Rooting theories of plant community ecology in microbial interactions

Sensitive responders among bacterial and fungal microbiome to pyrogenic organic matter (biochar) addition differed greatly between rhizosphere and bulk soils

Sensitive responses among bacterial and fungal communities to pyrogenic organic matter (PyOM) (biochar) addition in rhizosphere and bulk soils are poorly understood. We conducted a pot experiment with manure and straw PyOMs added to an acidic paddy soil, and identified the sensitive “responders” whose relative abundance was significantly increased/decreased among the whole microbial community following PyOM addition. Results showed that PyOMs significantly (p < 0.05) increased root growth, and simultaneously changed soil chemical parameters by decreasing soil acidity and increasing biogenic resource. PyOM-induced acidity and biogenic resource co-determined bacterial responder community structure whereas biogenic resource was the dominant parameter structuring fungal responder community. Both number and proportion of responders in rhizosphere soil was larger than in bulk soil, regardless of PyOM types and microbial domains, indicating the microbial community in rhizosphere soil was sensitive to PyOM addition than bulk soil. The significant increased root biomass and length caused by PyOM addition, associated with physiological processes, e.g. C exudates secretion, likely favored more sensitive responders in rhizosphere soil than in bulk soil. Our study identified the responders at fine taxonomic resolution in PyOM amended soils, improved the understanding of their ecological phenomena associated with PyOM addition, and examined their interactions with plant roots.

Effect of different root endophytic fungi on plant community structure in experimental microcosms

Understanding the effects of root-associated microbes in explaining plant community patterns represents a challenge in community ecology. Although typically overlooked, several lines of evidence point out that nonmycorrhizal, root endophytic fungi in the Ascomycota may have the potential to drive changes in plant community ecology given their ubiquitous presence, wide host ranges, and plant species-specific fitness effects. Thus, we experimentally manipulated the presence of root endophytic fungal species in microcosms and measured its effects on plant communities. Specifically, we tested whether (1) three different root endophyte species can modify plant community structure; (2) those changes can also modified the way plant respond to different soil types; and (3) the effects are modified when all the fungi are present. As a model system, we used plant and fungal species that naturally co-occur in a temperate grassland. Further, the soil types used in our experiment reflected a strong gradient in soil texture that has been shown to drive changes in plant and fungal community structure in the field. Results showed that each plant species responded differently to infection, resulting in distinct patterns of plant community structure depending on the identity of the fungus present. Those effects depended on the soil type. For example, large positive effects due to presence of the fungi were able to compensate for less nutrients levels in one soil type. Further, host responses when all three fungi were present were different from the ones observed in single fungal inoculations, suggesting that endophyte-endophyte interactions may be important in structuring plant communities. Overall, these results indicate that plant responses to changes in the species identity of nonmycorrhizal fungal community species and their interactions can modify plant community structure.

The Mutualistic Niche: Mycorrhizal Symbiosis and Community Dynamics

The niche is generally viewed in terms of species' intrinsic physiological potential and limitations due to competition. Although DNA sequencing has revealed the ubiquity of beneficial microbial symbioses, the role of mutualisms in shaping species niches is not broadly recognized. In this review, I use a widespread terrestrial mutualism, the ectomycorrhizal symbiosis, to help develop the mutualistic niche concept. Using contemporary niche theory, I show how mycorrhizal symbioses expand environmental ranges (requirement niche) and influence resource use (impact niche) for both plants and fungi. Simple niche models for competition between resource specialists and generalists also predict a range of ecological phenomena, from unexpected monodominance by some tropical trees to the functional biogeography of mycorrhizal symbiosis. A niche-based view of mutualism may also help explain stability of mutualisms even in the absence of clear benefits. The niche is a central concept in ecology, and better integration of mutualism will more accurately reflect the positive interactions experienced by nearly all species.

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.

