Two Invasive Plants Alter Soil Microbial Community Composition in Serpentine Grasslands

Plant invasions pose a serious threat to native ecosystem structure and function. However, little is known about the potential role that rhizosphere soil microbial communities play in facilitating or resisting the spread of invasive species into native plant communities. The objective of this study was to compare the microbial communities of invasive and native plant rhizospheres in serpentine soils. We compared rhizosphere microbial communities, of two invasive species, Centaurea solstitialis (yellow starthistle) and Aegilops triuncialis (barb goatgrass), with those of five native species that may be competitively affected by these invasive species in the field (Lotus wrangelianus, Hemizonia congesta, Holocarpha virgata, Plantago erecta, and Lasthenia californica). Phospholipid fatty acid analysis (PLFA) was used to compare the rhizosphere microbial communities of invasive and native plants. Correspondence analyses (CA) of PLFA data indicated that despite yearly variation, both starthistle and goatgrass appear to change microbial communities in areas they invade, and that invaded and native microbial communities significantly differ. Additionally, rhizosphere microbial communities in newly invaded areas are more similar to the original native soil communities than are microbial communities in areas that have been invaded for several years. Compared to native plant rhizospheres, starthistle and goatgrass rhizospheres have higher levels of PLFA biomarkers for sulfate reducing bacteria, and goatgrass rhizospheres have higher fatty acid diversity and higher levels of biomarkers for sulfur-oxidizing bacteria, and arbuscular mycorrhizal fungi. Changes in soil microbial community composition induced by plant invasion may affect native plant fitness and/or ecosystem function.     

黄顶菊对入侵地群落动态及植物生长生理特征的影响

为明确黄顶菊对入侵地植物群落和土著植物生理生长的影响机制,采用同质园试验对入侵和非入侵土壤的植物群落开展了整个生育期动态监测,并分析了黄顶菊入侵对狗尾草、羽叶鬼针草、灰绿藜、地肤4种土著植物生长和生理特征的影响规律。结果表明:黄顶菊入侵土壤植物群落多样性指数低于非入侵地,且有季节性差异,随生育期的推进差异逐渐减小;黄顶菊对本地植物的生长指标有显著影响(P0.05),随时间变化显著,但存在物种差异;4种植物的净光合速率(Pn)、气孔导度(Cd)、蒸腾速率(Tr)在非入侵土壤生长显著高于入侵地土壤(P0.05);而4种植物在入侵土壤生长的比叶面积(SLA)、比根长(SRL)、比根面积(SRA)显著高于本地土壤(P0.05)。综上,黄顶菊入侵抑制了本地植物的光合效率,减少了生物量的积累,导致本地植物群落的生物多样性水平降低,但表现出季节差异;不同物种对黄顶菊入侵胁迫的响应表现种间特异性,为理解入侵种对群落结构影响和实现入侵生境恢复提供了理论依据。     

Invasion by <Emphasis Type="Italic">Aegilops triuncialis</Emphasis> (Barb Goatgrass) Slows Carbon and Nutrient Cycling in a Serpentine Grassland

Invasive plant species alter plant community composition and ecosystem function. In the United States, California native grasslands have been displaced almost completely by invasive annual grasses, with serpentine grasslands being one of the few remaining refugia for California grasslands. This study examined how the invasive annual grass, Aegilops triuncialis, has altered decomposition processes in a serpentine annual grassland. Our objectives were to (1) assess howA. triuncialis alters primary productivity and litter tissue chemistry, (2) determine whether A. triuncialis litter is more recalcitrant to decomposition than native litter, and (3) evaluate whether differences in the soil microbial community in A. triuncialis-invaded and native-dominated areas result in different decomposition rates of invasive and/or native plant litter. In invaded plant patches, A. triuncialis was approximately 50% of the total plant cover, in contrast to native plant patches in which A. triuncialis was not detected and native plants comprised over 90% of the total plant cover. End-of-season aboveground biomass was 2-fold higher in A. triuncialis dominated plots compared to native plots; however, there was no significant difference in belowground biomass. Both above- and below-ground plant litter from A. triuncialis plots had significantly higher lignin:N and C:N ratios and lower total N, P, and K than litter from native plant plots. Aboveground litter from native plots decomposed more rapidly than litter from A. triuncialis plots, although there was no difference in decomposition of belowground tissues. Soil microbial community composition associated with different soil patch types had no effect on decomposition rates. These data suggest that plant invasion impacts decomposition and nutrient cycling through changes in plant community tissue chemistry and biomass production.     

