Nitrogen isotopes in ectomycorrhizal sporocarps correspond to belowground exploration types

Nitrogen isotope values (δ¹⁵N) are higher in ectomycorrhizal fungi than in their plant hosts but the wide variability in δ¹⁵N among sporocarps of different fungal taxa is unexplained. We propose that fungal δ¹⁵N reflects sequestration of fungal nitrogen to build fungal biomass, and should accordingly reflect fungal exploration strategies and hyphal properties. To test this, we compared δ¹⁵N to exploration types, hyphal hydrophobicity, and the presence of rhizomorphs in ectomycorrhizal species from surveys at four sites in temperate and boreal coniferous forests. Fungi with exploration types of high biomass, such as long-distance (e.g., Suillus), medium-distance mat (e.g., Hydnellum), and medium-distance fringe (e.g., Cortinarius) were 4–7‰ more enriched in ¹⁵N than fungi with exploration types of low biomass [medium-distance smooth (e.g., Amanita), short-distance (e.g., Inocybe), and contact (e.g., Hygrophorus)]. High biomass types comprised 79% (Åheden, northern Sweden), 65% (Deer Park, Pacific Northwest, USA), 45% (Stadsskogen, central Sweden), and 39% (Hoh, Pacific Northwest, USA) of ectomycorrhizal species, with these types more prevalent at sites of lower nitrogen availability. Species with hydrophobic hyphae or with rhizomorphs were 3–4‰ more enriched in ¹⁵N than taxa with hydrophilic hyphae or without rhizomorphs. The consistency of these patterns suggest that δ¹⁵N measurements could provide insights into belowground functioning of poorly known taxa of ectomycorrhizal fungi and into relative fungal biomass across ectomycorrhizal communities.     

Strategies of carbon and nitrogen acquisition by saprotrophic and ectomycorrhizal fungi in Finnish boreal Picea abies-dominated forests

We compared the δ¹³C and δ¹⁵N of forest material with an extensive sporocarp collection to elucidate the role of litter, wood and soil as fungal carbon and nitrogen sources in Finnish boreal Picea abies-dominated forests. Ectomycorrhizal Hydnum and Cortinarius had higher δ¹⁵N than other ectomycorrhizal fungi, suggesting use of ¹⁵N-enriched, deeper nitrogen. Russula had lower δ¹⁵N than other ectomycorrhizal fungi and resembled some litter decay genera, suggesting use of litter-derived nitrogen. There was little variation in δ¹⁵N among other genera of ectomycorrhizal fungi, indicating limited functional diversity in nitrogen use. Saprotrophic Leotia, Gymnopus, Hypholoma, Pholiota, Rhodocollybia and Calocera had δ¹⁵N values similar to ectomycorrhizal fungi, indicating overlap in use of older nitrogen from soil or roots or use of newly fixed nitrogen. Genera of litter and wood decay fungi varied up to 6‰ in δ¹³C and 10‰ in δ¹⁵N, suggesting large differences in carbon and nitrogen sources and processing. Similar δ¹³C between white and brown rot wood decay fungi also suggest that white rot fungi do not use lignin-derived carbon. Together, these δ¹³C and δ¹⁵N patterns of fungi from Finnish boreal forests enhance our knowledge of fungal functional diversity and indicate broad use of litter, wood and soil resources.     

Nitrogen Uptake by Trees and Mycorrhizal Fungi in a Successional Northern Temperate Forest: Insights from Multiple Isotopic Methods

Forest succession may cause changes in nitrogen (N) availability, vegetation and fungal community composition that affect N uptake by trees and their mycorrhizal symbionts. Understanding how these changes affect the functioning of the mycorrhizal symbiosis is of interest to ecosystem ecology because of the fundamental roles mycorrhizae play in providing nutrition to trees and structuring forest ecosystems. We investigated changes in tree and mycorrhizal fungal community composition, the availability and uptake of N by trees and mycorrhizal fungi in a forest undergoing a successional transition (age-related loss of early successional tree taxa). In this system, 82–96% of mycorrhizal hyphae were ectomycorrhizal (EM). As biomass production of arbuscular mycorrhizal (AM) trees increased, AM hyphae comprised a significantly greater proportion of total fungal hyphae, and the EM contribution to the N requirement of EM-associated tree taxa declined from greater than 75% to less than 60%. Increasing N availability was associated with lower EM hyphal foraging and ¹⁵N tracer uptake, yet the EM-associated later-successional species Quercus rubra was nonetheless a stronger competitor for ¹⁵N than AM-associated Acer rubrum, likely due to the more extensive nature of the persistent EM hyphal network. These results indicate that successional increases in N availability and co-dominance by AM-associated trees have increased the importance of AM fungi in the mycorrhizal community, while down-regulating EM N acquisition and transfer processes. This work advances understanding of linkages between tree and fungal community composition, and indicates that successional changes in N availability may affect competition between tree taxa with divergent resource acquisition strategies.     