Plant Identity Exerts Stronger Effect than Fertilization on Soil Arbuscular Mycorrhizal Fungi in a Sown Pasture

Arbuscular mycorrhizal (AM) fungi play key roles in plant nutrition and plant productivity. AM fungal responses to either plant identity or fertilization have been investigated. However, the interactive effects of different plant species and fertilizer types on these symbiotic fungi remain poorly understood. We evaluated the effects of the factorial combinations of plant identity (grasses Avena sativa and Elymus nutans and legume Vicia sativa) and fertilization (urea and sheep manure) on AM fungi following 2-year monocultures in a sown pasture field study. AM fungal extraradical hyphal density was significantly higher in E. nutans than that in A. sativa and V. sativa in the unfertilized control and was significantly increased by urea and manure in A. sativa and by manure only in E. nutans, but not by either fertilizers in V. sativa. AM fungal spore density was not significantly affected by plant identity or fertilization. Forty-eight operational taxonomic units (OTUs) of AM fungi were obtained through 454 pyrosequencing of 18S rDNA. The OTU richness and Shannon diversity index of AM fungi were significantly higher in E. nutans than those in V. sativa and/or A. sativa, but not significantly affected by any fertilizer in all of the three plant species. AM fungal community composition was significantly structured directly by plant identity only and indirectly by both urea addition and plant identity through soil total nitrogen content. Our findings highlight that plant identity has stronger influence than fertilization on belowground AM fungal community in this converted pastureland from an alpine meadow.

Co-occurring nonnative woody shrubs have additive and non-additive soil legacies

To maximize limited conservation funds and prioritize management projects that are likely to succeed, accurate assessment of invasive nonnative species impacts is essential. A common challenge to prioritization is a limited knowledge of the difference between the impacts of a single nonnative species compared to the impacts of nonnative species when they co-occur, and in particular predicting when impacts of co-occurring nonnative species will be non-additive. Understanding non-additivity is important for management decisions because the management of only one co-occurring invader will not necessarily lead to a predictable reduction in the impact or growth of the other nonnative plant. Nonnative plants are frequently associated with changes in soil biotic and abiotic characteristics, which lead to plant-soil interactions that influence the performance of other species grown in those soils. Whether co-occurring nonnative plants alter soil properties additively or non-additively relative to their effects on soils when they grow in monoculture is rarely addressed. We use a greenhouse plant-soil feedback experiment to test for non-additive soil impacts of two common invasive nonnative woody shrubs, Lonicera maackii and Ligustrum sinense, in deciduous forests of the southeastern United States. We measured the performance of each nonnative shrub, a native herbaceous community, and a nonnative woody vine in soils conditioned by each shrub singly or together in polyculture. Soils conditioned by both nonnative shrubs had non-additive impacts on native and nonnative performance. Root mass of the native herbaceous community was 1.5 times lower and the root mass of the nonnative L. sinense was 1.8 times higher in soils conditioned by both L. maackii and L. sinense than expected based upon growth in soils conditioned by either shrub singly. This result indicates that when these two nonnative shrubs co-occur, their influence on soils disproportionally favors persistence of the nonnative L. sinense relative to this native herbaceous community, and could provide an explanation of why native species abundance is frequently depressed in these communities. Additionally, the difference between native and nonnative performance demonstrates that invasive impact studies focusing on the impact only of single species can be insufficient for determining the impact of co-occurring invasive plant species.

Tree species effects on pathogen-suppressive capacities of soil bacteria across two tropical dry forests in Costa Rica

Antibiotic-producing bacteria in the genus Streptomyces can inhibit soil-borne plant pathogens, and have the potential to mediate the impacts of disease on plant communities. Little is known about how antibiotic production varies among soil communities in tropical forests, despite a long history of interest in the role of soil-borne pathogens in these ecosystems. Our objective was to determine how tree species and soils influence variation in antibiotic-mediated pathogen suppression among Streptomyces communities in two tropical dry forest sites (Santa Rosa and Palo Verde). We targeted tree species that co-occur in both sites and used a culture-based functional assay to quantify pathogen-suppressive capacities of Streptomyces communities beneath 50 focal trees. We also measured host-associated litter and soil element concentrations as potential mechanisms by which trees may influence soil microbes. Pathogen-suppressive capacities of Streptomyces communities varied within and among tree species, and inhibitory phenotypes were significantly related to soil and litter element concentrations. Average proportions of inhibitory Streptomyces in soils from the same tree species varied between 1.6 and 3.3-fold between sites. Densities and proportions of pathogen-suppressive bacteria were always higher in Santa Rosa than Palo Verde. Our results suggest that spatial heterogeneity in the potential for antibiotic-mediated disease suppression is shaped by tree species, site, and soil characteristics, which could have significant implications for understanding plant community composition and diversity in tropical dry forests.

