Foliar and fungal <Superscript>15</Superscript> N:<Superscript>14</Superscript> N ratios reflect development of mycorrhizae and nitrogen supply during primary succession: testing analytical models

Nitrogen isotopes (¹⁵N/¹⁴N ratios, expressed as δ¹⁵N values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against ¹⁵N during creation of transfer compounds within mycorrhizal fungi decreases the ¹⁵N/¹⁴N in plants (low δ¹⁵N) and increases the ¹⁵N/¹⁴N of the fungi (high δ¹⁵N). Analytical models of ¹⁵N distribution would be helpful in interpreting δ¹⁵N patterns in fungi and plants. To compare different analytical models, we measured nitrogen isotope patterns in soils, saprotrophic fungi, ectomycorrhizal fungi, and plants with different mycorrhizal habits on a glacier foreland exposed during the last 100 years of glacial retreat and on adjacent non-glaciated terrain. Since plants during early primary succession may have only limited access to propagules of mycorrhizal fungi, we hypothesized that mycorrhizal plants would initially be similar to nonmycorrhizal plants in δ¹⁵N and then decrease, if mycorrhizal colonization were an important factor influencing plant δ¹⁵N. As hypothesized, plants with different mycorrhizal habits initially showed similar δ¹⁵N values (−4 to −6‰ relative to the standard of atmospheric N₂ at 0‰), corresponding to low mycorrhizal colonization in all plant species and an absence of ectomycorrhizal sporocarps. In later successional stages where ectomycorrhizal sporocarps were present, most ectomycorrhizal and ericoid mycorrhizal plants declined by 5–6‰ in δ¹⁵N, suggesting transfer of ¹⁵N-depleted N from fungi to plants. The values recorded (−8 to −11‰) are among the lowest yet observed in vascular plants. In contrast, the δ¹⁵N of nonmycorrhizal plants and arbuscular mycorrhizal plants declined only slightly or not at all. On the forefront, most ectomycorrhizal and saprotrophic fungi were similar in δ¹⁵N (−1 to −3‰), but the host-specific ectomycorrhizal fungus Cortinarius tenebricus had values of up to 7‰. Plants, fungi and soil were at least 4‰ higher in δ¹⁵N from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher δ¹⁵N values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude:(1) large enrichments in ¹⁵N of many ectomycorrhizal fungi relative to co-occurring ectomycorrhizal plants are best explained by treating the plant-fungal-soil system as a closed system with a discrimination against ¹⁵N of 8–10‰ during transfer from fungi to plants, (2) based on models of ¹⁵N mass balance, ericoid and ectomycorrhizal fungi retain up to two-thirds of the N in the plant-mycorrhizal system under the N-limited conditions at forefront sites, (3) sporocarps are probably enriched in ¹⁵N by an additional 3‰ relative to available nitrogen, and (4) host-specific ectomycorrhizal fungi may transfer more N to plant hosts than non-host-specific ectomycorrhizal fungi. Our study confirms that nitrogen isotopes are a powerful tool for probing nitrogen dynamics between mycorrhizal fungi and associated plants.     

Fungal associations of Danish Calluna vulgaris roots with special reference to ericoid mycorrhiza

Fungi were isolated from young, serial-washed roots of Calluna sampled from a Danish heathland, Hjelm Hede. Of the 626 isolates, those that were dark, sterile and septate were divided into 13 morphological groups based on their appearance in culture on malt agar. Mycorrhizal synthesis in vitro showed that several groups formed typical ericoid mycorrhiza with seedlings of Calluna; these ericoid mycorrhizal fungi were morphologically similar to Hymenoscyphus ericae. The identities of the other dark, septate fungi are uncertain. Oidiodendron spp. were isolated in a very low frequency; these fungi also formed typical ericoid mycorrhiza. The Calluna root system on Hjelm Hede demonstrated a high morphological diversity among the associated dark, septate fungi suggesting that more than one fungus could coexist in the same host root system.     