Role of nutrient level and defoliation on symbiotic function: experimental evidence by tracing 14C/15N exchange in mycorrhizal birch seedlings

High nutrient availability and defoliation generally reduce ectomycorrhizal colonization levels in trees, but it is not known how this affects the functional aspects of mycorrhizal symbiosis. It was therefore investigated whether (1) defoliation or increasing substrate N availability reduce C allocation from the plant to the fungus and N allocation from the fungus to the plant (symbiotic resource exchange), (2) symbiotic resource exchange depends on relative N and P availability, and (3) fungal N translocation to plant and plant C allocation to fungus are interdependent. Birch (Betula pendula) seedlings were grown in symbiosis with the ectomycorrhizal fungus Paxillus involutus at five times excess N, or at five times excess N and P for 6 weeks. One-half of the plants were defoliated and the plant shoots were allowed to photosynthesize ¹⁴CO₂ while the fungal compartment was exposed to ¹⁴NH₄. After 3 days, the ¹⁴C of plant origin in fungal tissues and ¹⁵N of fungal origin in plant tissues were quantified. Nutrient availability had no observable effect on symbiotic resource exchange in non-defoliated systems. Defoliation reduced symbiotic N acquisition by plants at all levels of nutrient availability, with the reduction being most marked at higher N availability, indicating an increased tendency in the symbiotic system to discontinue resource exchange after defoliation at higher fertility levels. The concentration of ¹⁴C in extramatrical mycelium correlated significantly with the concentration of ¹⁵N in birch shoots. The results support the assumption that N delivery to the host by the mycorrhizal fungus is dependent on C flow from the plant to the fungus, and that exchanges between the partners are reciprocal. No significant reductions in root ¹⁴C content as a response to defoliation were observed, indicating that defoliation specifically reduced allocation to fungus, but not markedly to roots.     

Attributing functions to ectomycorrhizal fungal identities in assemblages for nitrogen acquisition under stress

Mycorrhizal fungi have a key role in nitrogen (N) cycling, particularly in boreal and temperate ecosystems. However, the significance of ectomycorrhizal fungal (EMF) diversity for this important ecosystem function is unknown. Here, EMF taxon-specific N uptake was analyzed via ¹⁵N isotope enrichment in complex root-associated assemblages and non-mycorrhizal root tips in controlled experiments. Specific ¹⁵N enrichment in ectomycorrhizas, which represents the N influx and export, as well as the exchange of ¹⁵N with the N pool of the root tip, was dependent on the fungal identity. Light or water deprivation revealed interspecific response diversity for N uptake. Partial taxon-specific N fluxes for ectomycorrhizas were assessed, and the benefits of EMF assemblages for plant N nutrition were estimated. We demonstrated that ectomycorrhizal assemblages provide advantages for inorganic N uptake compared with non-mycorrhizal roots under environmental constraints but not for unstressed plants. These benefits were realized via stress activation of distinct EMF taxa, which suggests significant functional diversity within EMF assemblages. We developed and validated a model that predicts net N flux into the plant based on taxon-specific ¹⁵N enrichment in ectomycorrhizal root tips. These results open a new avenue to characterize the functional traits of EMF taxa in complex communities.     