Plant-soil feedbacks promote negative frequency dependence in the coexistence of two aridland grasses

Understanding the mechanisms of species coexistence is key to predicting patterns of species diversity. Historically, the ecological paradigm has been that species coexist by partitioning resources: as a species increases in abundance, self-limitation kicks in, because species-specific resources decline. However, determining coexistence mechanisms has been a particular puzzle for sedentary organisms with high overlap in their resource requirements, such as plants. Recent evidence suggests that plant-associated microbes could generate the stabilizing self-limitation (negative frequency dependence) that is required for species coexistence. Here, we test the key assumption that plant-microbe feedbacks cause such self-limitation. We used competition experiments and modelling to evaluate how two common groups of soil microbes (rhizospheric microbes and biological soil crusts) influenced the self-limitation of two competing desert grass species. Negative feedbacks between the dominant plant competitor and its rhizospheric microbes magnified self-limitation, whereas beneficial interactions between both plant species and biological soil crusts partly counteracted this stabilizing effect. Plant-microbe interactions have received relatively little attention as drivers of vegetation dynamics in dry land ecosystems. Our results suggest that microbial mechanisms can contribute to patterns of plant coexistence in arid grasslands.

Plant–microbe interactions along a gradient of soil fertility in tropical dry forest

Theoretical models predict that plant interactions with free-living soil microbes, pathogens and fungal symbionts are regulated by nutrient availability. Working along a steep natural gradient of soil fertility in a Costa Rican tropical dry forest, we examined how soil nutrients affect plant–microbe interactions using two complementary approaches. First, we measured mycorrhizal colonization of roots and soil P availability in 18 permanent plots spanning the soil fertility gradient. We measured root production, root colonization by mycorrhizal fungi, phosphatase activity and Bray P in each of 144 soil cores. Next, in a full-factorial manipulation of soil type and microbial community origin, tree seedlings of Albizia guachapele and Swietenia macrophylla were grown in sterilized high-, intermediate- and low-fertility soils paired with microbial inoculum from each soil type. Seedling growth, biomass allocation and root colonization by mycorrhizas were quantified after 2 mo. In the field, root colonization by mycorrhizal fungi was unrelated to soil phosphorus across a five-fold gradient of P availability. In the shadehouse, inoculation with soil microbes had either neutral or positive effects on plant growth, suggesting that positive effects of mycorrhizal symbionts outweighed negative effects of soil pathogens. The presence of soil microbes had a greater effect on plant biomass than variation in soil nutrient concentrations (although both effects were modest), and plant responses to mycorrhizal inoculation were not dependent on soil nutrients. Taken together, our results emphasize that soil microbial communities can influence plant growth and morphology independently of soil fertility.

Performance of Endophyte Infected Tall Fescue in Europe and North America

Human assisted plant invasions from Europe to North America have been more common than the reverse. We tested endophyte-mediated performance of tall fescue in parallel three year experiments in Europe and the USA using endophyte infected and uninfected wild and cultivated plants. Experimental plants were subjected to nutrient and water treatments. Whereas endophyte infection increased tall fescue performance in general, the effects of endophytes on plant growth and reproduction varied among plant origins under different environmental conditions. Naturally endophyte-free Finnish cultivar ‘Retu’ performed equally well as ‘Kentucky-31’ in both geographic locations. All Eurasian origin plants performed well in the US. In Finland, plants established well and both cultivars survived over the first winter. However, winter mortality of ‘Kentucky-31’ plants was higher, particularly in fertilized soils in the subsequent winters. Our results suggest that tall fescue ecotype ‘Kentucky-31’ that flourishes in North America is poorly adapted to Northern European conditions.

Pervasive and strong effects of plants on soil chemistry: a meta-analysis of individual plant 'Zinke' effects

Plant species leave a chemical signature in the soils below them, generating fine-scale spatial variation that drives ecological processes. Since the publication of a seminal paper on plant-mediated soil heterogeneity by Paul Zinke in 1962, a robust literature has developed examining effects of individual plants on their local environments (individual plant effects). Here, we synthesize this work using meta-analysis to show that plant effects are strong and pervasive across ecosystems on six continents. Overall, soil properties beneath individual plants differ from those of neighbours by an average of 41%. Although the magnitudes of individual plant effects exhibit weak relationships with climate and latitude, they are significantly stronger in deserts and tundra than forests, and weaker in intensively managed ecosystems. The ubiquitous effects of plant individuals and species on local soil properties imply that individual plant effects have a role in plant-soil feedbacks, linking individual plants with biogeochemical processes at the ecosystem scale.

