The ectomycorrhizal status of a tropical black bolete, <Emphasis Type="Italic">Phlebopus portentosus</Emphasis>, assessed using mycorrhizal synthesis and isotopic analysis

Phlebopus portentosus is one of the most popular wild edible mushrooms in Thailand and can produce sporocarps in the culture without a host plant. However, it is still unclear whether Phlebopus portentosus is a saprotrophic, parasitic, or ectomycorrhizal (ECM) fungus. In this study, Phlebopus portentosus sporocarps were collected from northern Thailand and identified based on morphological and molecular characteristics. We combined mycorrhizal synthesis and stable isotopic analysis to investigate the trophic status of this fungus. In a greenhouse experiment, ECM-like structures were observed in Pinus kesiya at 1 year after inoculation with fungal mycelium, and the association of Phlebopus portentosus and other plant species showed superficial growth over the root surface. Fungus-colonized root tips were described morphologically and colonization confirmed by molecular methods. In stable isotope measurements, the δ¹³C and δ¹⁵N of natural samples of Phlebopus portentosus differed from saprotrophic fungi. Based on the isotopic patterns of Phlebopus portentosus and its ability to form ECM-like structures in greenhouse experiments, we conclude that Phlebopus portentosus could be an ECM fungus.     

Nutrient and water dynamics of Amazonian <Emphasis Type="Italic">canga</Emphasis> vegetation differ among physiognomies and from those of other neotropical ecosystems

Ferriferous savannas, also known as cangas in Brazil, are nutrient-impoverished ecosystems adapted to seasonal droughts. These ecosystems support distinctive vegetation physiognomies and high plant diversity, although little is known about how nutrient and water availability shape these ecosystems. Our study was carried out in the cangas from Carajás, eastern Amazonia, Brazil. To investigate the N cycling and drought adaptations of different canga physiognomies and compare the findings with those from other ecosystems, we analyzed nutrient concentrations and isotope ratios (δ¹³C and δ¹⁵N) of plants, litter, and soils from 36 plots distributed in three physiognomies: typical scrubland (SB), Vellozia scrubland (VL), and woodland (WD). Foliar δ¹⁵N values in cangas were higher than those in savannas but lower than those in tropical forests, indicating more conservative N cycles in Amazonian cangas than in forests. The lower δ¹⁵N in savanna formations may be due to a higher importance of mycorrhizal species in savanna vegetation than in canga vegetation. Elevated δ¹³C values indicate higher water shortage in canga ecosystems than in forests. Foliar and litter nutrient concentrations vary among canga physiognomies, indicating differences in nutrient dynamics. Lower nutrient availability, higher C:N ratios, and lower δ¹⁵N values characterize VL, whereas WD is delineated by lower δ¹³C values and higher soil P. These results suggest lower water restriction and lower P limitation in WD, whereas VL shows more conserved N cycles due to lower nutrient availability. Differences in nutrient and water dynamics among physiognomies indicate different ecological processes; thus, the conservation of all physiognomies is required to ensure the maintenance of functional diversity in this unique ecosystem.     

Imbalanced plant stoichiometry at contrasting geologic-derived phosphorus sites in subtropics: the role of microelements and plant functional group

Aims

In western North America ectomycorrhizal fungi are critical to establishment of conifers in low nitrogen soils. Fire can affect both ectomycorrhizal fungi and soil properties, and inoculation with ectomycorrhizal fungi is recommended when planting on burns for restoration. The aim of this study was to examine how Suillus species used in inoculation affect whitebark pine (Pinus albicaulis L.) seedlings planted in fire-impacted soil.

Methods

In a greenhouse experiment, Suillus-colonized and uncolonized whitebark pine seedlings were planted in unsterilized and sterilized (control) soil from a recent burn. After 6 months, foliar nitrogen and carbon content, concentration, and stable isotope values were assessed, along with growth parameters.

Results

When seedlings were colonized, biomass was 61% greater, foliar nitrogen content 25% higher, foliar nitrogen concentration 30–63% lower; needles had lower δ¹⁵N and higher δ¹³C. Differences were more pronounced in sterilized soil where colonization was higher. Foliar N content was negatively correlated with δ¹⁵N values.