Natural Abundance of <Superscript>15</Superscript>N in Nitrogen-Limited Forests and Tundra Can Estimate Nitrogen Cycling Through Mycorrhizal Fungi: A Review

The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural ¹⁵N:¹⁴N ratios (expressed as δ¹⁵N values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against ¹⁵N when they create compounds for transfer to plants; host plants are depleted in ¹⁵N, whereas mycorrhizal fungi are enriched in ¹⁵N. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant δ¹⁵N increases. Careful analyses of δ¹⁵N patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant–mycorrhizal symbioses caused by shifts in climate and N deposition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molecular detection,community structure and phylogeny of ericoid mycorrhizal fungi

Through traditional culturing and molecular characterization, we have determined that five putative species and 2 polyphyletic assemblages of fungi produce ericoid mycorrhizae in Gaultheria shallon, other Ericaceae and Epacridaceae. Using phylogenetic analysis of ITS2 sequences in GenBank, we have confirmed that most of these fungi occur in North America, Europe, and Australia. The low recovery rate of culturable ericoid mycorrhizal fungi from Gaultheria shallon may partly be explained by the fact that most mycorrhizal root segments contain an unculturable basidiomycete, revealed by direct amplification, cloning, and sequencing of LSU fungal DNA from root. Molecular characterization and phylogenetic analysis are powerful tools in revealing the geographic distribution and identity of ericoid mycorrhizal fungi.     

Contrasts Among Mycorrhizal Plant Guilds in Foliar Nitrogen Concentration and δ15N Along Productivity Gradients of a Boreal Forest

The distribution of plant species in boreal forest understories is hypothesized to reflect mycorrhizal guilds and associated adaptations for organic nitrogen (N) acquisition. In this study of a natural edaphic gradient, where supply rates of inorganic N increase with site productivity, we noted a decline in understory ectomycorrhizal, ericoid, and arbutoid plant communities on productive sites, in contrast to a positive response by most arbuscular species. We then assessed the rate of change in foliar N concentration (N_conc) and abundance of ¹⁵N (δ¹⁵N) of select plants from these mycorrhizal guilds. Two arbuscular plant species (Rubus parviflorus and Viburnum edule) had the sharpest increases in foliar N_conc with enhanced supplies of NH₄ ⁺ and NO₃ ⁻, but with no differences in foliar δ¹⁵N. An ectomycorrhizal species, Abies lasiocarpa, and ericoid species, Vaccinium membranaceum, had parallel increases in both N_conc and δ¹⁵N with soil N supply. The foliar δ¹⁵N of two arbutoid plants (Orthilia secunda and Pyrola asarifolia) were as enriched as ectomycorrhizal sporocarps, likely indicating N transfer from mycorrhizal networks. The depletion of foliar δ¹⁵N by ectomycorrhizal and ericoid plants on poorer sites likely reflected a high degree of N retention and photosynthate demand by fungi, whereas arbuscular plants may have had a less significant δ¹⁵N response because of a more passive role by fungi in scavenging organic N. The results suggest differences in how mycorrhiza exploit diverse soil N supplies (recalcitrant and labile organic N, NH₄ ⁺, NO₃ ⁻, and parasitized N) could be an important factor in boreal plant community composition.     

荧光蛋白在双色蜡蘑中的构建与表达

菌根是真菌与植物之间形成的互惠互利的营养共生体,对生态环境有重大的意义。外生菌根真菌与植物互作机制以及真菌基因功能的深入研究都需要对菌根真菌进行遗传转化,本研究以外生菌根真菌模式生物双色蜡蘑(Laccaria bicolor)为研究对象,选择细胞核中的核小体蛋白H₂B基因为目的基因,以pCEBN为表达载体,融合红色荧光蛋白,最终构建在真菌中表达的双元载体,使用根瘤农杆菌介导转化法转化双色蜡蘑菌丝,随后利用PCR对真菌转化子进行验证后,通过激光共聚焦显微镜观察到菌丝细胞核中的红色荧光,成功将融合荧光蛋白转化菌根真菌,为后续研究菌根真菌中基因的亚细胞定位提供了实验平台。结果表明,利用双元载体和农杆菌转化方法,建立了高效的双色蜡蘑转化体系(93.33%),在激光共聚焦显微镜下观察到菌丝细胞核中红色荧光信号,验证了融合荧光蛋白在双色蜡蘑中的成功表达。本研究成功地构建了菌根真菌中的核小体蛋白和红色荧光蛋白融合表达的真菌转化体系。     