Impact of alien pines on local arbuscular mycorrhizal fungal communities-evidence from two continents

The introduction of alien plants can influence biodiversity and ecosystems. However, its consequences for soil microbial communities remain poorly understood. We addressed the impact of alien ectomycorrhizal (EcM) pines on local arbuscular mycorrhizal (AM) fungal communities in two regions with contrasting biogeographic histories: in South Africa, where no native EcM plant species are present; and in Argentina, where EcM trees occur naturally. The effect of alien pines on AM fungal communities differed between these regions. In South Africa, plantations of alien EcM pines exhibited lower AM fungal richness and significantly altered community composition, compared with native fynbos. In Argentina, the richness and composition of local AM fungal communities were similar in plantations of alien EcM pines and native forest. However, the presence of alien pines resulted in slight changes to the phylogenetic structure of root AM fungal communities in both regions. In pine clearcut areas in South Africa, the richness and composition of AM fungal communities were intermediate between the native fynbos and the alien pine plantation, which is consistent with natural regeneration of former AM fungal communities following pine removal. We conclude that the response of local AM fungal communities to alien EcM pines differs between biogeographic regions with different histories of species coexistence.

Regulation of plants' phosphate uptake in common mycorrhizal networks: Role of intraradical fungal phosphate transporters

We have recently identified two genes coding for inorganic phosphate transporters (Pht) in sorghum (Sorghum bicolor) and flax (Linum usitatissimum) that were induced in roots colonized by arbuscular mycorrhizal (AM) fungi. Mycorrhizal acquisition of inorganic phosphorus (Pi) was strongly affected by the combination of plant and AM fungal species, but the expression level of these genes coding for AM-inducible Pi transporters did not explain differences in plant phosphorus acquisition where flax and sorghum are sharing a common mycorrhizal network. In the present study, we investigated the possible role of fungal Pi transporters in the regulation of mycorrhizal Pi acquisition by measuring their expression in roots of flax and sorghum. One Pi transporter of Rhizophagus irregularis (RiPT5) showed a positive correlation with mycorrhizal Pi acquisition of sorghum. This indicates that a possible involvement in the regulation of mycorrhizal Pi acquisition. In general, expression of AMF Pi transporters was more related to mycorrhizal Pi acquisition of sorghum than of flax, indicating plant species-specific differences in the regulation of mycorrhizal Pi acquisition.

The effect of soil-borne pathogens depends on the abundance of host tree species

The overarching issue for understanding biodiversity maintenance is how fitness advantages accrue to a species as it becomes rare, as this is the defining feature of stable coexistence mechanisms. Without these fitness advantages, average fitness differences between species will lead to exclusion. However, empirical evidence is lacking, especially for forests, due to the difficulty of manipulating density on a large-enough scale. Here we took advantage of naturally occurring contrasts in abundance between sites of a subtropical tree species, Ormosia glaberrima, to demonstrate how low-density fitness advantages accrue by the Janzen–Connell mechanism. The results showed that soil pathogens suppressed seedling recruitment of O. glaberrima when it is abundant but had little effect on the seedlings when it is at low density due to the lack of pathogens. The difference in seedling survival between abundant and low-density sites demonstrates strong dependence of pathogenic effect on the abundance of host species.

Fitness advantages conferred on species living at low density is thought to be one mechanism by which stable biodiversity is maintained. Here, Liu et al. show that recruitment of seedlings in high-density populations of a subtropical tree is suppressed by soil pathogens, with little effect at low-density.

Effect of combined microbes on plant tolerance to Zn-Pb contaminations

The presence and composition of soil microbial communities has been shown to have a large impact on plant-plant interactions and consequently plant diversity and composition. The goal of the present study was to evaluate impact of arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria, which constitutes an essential link between the soil and the plant's roots. A greenhouse pot experiment was conducted to evaluate the feasibility of using selected microbes to improve Hieracium pilosella and Medicago sativa growth on Zn-Pb-rich site. Results of studies revealed that biomass, the dry mass of shoots and roots, increased significantly when plants were inoculated with mycorrhizal fungi and nitrogen-fixing bacteria. The addition of Azospirillum sp. and Nostoc edaphicum without mycorrhiza suppressed plant growth. Single bacterial inoculation alone does not have a positive effect on M. sativa growth, while co-inoculation with AMF improved plant growth. Plant vitality (expressed by the performance index) was improved by the addition of microbes. However, our results indicated that even dry heat sterilization of the substratum created imbalanced relationships between soil-plant and plants and associated microorganisms. The studies indicated that AMF and N2-fixers can improve revegetation of heavy metal-rich industrial sites, if the selection of interacting symbionts is properly conducted.