Conclusions

Colonization by host-specific fungi produced larger seedlings with higher foliar nitrogen content in both burn soils. The hypothesis that ectomycorrhizal fungi on roots fractionate nitrogen isotopes leading to lower δ¹⁵N in needles is supported. This helps explain restoration outcomes, and bridges the gap between field and in vitro investigations.

     

Trophic plasticity among spring vs. cave populations of <Emphasis Type="Italic">Gammarus minus:</Emphasis> examining functional niches using stable isotopes and C/N ratios

In some environments, species may exhibit trophic plasticity, which allows them to extend beyond their assigned functional group. For Gammarus minus, a freshwater amphipod classified as a shredder or detritivore, cave populations have been observed consuming heterotrophs as well as shredding leaves, and therefore may be exhibiting trophic plasticity. To test this possibility, we examined the C and N stable isotope and C/N ratios for cave and spring populations of G. minus. A 15-day feeding experiment using leaves and G. minus from a spring population established that the diet-tissue discrimination factor was 3.2 ‰ for δ¹⁵N. Cave G. minus were 8 ‰ higher in δ¹⁵N relative to cave leaves, indicating they did not derive nitrogen from leaves, whereas field collected spring populations were 2–3 ‰ higher than spring leaves, indicating that they did. Cave G. minus were 2.6 ‰ higher in δ¹⁵N than the cave isopod, Caecidotea holsingeri. Relative to spring populations, Organ Cave G. minus were ¹⁵N enriched by 6 ‰, suggesting they occupied a different trophic level, or incorporated an isotopically distinct N source. While stable isotopes cannot tell what the cave G. minus are eating, the isotopes certainly show that G. minus are not eating leaves and are trophically distinct form the surface populations. Differences in C/N ratios were observed, but reflect the size of the G. minus examined and not feeding group or habitat. The isotope data strongly support the hypothesis that cave populations of G. minus have become generalist or omnivorous by including animal protein in their diet.     

Variations in the Carbon and Nitrogen Isotope Composition of the Crabs <Emphasis Type="Italic">Chionoecetes opilio</Emphasis> (Fabricius, 1788) and <Emphasis Type="Italic">Hyas coarctatus</Emphasis> Leach, 1816 (Crustacea: Decapoda) from the Chukchi Sea

Some regions of the Chukchi Sea shelf are characterized by an unusually high abundance of the benthos. The carbon and nitrogen isotope composition (δ¹³C and δ¹⁵N) of two crab species, Chionoecetes opilio and Hyas coarctatus, which occupy the upper trophic levels in bottom communities in the southern regions influenced by the Anadyr Water (AW) and in the regions exposed to the influence of the Siberian Coastal Current (SCC), was analyzed in order to compare between the trophic characteristics of their benthic communities. It has been found that these two species have similar spatial trends of stable isotope signature variations. As the effect of the productive AW on bottom communities decreases, a substantial depletion of δ¹³C with a simultaneous growth of δ¹⁵N values are observed in crab tissues. The trophic niches of C. opilio and H. coarctatus, which are represented by “isotope” niches as standard ellipse areas in the δ¹³C/δ¹⁵N biplot, significantly overlap in communities of the southern part of the shelf and become completely separated in those of the regions influenced by the SCC. The separation of the isotope niches of C. opilio and H. coarctatus and the simultaneous increase in δ¹⁵N values in both species by more than 3‰ indicate partitioning of the trophic resources between them and elongation of the food chains of both these carnivorous generalists by almost one trophic level in the communities that exist under the influence of less productive waters.     

The fatty-acid composition and nutrition of deep-sea holothurians from the Sea of Okhotsk