Ferricrocin - an ectomycorrhizal siderophore of Cenococcum geophilum

The ectomycorrhizal fungus Cenococcum geophilum was grown in low-iron medium and the excreted siderophores were extracted, purified and analyzed by HPLC. The principal hydroxamate siderophore produced, was identified as ferricrocin as confirmed by analytical HPLC, FAB-mass spectrometry and ¹H- and ¹³C-NMR spectra. Although the occurrence of ferricrocin has been shown earlier to occur in the ericoid mycorrhizal fungi, this is the first report of ferricrocin in a true ectomycorrhizal fungus which is taxonomically related to the ascomycetes.     

Decomposition of 14C-labelled lignin,holocellulose and lignocellulose by mycorrhizal fungi

Five different species of known ecto-mycorrhizal fungi: Cenococcum geophilum, Amanita muscaria, Tricholoma aurantium, Rhizopogon luteolus and Rhizopogon roseolus were studied for their ability to metabolize the major components of plant cell walls. All strains were able to decompose ¹⁴C-labelled plant lignin, ¹⁴C-lignocellulose and ¹⁴C-DHP-lignin at a rate which was lower than the one observed for the known white rot fungi Heterobasidion annosum and Sporotrichum pulverulentum. Also ¹⁴C-(U)-holocellulose was relatively less degradable for the mycorrhizal fungi than for the white rotters. On the other hand, aromatic monomers like ¹⁴C-vanillic acid were decomposed to a much higher extent by two species of mycorrhizal fungi compared to the activity observed for Heterobasidion annosum. The results of the experiments reveal that these stains of mycorrhizal fungi are well able to utilize the major components of plant material and thus can contribute to litter decomposition in the forest floor.     

Wood stimulates the demethoxylation of [O14CH3]-labeled lignin model compounds by the white-rot fungi Phanerochaete chrysosporium and Phlebia radiata

Mineralization of polymeric wood lignin and its substructures is a result of complex reactions involving oxidizing and reducing enzymes and radicals. The degradation of methoxyl groups is an essential part of this process. The presence of wood greatly stimulates the demethoxylation of a non-phenolic lignin model compound (a [O¹⁴CH₃]-labeled β-O-4 dimer) by the lignin-degrading white-rot fungi Phlebia radiata and Phanerochaete chrysosporium. When grown on wood, both fungi produced up to 47 and 40% ¹⁴CO₂ of the applied ¹⁴C activity, respectively, under air and oxygen in 8 weeks. Without wood, the demethoxylation of the dimer by both fungi was lower, varying between 0.5 and 35%. Addition of nutrient nitrogen together with glucose decreased demethoxylation when the fungi were grown on spruce wood under air. Because the evolution of ¹⁴CO₂ in the absence of wood was poor, the fungi may have preferably used wood as a carbon and nitrogen source. The amount of fungal mycelium, as determined by the ergosterol assay, did not show connection to demethoxylation. P. radiata also showed a high demethoxylation of [O¹⁴CH₃]-labeled vanillic acid in the presence of birch wood. The degradation of lignin and lignin-related substances should be studied in the presence of wood, the natural substrate for white-rot fungi.     