Regulation of resource exchange in the arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) fungi are one of the most important groups of plant symbionts. These fungi provide mineral nutrients to plants in exchange for carbon. Although substantial amounts of resources are exchanged, the factors that regulate trade in the AM symbiosis are poorly understood. Recent evidence for the reciprocally regulated exchange of resources by AM fungi and plants has led to the suggestion that these symbioses operate according to biological market dynamics, in which interactions are viewed from an economic perspective, and the most beneficial partners are favoured. Here we present five arguments that challenge the importance of reciprocally regulated exchange, and thereby market dynamics, for resource exchange in the AM symbiosis, and suggest that such reciprocity is only found in a subset of symbionts, under specific conditions. We instead propose that resource exchange in the AM symbiosis is determined by competition for surplus resources, functional diversity and sink strength.

Complementarity among plant growth promoting traits in rhizospheric bacterial communities promotes plant growth

An assessment of roles of rhizospheric microbial diversity in plant growth is helpful in understanding plant-microbe interactions. Using random combinations of rhizospheric bacterial species at different richness levels, we analysed the contribution of species richness, compositions, interactions and identity on soil microbial respiration and plant biomass. We showed that bacterial inoculation in plant rhizosphere enhanced microbial respiration and plant biomass with complementary relationships among bacterial species. Plant growth was found to increase linearly with inoculation of rhizospheric bacterial communities with increasing levels of species or plant growth promoting trait diversity. However, inoculation of diverse bacterial communities having single plant growth promoting trait, i.e., nitrogen fixation could not enhance plant growth over inoculation of single bacteria. Our results indicate that bacterial diversity in rhizosphere affect ecosystem functioning through complementary relationship among plant growth promoting traits and may play significant roles in delivering microbial services to plants.

Incorporating the soil environment and microbial community into plant competition theory

Plants affect microbial communities and abiotic properties of nearby soils, which in turn influence plant growth and interspecific interaction, forming a plant-soil feedback (PSF). PSF is a key determinant influencing plant population dynamics, community structure, and ecosystem functions. Despite accumulating evidence for the importance of PSF and development of specific PSF models, different models are not yet fully integrated. Here, we review the theoretical progress in understanding PSF. When first proposed, PSF was integrated with various mathematical frameworks to discuss its influence on plant competition. Recent theoretical models have advanced PSF research at different levels of ecological organizations by considering multiple species, applying spatially explicit simulations to examine how local-scale predictions apply to larger scales, and assessing the effect of PSF on plant temporal dynamics over the course of succession. We then review two foundational models for microbial- and litter-mediated PSF. We present a theoretical framework to illustrate that although the two models are typically presented separately, their behavior can be understood together by invasibility analysis. We conclude with suggestions for future directions in PSF theoretical studies, which include specifically addressing microbial diversity to integrate litter- and microbial-mediated PSF, and apply PSF to general coexistence theory through a trait-based approach.

Community Structure and Function of Amphibian Skin Microbes: An Experiment with Bullfrogs Exposed to a Chytrid Fungus

The vertebrate microbiome contributes to disease resistance, but few experiments have examined the link between microbiome community structure and disease resistance functions. Chytridiomycosis, a major cause of amphibian population declines, is a skin disease caused by the fungus, Batrachochytrium dendrobatidis (Bd). In a factorial experiment, bullfrog skin microbiota was reduced with antibiotics, augmented with an anti-Bd bacterial isolate (Janthinobacterium lividum), or unmanipulated, and individuals were then either exposed or not exposed to Bd. We found that the microbial community structure of individual frogs prior to Bd exposure influenced Bd infection intensity one week following exposure, which, in turn, was negatively correlated with proportional growth during the experiment. Microbial community structure and function differed among unmanipulated, antibiotic-treated, and augmented frogs only when frogs were exposed to Bd. Bd is a selective force on microbial community structure and function, and beneficial states of microbial community structure may serve to limit the impacts of infection.