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

Competitive context alters plant-soil feedback in an experimental woodland community

Recent findings on feedback between plants and soil microbial communities have improved our understanding of mechanisms underlying the success and consequences of invasions. However, additional studies to test for feedback in the presence and absence of interspecific competition, which may alter the strength or direction of feedbacks, are needed. We tested for soil microbial feedback in communities of the invasive grass Microstegium vimineum and commonly co-occurring native plant species. To incorporate competitive context, we used a factorial design with three plant treatments (M. vimineum alone, M. vimineum with the native plant community, and the native community without M. vimineum) and two soil inoculum treatments (experimentally invaded and uninvaded soil). When competing with M. vimineum, native communities were 27% more productive in invaded than uninvaded soil. In contrast, soil type did not significantly affect M. vimineum biomass or fecundity. At the community level, these results indicate a net negative soil microbial feedback when native plants and M. vimineum are grown in competitive mixture, but not when they are grown separately. Since positive, not negative, feedback is associated with dominance and invasion, our findings do not support plant–soil feedback as a driver of invasion in this species. Our results do show that the importance of soil feedback can change with competitive context. Such context-dependency implies that soil feedback may change when competitive interactions between natives and invading species shift as invasions progress.     

Invasive plants decrease microbial capacity to nitrify and denitrify compared to native California grassland communities

Exotic plant invasions are a major driver of global environmental change that can significantly alter the availability of limiting nutrients such as nitrogen (N). Beginning with European colonization of California, native grasslands were replaced almost entirely by annual exotic grasses, many of which are now so ubiquitous that they are considered part of the regional flora (“naturalized”). A new wave of invasive plants, such as Aegilops triuncialis (Barb goatgrass) and Elymus caput-medusae (Medusahead), continue to spread throughout the state today. To determine whether these new-wave invasive plants alter soil N dynamics, we measured inorganic N pools, nitrification and denitrification potentials, and possible mediating factors such as microbial biomass and soil pH in experimental grasslands comprised of A. triuncialis and E. caput-medusae. We compared these measurements with those from experimental grasslands containing: (1) native annuals and perennials and (2) naturalized exotic annuals. We found that A. triuncialis and E. caput-medusae significantly reduced ion-exchange resin estimates of nitrate (NO₃ ^?) availability as well as nitrification and denitrification potentials compared to native communities. Active microbial biomass was also lower in invaded soils. In contrast, potential measurements of nitrification and denitrification were similar between invaded and naturalized communities. These results suggest that invasion by A. triuncialis and E. caput-medusae may significantly alter the capacity for soil microbial communities to nitrify or denitrify, and by extension alter soil N availability and rates of N transformations during invasion of remnant native-dominated sites.     

Soil biota composition and the performance of a noxious weed across its invaded range

The success of invasive plant species is driven, in part, by feedback with soil ecosystems. Yet, how variation in belowground communities across latitudinal gradients affects invader distributions remains poorly understood. To determine the effect of soil communities on the performance of the noxious weed Cirsium arvense across its invaded range, we grew seedlings for 40 days in soils collected across a 699 km linear distance from both inside and outside established populations. We also described the mesofaunal and bacterial communities across all soil samples. We found that C. arvense typically performed better when grown in soils sourced from northern populations than from southern locations where it has a longer invasion history. We also found evidence that C. arvense performed best in soils sourced from outside invaded patches, although this was not consistent across all sites. The bacterial community showed a significant increase in the magnitude of compositional change in invaded sites at higher latitudes, while the mesofaunal community showed the opposite pattern. Bacterial community composition was significantly correlated with C. arvense performance, although mesofaunal community composition was not. Our results demonstrate that the interactions between an invasive plant and associated soil communities change across the invaded range, and the bacterial community in particular may affect variation in plant performance. Observed patterns may be caused by C.arvense presence and time since invasion allowing for an accumulation of species‐specific pathogens in southern soils, while the naïveté of northern soils to invasion results in a more responsive bacterial community. Although these interactions are difficult to predict, such effects could possibly facilitate the establishment of this exotic species to novel locations.     