Diversity and community structure of ectomycorrhizal fungi in a wooded meadow

Wooded meadows are seminatural plant communities that support high diversity of various taxa. Due to changes in land use, wooded meadows have severely declined during the last century. The dominant trees in wooded meadows acquire mineral nutrients via ectomycorrhizal fungi. Using anatomotyping and sequencing of root tips, interpolation and extrapolation methods, we studied the diversity and community structure of ectomycorrhizal fungi in two soil horizons of both managed and forested parts of a wooded meadow in Estonia. Species of Thelephoraceae, Sebacinaceae and the genus Inocybe dominated the whole ectomycorrhizal fungal community of 172 observed species. Forested and managed parts of the wooded meadow harboured different communities of ectomycorrhizal fungi, whereas soil horizon had a negligible effect on the fungal community composition. Diverse soil conditions and host trees likely support the high richness of ectomycorrhizal fungi in the wooded meadow ecosystem. Direct sequencing integrated with interpolation and extrapolation methods are promising to identify the fungi at the species level and to compare species richness between communities of ectomycorrhizal fungi.     

Differences in nitrogen redistribution between early and late plant colonizers through ectomycorrhizal fungi on the volcano Mount Koma

Relationships involving the transfer of nitrogen (N) among Salix reinii (willow), Larix kaempferi (larch), and mycorrhizal fungi were investigated in a ridge and hillslope on the volcano Mount Koma in northern Japan using a two-pool fungal model. This model estimated N transfer among the examined taxa by measuring changes in the stable isotope ratio of N (δ¹⁵N). Although N content in tephra was low at both sites, it was higher on the ridge than on the hillslope, and higher in the willow patch than on bare ground or in the larch understory. The non-mycorrhizal sedge (Carex oxyandra) exhibited non-significant differences between the two sites regarding δ¹⁵N for N obtained from tephra. Larches developed a relationship with larch-specific Suillus mycorrhizal fungal species in the roots, and had a lower foliar δ¹⁵N on the hillslope than on the ridge. The larch δ¹⁵N increased during the growing season, while the willow δ¹⁵N remained stable. The dependence of larch on mycorrhizal fungi for N uptake was 3–5 % on the ridge and 56–76 % on the hillslope in autumn. Therefore, larches exhibited a flexible symbiotic relationship with mycorrhizal fungi for obtaining N. Over 45 % of the N taken up by willow plants was obtained from mycorrhizal fungi at both sites. In conclusion, willow plants promoted N deposition in tephra through the litter supply, and formed a stable relationship with mycorrhizal fungi. This enabled successful revegetation with larch plants, which exhibited flexibility in terms of N uptake (i.e., dependent on mycorrhizae or from tephra).     

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.     

Nitrogen Isotope Patterns in Alaskan Black Spruce Reflect Organic Nitrogen Sources and the Activity of Ectomycorrhizal Fungi

Global patterns in soil, plant, and fungal stable isotopes of N (δ¹⁵N) show promise as integrated metrics of N cycling, particularly the activity of ectomycorrhizal (ECM) fungi. At small spatial scales, however, it remains difficult to differentiate the underlying causes of plant δ¹⁵N variability and this limits the application of such measurements to better understand N cycling. We conducted a landscape-scale analysis of δ¹⁵N values from 31 putatively N-limited monospecific black spruce (Picea mariana) stands in central Alaska to assess the two main hypothesized sources of plant δ¹⁵N variation: differing sources and ECM fractionation. We found roughly 20% of the variability in black spruce foliar N and δ¹⁵N values to be correlated with the concentration and δ¹⁵N values of soil NH₄ ⁺ and dissolved organic N (DON) pools, respectively. However, ¹⁵N-based mixing models from 24 of the stands suggested that fractionation by ECM fungi obscures the ¹⁵N signature of soil N pools. Models, regressions, and N abundance data all suggested that increasing dependence on soil DON to meet black spruce growth demands predicates increasing reliance on ECM-derived N and that black spruce, on average, received 53% of its N from ECM fungi. Future research should partition the δ¹⁵N values within the soil DON pool to determine how choice of soil δ¹⁵N values influence modeled ECM activity. The C balance of boreal forests is tightly linked to N cycling and δ¹⁵N values may be useful metrics of changes to these connections.     

