Extreme environments simplify reassembly of communities of arbuscular mycorrhizal fungi

The ecological impacts of long-term (press) disturbance on mechanisms regulating the relative abundance (i.e., commonness or rarity) and temporal dynamics of species within a community remain largely unknown. This is particularly true for the functionally important arbuscular mycorrhizal (AM) fungi; obligate plant-root endosymbionts that colonize more than two-thirds of terrestrial plant species. Here, we use high-resolution amplicon sequencing to examine how AM fungal communities in a specific extreme ecosystem-mofettes or natural CO2 springs caused by geological CO2 exhalations-are affected by long-term stress. We found that in mofettes, specific and temporally stable communities form as a subset of the local metacommunity. These communities are less diverse and dominated by adapted, "stress tolerant" taxa. Those taxa are rare in control locations and more benign environments worldwide, but show a stable temporal pattern in the extreme sites, consistently dominating the communities in grassland mofettes. This pattern of lower diversity and high dominance of specific taxa has been confirmed as relatively stable over several sampling years and is independently observed across multiple geographic locations (mofettes in different countries). This study implies that the response of soil microbial community composition to long-term stress is relatively predictable, which can also reflect the community response to other anthropogenic stressors (e.g., heavy metal pollution or land use change). Moreover, as AM fungi are functionally differentiated, with different taxa providing different benefits to host plants, changes in community structure in response to long-term environmental change have the potential to impact terrestrial plant communities and their productivity.IMPORTANCEArbuscular mycorrhizal (AM) fungi form symbiotic relationships with more than two-thirds of plant species. In return for using plant carbon as their sole energy source, AM fungi improve plant mineral supply, water balance, and protection against pathogens. This work demonstrates the importance of long-term experiments to understand the effects of long-term environmental change and long-term disturbance on terrestrial ecosystems. We demonstrated a consistent response of the AM fungal community to a long-term stress, with lower diversity and a less variable AM fungal community over time under stress conditions compared to the surrounding controls. We have also identified, for the first time, a suite of AM fungal taxa that are consistently observed across broad geographic scales in stressed and anthropogenically heavily influenced ecosystems. This is critical because global environmental change in terrestrial ecosystems requires an integrative approach that considers both above- and below-ground changes and examines patterns over a longer geographic and temporal scale, rather than just single sampling events.

Long-term tropospheric ozone pollution disrupts plant-microbe-soil interactions in the agroecosystem

Tropospheric ozone (O3 ) threatens agroecosystems, yet its long-term effects on intricate plant-microbe-soil interactions remain overlooked. This study employed two soybean genotypes of contrasting O3 -sensitivity grown in field plots exposed elevated O3 (eO3 ) and evaluated cause-effect relationships with their associated soil microbiomes and soil quality. Results revealed long-term eO3 effects on belowground soil microbiomes and soil health surpass damage visible on plants. Elevated O3 significantly disrupted belowground bacteria-fungi interactions, reduced fungal diversity, and altered fungal community assembly by impacting soybean physiological properties. Particularly, eO3 impacts on plant performance were significantly associated with arbuscular mycorrhizal fungi, undermining their contribution to plants, whereas eO3 increased fungal saprotroph proliferation, accelerating soil organic matter decomposition and soil carbon pool depletion. Free-living diazotrophs exhibited remarkable acclimation under eO3 , improving plant performance by enhancing nitrogen fixation. However, overarching detrimental consequences of eO3 negated this benefit. Overall, this study demonstrated long-term eO3 profoundly governed negative impacts on plant-soil-microbiota interactions, pointing to a potential crisis for agroecosystems. These findings highlight urgent needs to develop adaptive strategies to navigate future eO3 scenarios.

The interspecific competition of tree plants in the presence of AM fungi and litter facilitates root morphological development and nutrition when compared with intraspecific competition

Arbuscular mycorrhizal (AM) fungi can affect plant growth by regulating competition. Nutrient-deficient karst habitats contain abundant plants that compete for nutrients through interspecific or intraspecific competition, involving the nutritional transformation of litter decomposition. However, how plant competition in the presence of AM fungi and litter affects root development and nutrition remains unclear. A potted experiment was conducted, including AM fungus treatment with or without Glomus etunicatum, the competition treatment concerning intraspecific or interspecific competition through planting Broussonetia papyrifera and Carpinus pubescens seedlings, and the litter treatment with or without the mixture of B. papyrifera and C. pubescens litter leaves. The root morphological traits were analyzed, and nitrogen (N), phosphorus (P), and potassium (K) were measured. The results showed that AM fungus differently affected the root morphological development and nutrition of both competitive plants, significantly promoting B. papyrifera roots in the increase of dry weight, length, volume, surface area, tips, and branches as well as N, P, and K acquisitions regardless of litter addition. However, there was no apparent influence for C. pubescens roots, except for the diameter in the interspecific competition with litter. The root dry weight, length, volume, surface area, and tips of B. papyrifera under two competitive styles were significantly greater than C. pubescens regulated by AM fungus, presenting significant species differences. The responses of the relative competition intensity (RCI) on root morphological and nutritional traits indicated that AM fungus and litter both asymmetrically alleviated more competitive pressure for B. papyrifera than C. pubescens, and the interspecific competition facilitated more root morphological development and nutrition utilization by endowing B. papyrifera root superiority relative to C. pubescens compared with the intraspecific competition. In conclusion, interspecific competition is more beneficial for plant root development and nutrition than intraspecific competition in the presence of AM fungus and litter via asymmetrically alleviating competitive pressure for different plants.

Arbuscular mycorrhizal fungi community analysis revealed the significant impact of arsenic in antimony- and arsenic-contaminated soil in three Guizhou regions

Introduction

The lack of systematic investigations of arbuscular mycorrhizal fungi (AMF) community composition is an obstacle to AMF biotechnological applications in antimony (Sb)- and arsenic (As)-polluted soil.

Methods

Morphological and molecular identification were applied to study the AMF community composition in Sb- and As-contaminated areas, and the main influencing factors of AMF community composition in Sb- and As-contaminated areas were explored.

Results

(1) A total of 513,546 sequences were obtained, and the majority belonged to Glomeraceae [88.27%, 193 operational taxonomic units (OTUs)], followed by Diversisporaceae, Paraglomeraceae, Acaulosporaceae, Gigasporaceae, and Archaeosporaceae; (2) the affinity between AMF and plants was mainly related to plant species (F = 3.488, p = 0.022 < 0.050), which was not significantly correlated with the total Sb (TSb) and total As (TAs) in soil; (3) the AMF spore density was mainly related to the available nitrogen, available potassium, and total organic carbon; (4) The effect of soil nutrients on AMF community composition (total explanation: 15.36%) was greater than that of soil Sb and As content (total explanation: 5.80%); (5) the effect of TAs on AMF community composition (λ = -0.96) was more drastic than that of TSb (λ = -0.21), and the effect of As on AMF community composition was exacerbated by the interaction between As and phosphorus in the soil; and (6) Diversisporaceae was positively correlated with the TSb and TAs.

