for the transformation of labile and recalcitrant organic C, N, P and S compounds.     

Microbial Diversity and Community Structure in Two Different Agricultural Soil Communities

Abstract In this study, two different agricultural soils were investigated: one organic soil and one sandy soil, from Stend (south of Bergen), Norway. The sandy soil was a field frequently tilled and subjected to crop rotations. The organic soil was permanent grazing land, infrequently tilled. Our objective was to compare the diversity of the cultivable bacteria with the diversity of the total bacterial population in soil. About 200 bacteria, randomly isolated by standard procedures, were investigated. The diversity of the cultivable bacteria was described at phenotypic, phylogenetic, and genetic levels by applying phenotypical testing (Biolog) and molecular methods, such as amplified rDNA restriction analysis (ARDRA); hybridization to oligonucleotide probes; and REP-PCR. The total bacterial diversity was determined by reassociation analysis of DNA isolated from the bacterial fraction of environmental samples, combined with ARDRA and DGGE analysis. The relationship between the diversity of cultivated bacteria and the total bacteria was elucidated. Organic soil exhibited a higher diversity for all analyses performed than the sandy soil. Analysis of cultivable bacteria resulted in different resolution levels and revealed a high biodiversity within the population of cultured isolates. The difference between the two agricultural soils was significantly higher when the total bacterial population was analyzed than when the cultivable population was. Thus, analysis of microbial diversity must ultimately embrace the entire microbial community DNA, rather than DNA from cultivable bacteria.     

Early Development of Soil Microbial Communities on Rehabilitated Quarries

The ecological development of soil microbial communities was studied in terms of microbial composition and structure at different rehabilitated phases on three quarries, namely Turret Hill Quarry (TH), Lam Tei Quarry (LT), and Shek O Quarry (SO), in Hong Kong. Using individual fatty acid methylesters (FAMEs) as biomarkers, Gram^? bacteria dominated at all sites, in which cy19:0 represented more than 15% of the total extracted FAMEs in all soils tested. Redundancy analysis showed that the abundance of Gram^? bacteria and Gram⁺ bacteria correlated significantly with woody species richness, native species richness, organic C, total N, and extractable NO₃‐N. Fungi (18:1w9c and 18:2w6) and arbuscular mycorrhizal (AM) fungi (16:1w5c) decreased in abundance with increasing age in SO and LT, which were positively correlated with grass coverage, soil pH, extractable NH₄‐N, and extractable P. Our study suggested that soil aeration and C availability in soils played a dominant role driving the changes in the composition and structure of Gram^? bacteria and Gram⁺ bacteria communities. Available P was the limiting factor for regulating the fungal and AM fungal communities on our local quarries.     

Redox Fluctuation Structures Microbial Communities in a Wet Tropical Soil

Frequent high-amplitude redox fluctuation may be a strong selective force on the phylogenetic and physiological composition of soil bacterial communities and may promote metabolic plasticity or redox tolerance mechanisms. To determine effects of fluctuating oxygen regimens, we incubated tropical soils under four treatments: aerobic, anaerobic, 12-h oxic/anoxic fluctuation, and 4-day oxic/anoxic fluctuation. Changes in soil bacterial community structure and diversity were monitored with terminal restriction fragment length polymorphism (T-RFLP) fingerprints. These profiles were correlated with gross N cycling rates, and a Web-based phylogenetic assignment tool was used to infer putative community composition from multiple fragment patterns. T-RFLP ordinations indicated that bacterial communities from 4-day oxic/anoxic incubations were most similar to field communities, whereas those incubated under consistently aerobic or anaerobic regimens developed distinctly different molecular profiles. Terminal fragments found in field soils persisted either in 4-day fluctuation/aerobic conditions or in anaerobic/12-h treatments but rarely in both. Only 3 of 179 total fragments were ubiquitous in all soils. Soil bacterial communities inferred from in silico phylogenetic assignment appeared to be dominated by Actinobacteria (especially Micrococcus and Streptomycetes), “Bacilli,” “Clostridia,” and Burkholderia and lost significant diversity under consistently or frequently anoxic incubations. Community patterns correlated well with redox-sensitive processes such as nitrification, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification but did not predict patterns of more general functions such as N mineralization and consumption. The results suggest that this soil's indigenous bacteria are highly adapted to fluctuating redox regimens and generally possess physiological tolerance mechanisms which allow them to withstand unfavorable redox periods.     