The changing nature of plant–microbe interactions during a biological invasion

Invasive plant species can alter belowground microbial communities. Simultaneously, the composition of soil microbial communities and the abundance of key microbes can influence invasive plant success. Such reciprocal effects may cause plant–microbe interactions to change rapidly during the course of biological invasions in ways that either inhibit or promote invasive species growth. Here we use a space-for-time substitution to illustrate how effects of soil microbial communities on the exotic legume Vicia villosa vary across uninvaded sites, recently invaded sites, and sites invaded by V. villosa for over a decade. We find that soil microorganisms from invaded areas increase V. villosa growth compared to sterilized soil or live soils collected from uninvaded sites, likely because mutualistic nitrogen-fixing rhizobia are not abundant in uninvaded areas. Notably, the benefits resulting from inoculation with live soils were higher for soils from recently invaded sites compared to older invasions, potentially indicating that over longer time scales, soil microbial communities change in ways that may reduce the success of exotic species. These findings suggest that short-term changes to soil microbial communities following invasion may facilitate exotic legume growth likely because of increases in the abundance of mutualistic rhizobia, but also indicate that longer term changes to soil microbial communities may reduce the growth benefits belowground microbial communities provide to exotic species. Our results highlight the changing nature of plant–microbe interactions during biological invasions and illustrate how altered biotic interactions could contribute to both the initial success and subsequent naturalization of invasive legume species.     

An invasive aster (Ageratina adenophora) invades and dominates forest understories in China: altered soil microbial communities facilitate the invader and inhibit natives

Exotic plant invasion may alter underground microbial communities, and invasion-induced changes of soil biota may also affect the interaction between invasive plants and resident native species. Increasing evidence suggests that feedback of soil biota to invasive and native plants leads to successful exotic plant invasion. To examine this possible underlying invasion mechanism, soil microbial communities were studied where Ageratina adenophora was invading a native forest community. The plant–soil biota feedback experiments were designed to assess the effect of invasion-induced changes of soil biota on plant growth, and interactions between A. adenophora and three native plant species. Soil analysis showed that nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), and available P and K content were significantly higher in a heavily invaded site than in a newly invaded site. The structure of the soil microbial community was clearly different in all four sites. Ageratina adenophora invasion strongly increased the abundance of soil VAM (vesicular-arbuscular mycorrhizal fungi) and the fungi/bacteria ratio. A greenhouse experiment indicated that the soil biota in the heavily invaded site had a greater inhibitory effect on native plant species than on A. adenophora and that soil biota in the native plant site inhibited the growth of native plant species, but not of A. adenophora. Soil biota in all four sites increased A. adenophora relative dominance compared with each of the three native plant species and soil biota in the heavily invaded site had greater beneficial effects on A. adenophora relative dominance index (20% higher on average) than soil biota in the non-invaded site. Our results suggest that A. adenophora is more positively affected by the soil community associated with native communities than are resident natives, and once the invader becomes established it further alters the soil community in a way that favors itself and inhibits natives, helping to promote the invasion. Soil biota alteration after A. adenophora establishment may be an important part of its invasion process to facilitate itself and inhibit native plants.     

Virulence of oomycete pathogens from Phragmites australis‐invaded and noninvaded soils to seedlings of wetland plant species

Soil pathogens affect plant community structure and function through negative plant–soil feedbacks that may contribute to the invasiveness of non‐native plant species. Our understanding of these pathogen‐induced soil feedbacks has relied largely on observations of the collective impact of the soil biota on plant populations, with few observations of accompanying changes in populations of specific soil pathogens and their impacts on invasive and noninvasive species. As a result, the roles of specific soil pathogens in plant invasions remain unknown. In this study, we examine the diversity and virulence of soil oomycete pathogens in freshwater wetland soils invaded by non‐native Phragmites australis (European common reed) to better understand the potential for soil pathogen communities to impact a range of native and non‐native species and influence invasiveness. We isolated oomycetes from four sites over a 2‐year period, collecting nearly 500 isolates belonging to 36 different species. These sites were dominated by species of Pythium, many of which decreased seedling survival of a range of native and invasive plants. Despite any clear host specialization, many of the Pythium species were differentially virulent to the native and non‐native plant species tested. Isolates from invaded and noninvaded soils were equally virulent to given individual plant species, and no apparent differences in susceptibility were observed between the collective groups of native and non‐native plant species.     