Composition of fungal soil communities varies with plant abundance and geographic origin

Interactions of belowground fungal communities with exotic and native plant species may be important drivers of plant community structure in invaded grasslands. However, field surveys linking plant community structure with belowground fungal communities are missing. We investigated whether a selected number of abundant and relatively rare plants, either native or exotic, from an old-field site associate with different fungal communities. We also assessed whether these plants showed different symbiotic relationships with soil biota through their roots. We characterized the plant community and collected roots to investigate fungal communities using 454 pyrosequencing and assessed arbuscular mycorrhizal colonization and enemy-induced lesions. Differences in fungal communities were considered based on the assessment of α- and β diversity depending on plant 'abundance' and 'origin'. Plant abundance and origin determined the fungal community. Fungal richness was higher for native abundant as opposed to relatively rare native plant species. However, this was not observed for exotics of contrasting abundance. Regardless of their origin, β diversity was higher for rare than for abundant species. Abundant exotics in the community, which happen to be grasses, were the least mycorrhizal whereas rare natives were most susceptible to enemy attack. Our results suggest that compared with exotics, the relative abundance of remnant native plant species in our old-field site is still linked to the structure of belowground fungal communities. In contrast, exotic species may act as a disturbing agent contributing towards the homogenization of soil fungal communities, potentially changing feedback interactions.

Spatial heterogeneity of plant-soil feedback affects root interactions and interspecific competition

Plant-soil feedback is receiving increasing interest as a factor influencing plant competition and species coexistence in grasslands. However, we do not know how spatial distribution of plant-soil feedback affects plant below-ground interactions. We investigated the way in which spatial heterogeneity of soil biota affects competitive interactions in grassland plant species. We performed a pairwise competition experiment combined with heterogeneous distribution of soil biota using four grassland plant species and their soil biota. Patches were applied as quadrants of 'own' and 'foreign' soils from all plant species in all pairwise combinations. To evaluate interspecific root responses, species-specific root biomass was quantified using real-time PCR. All plant species suffered negative soil feedback, but strength was species-specific, reflected by a decrease in root growth in own compared with foreign soil. Reduction in root growth in own patches by the superior plant competitor provided opportunities for inferior competitors to increase root biomass in these patches. These patterns did not cascade into above-ground effects during our experiment. We show that root distributions can be determined by spatial heterogeneity of soil biota, affecting plant below-ground competitive interactions. Thus, spatial heterogeneity of soil biota may contribute to plant species coexistence in species-rich grasslands.

Niche Limits of Symbiotic Gut Microbiota Constrain the Salinity Tolerance of Brine Shrimp

Symbiosis generally causes an expansion of the niche of each partner along the axis for which a service is mutually provided. However, for other axes, the niche can be restricted to the intersection of each partner's niche and can thus be constrained rather than expanded by mutualism. We explore this phenomenon using Artemia as a model system. This crustacean is able to survive at very high salinities but not at low salinities, although its hemolymph's salinity is close to freshwater. We hypothesized that this low-salinity paradox results from poor performance of its associated microbiota at low salinity. We showed that, in sterile conditions, Artemia had low survival at all salinities when algae were the only source of carbon. In contrast, survival was high at all salinities when fed with yeast. We also demonstrated that bacteria isolated from Artemia's gut reached higher densities at high salinities than at low salinities, including when grown on algae. Taken together, our results show that Artemia can survive at low salinities, but their gut microbiota, which are required for algae digestion, have reduced fitness. Widespread facultative symbiosis may thus be an important determinant of niche limits along axes not specific to the mutualistic interaction.

Competitive interactions between a nonmycorrhizal invasive plant, Alliaria petiolata, and a suite of mycorrhizal grassland, old field, and forest species

The widespread invasion of the nonmycorrhizal biennial plant, Alliaria petiolata in North America is hypothesized to be facilitated by the production of novel biochemical weapons that suppress the growth of mycorrhizal fungi. As a result, A. petiolata is expected to be a strong competitor against plant species that rely on mycorrhizal fungi for nutrient uptake services. If A. petiolata is also a strong competitor for soil resources, it should deplete nutrients to levels lower than can be tolerated by weaker competitors. Because the negative effect of losing the fungal symbiont for mycorrhizal plants is greatest when nutrients are low, the ability of A. petiolata to simultaneously suppress fungi and efficiently take up soil nutrients should further strengthen its competitive ability against mycorrhizal plants. To test this hypothesis, we grew 27 mycorrhizal tree, forb and grass species that are representative of invaded habitats in the absence or presence of competition with A. petiolata in soils that had previously been experimentally planted with the invader or left as a control. A history of A. petiolata in soil reduced plant available forms of nitrogen by >50% and phosphorus by 17% relative to control soil. Average mycorrhizal colonization of competitor species was reduced by >50% in A. petiolata history versus control soil. Contrary to expectations, competition between A. petiolata and other species was stronger in control than history soil. The invader suppressed the biomass of 70% of competitor species in control soil but only 26% of species in history soil. In addition, A. petiolata biomass was reduced by 56% in history versus control soil, whereas the average biomass of competitor species was reduced by 15%. Thus, our results suggest that the negative effect of nutrient depletion on A. petiolata was stronger than the negative effect of suppressing mycorrhizal colonization on competitor species. These findings indicate that the inhibitory potential of A. petiolata on competitor species via mycorrhizal suppression is not enhanced under nutrient limitation.