The results of a comparative study of the fatty-acid composition in eight species of holothurians that were collected in the Sea of Okhotsk in the area of the Kuril Islands (depths of 90–560 m) are presented. It is shown that interspecific differences in the fatty-acid compositions of the holothurians were consistent with the isotope composition (δ¹³C and δ¹⁵N) and the structural features of the tentacles and the lifestyle of holothurians, as indicators of trophic resources used by these holothurians. According to the results of the cluster analysis, the holothurians were divided into three groups. The first group included suspension-feeding dendrochirotides Eupentacta pusilla and Pseudocnus fallax with a high content of the fatty acid 20:5(n-3), which is a marker of diatoms, and with the δ¹⁵N values that are typical of consumers of suspended organic matter. The second group consisted of the dendrochirotides Psolus chitonoides and Psolidium sp. with a much lower content of 20:5 (n-3) and higher contents of 20:4 (n-6) and 22:6 (n-3), as well as high values of δ¹⁵N, which are typical of surface deposit-feeders. The third group consisted of surface and subsurface depositfeeders, Chiridota sp., Molpadia orientalis, Pseudostichopus mollis, and Synallactes nozawai. Very high contents of 20: 4 (n-6) and 21: 4 (n-7) were typical of the third group and the highest values of δ¹⁵N, indicating feeding on repeatedly recycled organic matter.     

Can tree-ring δ<Superscript>15</Superscript>N be used as a proxy for foliar δ<Superscript>15</Superscript>N in European beech and Norway spruce?

Key message

For long-term environmental investigations, tree-ring δ ¹⁵ N values are inappropriate proxies for foliar δ ¹⁵ N for both Fagus sylvatica and Picea abies under moderate N loads.

Abstract

Currently it is unclear whether stable nitrogen isotope signals of tree-rings are related to those in foliage, and whether they can be used to infer tree responses to environmental changes. We studied foliar and tree-ring nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope ratios in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) from six long-term forest monitoring sites in Switzerland together with data on N deposition and soil N availability, as well as a drought response index over the last two decades. For both species, tree-ring δ¹⁵N and δ¹³C values were less negative compared to foliar δ¹⁵N and δ¹³C values, most likely due to recycling and reallocation of N within the tree and fractionation processes associated with the transport of sucrose and the formation of tree-rings, respectively. Temporal trends recorded in foliar δ¹⁵N were not reflected in tree-ring δ¹⁵N, with much higher variations in tree-rings compared to foliage. Soil N availability and N deposition were partially able to explain changes in foliar δ¹³C, while there were no significant correlations between environmental variables and either tree-ring or foliar δ¹⁵N. Our results suggest an uncoupling between the N isotopic composition of tree-rings and foliage. Consequently, tree-ring δ¹⁵N values are inappropriate proxies of foliar δ¹⁵N values under low-to-moderate N deposition loads. Furthermore, at such low levels of deposition, tree-ring δ¹⁵N values are not recommended as archives of tree responses to soil C/N or bulk N deposition.

     

Non-specific symbiotic germination of <Emphasis Type="Italic">Cynorkis purpurea</Emphasis> (Thouars) Kraezl., a habitat-specific terrestrial orchid from the Central Highlands of Madagascar

Orchids, particularly terrestrial taxa, rely mostly on basidiomycete fungi in the Cantharellales and Sebacinales that trigger the process of seed germination and/or initiate the full development of the seedling. During the course of development, orchids may associate with the same fungus, or they may enlist other types of fungi for their developmental needs leading to resilience in a natural setting. This study examined in vitro seed germination and seedling developmental behavior of Cynorkis purpurea, a terrestrial orchid from the Central Highlands of Madagascar. This species is mostly restricted to gallery forests in the Itremo Massif, in moist substrate between rocks bordering streams. The main objective was to understand the influence of diverse mycorrhizal fungi on seed germination and further development of C. purpurea. The study aims to compare symbiotic versus asymbiotic germination and seedling development with seeds and fungi collected from a 13-km² area in the Itremo region. Seeds collected from the wild were sown with diverse orchid mycorrhizal fungi (OMF) spanning 12 operational taxonomic units (OTUs) in three genera (Tulasnella, Ceratobasidium, and Sebacina) acquired from different habitats. Treatments were assessed in terms of the percentage of germinated seeds and fully developed seedlings against those in asymbiotic control media treatments. Overall, OMF significantly improved seedling development within the 12-week experiment period. Sebacina as a genus was the most effective at promoting seedling development of C. purpurea, as well as having the ability to enter into successful symbiotic relationships with orchids of different life forms; this new knowledge may be especially useful for orchid conservation practiced in tropical areas like Madagascar. A Sebacina isolate from an epiphytic seedling of Polystachya concreta was the most effective at inducing rapid seedling development and was among the five that outperformed fungi isolated from roots of C. purpurea. C. purpurea was found to be a mycorrhizal generalist, despite its specific habitat preference, highlighting the complex interaction between the plant, fungi, and the environment. The potential impact on conservation strategies of understanding the requirements for orchid seed germination and development by identifying and using OMF from diverse sources is discussed in detail.     