Relationships between Methylobacteria and Glyphosate with Native and Invasive Plant Species: Implications for Restoration

After removing invasive plants, whether by herbicides or other means, typical restoration design focuses on rebuilding native plant communities while disregarding soil microbial communities. However, microbial–plant interactions are known to influence the relative success of native versus invasive plants. Therefore, the abundance and composition of soil microorganisms may affect restoration efforts. We assessed the effect of herbicide treatment on phytosymbiotic pink‐pigmented facultative methylotrophic (PPFM) bacteria and the potential consequences of native and invasive species establishment post‐herbicide treatment in the lab and in a coastal sage scrub (CSS)/grassland restoration site. Lab tests showed that 4% glyphosate reduced PPFM abundance. PPFM addition to seeds increased seedling length of a native plant (Artemisia californica) but not an invasive plant (Hirschfeldia incana). At the restoration site, methanol addition (a PPFM substrate) improved native bunchgrass (Nassella pulchra) germination and size by 35% over controls. In a separate multispecies field experiment, PPFM addition stimulated the germination of N. pulchra, but not that of three invasive species. Neither PPFM nor methanol addition strongly affected the growth of any plant species. Overall, these results are consistent with the hypothesis that PPFMs have a greater benefit to native than invasive species. Together, these experiments suggest that methanol or PPFM addition could be useful in improving CSS/grassland restorations. Future work should test PPFM effects on additional species and determine how these results vary under different environmental conditions.     

No consistent legacy effects of invasion by giant goldenrod (Solidago gigantea) via soil biota on native plant growth

Aims Changes in soil microbial communities after occupation by invasive alien plants can represent legacy effects of invasion that may limit recolonization and establishment of native plant species in soils previously occupied by the invader. In this study, for three sites in southern Germany, we investigated whether invasion by giant goldenrod (Solidago gigantea) leads to changes in soil biota that result in reduced growth of native plants compared with neighbouring uninvaded soils.Methods We grew four native plant species as a community and treated those plants with soil solutions from invaded or uninvaded soils that were sterilized, or live, with live solutions containing different fractions of the soil biota using a decreasing sieve mesh-size approach. We measured aboveground biomass of the plants in the communities after a 10-week growth period.Main Findings Across all three sites and regardless of invasion, communities treated with <20 μm soil biota or sterilized soil solutions had significantly greater biomass than communities treated with the complete soil biota solution. This indicates that soil biota>20 μm are more pathogenic to the native plants than smaller organisms in these soils. Across all three sites, there was only a non-significant tendency for the native community biomass to differ among soil solution types, depending on whether or not the soil was invaded. Only one site showed significant differences in community biomass among soil solution types, depending on whether or not the soil was invaded; community biomass was significantly lower when treated with the complete soil biota solution than with soil biota <20 μm or sterilized soil solutions, but only for the invaded soil. Our findings suggest that efforts to restore native communities on soils previously invaded by Solidago gigantea are unlikely to be hindered by changes in soil microbial community composition as a result of previous invasion.     

Size, activity and catabolic diversity of the soil microbial biomass in a wetland complex invaded by reed canary grass

Reed canary grass (Phalaris arundinacea, L.) invasion of wetlands is an ecological issue that has received attention, but its impact on soil microbial diversity is not well documented. The present study assessed the size (substrate-induced respiration), catabolic diversity (CLPP, community level physiological profiles) and composition (selective inhibition) of the soil microbial community in invaded (>95% P. arundinacea cover) and in non-invaded areas of a wetland occupied by native species grown either as a mixed assemblage (22 species) or as quasi-monotypic stands of Scirpus cyperinus (74% cover). The study also tested the hypothesis that decomposition of lignin- and phenolics-rich plant tissues would be fastest in soils exhibiting high catabolic diversity. Results showed that soil respiration, microbial biomass and diversity were significantly higher (P?<?0.03; 1.5 to 3 fold) in P. arundinacea-invaded soils than in soils supporting native plant species. Fungal to bacterial ratios were also higher in invaded (0.6) than in non-invaded (0.4) plots. Further, canonical discriminant analysis of CLPP data showed distinct communities of soil decomposers associated with each plant community. However, these differences in microbial attributes had no effect on decomposition of plant biomass which was primarily controlled by its chemical composition. While P. arundinacea invasion has substantially reduced plant diversity, this study found no parallel decline in the size and diversity of the soil microbial community in the invaded areas.     

Allelopathy and its coevolutionary implications between native and non‐native neighbors of invasive Cynara cardunculus L.