Plant-soil feedbacks from 30-year family-specific soil cultures: phylogeny, soil chemistry and plant life stage

Intraspecific negative feedback effects, where performance is reduced on soils conditioned by conspecifics, are widely documented in plant communities. However, interspecific feedbacks are less well studied, and their direction, strength, causes, and consequences are poorly understood. If more closely related species share pathogens, or have similar soil resource requirements, plants may perform better on soils conditioned by more distant phylogenetic relatives. There have been few empirical tests of this prediction across plant life stages, and none of which attempt to account for soil chemistry. Here, we test the utility of phylogeny for predicting soil feedback effects on plant survival and performance (germination, seedling survival, growth rate, biomass). We implement a full factorial experiment growing species representing five families on five plant family-specific soil sources. Our experiments exploit soils that have been cultured for over 30 years in plant family-specific beds at Oxford University Botanic Gardens. Plant responses to soil source were idiosyncratic, and species did not perform better on soils cultured by phylogenetically more distant relatives. The magnitude and sign of feedback effects could, however, be explained by differences in the chemical properties of "home" and "away" soils. Furthermore, the direction of soil chemistry-related plant-soil feedbacks was dependent on plant life stage, with the effects of soil chemistry on germination success and accumulation of biomass inversely related. Our results (1) suggest that the phylogenetic distance between plant families cannot predict plant-soil feedbacks across multiple life stages, and (2) highlight the need to consider changes in soil chemistry as an important driver of population responses. The contrasting responses at plant life stages suggest that studies focusing on brief phases in plant demography (e.g., germination success) may not give a full picture of plant-soil feedback effects.

Putative linkages between below- and aboveground mutualisms during alien plant invasions

Evidence of the fundamental role of below-aboveground links in controlling ecosystem processes is mostly based on studies done with soil herbivores or mutualists and aboveground herbivores. Much less is known about the links between belowground and aboveground mutualisms, which have been studied separately for decades. It has not been until recently that these mutualisms-mycorrhizas and legume-rhizobia on one hand, and pollinators and seed dispersers on the other hand-have been found to influence each other, with potential ecological and evolutionary consequences. Here we review the mechanisms that may link these two-level mutualisms, mostly reported for native plant species, and make predictions about their relevance during alien plant invasions. We propose that alien plants establishing effective mutualisms with belowground microbes might improve their reproductive success through positive interactions between those mutualists and pollinators and seed dispersers. On the other hand, changes in the abundance and diversity of soil mutualists induced by invasion can also interfere with below-aboveground links for native plant species. We conclude that further research on this topic is needed in the field of invasion ecology as it can provide interesting clues on synergistic interactions and invasional meltdowns during alien plant invasions.

Inhibitory effects of soil biota are ameliorated by high plant diversity

The idea that plant communities with high species diversity are more stable, productive, and resistant to invasion at small spatial scales has become an important ecological paradigm. Recently, the role of soil biota has emerged as a major driver of this relationship between plant species diversity and ecosystem function. In greenhouse experiments, we found that soil collected from experimentally constructed species-rich plant assemblages (that originally contained between 10 and 16 species) promoted the growth of 4 native target plant species more than soil from species-poor communities (that originally contained between 2 and 5 species). Sterilization of soils from species-poor communities improved the growth of these target species more than sterilization of soils from species-rich plant communities, indicative that inhibitory soil biota had greater negative impacts on plant growth in low versus high diversity soils. These results suggest that strong soil biota effects in soils do not simply accrue in experimental monocultures, but can occur in low diversity assemblages that are more realistic of what occurs in nature. Our findings suggest a mechanistic explanation for the diversity-productivity relationship, and further support the importance of inhibitory soil biota as significant contributors to spatial and temporal patterns of abundance in natural plant communities through negative plant-soil feedback.