Variation in mycorrhizal performance in the epiphytic orchid <Emphasis Type="Italic">Tolumnia variegata in vitro</Emphasis>: the potential for natural selection

Symbiotic seed germination is a critical stage in orchid life histories. Natural selection may act to favor plants that efficiently use mycorrhizal fungi. However, the necessary conditions for natural selection – variation, heritability, and differences in fitness – have not been demonstrated for either orchid or fungus. With the epiphytic orchid Tolumnia variegata as a model system, we ask the following questions: (1) Do seeds from different individuals in a population differ in germination and seedling development in the presence of the same fungi? (2) Do different mycorrhizal fungi (Ceratobasidium spp.) differ in ability to stimulate seed germination and growth in T. variegata? And (3) are the Ceratobasidium isolates that best induce seed germination and seedling development more closely related to each other than to isolates that are less effective? We performed symbiotic seed germination experiments in vitro. The experiments were done using mycorrhizal fungi isolated from T. variegata; relationships among the fungi were inferred from nuclear ribosomal ITS sequences. We found significant variation for both symbiotic germination and seedling growth among biparental seed crops obtained from a population of T. variegata plants. Differences among Ceratobasidium fungi in seed germination were significant. The fungi that induced highest seed germination and seedling development belonged to two of four clades of Ceratobasidium. The two experiments show that there is potential for natural selection to act on orchid–fungus relationships. Given that orchids vary in performance, and that mycorrhizal fungi are not geographically distributed homogeneously, mycorrhizae may affect population size, distribution and evolution of orchids.     

Arbuscular mycorrhizal colonization in field-collected terrestrial cordate gametophytes of pre-polypod leptosporangiate ferns (Osmundaceae,Gleicheniaceae, Plagiogyriaceae,Cyatheaceae)

To determine the mycorrhizal status of pteridophyte gametophytes in diverse taxa, the mycorrhizal colonization of wild gametophytes was investigated in terrestrial cordate gametophytes of pre-polypod leptosporangiate ferns, i.e., one species of Osmundaceae (Osmunda banksiifolia), two species of Gleicheniaceae (Diplopterygium glaucum, Dicranopteris linearis), and four species of Cyatheales including tree ferns (Plagiogyriaceae: Plagiogyria japonica, Plagiogyria euphlebia; Cyatheaceae: Cyathea podophylla, Cyathea lepifera). Microscopic observations revealed that 58 to 97 % of gametophytes in all species were colonized with arbuscular mycorrhizal (AM) fungi. Fungal colonization was limited to the multilayered midrib (cushion) tissue in all gametophytes examined. Molecular identification using fungal SSU rDNA sequences indicated that the AM fungi in gametophytes primarily belonged to the Glomeraceae, but also included the Claroideoglomeraceae, Gigasporaceae, Acaulosporaceae, and Archaeosporales. This study provides the first evidence for AM fungal colonization of wild gametophytes in the Plagiogyriaceae and Cyatheaceae. Taxonomically divergent photosynthetic gametophytes are similarly colonized by AM fungi, suggesting that mycorrhizal associations with AM fungi could widely occur in terrestrial pteridophyte gametophytes.     

Is the root-colonizing endophyte <Emphasis Type="Italic">Acremonium strictum</Emphasis> an ericoid mycorrhizal fungus?