Invasive plants apply new selection pressures on neighbor plant species by different means including allelopathy. Recent evidence shows allelopathy functions as remarkably influential mediator for invaders to be successful in their invaded range. However, few studies have determined whether native and non‐native species co‐occurring with invaders have evolved tolerance to allelopathy. In this study, we conducted germination and growth experiments to evaluate whether co‐occurring native Juncus pallidus and non‐native Lolium rigidum species may evolve tolerance to the allelochemicals induced by Cyanara cardunculus in Australian agricultural fields. The test species were germinated and grown in pots filled with collected invaded and uninvaded rhizosphere soil of C. cardunculus with and without activated carbon (AC). Additionally, a separate experiment was done to differentiate the direct effects of AC on the test species. The soil properties showed invaded rhizosphere soils had higher total phenolic and lower pH compared with uninvaded soils. We found significant reduction of germination percentage and seedling growth in terms of above‐ and belowground biomass, and maximum plant height and root length of native in the invaded rhizosphere soil of C. cardunculus, but little effect on non‐native grass species. Even soil manipulated with AC showed no significant differences in the measured parameters of non‐native except aboveground biomass. Taken together, the results indicate allelochemicals induced by C. cardunculus exert more suppressive effects on native than non‐native linking the coevolved tolerance of those.     

Biogeographic differences in the effects of <Emphasis Type="Italic">Centaurea stoebe</Emphasis> on the soil nitrogen cycle: novel weapons and soil microbes

The success of some invasive plants may be due in part to native organisms lacking adaptation to species-specific biochemical traits of invaders—the Novel Weapons Hypothesis. We tested this hypothesis in the context of soil microbial communities by comparing the effects of Centaurea stoebe and the root exudate (±)-catechin, on ammonification and nitrification in both the non-native and native ranges of this species. In a non-native range (Montana), soil nitrate (NO₃ ⁻) concentrations were lower in invaded than uninvaded grasslands. This did not appear to be due only to higher uptake rates as both C. stoebe plants and catechin significantly reduced resin extractable NO₃ ⁻, the maximum rate of nitrification, and gross nitrification in Montana soils. Thus, reduced NO₃ ⁻ in invaded communities may be due in part to the inhibition of nitrifying bacteria by secondary metabolites produced by C. stoebe. The effects of C. stoebe on N-related processes were different in Romanian grasslands, where C. stoebe is native. In Romanian soil, C. stoebe had no effect on resin extractable NH₄ ⁺ or NO₃ ⁻ (compared to other plant species), the maximum rate of nitrification, nor gross nitrification. A relatively high concentration of catechin reduced the maximum rate of nitrification in situ, but substantially less than in Montana. In vivo, gross ammonification was lowest when treated with catechin. Our results suggest biogeographic differences in the way a plant species alters nitrogen cycling through the direct effects of root exudates and adds to a growing body of literature demonstrating the important belowground effects of invasive plants.     

Soils as agents of selection: feedbacks between plants and soils alter seedling survival and performance

Soils are one of the first selective environments a seed experiences and yet little is known about the evolutionary consequences of plant-soil feedbacks. We have previously found that plant phytochemical traits in a model system, Populus spp., influence rates of leaf litter decay, soil microbial communities and rates of soil net nitrogen mineralization. Utilizing this natural variation in plant-soil linkages we examined two related hypotheses: (1) Populus angustifolia seedlings are locally adapted to their native soils; and (2) Soils act as agents of selection, differentially affecting seedling survival and the heritability of plant traits. We conducted a greenhouse experiment by planting seedlings from 20 randomly collected P. angustifolia genetic families in soils conditioned by various Populus species and measured subsequent survival and performance. Even though P. angustifolia soils are less fertile overall, P. angustifolia seedlings grown in these soils were twice as likely to survive, grew 24% taller, had 27% more leaves, and 29% greater above-ground biomass than P. angustifolia seedlings grown in non-native P. fremontii or hybrid soils. Increased survival resulted in higher trait variation among seedlings in native soils compared to seedlings grown in non-native soils. Soil microbial biomass varied significantly across soil environments which could explain more of the variation in seedling performance than soil texture, pH, or nutrient availability, suggesting strong microbial interactions and feedbacks between plants, soils, and associated microorganisms. Overall, these data suggest that a “home-field advantage” or a positive plant soil feedback helps maintain genetic variance in P. angustifolia seedlings.     