Discussion

The potential impact of As on the effective application of mycorrhizal technology should be further considered when applied to the ecological restoration of Sb- and As-contaminated areas.

Soil Mercury Pollution Changes Soil Arbuscular Mycorrhizal Fungal Community Composition

Remediation of mercury (Hg)-contaminated soil by mycorrhizal technology has drawn increasing attention because of its environmental friendliness. However, the lack of systematic investigations on arbuscular mycorrhizal fungi (AMF) community composition in Hg-polluted soil is an obstacle for AMF biotechnological applications. In this study, the AMF communities within rhizosphere soils from seven sites from three typical Hg mining areas were sequenced using an Illumina MiSeq platform. A total of 297 AMF operational taxonomic units (OTUs) were detected in the Hg mining area, of which Glomeraceae was the dominant family (66.96%, 175 OTUs). AMF diversity was significantly associated with soil total Hg content and water content in the Hg mining area. Soil total Hg showed a negative correlation with AMF richness and diversity. In addition, the soil properties including total nitrogen, available nitrogen, total potassium, total phosphorus, available phosphorus, and pH also affected AMF diversity. Paraglomeraceae was found to be negatively correlated to Hg stress. The wide distribution of Glomeraceae in Hg-contaminated soil makes it a potential candidate for mycorrhizal remediation.

Dirt cheap: an experimental test of controls on resource exchange in an ectomycorrhizal symbiosis

To distinguish among hypotheses on the importance of resource-exchange ratios in outcomes of mutualisms, we measured resource (carbon (C), nitrogen (N), and phosphorus (P)) transfers and their ratios, between Pinus taeda seedlings and two ectomycorrhizal (EM) fungal species, Rhizopogon roseolus and Pisolithus arhizus in a laboratory experiment. We evaluated how ambient light affected those resource fluxes and ratios over three time periods (10, 20, and 30 wk) and the consequences for plant and fungal biomass accrual, in environmental chambers. Our results suggest that light availability is an important factor driving absolute fluxes of N, P, and C, but not exchange ratios, although its effects vary among EM fungal species. Declines in N : C and P : C exchange ratios over time, as soil nutrient availability likely declined, were consistent with predictions of biological market models. Absolute transfer of P was an important predictor of both plant and fungal biomass, consistent with the excess resource-exchange hypothesis, and N transfer to plants was positively associated with fungal biomass. Altogether, light effects on resource fluxes indicated mixed support for various theoretical frameworks, while results on biomass accrual better supported the excess resource-exchange hypothesis, although among-species variability is in need of further characterization.

Identifying thresholds of nitrogen enrichment for substantial shifts in arbuscular mycorrhizal fungal community metrics in a temperate grassland of northern China

Nitrogen (N) enrichment poses threats to biodiversity and ecosystem stability, while arbuscular mycorrhizal (AM) fungi play important roles in ecosystem stability and functioning. However, the ecological impacts, especially thresholds of N enrichment potentially causing AM fungal community shifts have not been adequately characterized. Based on a long-term field experiment with nine N addition levels ranging from 0 to 50 g N m^-2  yr^-1 in a temperate grassland, we characterized the community response patterns of AM fungi to N enrichment. Arbuscular mycorrhizal fungal biomass continuously decreased with increasing N addition levels. However, AM fungal diversity did not significantly change below 20 g N m^-2  yr^-1 , but dramatically decreased at higher N levels, which drove the AM fungal community to a potentially unstable state. Structural equation modeling showed that the decline in AM fungal biomass could be well explained by soil acidification, whereas key driving factors for AM fungal diversity shifted from soil nitrogen : phosphorus (N : P) ratio to soil pH with increasing N levels. Different aspects of AM fungal communities (biomass, diversity and community composition) respond differently to increasing N addition levels. Thresholds for substantial community shifts in response to N enrichment in this grassland ecosystem are identified.

Effects of nitrogen addition and seasonal change on arbuscular mycorrhizal fungi community diversity in a poplar plantation

Arbuscular mycorrhizal (AM) fungi play a crucial role in carbon (C), nitrogen (N), and phosphorous (P) biogeochemical cycling. Therefore, it is essential to determine the seasonal responses of the AM fungal community to N addition to understanding better the ecological processes against a background of intensified N deposition. Based on an ongoing field simulation experiment with five N addition levels (0, 5, 10, 15, and 30 gN·m−2·a−1) in a 5-year-old poplar plantation at Dongtai Forest Farm in Yancheng, Jiangsu province, eastern China, soil physicochemical properties, the root colonization rate, and the rhizosphere soil AM fungal community diversity and composition in four seasons (summer, autumn, winter, and spring) were investigated. Meanwhile, the relationships between the characteristics of the AM fungal community and soil environmental factors were analyzed. High-throughput sequencing showed that the dominant genera in the poplar plantation were Glomus (average relative abundance 87.52%), Diversispora (9.62%), and Acaulospora (1.85%). The addition of N significantly increased the root colonization rate in spring. The diversity of the AM fungal community (Chao and Shannon indexes) was primarily affected by seasonal change rather than N addition, and the diversity in summer was significantly lower than in the other three seasons. Redundancy analysis showed that soil temperature, available P, total P, and pH significantly affected the structure of the AM fungal community. It can be concluded N addition primarily influenced the root colonization rate, whereas seasonal change had a notable effect on the AM fungal community diversity. Although seasonal change and N addition greatly influenced the composition, seasonal change exerted a more substantial effect than N addition. These results will improve our understanding of the underground ecological processes in poplar plantation ecosystems.

Mycorrhizal inoculation effects on growth and the mycobiome of poplar on two phytomanaged sites after 7-year-short rotation coppicing

AIMS

Afforestation of trace-element contaminated soils, notably with fast growing trees, has been demonstrated to be an attractive option for bioremediation due to the lower costs and dispersion of contaminants than conventional cleanup methods. Mycorrhizal fungi form symbiotic associations with plants, contributing to their tolerance towards toxic elements and actively participating to the biorestoration processes. The aim of this study was to deepen our understanding on the effects of mycorrhizal inoculation on plant development and fungal community at two trace-element contaminated sites (Pierrelaye and Fresnes-sur-Escaut, France) planted with poplar (Populus trichocarpa x Populus maximowiczii).

METHODS

The 2 sites were divided into 4 replicated field blocks with a final plant density of 2200 tree h^-1. Half of the trees were inoculated with a commercial inoculum made of a mix of mycorrhizal species. The sites presented different physico-chemical characteristics (e.g., texture: sandy soil versus silty-loam soil and organic matter: 5.7% versus 3.4% for Pierrelaye and Fresnes-sur-Escaut, respectively) and various trace element contamination levels.

RESULTS

After 7 years of plantation, inoculation showed a significant positive effect on poplar biomass production at the two sites. Fungal composition study demonstrated a predominance of the phylum Ascomycota at both sites, with a dominance of Geopora Arenicola and Mortierella elongata, and a higher proportion of ectomycorrhizal and endophytic fungi (with the highest values observed in Fresnes-sur-Escaut: 45% and 28% for ECM and endophytic fungi, respectively), well known for their capacity to have positive effects on plant development in stressful conditions. Furthermore, Pierrelaye site showed higher frequency (%) of mycorrhizal tips for ectomycorrhizal fungi (ECM) and higher intensity (%) of mycorrhizal root cortex colonization for arbuscular mycorrhizal fungi (AMF) than Fresnes-sur-Escaut site, which translates in a higher level of diversity.