Nitrogen nutrition,photosynthesis and carbon allocation in ectomycorrhizal pine

Summary Studies examined net photosynthesis (Pn) and dry matter production of mycorrhizal and nonmycorrhizalPinus taeda at 6 intervals over a 10-month period. Pn rates of mycorrhizal plants were consistently greater than nonmycorrhizal plants, and at 10 months were 2.1-fold greater. Partitioning of current photosynthate was examined by pulse-labelling with¹⁴CO₂ at each of the six time intervals. Mycorrhizal plants assimilated more¹⁴CO₂, allocated a greater percentage of assimilated¹⁴C to the root systems, and lost a greater percentage of¹⁴C by root respiration than did nonmycorrhizal plants. At 10 months, the quantity of¹⁴CO₂ respired by roots per unit root weight was 3.6-fold greater by mycorrhizal than nonmycorrhizal plants. Although the stimulation of photosynthesis and translocation of current photosynthate to the root system by mycorrhiza formation was consistent with the source-sink concept of sink demand, foliar N and P concentrations were also greater in mycorrhizal plants.Further studies examined Pn and dry matter production ofPinus contorta in response to various combinations of N fertilization (3, 62, 248 ppm), irradiance and mycorrhizal fungi inoculation. At 16 weeks of age, 6 weeks following inoculation with eitherPisolithus tinctorius orSuillus granulatus, Pn rates and biomass were significantly greater in mycorrhizal than nonmycorrhizal plants. Mycorrhizal plants had significantly greater foliar %P, but not %N, than did nonmycorrhizal plants. Fertilization with 62 ppm N resulted in greater mycorrhiza formation than either 3 or 248 ppm. Increased irradiance resulted in increased mycorrhiza formation.     

Respiration activity of ectomycorrhizas from Cenococcum geophilum and Lactarius sp. in relation to soil water potential in five beech forests

Forest trees are involved in root symbioses with hundreds of species of ectomycorrhizal fungi which constitute functional guilds able to improve the water and mineral nutrition of host trees. In temperate ecosystems, water shortage is a main factor limiting tree vitality. To assess how soil water conditions affected the physiological state of beech (Fagus silvatica L.) ectomycorrhizal roots, we monitored glucose respiration of two ectomycorrhizal types (Lactarius sp. and Cenococcum geophilum) during two complete growing seasons. Five stands of contrasting soil conditions were chosen in north-eastern France. The top soil horizons were equipped with micropsychrometers for measuring water potential and temperature. Glucose respiration on individual ectomycorrhizas was measured in vitro by trapping [¹⁴C]-CO₂ from radiolabelled glucose. For soil water potential <-0.2 MPa, the potential respiration activity of C. geophilumectomycorrhizas was significantly less altered than that of Lactariussp. ectomycorrhizas, indicating that C. geophilumis more likely than Lactariussp. to maintain the physiological integrity of beech roots facing drought stress.     

Salt stress affects in vitro growth and in situ symbioses of ectomycorrhizal fungi

Ten isolates of six species of ectomycorrhizal fungi were grown in vitro at nine concentrations of three sodium salts (NaCl, Na₂SO₄, Na₃C₆H₅O₇) for 4 weeks. Colony diamater, biomass and protein content of fungi were evaluated. Isolates of Pisolithus tinctorius and Suillus luteus were more tolerant of NaCl and Na₂SO₄ than of Na₃C₆H₅O₇. Fungi in the genera Cenococcum, Laccaria, and Thelephora were highly intolerant of Na₃C₆H₅O₇ and Na₂SO₄ in vitro. Biomass and protein content of fungi generally declined with increasing substrate salinity in solution culture. In situ ectomycorrhizal colonization by Laccara laccata and P. tinctorius and the dry weight of Pinus taeda seedlings were significantly reduced by 80 mM NaCl after 14 weeks. Only select ectomycorrhizal fungi appear capable of growth and symbiosis in saline soils.     

Occurrence of carpophores of ectomycorrhizal fungi in selected stands ofPinus sylvestris in the Netherlands in relation to stand vitality and air pollution

