Rhizosphere microbial community structure differs between constant subzero and freeze-thaw temperature regimes in a subarctic soil.

In the Arctic and subarctic, climate change is causing reduced snowpack extent and earlier snowmelt. Shallower snowpack decreases the thermal insulation of underlying soil and results in more freeze-thaw conditions reflective of dynamic air temperatures. The aim of this study was to determine the effect of alternative temperature regimes on overall microbial community structure and rhizosphere recruitment across representatives of three subarctic plant functional groups. We hypothesized that temperature regime would influence rhizosphere community structure more than plant type. Planted microcosms were established using a tree, forb, grass, or no plant control and subjected to either freeze-thaw cycling or static subzero temperatures. Our results showed rhizosphere communities exhibited reduced diversity compared to bulk soils, and were influenced by temperature conditions and to a lesser extent plant type. We found that plants have a core microbiome that is persistent under different winter temperature scenarios but also have temperature regime-specific rhizosphere microbes. Freeze-thaw cycling resulted in greater community shifts from the pre-incubation soils when compared to constant subzero temperature. This finding suggests that wintertime snowpack conditions may be a significant factor for plant-microbe interactions upon spring thaw.

Solid-phase microextraction (SPME) for determination of geosmin and 2-methylisoborneol in volatile emissions from soil disturbance.

A method is described here for the concentration and determination of geosmin and 2-methylisoborneol (2-MIB) from the gaseous phase, with translation to field collection and quantification from soil disturbances in situ. The method is based on the use of solid-phase microextraction (SPME) fibers for adsorption of volatile chemicals from the vapor phase, followed by desorption into a gas chromatograph-mass spectrometer (GC-MS) for analysis. The use of a SPME fiber allows simple introduction to the GC-MS without further sample preparation. Several fiber sorbent types were studied and the 50/30 µm DVB/CAR/PDMS was the best performer to maximize the detected peak areas of both analytes combined. Factors such as extraction temperature and time along with desorption temperature and time were explored with respect to analyte recovery. An extraction temperature of 30 °C for 10 min, with a desorption temperature of 230 °C for 4 min was best for the simultaneous analysis of both geosmin and 2-MIB without complete loss of either one. The developed method was used successfully to measure geosmin and 2-MIB emission from just above disturbed and undisturbed soils, indicating that this method detects both compounds readily from atmospheric samples. Both geosmin and 2-MIB were present as background concentrations in the open air, while disturbed soils emitted much higher concentrations of both compounds. Surprisingly, 2-MIB was always detected at higher concentrations than geosmin, indicating that a focus on its detection may be more useful for soil emission monitoring and more sensitive to low levels of soil disturbance.

Metal accumulation capacity in indigenous Alaska vegetation growing on military training lands.

Permafrost thawing could increase soil contaminant mobilization in the environment. Our objective was to quantify metal accumulation capacities for plant species and functional groups common to Alaskan military training ranges where elevated soil metal concentrations were likely to occur. Plant species across multiple military training range sites were collected. Metal content in shoots and roots was compared to soil metal concentrations to calculate bioconcentration and translocation factors. On average, grasses accumulated greater concentrations of Cr, Cu, Ni, Pb, Sb, and Zn relative to forbs or shrubs, and bioconcentrated greater concentrations of Ni and Pb. Shrubs bioconcentrated greater concentrations of Sb. Translocation to shoots was greatest among the forbs. Three native plants were identified as candidate species for use in metal phytostabilization applications. Elymus macrourus, a grass, bioconcentrated substantial concentrations of Cu, Pb, and Zn in roots with low translocation to shoots. Elaeagnus commutata, a shrub, bioconcentrated the greatest amounts of Sb, Ni, and Cr, with a low translocation factor. Solidago decumbens bioconcentrated the greatest amount of Sb among the forbs and translocated the least amount of metals. A combination of forb, shrub, and grass will likely enhance phytostabilization of heavy metals in interior Alaska soils through increased functional group diversity.

Ecotoxicity of 2,4-dinitrotoluene to cold tolerant plant species in a sub-Arctic soil.

Decades of live-fire training exercises have left millions of acres of military training lands contaminated with various munitions constituents such as dinitrotoluene. Those that pose a threat to higher organisms due to their toxicity and mobility in the soil are of particular concern. Plants aid in the biodegradation and phytoextraction of contaminants, and site-specific ecotoxicity determinations are critical to inform effective remediation strategy. These ecotoxicity determinations are lacking in cold-adapted plants and would be very informative for contaminated training lands in cold regions. Therefore, we conducted a phytotoxicity study to determine the median effective concentration (EC50) of 2,4-dinitrotoluene (2,4-DNT) to four native Alaskan plant species in a sub-Arctic soil at two different temperatures. Plant species investigated were white spruce (Picea glauca), field locoweed (Oxytropis campestris), bluejoint grass (Calamagrostis canadensis), and Jacob's ladder (Polemonium pulcherrimum). Seedling emergence, fresh plant mass, and dry plant mass were used to model plant response to 2,4-DNT contamination. White spruce was most tolerant to 2,4-DNT contamination (EC50 = 130.8 mg kg-1) and field locoweed was least tolerant (EC50 = 0.38 mg kg-1). In general, Arctic plant species were more vulnerable to 2,4-DNT when compared to plant types native to temperate or tropical regions.