CONCLUSIONS

Finally, this study demonstrated that this biofertilization approach could be recommended as an appropriate phytomanagement strategy, due to its capacity to significantly improve poplar productivity without any perturbations in soil mycobiomes.

Assessment of mycorrhizal association of a threatened medicinal plant Clerodendrum indicum (L.) O. Kuntze (Verbenaceae) in different ecological variations

Mycorrhizae association is reported to enhance the survivability of the host plant under adverse environmental conditions. The present study aims to explore the mycorrhizal association in the roots of different ecotypes of a threatened medicinal plant, Clerodendrum indicum (L.) O. Kuntze (Verbenaceae), collected from W.B., India, which correlates the degree of root colonization to the nutritional status of the native soil. Ten ecotypes of C. indicum having diverse morphological variations were collected. The mycorrhizae were characterized by both morphological and molecular methods. The nutritional status of the native soils was estimated. The study revealed that all the ecotypes have an association with mycorrhizal forms like hyphae, arbuscules, and vesicles. The molecular analysis showed Glomus intraradices and Rhizophagus irregularis as the associated arbuscular mycorrhizal fungi (AMF). A significant variation in arbuscule and vesicle formation was found growing in the varied nutritional statuses concerning soil parameters. The arbuscule was found negatively correlated with pH, conductivity, and potassium and positively correlated with organic carbon, nitrogen, and phosphorus. The vesicle was found positively correlated with pH, organic carbon, and potassium and negatively correlated with conductivity, nitrogen, and phosphorus. The interaction between conductivity: nitrogen, conductivity: phosphorus, organic-carbon: nitrogen, and pH: conductivity was significant in influencing vesicle formation. However, none of the interactions between parameters was found significant in influencing arbuscule formation. Thus, the study concludes that G. intraradices and R. irregularis are the principle mycorrhizae forming the symbiotic association with the threatened medicinal plant, C. indicum. They form vesicles and arbuscules based on their soil nutritive factors. Therefore, a large-scale propagation through a selective AMF association would help in the conservation of this threatened species from extinction.

The Diversity of Soil Bacteria and Fungi Under Altered Nitrogen and Rainfall Patterns in a Temperate Steppe

The response of soil microorganisms to altered nitrogen (N) and rainfall patterns plays an important role in understanding ecosystem carbon and nitrogen cycling processes under global change. Previous studies have separately focused on the effects of N addition and rainfall on soil microbial diversity and community composition. However, the combined and interactive impact of N addition and rainfall on soil microbial diversity and function mediated by plant and soil processes have been poorly investigated for grassland ecosystems. Here, we conducted a field experiment with simulated N addition (N addition: 10 g N m^–2yr^–1) and altered rainfall pattern [control, rainfall reduction (compared to control –50%); rainfall addition (compared to control + 50%)] to study their interactive effects on soil microbial diversity and function in a temperate steppe of Inner Mongolia. Our results showed that N addition and rainfall addition significantly increased soil bacterial diversity, and the bacterial diversity was positively correlated with soil microbial biomass nitrogen, inorganic nitrogen, and Stipa krylovii root exudate C:N ratio, Allium polyrhizum root exudate C and N, and A. polyrhizum root exudate C:N ratio. N addition and rainfall reduction significantly reduced fungal diversity, which correlated closely with soil microbial biomass carbon and the C:N ratio of A. polyrhizum root exudates. Bacteria were mainly eutrophic r-strategists, and the responses of bacterial function guilds to the interaction between N addition and rainfall pattern were not significant. However, the arbuscular mycorrhizal fungi (AMF), in the functional classification of fungi, were significantly reduced under the condition of N addition and rainfall reduction, and the absolute abundance of the phylum Glomeromycota increased under rainfall addition, suggesting that AMFs are sensitive to altered N and rainfall patterns over short timescales (1 year). Collectively, our results have important implications for understanding the plant–soil–microbe system of grasslands under climate change.

Atmospheric pollution, soil nutrients and climate effects on Mucoromycota arbuscular mycorrhizal fungi

Fine root endophyte mycorrhizal fungi in the Endogonales (Mucoromycota arbuscular mycorrhizal fungi, M‐AMF) are now recognized as at least as important globally as Glomeromycota AMF (G‐AMF), yet little is known about the environmental factors which influence M‐AMF diversity and colonization, partly because they typically only co‐colonize plants with G‐AMF. Wild populations of Lycopodiella inundata predominantly form mycorrhizas with M‐AMF and therefore allow focussed study of M‐AMF environmental drivers. Using microscopic examination and DNA sequencing we measured M‐AMF colonization and diversity over three consecutive seasons and modelled interactions between these response variables and environmental data. Significant relationships were found between M‐AMF colonization and soil S, P, C:N ratio, electrical conductivity, and the previously overlooked micronutrient Mn. Estimated N deposition was negatively related to M‐AMF colonization. Thirty‐nine Endogonales Operational Taxonomic Units (OTUs) were identified in L. inundata roots, a greater diversity than previously recognized in this plant. Endogonales OTU richness correlated negatively with soil C:N while community composition was mostly influenced by soil P. This study provides first evidence that M‐AMF have distinct ecological preferences in response to edaphic variables also related to air pollution. Future studies require site‐level atmospheric pollution monitoring to guide critical load policy for mycorrhizal fungi in heathlands and grasslands.

Nutrients Regulate the Effects of Arbuscular Mycorrhizal Fungi on the Growth and Reproduction of Cherry Tomato

Arbuscular mycorrhizal fungi (AMF) colonize the rhizosphere of plants and form a symbiotic association with plants. Mycorrhizal symbionts have diversified ecological roles and functions which are affected by soil conditions. Understanding the effects of different AMF inoculation on plants under varied nutritional conditions is of great significance for further understanding the effects of the external environment regulating mycorrhizal symbiosis on plant phenotypic traits. In this study, the effects of four AMF inoculation treatments on the growth and reproductive performance of cherry tomato (Solanum lycopersicum var. cerasiforme) were investigated under three nutrient levels by pot experiment. It was found that the growth-promoting effect of AMF on cherry tomato decreased with nutrient reduction, and the effects of the same AMF inoculation treatment on cherry tomato were different at different nutrient levels. Nutrient levels and AMF had interactive effects on flower characteristics, fruit yield, resource allocation, and seed germination of the cherry tomato. In addition, AMF could promote sexual reproductive investment. Nutrient levels and AMF also affected the accumulation of nitrogen and phosphorus in cherry tomato, and there were significant differences among different AMF inoculation treatments. The results indicated that nutrient differences could affect the symbiosis between AMF and plants, and confirmed that there were differences in the effects of the four AMF inoculation treatments on the growth and reproductive traits of plants. The differences in growth and reproduction characteristics of cherry tomato between different AMF inoculation treatments at different nutrient levels indicated that the effects of AMF mycorrhizal on the traits of cherry tomato were regulated by nutrients.