The relation between the occurrence of carpophores of mycorrhizal fungi, tree vitality, and air pollution in ‘young’ (5–10 years) and ‘old’ (50–80 years) stands ofPinus sylvestris L. was investigated. In the Netherlands 21 homogeneous plots of 1000 m² were selected on dry, sandy soils. Tree vitality was assessed in the summer of 1985 and during the autumn the plots were searched for carpophores of mycorrhizal fungi. In the young plots 3 times more species of ectomycorrhizal fungi and 20 times more carpophores per plot were found than in the old plots. In the old plots, the number of carpophores and the number of species showed significant positive correlations with the average number of needleyears of the trees. The number of carpophores and the number of species were significantly negative correlated with the concentrations of SO₂ and O₃ in the air and the amount of NH₃ emission. The number of needleyears, the crown density and the overall tree vitality were found to be significantly negative correlated with the amount of NH₃ emission, and the number of needleyears also with the concentration of O₃ in the air. Such correlations were not found for the young plots. No differences were found in soil chemical properties between the plots. Evidence was obtained that the fructification of mycorrhizal fungi of old stands ofPinus sylvestris is influenced by air pollution either directly, or indirectly by way of tree vitality. It is hypothesized that the young plots offer more advantageous circumstances for fructification of the fungi because of the disturbance of the upper soil-layer at the time of planting.     

Mycorrhizal colonization mediated by species interactions in arctic tundra

The Alaskan tussock tundra is a strongly nutrient-limited ecosystem, where almost all vascular plant species are mycorrhizal. We established a long-term removal experiment to document effects of arctic plant species on ecto- and ericoid mycorrhizal fungi and to investigate whether species interactions and/or nutrient availability affect mycorrhizal colonization. The treatments applied were removal of Betula nana (Betulaceae, dominant deciduous shrub species), removal of Ledum palustre (Ericaceae, dominant evergreen shrub species), control (no removal), and each of these three treatments with the addition of fertilizer. After 3 years of Ledum removal and fertilization, we found that overall ectomycorrhizal colonization in Betula was significantly reduced. Changes in ectomycorrhizal morphotype composition in removal and fertilized treatments were also observed. These results suggest that the effect of Ledum on Betula 's mycorrhizal roots is due to sequestration of nutrients by Ledum, leading to reduced nutrient availability in the soil. In contrast, ericoid mycorrhizal colonization was not affected by fertilization, but the removal of Betula and to a lower degree of Ledum resulted in a reduction of ericoid mycorrhizal colonization suggesting a direct effect of these species on ericoid mycorrhizal colonization. Nutrient availability was only higher in fertilized treatments, but caution should be taken with the interpretation of these data as soil microbes may effectively compete with the ion exchange resins for the nutrients released by plant removal in these nutrient-limited soils.     

Introduction of asymbiotically propagated seedlings of <Emphasis Type="Italic">Cephalanthera falcata</Emphasis> (Orchidaceae) into natural habitat and investigation of colonized mycorrhizal fungi

Asymbiotic seedling propagation and introduction of seedlings into a natural habitat were achieved for Cephalanthera falcata. For immature seeds collected 65 days after pollination, high germination rate (av. 50%) was achieved on Hyponex agar medium plates. Root development occurred in about 10% of the protocorms 5 months after seed sowing. Rooted protocorms were transferred to a culture bottle containing 100 ml of the Hyponex agar medium and incubated continually. In about 30% of the transferred individuals, shoot height reached 1.5–2 cm 8 months after the transfer. After acclimatization in wet vermiculite at 4°C for 6 months, 135 individuals were planted in a natural stand of C. falcata in mid February 2001. Shoot appearance rate was 44.4% at the first year and flowering was noted in some plants. At the fifth year, shoots with an average height of 21.6 cm still appeared in four plants, and flowering was noted in three of them. Colonization of mycorrhizal fungi was examined in two of them as well as one wild plant, in which the mycorrhizal fungi were identified to be in Thelephoraceae or Russulaceae. These fungi are known to form ectomycorrhiza with trees, and thus a tripartnership symbiosis consisting of C. falcata, mycorrhizal fungi and trees was suggested. The involvement of ectomycorrhizal fungi might be the reason for the low survival rate in the field experiment, because the distribution of ectomycorrhizal fungi relevant to this orchid is assumed to be uneven. The possibility of introducing artificially propagated orchids into natural habitats was discussed.     