Competition overwhelms the positive plant–soil feedback generated by an invasive plant

Invasive plant species can modify soils in a way that benefits their fitness more than the fitness of native species. However, it is unclear how competition among plant species alters the strength and direction of plant–soil feedbacks. We tested how community context altered plant–soil feedback between the non-native invasive forb Lespedeza cuneata and nine co-occurring native prairie species. In a series of greenhouse experiments, we grew plants individually and in communities with soils that differed in soil origin (invaded or uninvaded by L. cuneata) and in soils that were live vs. sterilized. In the absence of competition, L. cuneata produced over 60% more biomass in invaded than uninvaded soils, while native species performance was unaffected. The absence of a soil origin effect in sterile soil suggests that the positive plant–soil feedback was caused by differences in the soil biota. However, in the presence of competition, the positive effect of soil origin on L. cuneata growth disappeared. These results suggest that L. cuneata may benefit from positive plant–soil feedback when establishing populations in disturbed landscapes with few interspecific competitors, but does not support the hypothesis that plant–soil feedbacks influence competitive outcomes between L. cuneata and native plant species. These results highlight the importance of considering whether competition influences the outcome of interactions between plants and soils.     

Biogeographical variation in community response to root allelochemistry: novel weapons and exotic invasion

Centaurea diffusa is one of the most destructive invasive weeds in the western USA and allelopathy appears to contribute to its invasiveness ( Callaway & Aschehoug 2000 ). Here we identify a chemical from the root exudates of C. diffusa, 8‐hydroxyquinoline, not previously reported as a natural product, and find that it varies biogeographically in its natural concentration and its effect as an allelochemical. 8‐Hydroxyquinoline is at least three times more concentrated in C. diffusa‐invaded North American soils than in this weed's native Eurasian soils and has stronger phytotoxic effects on grass species from North America than on grass species from Eurasia. Furthermore, experimental communities built from North American plant species are far more susceptible to invasion by C. diffusa than communities built from Eurasian species, regardless of the biogeographical origin of the soil biota. Sterilization of North American soils suppressed C. diffusa more than sterilization of Eurasian soils, indicating that North American soil biota may also promote invasion by C. diffusa. Eurasian plants and soil microbes may have evolved natural resistance to 8‐hydroxyquinoline while North American plants have not, suggesting a remarkable potential for evolutionary compatibility and homeostasis among plants within natural communities and a mechanism by which exotic weeds destroy these communities.     

Patterns of introduction and adaptation during the invasion of Aegilops triuncialis (Poaceae) into Californian serpentine soils

Multiple introductions can play a prominent role in explaining the success of biological invasions. One often cited mechanism is that multiple introductions of invasive species prevent genetic bottlenecks by parallel introductions of several distinct genotypes that, in turn, provide heritable variation necessary for local adaptation. Here, we show that the invasion of Aegilops triuncialis into California, USA, involved multiple introductions that may have facilitated invasion into serpentine habitats. Using microsatellite markers, we compared the polymorphism and genetic structure of populations of Ae. triuncialis invading serpentine soils in California to that of accessions from its native range. In a glasshouse study, we also compared phenotypic variation in phenological and fitness traits between invasive and native populations grown on loam soil and under serpentine edaphic conditions. Molecular analysis of invasive populations revealed that Californian populations cluster into three independent introductions (i.e. invasive lineages). Our glasshouse common garden experiment found that all Californian populations exhibited higher fitness under serpentine conditions. However, the three invasive lineages appear to represent independent pathways of adaptation to serpentine soil. Our results suggest that the rapid invasion of serpentine habitats in California may have been facilitated by the existence of colonizing Eurasian genotypes pre‐adapted to serpentine soils.     

Positive effects of plant diversity on soil microbial biomass and activity are associated with more root biomass production

This study aims to explore relationships between plant diversity and soil microbial function and the factors that mediate the relationships. Artificial plant communities (1, 2, 4 and 8 species) were established filled with natural and mine tailing soils, respectively. After 12 months, the plant species richness positively affected the soil microbial functional diversity in both soil environments but negatively affected microbial biomass and soil basal respiration in the natural soil. The root biomass positively correlated with the microbial biomass, cultural bacterial activity and soil basal respiration in both soil environments. Moreover, the D_i (deviations between observed performances and expected performances from the monoculture performance of each species of mixture) of microbial biomass, cultural bacterial activity and soil basal respiration positively correlated with the D_i of root biomass in both soil environments. Consistent with stress-gradient hypothesis, the D_mix (over-function index) of aboveground biomass positively correlated plant species richness in the mine tailing soil. Results suggest that the root biomass production is an important mechanism that affects the effects of plant diversity on soil microbial functions. Different responses of soil microbial function to increasing plant diversity may be due to root biomass production mediated by other factors.