In previous investigations, we found that Acremonium strictum (strain DSM 100709) developed intracellular structures with similarity to mycelia of ericoid mycorrhizal fungi in the rhizodermal cells of flax plants and in hair roots of Rhododendron plantlets. A. strictum had also been isolated from roots of ericaceous salal plants and was described as an unusual ericoid mycorrhizal fungus (ERMF). As its mycorrhizal traits were doubted, we revised the hypothesis of a mycorrhizal nature of A. strictum. A successful synthesis of mycorrhiza in hair roots of inoculated ericaceous plants was a first step of evidence, followed by fluorescence microscopy with FUN^®1 cell stain to observe the vitality of the host cells at the early infection stage. In inoculation trials with in vitro-raised mycorrhiza-free Rhododendron plants in axenic liquid culture and in greenhouse substrate culture, A. strictum was never observed in living hair root cells. As compared to the ERMF Oidiodendron maius and Rhizoscyphus ericae that invaded metabolically active host cells and established a symbiotic unit, A. strictum was only found in cells that were dead or in the process of dying and in the apoplast. In conclusion, A. strictum does not behave like a common ERMF—if it is one at all. A comparison of A. strictum isolates from ericaceous and non-ericaceous hosts could reveal further identity details to generalize or specify our findings on the symbiotic nature of A. strictum. At least, the staining method enables to discern between true mycorrhizal and other root endophytes—a tool for further studies.     

Invasive species alter ontogenetic shifts in the trophic ecology of Lake Sturgeon (<Emphasis Type="Italic">Acipenser fulvescens</Emphasis>) in the Niagara River and Lake Ontario

Species invasions can alter food web structure and change ecosystem-level functioning, but it is often unclear how these invasions may affect the life history of native species. The Lake Sturgeon (Acipenser fulvescens), a large long-lived native fish species in the Great Lakes, has increased in abundance in the lower Niagara River and nearby Lake Ontario during a period of invasive species-induced ecosystem change precipitated most recently by Dreissenid mussels (Driessena polymorpha and Driessena bugensis) and Round Goby (Neogobius melanostomus). Material taken from cross-sections of archived pectoral spines from Niagara River Lake Sturgeon captured in 1998–2000 and 2010–2012 were analyzed for stable isotopes across discrete growth zones to provide an ontogenetic assessment of diet, and diet analysis of Lake Sturgeon captured in 2014 was conducted to assess the contribution of invasive prey. Round Goby was the most important Lake Sturgeon prey item (86% by weight) in 2014, which corroborated results of δ¹⁵N and δ¹³C. Lake Sturgeon captured after the invasion of Round Goby exhibited ontogenetic changes in δ¹⁵N that differed from pre-Round Goby patterns, though this effect was weaker for δ¹³C. Values of δ¹⁵N from spine growth zones indicated non-linear increases in trophic position with age and increased rate of δ¹⁵N enrichment after the Round Goby invasion. We conclude that Round Goby establishment in western Lake Ontario changed the feeding ecology of Lake Sturgeon, which may have a positive effect on population growth for this native species.     

Fatty-Acid and Stable-Isotope Compositions in Shallow-Water Bivalve Mollusks and their Food

We conducted a comparative analysis of the fatty acid (FA) composition and the ratios of stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) in soft tissues of ten species of bivalve mollusks collected simultaneously on adjacent biotopes in shallow Vostok Bay (the Sea of Japan). Comparison of the FA composition of the lipids of digestive gland and all soft tissues showed that the percentages of C₁₆ and C₁₈ marker FAs were greater in the digestive gland and the levels of marker C₂₀ and C₂₂ FAs were, in most cases, higher in soft tissues. According to the results of cluster analysis and principal component analysis, four groups of samples were identified with a similarity of the FA composition of more than 80% within groups. The carbon stableisotope ratios varied within very wide limits in the studied species of bivalves; the range of δ¹³C variations was 8.1‰. The range of δ15N variations was much smaller, 2.5‰. Two pairs of species of mollusks (Saxidomus purpurata–Protothaca euglypta and P. jedoensis–Diplodonta semiasperoides) did not differ in the values of both δ¹⁵N and δ¹³C, the remaining species differed in at least one of these parameters. The greatest similarity of the FA composition and stable-isotope ratios was found in species that inhabit similar substrates, except Macoma irus and D. semiasperoides. Particularly marked differences in the FA composition and stable-isotope ratios were found between a filter-/surface deposit-feeder M. irus and filter-feeders Arca boucardi and Mytilus coruscus that live next to this species.     

Insights into root growth, function, and mycorrhizal abundance from chemical and isotopic data across root orders

Background and aims

Detailed analyses of root chemistry by branching order may provide insights into root function, root lifespan and the abundance of root-associated mycorrhizal fungi in forest ecosystems.