Field Released Transgenic Papaya Affects Microbial Communities and Enzyme Activities in Soil

Soil properties, microbial communities, and enzyme activities were studied in soil planted with transgenic or nontransgenic papaya under field conditions. The transgenic papaya contained a replicase (RP) mutant gene of the papaya ringspot virus (PRSV), which conferred resistance to the virus, the neomycin phosphotransferase II (NPT II) marker gene, which conferred Km resistance, and a cauliflower mosaic virus 35S promoter (CaMV 35S). There were significant differences (P < 0.05) in the total number of colony forming units (CFUs) of bacteria, actinomycetes, and fungi between soils planted with RP-transgenic and nontransgenic plants; total CFUs of bacteria, actinomycetes, and fungi in soil planted with transgenic papaya were significantly higher by 0.43, 0.80, and 0.46 times, respectively. Significantly higher (P < 0.05) CFUs of bacteria, actinomycetes, and fungi resistant to kanamycin (Km) were present in soils planted with the transgenic papaya than in those planted with nontransgenic papaya. Resistance quotients (CFU in the presence of a chemical relative to that without) of Km-resistant bacteria, actinomycetes and fungi were higher in soil planted with transgenic papaya, and the resistance quotients of Km-resistant bacteria, actinomycetes, and fungi in soils planted with transgenic papaya increased statistically significantly (P<0.05) from 1.5 to 2.5, from 1.2 to 2.6, and from 0.9 to 2.8 times, respectively. Soils planted with transgenic papaya had significantly higher enzyme activities of arylsulfatases (+5.4 times), alkaline phosphatases (+0.5 time), invertase (+0.5 time) and phosphodiesterases (+0.2 time), but lower enzyme activities of proteases (−2.1 times), polyphenol oxidases (−1.4 times), urease (−0.2 time) than the soils planted with nontransgenic papaya. Our results suggest that transgenic papaya could alter chemical properties, enzyme activities, and microbial communities in soil.     

Microbial Communities Involved in Fe Reduction and Mobility During Soil Organic Matter (SOM) Mineralization in Two Contrasted Paddy Soils

Lowland rice fields of West Africa (Ivory Coast) and South Asia (Thailand) are affected by ferrous toxicity or salinity, respectively, and their soil waters contain large amounts of ferrous iron, depending on reducing irrigation condition and suggesting occurrence of bacterial reducing processes. To determine the involvement, dynamic and activities of bacterial communities in Fe(III) reduction and mobilization during anaerobic degradation and mineralization of soil organic matter (SOM), different experiments and analyses have been performed. Results demonstrated that the utilization of SOM as sole carbon, nutrient and energy sources favored the presence of large bacterial communities: facultative anaerobic and anaerobic bacteria, Fe(III)-reducing bacteria (FeRB) (fermentative and Fe respiring), sulfate reducing bacteria (SRB) which are involved in carbon, nitrogen, iron and sulfur cycling. The larger functional diversity is observed in the Ivory Coast paddy soils containing larger amounts of organic matter and sulfur compounds. These communities contained complementary populations (chemoorganotrophic, chemolitotrophic, aerobic, facultative anaerobic and anaerobic) that can be active at different steps of iron solubilization with simultaneous organic matter mineralization. Our results indicate that the pH controlled by bacterial activity, the nature much more than the content of organic matter, and consequently the structure and activity of bacterial communities influence significantly the availability and dynamic of iron in paddy fields which affect the soil quality.     

The Structure of Microbial Communities in Soil and the Lasting Impact of Cultivation

The structure of microbial communities was examined as a function of community composition and the relative abundance of specific microbial groups to examine the effects that plant community composition and land-use history have on microbial communities in the soil. The sites sampled were part of the Long Term Ecological Research (LTER) project in agricultural ecology at the W.K. Kellogg Biological Station of Michigan State University (Hickory Corners, MI) and included both active and abandoned agricultural fields as well as nearby fields that had never been cultivated. Microbial community structure was assessed by extracting total RNA from soil samples and using 16S rRNA-targeted oligonucleotide probes to quantify the abundance of rRNA from the alpha, beta, and gamma Proteobacteria, the Actinobacteria (Gram positive bacteria with a high mol % G+C genome), the Bacteria, and the Eukarya. In addition, soil microbial communities were characterized by examining fluorescently tagged terminal restriction fragment length polymorphisms (T-RFLP) in PCR amplified 16S rDNA. Microbial community structure was observed to be remarkably similar among plots that shared a long-term history of agricultural management despite differences in plant community composition and land management that have been maintained on the plots in recent years. In contrast, microbial community structure differed significantly between fields that had never been cultivated and those having a long-term history of cultivation.