Species-specific measurements of ectomycorrhizal turnover under N-fertilization: combining isotopic and genetic approaches

Ectomycorrhizal fungi play a significant role in the transfer of nutrients between plant and soil pools. Here we combine natural abundance ¹⁴C measurements with restriction fragment length polymorphism (RFLP) to study the effects of nitrogen fertilization on the residence time of carbon within ectomycorrhizal species. We show that the carbon in ectomycorrhizal fungi turns over every 4–5 years, indicating that these fungi are relatively long-lived. Moreover, ectomycorrhizal fungi responded in a species-specific way to fertilization. Cenococcum geophilum contained younger carbon on average in nitrogen-fertilized plots than in control plots, even though turnover rates of the community as a whole did not shift significantly. Our results suggest that the response of ectomycorrhizal fungi to N availability is complex, and alterations in tissue turnover within this microbial pool may vary depending on community structure.     

Limited transfer of nitrogen between wood decomposing and ectomycorrhizal mycelia when studied in the field

Transfer of ¹⁵N between interacting mycelia of a wood-decomposing fungus (Hypholoma fasciculare) and an ectomycorrhizal fungus (Tomentellopsis submollis) was studied in a mature beech (Fagus sylvatica) forest. The amount of ¹⁵N transferred from the wood decomposer to the ectomycorrhizal fungus was compared to the amount of ¹⁵N released from the wood-decomposing mycelia into the soil solution as ¹⁵N-NH₄. The study was performed in peat-filled plastic containers placed in forest soil in the field. The wood-decomposing mycelium was growing from an inoculated wood piece and the ectomycorrhizal mycelium from an introduced root from a mature tree. The containers were harvested after 41 weeks when physical contact between the two foraging mycelia was established. At harvest, ¹⁵N content was analyzed in the peat (total N and ¹⁵NH₄ ⁺) and in the mycorrhizal roots. A limited amount of ¹⁵N was transferred to the ectomycorrhizal fungus and this transfer could be explained by ¹⁵NH₄ ⁺ released from the wood-decomposing fungus without involving any antagonistic interactions between the two mycelia. Using our approach, it was possible to study nutritional interactions between basidiomycete mycelia under field conditions and this and earlier studies suggest that the outcomes of such interactions are highly species-specific and depend on environmental conditions such as resource availability.     

Leaf 15N abundance of subarctic plants provides field evidence that ericoid,ectomycorrhizal and non-and arbuscular mycorrhizal species access different sources of soil nitrogen

The natural abundance of the nitrogen isotope 15, ¹⁵N, was analysed in leaves of 23 subarctic vascular plant species and two lichens from a tree-line heath at 450 m altitude and a fellfield at 1150 m altitude close to Abisko in N. Sweden, as well as in soil, rain and snow. The aim was to reveal if plant species with different types of mycorrhizal fungi also differ in their use of the various soil N sources. The dwarf shrubs and the shrubs, which in combination formed more than 65% of the total above-ground biomass at both sites, were colonized by ericoid or ectomycorrhizal fungi. Their leaf ¹⁵N was between–8.8 and–5.5 at the heath and between–6.1 and –3.3 at the fellfield. The leaf ¹⁵N of non- or arbuscular mycorrhizal species was markedly different, ranging from –4.1 to –0.4 at the heath, and from –3.4 to+2.2 at the fellfield. We conclude that ericoid and ectomycorrhizal dwarf shrubs and shrubs utilize a distinct N source, most likely a fraction of the organic N in fresh litter, and not complexed N in recalcitrant organic matter. The latter is the largest component of soil total N, which had a ¹⁵N of –0.7 at the heath and +0.5 at the fellfield. Our field-based data thus support earlier controlled-environment studies and studies on the N uptake of excised roots, which have demonstrated protease activity and amino acid uptake by ericoid and ectomycorrhizal tundra species. The leaves of ectomycorrhizal plants had slightly higher ¹⁵N (fellfield) and N concentration than leaves of the ericoids, and Betula nana, Dryas octopetala and Salix spp. also showed NO _inf3 ^sup- reductase activity. These species may depend more on soil inorganic N than the ericoids. The ¹⁵N of non- or arbuscular mycorrhizal species indicates that the ¹⁵N of inorganic N available to these plants was higher than that of average fresh litter, probably due to high microbial immobilization of inorganic N. The ¹⁵N of NH _inf4 ^sup+ -N was +12.3 in winter snow and +1.9 in summer rain. Precipitation N might be a major contributer in species with poorly developed root systems, e.g. Lycopodium selago. Our results show that coexisting plant species under severe nutrient limitation may tap several different N sources: NH _inf4 ^sup+ , NO _inf3 ^sup- and organic N from the soil, atmospheric N₂, and N in precipitation. Ericoid and ectomycorrhizal fungi are of major importance for plant N uptake in tundra ecosystems, and mycorrhizal fungi probably exert a major control on plant ¹⁵N in organic soils.     