Methods

We examined the nitrogen and carbon stable isotopes (δ¹⁵N and δ¹³C) and concentration (%N and %C) in the fine roots of an arbuscular mycorrhizal tree, Fraxinus mandshurica, and an ectomycorrhizal tree, Larix gmelinii, over depth, time, and across five root branching orders.

Results and conclusions

Larix δ¹⁵N increased by 2.3?‰ from 4th order to 1st order roots, reflecting the increased presence of ¹⁵N-enriched ECM fungi on the lower root orders. In contrast, arbuscular mycorrhizal Fraxinus only increased by 0.7?‰ from 4th order to 1st order roots, reflecting the smaller ¹⁵N enrichment and lower fungal mass on arbuscular mycorrhizal fine roots. Isotopic and anatomical mass balance calculations indicate that first, second, and third order roots in ectomycorrhizal Larix averaged 36 %, 23 %, and 8 % fungal tissue by mass, respectively. Using literature values of root production by root branching order, we estimate that about 25 % of fine root production in ECM species like Larix is actually of fungal sheaths. In contrast to %N, %C, and δ¹⁵N, δ¹³C changed minimally across depth, time, and branching order. The homogeneity of δ¹³C suggests root tissues are constructed from a large well-mixed reservoir of carbon, although compound specific δ¹³C data is needed to fully interpret these patterns. The measurements developed here are an important step towards explicitly including mycorrhizal production in forest ecosystem carbon budgets.     

Contribution of different arbuscular mycorrhizal fungal inoculum to <Emphasis Type="Italic">Elymus nutans</Emphasis> under nitrogen addition

Arbuscular mycorrhizal (AM) fungi are known to promote plant growth and nutrient uptake, but their role in nitrogen (N) uptake still remains unclear. Therefore, a pot experiment was set up to evaluate the impacts of N addition and AM inoculation (Diversispora eburnea, Claroideoglomus etunicatum, Paraglomus occultum, and their mixture) on AM root colonization, plant biomass, N and P nutrition in Elymus nutans. Our results showed that AM root colonization was unaffected by N addition but was significantly affected by different AM fungal species. D. eburnea and C. etunicatum showed significant higher root colonization than P. occultum. The E. nutans exhibited the highest biomass when inoculated with D. eburnea and significantly higher than non-mycorrhizal (the control) regardless of N addition. Under N addition treatment, D. eburnea significantly enhanced P content of roots, N content of shoots and roots, while AM mixture significantly enhanced shoot P content compared with non-mycorrhizal. However, N and P content in shoots and roots did not significantly vary among treatments when no N was added. In addition, inoculation with C. etunicatum and P. occultum showed no significant effect on plant biomass, N and P content regardless of N addition. In conclusion, this study revealed that the plant response to N addition depends on AM fungal species and also confirmed that significant functional diversity exists among AM fungal species.     

Do novel weapons that degrade mycorrhizal mutualisms promote species invasion?

Non-native plants often dominate novel habitats where they did not co-evolve with the local species. The novel weapons hypothesis suggests that non-native plants bring competitive traits against which native species have not adapted defenses. Novel weapons may directly affect plant competitors by inhibiting germination or growth, or indirectly by attacking competitor plant mutualists (degraded mutualisms hypothesis). Japanese knotweed (Fallopia japonica) and European buckthorn (Rhamnus cathartica) are widespread plant invaders that produce potent secondary compounds that negatively impact plant competitors. We tested whether their impacts were consistent with a direct effect on the tree seedlings (novel weapons) or an indirect attack via degradation of seedling mutualists (degraded mutualism). We compared recruitment and performance using three Ulmus congeners and three Betula congeners treated with allelopathic root macerations from allopatric and sympatric ranges. Moreover, given that the allelopathic species would be less likely to degrade their own fungal symbiont types, we used arbuscular mycorrhizal (AMF) and ectomycorrhizal (ECM) tree species to investigate the effects of F. japonica (no mycorrhizal association) and Rhamnus cathartica (ECM association) on the different fungal types. We also investigated the effects of F. japonica and R. cathartica exudates on AMF root colonization. Our results suggest that the allelopathic plant exudates impact seedlings directly by inhibiting germination and indirectly by degrading fungal mutualists. Novel weapons inhibited allopatric seedling germination but sympatric species were unaffected. However, seedling survivorship and growth appeared more dependent on mycorrhizal fungi, and mycorrhizal fungi were inhibited by allopatric species. These results suggest that novel weapons promote plant invasion by directly inhibiting allopatric competitor germination and indirectly by inhibiting mutualist fungi necessary for growth and survival.     