Differences in 15N uptake amongst spruce seedlings colonized by three pioneer ectomycorrhizal fungi in the field

Amongst the factors hypothesized to be responsible for high ectomycorrhizal fungal diversity are resource partitioning and niche differentiation. However, functional differences amongst ectomycorrhizal fungi, which are pre-requisites for resource partitioning, are known primarily from lab studies; now realistic field experiments are needed in order to establish that these differences exist under field conditions. In this study, Picea engelmannii seedlings planted in a subalpine clearcut became naturally colonized over the course of 1 y. Then a defined volume of soil around each seedling was injected with ¹⁵N-labelled nitrate, ammonium or aspartate. Seedling biomass and N content increased, but N concentration decreased, with percent colonization of root systems. Accumulation of ¹⁵N per unit dry weight was not affected by the proportion of roots colonized but, rather, was influenced by the primary ectomycorrhizal fungus colonizing the seedling. Seedlings colonized by a Wilcoxina sp. accumulated more ¹⁵N per g than seedlings colonized by a Cenococcum sp. The presence of dark septate hyphae in the mantle was associated with lower accumulation of ¹⁵N by seedlings colonized by Amphinema byssoides. Our results demonstrate that the physiological differences required to support the concept of niche differentiation amongst ectomycorrhizal fungi exist in the field.     

Nitrogen form, availability, and mycorrhizal colonization affect biomass and nitrogen isotope patterns in Pinus sylvestris

Nitrogen (N) isotope patterns are useful for understanding carbon and nitrogen dynamics in mycorrhizal systems but questions remain about how different N forms, fungal symbionts, and N availabilities influence δ¹⁵N signatures. Here, we studied how biomass allocation and δ¹⁵N patterns in Pinus sylvestris L. cultures were affected by nitrogen supply rate (3% per day or 4% per day relative to the nitrogen already present), nitrogen form (ammonium versus nitrate), and mycorrhizal colonization by fungi with a greater (Laccaria laccata) or lesser (Suillus bovinus) ability to assimilate nitrate. Mycorrhizal (fungal) biomass was greater with ammonium than with nitrate nutrition for Suillus cultures but similar for Laccaria cultures. Total biomass was less with nitrate nutrition than with ammonium nutrition for nonmycorrhizal cultures and was less in mycorrhizal cultures than in nonmycorrhizal cultures. The sequestration of available N by mycorrhizal fungi limited plant N supply. This limitation and the higher energetic cost of nitrate reduction than ammonium assimilation appeared to control plant biomass accumulation. Colonization decreased foliar δ¹⁵N by 0.5 to 2.2‰ (nitrate) or 1.7 to 3.5‰ (ammonium) and increased root tip δ¹⁵N by 0 to 1‰ (nitrate) or 0.6 to 2.3‰ (ammonium). Root tip δ¹⁵N and fungal biomass on root tips were positively correlated in ammonium treatments (r ²?=?0.52) but not in nitrate treatments (r ²?=?0.00). Fungal biomass on root tips was enriched in ¹⁵N an estimated 6–8‰ relative to plant biomass in ammonium treatments. At high nitrate availability, Suillus colonization did not reduce plant δ¹⁵N. We conclude that: (1) transfer of ¹⁵N-depleted N from mycorrhizal fungi to plants produces low plant δ¹⁵N signatures and high root tip and fungal δ¹⁵N signatures; (2) limited nitrate reduction in fungi restricted transfer of ¹⁵N-depleted N to plants when nitrate is supplied and may account for many field observations of high plant δ¹⁵N under such conditions; (3) plants could transfer assimilated nitrogen to fungi at high nitrate supply but such transfer was without ¹⁵N fractionation. These factors probably control plant δ¹⁵N patterns across N availability gradients and were here incorporated into analytical equations for interpreting nitrogen isotope patterns in mycorrhizal fungi and plants.     

Radiocarbon evidence for the mining of organic nitrogen from soil by mycorrhizal fungi