Climate warming promotes deterministic assembly of arbuscular mycorrhizal fungal communities

Arbuscular mycorrhizal fungi (AMF) significantly contribute to plant resource acquisition and play important roles in mediating plant interactions and soil carbon (C) dynamics. However, it remains unclear how AMF communities respond to climate change. We assessed impacts of warming and precipitation alterations (30% increase or decrease) on soil AMF communities, and examined major ecological processes shaping the AMF community assemblage in a Tibetan alpine meadow. Our results showed that warming significantly increased root biomass, and available nitrogen (N) and phosphorus (P) in soil. While precipitation alterations increased AMF abundances, they did not significantly affect the composition or diversity of AMF communities. In contrast, warming altered the composition of AMF communities and reduced their Shannon-Wiener index and Pielou's evenness. In particular, warming shifted the AMF community composition in favor of Diversisporaceae over Glomeraceae, likely through its impact on soil N and P availability. In addition, AMF communities were phylogenetically random in the unwarmed control but clustered in warming plots, implying more deterministic community assembly under climate warming. Warming enhancement of root growth, N and P availability likely reduced plant C-allocation to AMF, imposing stronger environmental filtering on AMF communities. We further proposed a conceptual framework that integrates biological and geochemical processes into a mechanistic understanding of warming and precipitation changes' effects on AMF. Taken together, these results suggest that soil AMF communities may be more sensitive to warming than expected, highlighting the need to monitor their community structure and associated functional consequences on plant communities and soil C dynamics under the future warmer climate.

Symbiosis and the Anthropocene

Recent human activity has profoundly transformed Earth biomes on a scale and at rates that are unprecedented. Given the central role of symbioses in ecosystem processes, functions, and services throughout the Earth biosphere, the impacts of human-driven change on symbioses are critical to understand. Symbioses are not merely collections of organisms, but co-evolved partners that arise from the synergistic combination and action of different genetic programs. They function with varying degrees of permanence and selection as emergent units with substantial potential for combinatorial and evolutionary innovation in both structure and function. Following an articulation of operational definitions of symbiosis and related concepts and characteristics of the Anthropocene, we outline a basic typology of anthropogenic change (AC) and a conceptual framework for how AC might mechanistically impact symbioses with select case examples to highlight our perspective. We discuss surprising connections between symbiosis and the Anthropocene, suggesting ways in which new symbioses could arise due to AC, how symbioses could be agents of ecosystem change, and how symbioses, broadly defined, of humans and “farmed” organisms may have launched the Anthropocene. We conclude with reflections on the robustness of symbioses to AC and our perspective on the importance of symbioses as ecosystem keystones and the need to tackle anthropogenic challenges as wise and humble stewards embedded within the system.

Nutrient trade-offs mediated by ectomycorrhizal strategies in plants: Evidence from an Abies species in subalpine forests

Ectomycorrhizal (ECM) symbiosis is an evolutionary biological trait of higher plants for effective nutrient uptakes. However, little is known that how the formation and morphological differentiations of ECM roots mediate the nutrients of below- and aboveground plant tissues and the balance among nutrient elements across environmental gradients. Here, we investigated the effects of ECM foraging strategies on root and foliar N and P concentrations and N:P ratio Abies faxoniana under variations of climate and soil conditions.The ECM symbionts preferentially mediated P uptake under both N and P limitations. The uptake efficiency of N and P was primarily associated with the ECM root traits, for example, ECM root tip density, superficial area of ECM root tips, and the ratio of living to dead root tips, and was affected by the ECM proliferations and morphological differentiations. The tissue N and P concentrations were positively associated with the abundance of the contact exploration type and negatively with that of the short-distance exploration type.Our findings indicate that the nutritional status of both below- and aboveground plant tissues can be strongly affected by ECM symbiosis in natural environments. Variations in the ECM strategies in response to varying environmental conditions significantly influence plant nutrient uptakes and trade-offs.

Microbe-mediated adaptation in plants

Interactions with microbial symbionts have yielded great macroevolutionary innovations across the tree of life, like the origins of chloroplasts and the mitochondrial powerhouses of eukaryotic cells. There is also increasing evidence that host-associated microbiomes influence patterns of microevolutionary adaptation in plants and animals. Here we describe how microbes can facilitate adaptation in plants and how to test for and differentiate between the two main mechanisms by which microbes can produce adaptive responses in higher organisms: microbe-mediated local adaptation and microbe-mediated adaptive plasticity. Microbe-mediated local adaptation is when local plant genotypes have higher fitness than foreign genotypes because of a genotype-specific affiliation with locally beneficial microbes. Microbe-mediated adaptive plasticity occurs when local plant phenotypes, elicited by either the microbial community or the non-microbial environment, have higher fitness than foreign phenotypes as a result of interactions with locally beneficial microbes. These microbial effects on adaptation can be difficult to differentiate from traditional modes of adaptation but may be prevalent. Ignoring microbial effects may lead to erroneous conclusions about the traits and mechanisms underlying adaptation, hindering management decisions in conservation, restoration, and agriculture.

Global negative effects of nutrient enrichment on arbuscular mycorrhizal fungi, plant diversity and ecosystem multifunctionality

Despite widespread anthropogenic nutrient enrichment, it remains unclear how nutrient enrichment influences plant-arbuscular mycorrhizal fungi (AMF) symbiosis and ecosystem multifunctionality at the global scale. Here, we conducted a meta-analysis to examine the worldwide effects of nutrient enrichment on AMF and plant diversity and ecosystem multifunctionality using data of field experiments from 136 papers. Our analyses showed that nutrient addition simultaneously decreased AMF diversity and abundance belowground and plant diversity aboveground at the global scale. The decreases in AMF diversity and abundance associated with nutrient addition were more pronounced with increasing experimental duration, mean annual temperature (MAT) and mean annual precipitation (MAP). Nutrient addition-induced changes in soil pH and available phosphorus (P) predominantly regulated the responses of AMF diversity and abundance. Furthermore, AMF diversity correlated with ecosystem multifunctionality under nutrient addition worldwide. Our findings identify the negative effects of nutrient enrichment on AMF and plant diversity and suggest that AMF diversity is closely linked with ecosystem function. This study offers an important advancement in our understanding of plant-AMF interactions and their likely responses to ongoing global change.