Ectomycorrhizal responses to organic and inorganic nitrogen sources when associating with two host species

While it is established that increasing atmospheric inorganic nitrogen (N) deposition reduces ectomycorrhizal fungal biomass and shifts the relative abundances of fungal species, little is known about effects of organic N deposition. The effects of organic and inorganic N deposition on ectomycorrhizal fungi may differ because responses to inorganic N deposition may reflect C-limitation. To compare the effects of organic and inorganic N additions on ectomycorrhizal fungi, and to assess whether host species may influence the response of ectomycorrhizal fungi to N additions, we conducted an N addition experiment at a field site in the New Jersey pine barrens. Seedlings of two host species, Quercus velutina (black oak) and Pinus rigida (pitch pine), were planted at the base of randomly-selected mature pitch pine trees. Nitrogen was added as glutamic acid, ammonium, or nitrate at a rate equivalent to 227.5 kg ha⁻¹ y⁻¹ for eight weeks, to achieve a total application of 35 kg ha⁻¹ during the 10-week study period. Organic and inorganic N additions differed in their effects on total ectomycorrhizal root tip abundance across hosts, and these effects differed for individual morphotypes between oak and pine seedlings. Mycorrhizal root tip abundance across hosts was 90 % higher on seedlings receiving organic N compared to seedlings in the control treatment, while abundances were similar among seedlings receiving the inorganic N treatments and seedlings in the control. On oak, 33–83 % of the most-common morphotypes exhibited increased root tip abundances in response to the three forms of N, relative to the control. On pine, 33–66 % of the most-common morphotypes exhibited decreased root tip abundance in response to inorganic N, while responses to organic N were mixed. Plant chemistry and regression analyses suggested that, on oak seedlings, mycorrhizal colonization increased in response to N limitation. In contrast, pine root and shoot N and C contents did not vary in response to any form of N added, and mycorrhizal root tip abundance was not associated with seedling N or C status, indicating that pine received sufficient N. These results suggest that in situ organic and inorganic N additions differentially affect ectomycorrhizal root tip abundance and that ectomycorrhizal fungal responses to N addition may be mediated by host tree species.     

连续三年夜间增温和施氮对云杉外生菌根及菌根真菌多样性的影响

以西南亚高山针叶林建群种粗枝云杉(Picea asperata)为研究对象,采用红外加热模拟增温结合外施氮肥(NH₄NO₃ 25 g N m^-2 a^-1)的方法,研究连续3a夜间增温和施肥对云杉幼苗外生菌根侵染率、土壤外生菌根真菌生物量及其群落多样性的影响。结果表明:夜间增温对云杉外生菌根侵染率的影响具有季节性及根级差异。夜间增温对春季(2011年5月)云杉1级根,夏季(2011年7月)和秋季(2010年10月)云杉2级根侵染率影响显著。除2011年7月1级根外,施氮对云杉1、2级根侵染率无显著影响。夜间增温对土壤中外生菌根真菌的生物量和群落多样性无显著影响,施氮及增温与施氮联合处理使土壤中外生菌根真菌生物量显著降低,但却提高了外生菌根真菌群落的多样性。这说明云杉幼苗外生菌根侵染率对温度较敏感,土壤外生菌根真菌生物量及其群落多样性对施氮较敏感。这为进一步研究该区域亚高山针叶林地下过程对全球气候变化的响应机制提供了科学依据。