Organic nitrogen use by mycorrhizal fungi and associated plants could fuel productivity in nitrogen-limited systems. To test whether fungi assimilated soil-derived organic nitrogen, we compared the ¹⁴C signal (expressed as Δ¹⁴C) from 1950s to 1960s thermonuclear testing in protein and structural carbon of ectomycorrhizal fungi. As expected, structural carbon had Δ¹⁴C similar to recent photosynthesis; however, protein Δ¹⁴C was either higher or lower than structural carbon depending on the fungal taxa. This suggests that some protein carbon derived from uptake of organic nitrogen with different Δ¹⁴C signals. Specimens from two taxa (Lactarius and Russula) adapted to taking up soluble nutrients had protein higher than structural carbon in Δ¹⁴C, indicating uptake of young, post-bomb organic nitrogen, whereas two taxa (Cortinarius and Leccinum) adapted to using insoluble, complex organic nutrients had protein lower than structural carbon in Δ¹⁴C, indicating uptake of old, pre-bomb organic nitrogen. Tuber, a genus common in mineral soil, was also consistently lower in Δ¹⁴C for protein than for structural carbon, with an estimated 10 % of protein carbon originating from old, deep organic nitrogen for this taxon. Our results indicate that radiocarbon can provide evidence of organic nitrogen use in ectomycorrhizal fungi and reflects the exploration depth of different taxa.     

Isotopic signatures and trophic status of Ramaria

The genus Ramaria is composed of several subgenera that often correspond to specific trophic strategies. Because carbon and nitrogen isotopes can be used to assess fungal trophic status and nitrogen sources, we accordingly carried out an extensive survey of isotopic patterns in archived specimens of Ramaria from Germany and other locations. Isotopic patterns in species generally corresponded to subgeneric affiliations and to the range of different potential substrates, with fungi fruiting on wood and litter (subgenera Asteroramaria and Lentoramaria) much lower in δ¹⁵N (≈−3‰) than ectomycorrhizal taxa (≈12‰) (subgenus Ramaria) or taxa fruiting on soil (≈13‰) (subgenus Echinoramaria). Conversely, fungi fruiting on wood and litter were higher in δ¹³C (−23‰) than those fruiting on soil (≈−27‰), with ectomycorrhizal fungi intermediate (≈−24.5‰). Fungi colonizing mineral soil horizons were about 3‰ enriched in ¹⁵N relative to those colonizing both mineral and organic horizons. The high δ¹⁵N and low δ¹³C signatures of taxa fruiting on soil remains unexplained. The high degree of fidelity of isotopic signatures with subgeneric classifications and life history traits suggests that sporocarps are good integrators of patterns of carbon and nitrogen cycling for specific taxa. Archived specimens represent a useful trove of life history information that could be mined without requiring extensive supporting isotopic data from other ecosystem pools.     

Ectomycorrhizal fungi are shared between seedlings and adults in a monodominant Gilbertiodendron dewevrei rain forest in Cameroon

Ectomycorrhizal networks may facilitate the establishment and survival of seedlings regenerating under the canopies of tropical forests and are often invoked as a potential contributor to monodominance. We identified ectomycorrhizal fungi in a monodominant Gilbertiodendron dewevrei (Fabaceae) rain forest in Cameroon, using sporocarps and ectomycorrhizae of three age categories (seedlings, intermediate trees, and large trees) and tentatively revealed nutrient transfer through ectomycorrhizal networks by measuring spontaneous isotopic (¹³C and ¹⁵N) abundances in seedlings. Sporocarp surveys revealed fewer ectomycorrhizal fungal taxa (59 species from 1030 sporocarps) than molecular barcoding of ectomycorrhizal roots (75 operational taxonomic units from 828 ectomycorrhizae). Our observations suggest that ectomycorrhizal fungal diversity is similar to that in other mixed tropical forests and provide the first report of the Tuber‐Helvella lineage in a tropical forest. Despite some differences, all age categories of G. dewevrei had overlapping ectomycorrhizal fungal communities, with families belonging to Thelephoraceae, Russulaceae, Sebacinaceae, Boletaceae, and Clavulinaceae. Of the 49 operational taxonomic units shared by the three age categories (65.3% of the ectomycorrhizal fungal community), 19 were the most abundant on root tips of all categories (38.7% of the shared taxa), supporting the likelihood of ectomycorrhizal networks. However, we obtained no evidence for nutrient transfer from trees to seedlings. We discuss the composition of the ectomycorrhizal fungal community among the G. dewevrei age categories and the possible role of common ectomycorrhizal networks in this rain forest.     