Growth and photosynthetic response of two larches exposed to O<Subscript>3</Subscript> mixing ratios ranging from preindustrial to near future

In this study, we questioned whether ground-level ozone (O₃) induces hormesis in Japanese larch (Larix kaempferi) and its hybrid F₁ (L. gmelinii var. japonica × L. kaempferi). In order to answer the question, we exposed seedlings of both taxa to four O₃ treatments [ranging from ≈10 to 60 nmol(O₃) mol^–1] in open-top chambers for two consecutive growing seasons. We found a hormetic response in maximum photosynthetic rate (P_Nmax) at 1700 μmol(CO₂) mol^–1 and maximum rates of carboxylation (V_cmax) and electron transport (J_max) in both larches. Stimulation of P_Nmax, V_cmax, and J_max did not lead to suppressed plant productivity in Japanese larch, which followed a stress-tolerant strategy, but it did lead to suppressed plant productivity in hybrid larch which followed a competitive strategy. These findings are the first to suggest that stimulation of physiological functions by low O₃ exposures may have negative consequences for larch reproduction.     

Contrasting δ<Superscript>15</Superscript>N Values of Atmospheric Deposition and <Emphasis Type="Italic">Sphagnum</Emphasis> Peat Bogs: N Fixation as a Possible Cause

Nitrogen (N) isotope systematics were investigated at two high-elevation ombrotrophic peat bogs polluted by farming and heavy industry. Our objective was to identify N sources and sinks for isotope mass balance considerations. For the first time, we present a time-series of δ¹⁵Ν values of atmospheric input at the same locations as δ¹⁵Ν values of living Sphagnum and peat. The mean δ¹⁵Ν values systematically increased in the order: input NH₄ ⁺ (?10.0‰) < input NO₃ ^? (?7.9‰) < peat porewater (?5.6‰) < Sphagnum (?5.0‰) < shallow peat (?4.2‰) < deep peat (?2.2‰) < runoff (?1.4‰) < porewater N₂O (1.4‰). Surprisingly, N of Sphagnum was isotopically heavier than N of the atmospheric input (P < 0.001). If partial incorporation of reactive N from the atmosphere into Sphagnum was isotopically selective, the residual N would have to be isotopically extremely light. Such N, however, was not identified anywhere in the ecosystem. Alternatively, Sphagnum may have contained an admixture of isotopically heavier N. Ambient air contains such N in the form of N₂ (δ¹⁵Ν_N2 = 0‰). Because high energy is required to break the triple bond, microbial N fixation is likely to proceed only under limited availability of pollutant N. Also for the first time, a δ¹⁵Ν comparison is presented between anoxic deeper peat and porewater N₂O. Isotopically light N is removed from anoxic substrate by denitrification, whose final product, N₂, escapes into the atmosphere. Porewater N₂O is an isotopically heavy residuum following partial N₂O reduction to N₂.     

Stable nitrogen and carbon isotope ratios in wild native honeybees: the influence of land use and climate

The eastern hive bee Apis cerana is a major honeybee species in Asia providing numerous ecosystem services. Understanding how much the honeybees depend on natural and human-influenced plants and landscapes in different climates is important could contribute to evaluate how wild honeybees use food resources and to measure the ecosystem services. We investigated the effects of land use and climate changes on stable nitrogen and carbon isotope ratios in wild populations of A. cerana. In populations from 139 individual sites throughout Japan, we measured nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotope ratios and analyzed the effects of land use and climate. Our results showed that forested areas and annual precipitation had significant effects on δ¹⁵N, and that paddy fields and urban areas had significant effects on δ¹³C. These results suggest that A. cerana sensibly uses available food resources in the various environments and that stable nitrogen and carbon isotope ratios clearly reflect the effects of land use and climate changes on the populations of A. cerana. Thus, stable nitrogen and carbon isotope ratios in A. cerana, which widely distributes in Asia, can be used as indicators of the environments, such as land use and climate, of an area within its foraging range.