Responses of arbuscular mycorrhizal fungi to nitrogen addition: A meta-analysis

Arbuscular mycorrhizal (AM) fungi play important roles in carbon (C), nitrogen (N) and phosphorus (P) cycling of terrestrial ecosystems. The impact of increasing N deposition on AM fungi will inevitably affect ecosystem processes. However, generalizable patterns of how N deposition affects AM fungi remains poorly understood. Here we conducted a global-scale meta-analysis from 94 publications and 101 sites to investigate the responses of AM fungi to N addition, including abundance in both intra-radical (host roots) and extra-radical portion (soil), richness and diversity. We also explored the mechanisms of N addition affecting AM fungi by the trait-based guilds method. Results showed that N addition significantly decreased AM fungal overall abundance (-8.0%). However, the response of abundance in intra-radical portion was not consistent with that in extra-radical portion: root colonization decreased (-11.6%) significantly, whereas extra-radical hyphae length density did not change significantly. Different AM fungal guilds showed different responses to N addition: both the abundance (spore density) and relative abundance of the rhizophilic guild decreased significantly under N addition (-29.8% and -12.0%, respectively), while the abundance and relative abundance of the edaphophilic guild had insignificant response to N addition. Such inconsistent responses of rhizophilic and edaphophilic guilds were mainly moderated by the change of soil pH and the response of root biomass, respectively. Moreover, N addition had an insignificant negative effect on AM fungal richness and diversity, which was strongly related with the relative availability of soil P (i.e. soil available N/P ratio). Collectively, this meta-analysis highlights that considering trait-based AM fungal guilds, soil P availability and host plant C allocation can greatly improve our understanding of the nuanced dynamics of AM fungal communities under increasing N deposition.

Divergent biotic and abiotic filtering of root endosphere and rhizosphere soil fungal communities along ecological gradients

Plant roots assemble in two distinct microbial compartments: the rhizosphere (microbes in soil surrounding roots) and the endosphere (microbes within roots). Our knowledge of fungal community assembly in these compartments is limited, especially in wetlands. We tested the hypothesis that biotic factors would have direct effects on rhizosphere and endosphere assembly, while abiotic factors would have direct and indirect effects. Using a field study, we examined the influences of salinity, water level and biotic factors on baldcypress (Taxodium distichum) fungal communities. We found that endosphere fungi, unlike rhizosphere fungi, were correlated with host density and canopy cover, suggesting that hosts can impose selective filters on fungi colonizing their roots. Meanwhile, local abiotic conditions strongly influenced both rhizosphere and endosphere diversity in opposite patterns, e.g. highest endosphere diversity (hump-shaped) and lowest rhizosphere diversity (U-shaped) at intermediate salinity levels. These results indicate that the assembly and structure of the root endosphere and rhizosphere within a host can be shaped by different processes. Our results also highlight the importance of assessing how environmental changes affect plant and plant-associated fungal communities in wetland ecosystems where saltwater intrusion and sea level rise are major threats to both plant and fungal communities.

Climate Disruption of Plant-Microbe Interactions

Interactions between plants and microbes have important influences on evolutionary processes, population dynamics, community structure, and ecosystem function. We review the literature to document how climate change may disrupt these ecological interactions and develop a conceptual framework to integrate the pathways of plant-microbe responses to climate over different scales in space and time. We then create a blueprint to aid generalization that categorizes climate effects into changes in the context dependency of plant-microbe pairs, temporal mismatches and altered feedbacks over time, or spatial mismatches that accompany species range shifts. We pair a new graphical model of how plant-microbe interactions influence resistance to climate change with a statistical approach to predictthe consequences of increasing variability in climate. Finally, we suggest pathways through which plant-microbe interactions can affect resilience during recovery from climate disruption. Throughout, we take a forward-looking perspective, highlighting knowledge gaps and directions for future research.

Microbe-mediated local adaptation to limestone barrens is context dependent

Plant-root-associated microbes influence plant phenotype and tolerance to environmental stress, and thus have been hypothesized to play a role in plant local adaptation. Here, we test this hypothesis with factorial experiments addressing the role of microbes in local adaptation of Hypericum perforatum (St. John's wort) to stressful limestone barrens (alvars) compared to neighboring old-fields. Alvar plants benefited more from microbes in early life history stages, while at later growth stages, alvar and old-field plants benefited equally from microbes but only in the old-field habitat. Patterns of local adaptation were changed by the presence of microbes. Alvar plants grown in association with alvar microbes outperformed old-field plants in the alvar habitat, whereas old-field plants showed patterns of maladaptation when grown with microbes. In this demonstration of microbe-mediated adaptation, we show that rhizosphere microbes can be important for plant fitness and patterns of local adaptation but that those effects are dependent on life-history stage and habitat.

Role of Leaf Litter in Above-Ground Wood Decay

The effects of leaf litter on moisture content and fungal decay development in above-ground wood specimens were assessed. Untreated southern pine specimens were exposed with or without leaf litter contact. Two types of leaf litter were evaluated; aged (decomposed) and young (early stages of decomposition). The moisture content of specimens was monitored, and specimens were periodically removed for visual evaluation of decay development. In addition, amplicon-based sequencing analysis of specimens and associated leaf litter was conducted at two time points. Contact with either type of leaf litter resulted in consistently higher moisture contents than those not in contact with leaf litter. Visually, evident decay developed most rapidly in specimens in contact with the aged leaf litter. Analysis of amplicon-based sequencing revealed that leaf litter contributes a significant amount of the available wood decay fungal community with similar communities found in the litter exposed wood and litter itself, but dissimilar community profiles from unexposed wood. Dominant species and guild composition shifted over time, beginning initially with more leaf saprophytes (ascomycetes) and over time shifting to more wood rotting fungi (basidiomycetes). These results highlight the importance of the contributions of leaf litter to fungal colonization and subsequent decay hazard for above-ground wood.

Non-structural carbohydrate concentrations of Fagus sylvatica and Pinus sylvestris fine roots are linked to ectomycorrhizal enzymatic activity during spring reactivation

We evaluated whether changes in fine root non-structural carbohydrate reserves of Fagus sylvatica and Pinus sylvestris trees influence potential enzymatic activities of their ectomycorrhizal symbionts from winter towards spring reactivation, and whether these changes influence potential soil enzymatic activities. We analyzed sugar and starch concentrations in the fine roots of Fagus sylvatica and Pinus sylvestris and potential activities of ß-glucosidase, ß-xylosidase, and cellobiohydrolase (as proxies for carbon-degrading enzymes) as well as leucine aminopeptidase and chitinase (as proxies for nitrogen-degrading enzymes) of their dominant ectomycorrhizal symbionts as well as in the soil. Sugar concentrations in the fine roots were significantly positively correlated with enzymatic activities of the ectomycorrhizal symbionts. In Pinus sylvestris, both carbon- and nitrogen-degrading enzyme activities showed significant positive correlations with fine root sugar concentrations. In Fagus sylvatica, fine root sugar concentrations were explicitly positively correlated with the activity of nitrogen-degrading enzymes. The chitinase activity in the soil was found to be strongly positively correlated with the enzymatic activity of the ectomycorrhizal symbionts as well as with fine root sugar concentrations. Fine root carbohydrate concentrations of Fagus sylvatica and Pinus sylvestris trees and enzymatic activities of their associated ectomycorrhizal fungi are connected. The specific nutrient demand of the tree species during spring reactivation may affect ectomycorrhizal enzymatic activity via carbon mobilization in the fine roots of Fagus sylvatica and Pinus sylvestris. Moreover, our results suggest that trees indirectly contribute to the degradation of fungal necromass by stimulating ectomycorrhizal chitinase activity in the soil.

