Soil microbes may affect the way exotic invasive plants interact with native neighbors. We investigated the effects of soil fungi on interactions between the invasive weed Centaurea maculosa (spotted knapweed) and six species native to the intermountain prairies of the northwestern United States. We also compared the effect of C. maculosa on the composition of the soil microbial community to that of the native species. In the field, fungicide (Benomyl) reduced AM mycorrhizal colonization of C. maculosa roots by >80%. Fungicide did not significantly reduce non‐AM fungi. When grown alone, the biomass of C. maculosa was not affected by the fungicide application. However, depending on the combination of native competitor and fungicide, C. maculosa biomass varied from 10‐fold decreases to 1.9‐fold increases. In untreated soils, C. maculosa grew larger in the presence of Festuca idahoensis or Koeleria cristata than when alone. When fungicide was applied these positive effects of Festuca and Koeleria on C. maculosa did not occur. A third native grass, Pseudoroegneria spicata, had much stronger competitive effects on C. maculosa than Festuca or Koeleria, and fungicide reduced the competitive effects of Pseudoroegneria. Fungicide increased Centaurea biomass when competing with the forb Gallardia aristata. However, fungicide did not affect the way two other forbs; Achillea millefolium and Linum lewisii, interacted with C. maculosa. Rhizosphere microbial communities in the root zones of the three native bunchgrass species differed from that of C. maculosa. However, despite the strong effects of soil fungi in field interactions and differences in microbial community composition, soil biota from different plant rhizospheres did not affect the growth of C. maculosa in the absence of native competitors in greenhouse experiments. Our results suggest that successful invasions by exotic plant species can be affected by complex and often beneficial effects of local soil microbial communities. These effects were not manifest as simple direct effects, but become apparent only when native plants, invasive plants, and soil microbial communities were interacting at the same time.

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The hyporheic zone of a river is nonphotic, has steep chemical and redox gradients, and has a heterotrophic food web based on the consumption of organic carbon entrained from downwelling surface water or from upwelling groundwater. The microbial communities in the hyporheic zone are an important component of these heterotrophic food webs and perform essential functions in lotic ecosystems. Using a suite of methods (denaturing gradient gel electrophoresis, 16S rRNA phylogeny, phospholipid fatty acid analysis, direct microscopic enumeration, and quantitative PCR), we compared the microbial communities inhabiting the hyporheic zone of six different river sites that encompass a wide range of sediment metal loads resulting from large base-metal mining activity in the region. There was no correlation between sediment metal content and the total hyporheic microbial biomass present within each site. However, microbial community structure showed a significant linear relationship with the sediment metal loads. The abundances of four phylogenetic groups (groups I, II, III, and IV) most closely related to ␣-, ␤-, and ␥-proteobacteria and the cyanobacteria, respectively, were determined. The sediment metal content gradient was positively correlated with group III abundance and negatively correlated with group II abundance. No correlation was apparent with regard to group I or IV abundance. This is the first documentation of a relationship between fluvially deposited heavy-metal contamination and hyporheic microbial community structure. The information presented here may be useful in predicting long-term effects of heavy-metal contamination in streams and provides a basis for further studies of metal effects on hyporheic microbial communities.The hyporheic zone is a spatially and temporally dynamic ecotone which provides connectivity between terrestrial, groundwater, and lotic habitats (12,31,69,72,73). It lies beneath the channel of a stream (46), often extending great distances laterally in the subsurface, and is an essential part of lotic ecosystems (22,57,59). The microbial transformations of dissolved and particulate nutrients taking place in the hyporheic zone have been shown to influence both macroinvertebrate and algal assemblages and may play a role in the productivity of riparian vegetation (4,36,58). Therefore, alterations in the hyporheic ecosystem that result in changes in the resident microbial community structure may be translated to higher trophic levels. In addition, alterations in the structure of microbial communities may be a useful indicator of the effects and extent of anthropogenic contamination. Previous work in our laboratory has focused on describing the types and seasonal dynamics of microorganisms in the hyporheic zone (21). This investigation explores the effects of heavy-metal contamination on hyporheic-zone microbial community structure.Heavy metals contaminate numerous aquatic environments worldwide as a result of large-scale mining and other activities (49). Heavy metals reduce water quality a...

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Restoration of metals-contaminated environments requires a functional microbial community for successful plant community establishment, soil development, and biogeochemical cycling . Our research measured microbial community structure and carbon-utilization diversity in treatment plots from a mine waste revegetation project near Butte, Montana. Treatments included two controls (raw tailings) either (1) with or (2) without tilling, (3) shallow-tilled lime addition, (4) deep-tilled lime addition, (5) lime slurry injection, (6) topsoil addition, and (7) an undisturbed area near the tailings. Microbial community structural differences were assayed by plate counts of heterotrophic bacteria, actinomycetes, fungi, and bacterial endospores, and quantification of arbuscular mycorrhizae colonization. Metabolic diversity differences were assessed by carbon-utilization profiles generated with Biolog microtiter plates. Heterotrophic bacteria counts were significantly higher in the limed and topsoil treatment plots than the control plots, and the actinomycete and fungal counts increased in the tilled control plot as well. Endospore counts were significantly higher in the topsoil addition and the undisturbed plots than the other treatment plots. Carbon-utilization activity was very low in untreated plots, intermediate in lime-treated plots, and very high in topsoil and undisturbed plots. Arbuscular mycorrhizae (AM) colonization levels of two grass species showed low levels of colonization on control, shallow-limed, and lime slurry-injected plots, and high levels on the deep-limed and topsoil-addition plots. Plant and soil system components increased across the treatment plots, but individual components responded differently to changing environmental conditions.