Arbuscular mycorrhizal fungal community differs between a coexisting native shrub and introduced annual grass.

Arbuscular mycorrhizal fungi (AMF) have been implicated in non-native plant invasion success and persistence. However, few studies have identified the AMF species associating directly with plant invaders, or how these associations differ from those of native plant species. Identifying changes to the AMF community due to plant invasion could yield key plant-AMF interactions necessary for the restoration of native plant communities. This research compared AMF associating with coexisting Bromus tectorum, an invasive annual grass, and Artemisia tridentata, the dominant native shrub in western North America. At three sites, soil and root samples from Bromus and Artemisia were collected. Sporulation was induced using trap cultures, and spores were identified using morphological characteristics. DNA was extracted from root and soil subsamples and amplified. Sequences obtained were aligned and analyzed to compare diversity, composition, and phylogenetic distance between hosts and sites. Richness of AMF species associated with Artemisia in cultures was higher than AMF species associated with Bromus. Gamma diversity was similar and beta diversity was higher in AMF associated with Bromus compared to Artemisia. AMF community composition differed between hosts in both cultures and roots. Two AMF species (Archaeospora trappei and Viscospora viscosum) associated more frequently with Artemisia than Bromus across multiple sites. AMF communities in Bromus roots were more phylogenetically dispersed than in Artemisia roots, indicating a greater competition for resources within the invasive grass. Bromus associated with an AMF community that differed from Artemisia in a number of ways, and these changes could restrict native plant establishment.

Soil Bacteria and Fungi Respond on Different Spatial Scales to Invasion by the Legume Lespedeza cuneata.

The spatial scale on which microbial communities respond to plant invasions may provide important clues as to the nature of potential invader-microbe interactions. Lespedeza cuneata (Dum. Cours.) G. Don is an invasive legume that may benefit from associations with mycorrhizal fungi; however, it has also been suggested that the plant is allelopathic and may alter the soil chemistry of invaded sites through secondary metabolites in its root exudates or litter. Thus, L. cuneata invasion may interact with soil microorganisms on a variety of scales. We investigated L. cuneata-related changes to soil bacterial and fungal communities at two spatial scales using multiple sites from across its invaded N. American range. Using whole-community DNA fingerprinting, we characterized microbial community variation at the scale of entire invaded sites and at the scale of individual plants. Based on permutational multivariate analysis of variance, soil bacterial communities in heavily invaded sites were significantly different from those of uninvaded sites, but bacteria did not show any evidence of responding at very local scales around individual plants. In contrast, soil fungi did not change significantly at the scale of entire sites, but there were significant differences between fungal communities of native versus exotic plants within particular sites. The differential scaling of bacterial and fungal responses indicates that L. cuneata interacts differently with soil bacteria and soil fungi, and these microorganisms may play very different roles in the invasion process of this plant.

Grassland plant composition alters vehicular disturbance effects in Kansas, USA.

Many "natural" areas are exposed to military or recreational off-road vehicles. The interactive effects of different types of vehicular disturbance on vegetation have rarely been examined, and it has been proposed that some vegetation types are less susceptible to vehicular disturbance than others. At Fort Riley, Kansas, we experimentally tested how different plant community types changed after disturbance from an M1A1 Abrams tank driven at different speeds and turning angles during different seasons. The greatest vegetation change was observed because of driving in the spring in wet soils and the interaction of turning while driving fast (vegetation change was measured with Bray-Curtis dissimilarity). We found that less vegetation change occurred in communities with high amounts of native prairie vegetation than in communities with high amounts of introduced C(3) grasses, which is the first experimental evidence we are aware of that suggests plant communities dominated by introduced C(3) grasses changed more because of vehicular disturbance than communities dominated by native prairie grasses. We also found that vegetation changed linearly with vehicular disturbance intensity, suggesting that at least initially there was no catastrophic shift in vegetation beyond a certain disturbance intensity threshold. Overall, the intensity of vehicular disturbance appeared to play the greatest role in vegetation change, but the plant community type also played a strong role and this should be considered in land use planning. The reasons for greater vegetation change in introduced C(3) grass dominated areas deserve further study.