Conifers depend on established roots during drought: results from a coupled model of carbon allocation and hydraulics

Trees may survive prolonged droughts by shifting water uptake to reliable water sources, but it is unknown if the dominant mechanism involves activating existing roots or growing new roots during drought, or some combination of the two. To gain mechanistic insights on this unknown, a dynamic root-hydraulic modeling framework was developed that set up a feedback between hydraulic controls over carbon allocation and the role of root growth on soil-plant hydraulics. The new model was tested using a 5 yr drought/heat field experiment on an established piñon-juniper stand with root access to bedrock groundwater. Owing to the high carbon cost per unit root area, modeled trees initialized without adequate bedrock groundwater access experienced potentially lethal declines in water potential, while all of the experimental trees maintained nonlethal water potentials. Simulated trees were unable to grow roots rapidly enough to mediate the hydraulic stress, particularly during warm droughts. Alternatively, modeled trees initiated with root access to bedrock groundwater matched the hydraulics of the experimental trees by increasing their water uptake from bedrock groundwater when soil layers dried out. Therefore, the modeling framework identified a critical mechanism for drought response that required trees to shift water uptake among existing roots rather than growing new roots.

Arbuscular mycorrhizal fungi improve the growth and disease resistance of the invasive plant Wedelia trilobata

AIMS

Arbuscular mycorrhizal fungi (AMF) are symbiotic partners of many invasive plants, however, it is still unclear how AMF contribute to traits that are important for the successful invasion of their host and how environmental factors, such as nutrient conditions, influence this. This study was to explore the effects of Glomus versiforme (GV) and Glomus mosseae (GM) on the growth and disease resistance of the invasive plant Wedelia trilobata under different nutrient conditions.

METHODS AND RESULTS

We found that GV and GM had higher root colonization rates resulting in faster W. trilobata growth under both low-N and low-P nutrient conditions compared to the normal condition. Also, the colonization of W. trilobata by GV significantly reduced the infection area of the pathogenic fungus Rhizoctonia solani under low-N conditions.

CONCLUSIONS

These results demonstrated that AMF can promote the growth and pathogenic defence of W. trilobata in a nutrient-poor environment, which might contribute to their successful invasion into certain type of habitats.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, we report for the first time that AMF can promote growth and disease resistance of W. trilobata under nutrient-poor environment, which contribute to a better understanding of plant invasion.

Arbuscular mycorrhizal fungi alleviate phosphorus limitation by reducing plant N:P ratios under warming and nitrogen addition in a temperate meadow ecosystem

Global change apart from ecosystem processes also influences the community structure of key organisms, such as arbuscular mycorrhizal fungi (AMF). We conducted a 3-year experiment where we suppressed with benomyl mycorrhiza to understand how AMF alter the plant community structure under warming and nitrogen (N) addition. The elemental content and foliar tissue stoichiometry of the dominant species Leymus chinensis and the subordinate species Puccinellia tenuiflora were studied along with soil nutrient stoichiometries. Overall, N addition enhanced plant N: phosphorus (P) ratios at a greater level than experimental warming did. Under global change conditions, AMF symbionts significantly increased soil available P concentrations, promoted plant P absorption and decreased the plant N:P ratios. AMF alleviate P limitation by reducing plant N:P ratios. Our results highlight that the negative influence of global change on plant productivity might cancel each other out through the additive effects of AMF and that global change will increase the dependency of plants on their mycorrhizal symbionts.

Impacts of long-term elevated atmospheric CO2 concentrations on communities of arbuscular mycorrhizal fungi

The ecological impacts of long-term elevated atmospheric CO2 (eCO2 ) levels on soil microbiota remain largely unknown. This is particularly true for the arbuscular mycorrhizal (AM) fungi, which form mutualistic associations with over two-thirds of terrestrial plant species and are entirely dependent on their plant hosts for carbon. Here, we use high-resolution amplicon sequencing (Illumina, HiSeq) to quantify the response of AM fungal communities to the longest running (>15 years) free-air carbon dioxide enrichment (FACE) experiment in the Northern Hemisphere (GiFACE); providing the first evaluation of these responses from old-growth (>100 years) semi-natural grasslands subjected to a 20% increase in atmospheric CO2 . eCO2 significantly increased AM fungal richness but had a less-pronounced impact on the composition of their communities. However, while broader changes in community composition were not observed, more subtle responses of specific AM fungal taxa were with populations both increasing and decreasing in abundance in response to eCO2 . Most population-level responses to eCO2 were not consistent through time, with a significant interaction between sampling time and eCO2 treatment being observed. This suggests that the temporal dynamics of AM fungal populations may be disturbed by anthropogenic stressors. As AM fungi are functionally differentiated, with different taxa providing different benefits to host plants, changes in population densities in response to eCO2 may significantly impact terrestrial plant communities and their productivity. Thus, predictions regarding future terrestrial ecosystems must consider changes both aboveground and belowground, but avoid relying on broad-scale community-level responses of soil microbes observed on single occasions.

Exploring the role of ectomycorrhizal fungi in soil carbon dynamics

The extent to which ectomycorrhizal (ECM) fungi enable plants to access organic nitrogen (N) bound in soil organic matter (SOM) and transfer this growth-limiting nutrient to their plant host, has important implications for our understanding of plant-fungal interactions, and the cycling and storage of carbon (C) and N in terrestrial ecosystems. Empirical evidence currently supports a range of perspectives, suggesting that ECM vary in their ability to provide their host with N bound in SOM, and that this capacity can both positively and negatively influence soil C storage. To help resolve the multiplicity of observations, we gathered a group of researchers to explore the role of ECM fungi in soil C dynamics, and propose new directions that hold promise to resolve competing hypotheses and contrasting observations. In this Viewpoint, we summarize these deliberations and identify areas of inquiry that hold promise for increasing our understanding of these fundamental and widespread plant symbionts and their role in ecosystem-level biogeochemistry.

Utilizing herbarium specimens to quantify historical mycorrhizal communities

PREMISE OF THE STUDY

Mycorrhiza are critical to ecosystem functioning, but a lack of historical baseline data limits our understanding of the long-term belowground effects of global change. Herbarium specimens may provide this needed insight. However, it is unknown whether DNA of arbuscular mycorrhizal fungi (AMF) can be reliably extracted from vascular plant specimen roots.

METHODS

We sampled roots from herbarium specimens of four herbaceous forest species collected in western Pennsylvania between 1881-2008. Using molecular methods (terminal restriction fragment length polymorphism and sequence analysis), we quantified AMF communities from specimen roots and tested for contamination.

RESULTS

We successfully amplified AMF DNA from 44% (21/48) of the root but not leaf samples, indicating specimen contamination was negligible. As expected, there were significant differences in AMF composition between plant species (P < 0.05). However, no differences in AMF communities were detected through time, possibly due to limited sample size and low amplification rates in recent collections.

DISCUSSION

Herbaria have potential as sources of valuable belowground microbial data to answer questions across geographic, temporal, and taxonomic scales otherwise not feasible. Ongoing methodological developments will only magnify this potential. Further tests are needed to determine curatorial practices that maximize this innovative use of herbarium specimens.