Arbuscular mycorrhizal fungal diversity associated with Eleocharis obtusa and Panicum capillare growing in an extreme petroleum hydrocarbon-polluted sedimentation basin

Arbuscular mycorrhizal fungi (AMF) have been extensively studied in natural and agricultural ecosystems, but little is known about their diversity and community structure in highly petroleum-polluted soils. In this study, we described an unexpected diversity of AMF in a sedimentation basin of a former petrochemical plant, in which petroleum hydrocarbon (PH) wastes were dumped for many decades. We used high-throughput PCR, cloning and sequencing of 18S rDNA to assess the molecular diversity of AMF associated with Eleocharis obtusa and Panicum capillare spontaneously inhabiting extremely PH-contaminated sediments. The analyses of rhizosphere and root samples over two years showed a remarkable AMF richness comparable with that found in temperate natural ecosystems. Twenty-one taxa, encompassing the major families within Glomeromycota, were detected. The most abundant OTUs belong to genera Claroideoglomus, Diversispora, Rhizophagus and Paraglomus. Both plants had very similar overall community structures and OTU abundances; however, AMF community structure differed when comparing the overall OTU distribution across the two years of sampling. This could be likely explained by variations in precipitations between 2011 and 2012. Our study provides the first view of AMF molecular diversity in soils extremely polluted by PH, and demonstrated the ability of AMF to colonize and establish in harsh environments.

Soil biota effects on local abundances of three grass species along a land-use gradient

Biotic plant-soil interactions and land-use intensity are known to affect plant individual fitness as well as competitiveness and therefore plant-species abundances in communities. Therefore, a link between soil biota and land-use intensity on local abundance of plant species in grasslands can be expected. In two greenhouse experiments, we investigated the effects of soil biota from grassland sites differing in land-use intensity on three grass species that vary in local abundances along this land-use gradient. We were interested in those soil-biota effects that are associated with land-use intensity, and whether these effects act directly or indirectly. Therefore, we grew the three plant species in two separate experiments as single individuals and in mixtures and compared their performance. As single plants, all three grasses showed a similar performance with and without soil biota. In contrast, in mixtures growth of the species in response to the presence or absence of soil biota differed. This resulted in different soil-biota effects that tend to correspond with patterns of species-specific abundances in the field for two of the three species tested. Our results highlight the importance of indirect interactions between plants and soil microorganisms and suggest that combined effects of soil biota and plant-plant interactions are involved in structuring plant communities. In conclusion, our experiments suggest that soil biota may have the potential to alter effects of plant-plant interactions and therefore influence plant-species abundances and diversity in grasslands.

Responses of soil fungi to logging and oil palm agriculture in Southeast Asian tropical forests

Human land use alters soil microbial composition and function in a variety of systems, although few comparable studies have been done in tropical forests and tropical agricultural production areas. Logging and the expansion of oil palm agriculture are two of the most significant drivers of tropical deforestation, and the latter is most prevalent in Southeast Asia. The aim of this study was to compare soil fungal communities from three sites in Malaysia that represent three of the most dominant land-use types in the Southeast Asia tropics: a primary forest, a regenerating forest that had been selectively logged 50 years previously, and a 25-year-old oil palm plantation. Soil cores were collected from three replicate plots at each site, and fungal communities were sequenced using the Illumina platform. Extracellular enzyme assays were assessed as a proxy for soil microbial function. We found that fungal communities were distinct across all sites, although fungal composition in the regenerating forest was more similar to the primary forest than either forest community was to the oil palm site. Ectomycorrhizal fungi, which are important associates of the dominant Dipterocarpaceae tree family in this region, were compositionally distinct across forests, but were nearly absent from oil palm soils. Extracellular enzyme assays indicated that the soil ecosystem in oil palm plantations experienced altered nutrient cycling dynamics, but there were few differences between regenerating and primary forest soils. Together, these results show that logging and the replacement of primary forest with oil palm plantations alter fungal community and function, although forests regenerating from logging had more similarities with primary forests in terms of fungal composition and nutrient cycling potential. Since oil palm agriculture is currently the mostly rapidly expanding equatorial crop and logging is pervasive across tropical ecosystems, these findings may have broad applicability.