Plant-mycorrhizal interactions mediate plant community coexistence by altering resource demand

As the diversity of plants increases in an ecosystem, so does resource competition for soil nutrients, a process that mycorrhizal fungi can mediate. The influence of mycorrhizal fungi on plant biodiversity likely depends on the strength of the symbiosis between the plant and fungi, the differential plant growth responses to mycorrhizal inoculation, and the transfer rate of nutrients from the fungus to plant. However, our current understanding of how nutrient-plant-mycorrhizal interactions influence plant coexistence is conceptual and thus lacks a unified quantitative framework. To quantify the conditions of plant coexistence mediated by mycorrhizal fungi, we developed a mechanistic resource competition model that explicitly included plant-mycorrhizal symbioses. We found that plant-mycorrhizal interactions shape plant coexistence patterns by creating a tradeoff in resource competition. Especially, a tradeoff in resource competition was caused by differential payback in the carbon resources that plants invested in the fungal symbiosis and/or by the stoichiometric constraints on plants that required additional, less-beneficial, resources to sustain growth. Our results suggested that resource availability and the variation in plant-mycorrhizal interactions act in concert to drive plant coexistence patterns. Applying our framework, future empirical studies should investigate plant-mycorrhizal interactions under multiple levels of resource availability.

Responses of arbuscular mycorrhizal fungi to long-term inorganic and organic nutrient addition in a lowland tropical forest

Improved understanding of the nutritional ecology of arbuscular mycorrhizal (AM) fungi is important in understanding how tropical forests maintain high productivity on low-fertility soils. Relatively little is known about how AM fungi will respond to changes in nutrient inputs in tropical forests, which hampers our ability to assess how forest productivity will be influenced by anthropogenic change. Here we assessed the influence of long-term inorganic and organic nutrient additions and nutrient depletion on AM fungi, using two adjacent experiments in a lowland tropical forest in Panama. We characterised AM fungal communities in soil and roots using 454-pyrosequencing, and quantified AM fungal abundance using microscopy and a lipid biomarker. Phosphorus and nitrogen addition reduced the abundance of AM fungi to a similar extent, but affected community composition in different ways. Nutrient depletion (removal of leaf litter) had a pronounced effect on AM fungal community composition, affecting nearly as many OTUs as phosphorus addition. The addition of nutrients in organic form (leaf litter) had little effect on any AM fungal parameter. Soil AM fungal communities responded more strongly to changes in nutrient availability than communities in roots. This suggests that the 'dual niches' of AM fungi in soil versus roots are structured to different degrees by abiotic environmental filters, and biotic filters imposed by the plant host. Our findings indicate that AM fungal communities are fine-tuned to nutrient regimes, and support future studies aiming to link AM fungal community dynamics with ecosystem function.

The Effects of Arbuscular Mycorrhizal Fungal Colonisation on Nutrient Status, Growth, Productivity, and Canker Resistance of Apple (Malus pumila)

We assess whether arbuscular mycorrhizal fungi (AMF) improve growth, nutritional status, phenology, flower and fruit production, and disease resistance in woody perennial crops using apple (Malus pumila) as a study system. In a fully factorial experiment, young trees were grown for 3 years with or without AMF (Funneliformis mosseae and Rhizophagus irregularis), and with industrial standard fertiliser applications or restricted fertiliser (10% of standard). We use two commercial scions (Dabinett and Michelin) and rootstocks (MM111 and MM106). Industrial standard fertiliser applications reduced AMF colonisation and root biomass, potentially increasing drought sensitivity. Mycorrhizal status was influenced by above ground genotypes (scion type) but not rootstocks, indicating strong interactions between above and below ground plant tissue. The AMF inoculation significantly increased resistance to Neonectria ditissima, a globally economically significant fungal pathogen of apple orchards, but did not consistently alter leaf nutrients, growth, phenology or fruit and flower production. This study significantly advances understanding of AMF benefits to woody perennial crops, especially increased disease resistance which we show is not due to improved tree nutrition or drought alleviation. Breeding programmes and standard management practises can limit the potential for these benefits.

Arbuscular Mycorrhizal Symbiosis as a Promising Resource for Improving Berry Quality in Grapevines Under Changing Environments

Climate change and their resulting impacts are becoming a concern for winegrowers due to the high socioeconomic relevance of the winemaking sector worldwide. In fact, the projected climate change is expected to have detrimental impacts on the yield of grapevines, as well as on the quality and properties of grapes and wine. It is well known that arbuscular mycorrhizal fungi (AMF) can improve the nutritional quality of edible parts of crops and play essential roles in the maintenance of host plant fitness under stressed environments, including grapevines. The future scenarios of climate change may also modify the diversity and the growth of AMF in soils as well as the functionality of the mycorrhizal symbiosis. In this review, we summarize recent research progress on the effects of climate change on grapevine metabolism, paying special attention to the secondary compounds involved in the organoleptic properties of grapes and wines and to the levels of the phytohormones implied in the control of berry development and fruit ripening. In this context, the potential role of AMF for maintaining fruit quality in future climate change scenarios is discussed.

Chronic fertilization of 37 years alters the phylogenetic structure of soil arbuscular mycorrhizal fungi in Chinese Mollisols

Arbuscular mycorrhizal fungi (AMF) play vital roles in sustaining soil productivity and plant communities. However, adaption and differentiation of AMF in response to commonly used fertilization remain poorly understood. In this study, we showed that the AMF community composition was primarily driven by soil physiochemical changes associated with chronic inorganic and organic fertilization of 37 years in Mollisols. High-throughput sequencing indicated that inorganic fertilizer negatively affected AMF diversity and richness, implying a reduction of mutualism in plant-AMF symbiosis; however, a reverse trend was observed for the application of inorganic fertilizer combined with manure. With regards to AMF community composition, order Glomerales was dominant, but varied significantly among different fertilization treatments. All fertilization treatments decreased family Glomeraceae and genus Funneliformis, while Rhizophagus abundance increased. Plant-growth-promoting-microorganisms of family Claroideoglomeraceae and genus Claroideoglomus were stimulated by manure application, and likely benefited pathogen suppression and phosphorus (P) acquisition. Family Gigasporaceae and genus Gigaspora were negatively correlated with available P in soil. Additionally, redundancy analysis further suggested that soil available P, organic matter and pH were the most important factors in shaping AMF community composition. These results provide strong evidence for niche differentiation of phylogenetically distinct AMF populations under different fertilization regimes. Manure likely contributes to restoration and maintenance of plant-AMF symbiosis, and the balanced fertilization would favor the growth of beneficial AMF communities as one optimized management in support of sustainable agriculture in Mollisols.

Tracking plant preference for higher-quality mycorrhizal symbionts under varying CO2 conditions over multiple generations

The symbiosis between plants and root-colonizing arbuscular mycorrhizal (AM) fungi is one of the most ecologically important examples of interspecific cooperation in the world. AM fungi provide benefits to plants; in return plants allocate carbon resources to fungi, preferentially allocating more resources to higher-quality fungi. However, preferential allocations from plants to symbionts may vary with environmental context, particularly when resource availability affects the relative value of symbiotic services. We ask how differences in atmospheric CO 2-levels influence root colonization dynamics between AMF species that differ in their quality as symbiotic partners. We find that with increasing CO 2-conditions and over multiple plant generations, the more beneficial fungal species is able to achieve a relatively higher abundance. This suggests that increasing atmospheric carbon supply enables plants to more effectively allocate carbon to higher-quality mutualists, and over time helps reduce lower-quality AM abundance. Our results illustrate how environmental context may affect the extent to which organisms structure interactions with their mutualistic partners and have potential implications for mutualism stability and persistence under global change.

The mycorrhizal type governs root exudation and nitrogen uptake of temperate tree species

Even though the two dominant mycorrhizal associations of temperate tree species differentially couple carbon (C) and nitrogen (N) cycles in temperate forests, systematic differences between the biogeochemical cycles of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree species remain poorly described. A classification according to the mycorrhizal type offers the chance, though, to develop a global frame concept for the prediction of temperate ecosystem responses to environmental change. Focusing on the influence of mycorrhizal types on two key plant processes of biogeochemical cycling (root exudation and N acquisition), we investigated four temperate deciduous tree species per mycorrhizal type in a drought experiment in large mesocosms. We hypothesized that (H1) C loss by root exudation is higher in ECM than in AM trees, (H2) drought leads to higher reductions in root exudation of drought-sensitive ECM trees and (H3) inorganic N uptake is higher in AM than in ECM trees. In contradiction to H2, we found no systematic difference in root exudation between the mycorrhizal types at ample soil moisture, but almost twofold higher exudation in ECM trees when exposed to soil drought. In addition, photosynthetic C cost of root exudation strongly increased by ~10-fold in drought-treated ECM trees, while it only doubled in AM trees, which confirms H1. With respect to H3, we corroborated that AM trees had higher absolute and relative inorganic N acquisition rates than ECM trees, while the organic N uptake did not differ between mycorrhizal types. We conclude that ECM trees are less efficient in inorganic N uptake than AM trees, but ECM trees increase root C release as an adaptive response to dry soil to maintain hydraulic conductivity and/or nutrient availability. These systematic differences in key biogeochemical processes support hints on the key role of the mycorrhizal types in coupling C and N cycles in temperate forests.

Temperature drives the assembly of endophytic communities' seasonal succession

Endophytic microorganisms asymptomatically colonise plant tissues. Exploring the assembly dynamics of bacterial endophytic communities is essential to understand the functioning of the plant holobiont and to optimise their possible use as biopesticides or plant biostimulants. The variation in endophytic communities in above and below-ground organs in Vitis vinifera in the field were studied. To understand the specific effect of temperature on endophytic communities, a separate experiment was set up where grapevine cuttings were grown under controlled conditions at three different temperatures. The findings revealed the succession of endophytic communities over the year. Endophytic communities of roots and stems differ in terms of composition and dynamic response to temperature. Noticeably, compositional differences during the seasons affected bacterial taxa more in stems than in roots, suggesting that roots offer a more stable and less easily perturbed environment. Correlation abundance networks showed that the presence of several taxa (including Bradyrhizobium, Burkholderia, Dyella, Mesorhizobium, Propionibacterium and Ralstonia) is linked in both the field and the greenhouse.

Arbuscular mycorrhizal fungal community composition is altered by long-term litter removal but not litter addition in a lowland tropical forest

Tropical forest productivity is sustained by the cycling of nutrients through decomposing organic matter. Arbuscular mycorrhizal (AM) fungi play a key role in the nutrition of tropical trees, yet there has been little experimental investigation into the role of AM fungi in nutrient cycling via decomposing organic material in tropical forests. We evaluated the responses of AM fungi in a long-term leaf litter addition and removal experiment in a tropical forest in Panama. We described AM fungal communities using 454-pyrosequencing, quantified the proportion of root length colonised by AM fungi using microscopy, and estimated AM fungal biomass using a lipid biomarker. AM fungal community composition was altered by litter removal but not litter addition. Root colonisation was substantially greater in the superficial organic layer compared with the mineral soil. Overall colonisation was lower in the litter removal treatment, which lacked an organic layer. There was no effect of litter manipulation on the concentration of the AM fungal lipid biomarker in the mineral soil. We hypothesise that reductions in organic matter brought about by litter removal may lead to AM fungi obtaining nutrients from recalcitrant organic or mineral sources in the soil, besides increasing fungal competition for progressively limited resources.

Temperature-mediated local adaptation alters the symbiotic function in arbuscular mycorrhiza

Variation in the symbiotic function of arbuscular mycorrhizal fungi (AM fungi) has been demonstrated among distinct biotic and abiotic interactions. However, there is little knowledge on how local temperature conditions influence the functional divergence of AM symbionts in alpine ecosystems. Here, we conduct a reciprocal inoculation experiment to explore the three-way interactions among plants, AM fungal inoculum and temperature at sites of contrasting elevation. Evidence of local adaptation of plant growth was found only under low temperature conditions, with no consistent local versus foreign effect found in AM fungal performance. The origin of either the plant or the inoculum relative to the temperature was important in explaining symbiotic function. Specifically, when inoculum and temperature were sympatric but allopatric to the plant, poor adaptation by the plant to the novel environment was clearly found under both temperature conditions. Further analysis found that the symbiotic function was inversely related to fungal diversity under high temperature conditions. These results suggest that local adaptation represents a powerful factor in the establishment of novel combinations of plant, inoculum and temperature, and confirms the importance of taking into account both biotic and abiotic interactions in the prediction of the response of symbionts to global environmental change.

A phosphorus threshold for mycoheterotrophic plants in tropical forests

The majority of terrestrial plants associate with arbuscular mycorrhizal (AM) fungi, which typically facilitate the uptake of limiting mineral nutrients by plants in exchange for plant carbon. However, hundreds of non-photosynthetic plant species-mycoheterotrophs-depend entirely on AM fungi for carbon as well as mineral nutrition. Mycoheterotrophs can provide insight into the operation and regulation of AM fungal relationships, but little is known about the factors, fungal or otherwise, that affect mycoheterotroph abundance and distribution. In a lowland tropical forest in Panama, we conducted the first systematic investigation into the influence of abiotic factors on the abundance and distribution of mycoheterotrophs, to ask whether the availability of nitrogen and phosphorus altered the occurrence of mycoheterotrophs and their AM fungal partners. Across a natural fertility gradient spanning the isthmus of Panama, and also in a long-term nutrient-addition experiment, mycoheterotrophs were entirely absent when soil exchangeable phosphate concentrations exceeded 2 mg P kg-1 Experimental phosphorus addition reduced the abundance of AM fungi, and also reduced the abundance of the specific AM fungal taxa required by the mycoheterotrophs, suggesting that the phosphorus sensitivity of mycoheterotrophs is underpinned by the phosphorus sensitivity of their AM fungal hosts. The soil phosphorus concentration of 2 mg P kg-1 also corresponds to a marked shift in tree community composition and soil phosphatase activity across the fertility gradient, suggesting that our findings have broad ecological significance.