Controls of nitrogen isotope patterns in soil profiles

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (δ¹⁵N) data, patterns of ¹⁵N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum ¹⁵N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4‰ in ectomycorrhizal systems and 4.6 ± 0.5‰ in arbuscular mycorrhizal systems. The ¹⁵N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling ¹⁵N in soil profiles, fractionation against ¹⁵N during N transfer by mycorrhizal fungi to host plants, leads to ¹⁵N-depleted plant litter at the soil surface and ¹⁵N-enriched nitrogen of fungal origin at depth. The preferential preservation of ¹⁵N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large ¹⁵N enrichments with depth in less N-limited forests and may account for soil profiles where maximum δ¹⁵N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site.     

Ectomycorrhizal Fungal Protein Degradation Ability Predicted by Soil Organic Nitrogen Availability

In temperate and boreal forest ecosystems, nitrogen (N) limitation of tree metabolism is alleviated by ectomycorrhizal (ECM) fungi. As forest soils age, the primary source of N in soil switches from inorganic (NH₄⁺ and NO₃⁻) to organic (mostly proteins). It has been hypothesized that ECM fungi adapt to the most common N source in their environment, which implies that fungi growing in older forests would have greater protein degradation abilities. Moreover, recent results for a model ECM fungal species suggest that organic N uptake requires a glucose supply. To test the generality of these hypotheses, we screened 55 strains of 13 Suillus species with different ecological preferences for their in vitro protein degradation abilities. Suillus species preferentially occurring in mature forests, where soil contains more organic matter, had significantly higher protease activity than those from young forests with low-organic-matter soils or species indifferent to forest age. Within species, the protease activities of ecotypes from soils with high or low soil organic N content did not differ significantly, suggesting resource partitioning between mineral and organic soil layers. The secreted protease mixtures were strongly dominated by aspartic peptidases. Glucose addition had variable effects on secreted protease activity; in some species, it triggered activity, but in others, activity was repressed at high concentrations. Collectively, our results indicate that protease activity, a key ectomycorrhizal functional trait, is positively related to environmental N source availability but is also influenced by additional factors, such as carbon availability.     

Jack pine foliar δ15N indicates shifts in plant nitrogen acquisition after severe wildfire and through forest stand development

Background and aims

Natural abundance of the stable nitrogen (N) isotope ¹⁵N can elucidate shifts in plant N acquisition and ecosystem N cycling following disturbance events. This study examined the potential relationship between foliar δ¹⁵N and depth of plant N acquisition (surface organic vs. mineral soil) and nitrification as conifer stands develop following stand-replacing wildfire.

Methods

We measured foliar δ¹⁵N along an 18-site chronosequence of jack pine (Pinus banksiana) stands, 1 to 72 years in age post-wildfire. Foliar δ¹⁵N was compared to total δ¹⁵N of the organic (Oe + Oa) and mineral (0–15 cm) soil horizons, and organic horizon N mineralization and nitrification as functions of total mineralization.

Results

Foliar δ¹⁵N declined with stand age, yet wildfire effects were heterogeneous. Jack pine seedlings on burned, mineral soil patches in the youngest stand were significantly more enriched than those on unburned, organic patches (P?=?0.007). High foliar values in the youngest stands relative to mineral-horizon δ¹⁵N indicate that nitrification also likely contributed to seedling enrichment.

Conclusions

Our results suggest jack pine seedlings on burned patches obtain N from the mineral soil with potentially high nitrification rates, whereas seedlings on unburned patches and increasingly N-limited, mature jack pine acquire relatively more N from organic horizons.     

An Assessment of Contemporary and Historic Nitrogen Availability in Contrasting Coastal Douglas-Fir Forests Through δ15N of Tree Rings

Wood nitrogen isotope composition (δ¹⁵N) provides a potential retrospective evaluation of ecosystem N status but refinement of this index is needed. We calibrated current wood δ¹⁵N of Douglas-fir (Pseudotsuga menziesii), an ectomycorrhizal tree species, against a productivity gradient of contrasting coastal forests of southern Vancouver Island (Canada). We then examined historical δ¹⁵N via increment cores, and tested whether wood δ¹⁵N corresponded with climatic fluctuations. Extractable soil N ranged from 11 to 43 kg N ha^?1 along the productivity gradient, and was characterized by a progressive replacement of N forms (amino acids, NH₄ ⁺ and NO₃ ^?). Current wood δ¹⁵N was significantly less depleted (?5.0 to ?2.6 ‰) with increasing productivity, although linear correlations were stronger with Δδ¹⁵N (the difference between wood and soil δ¹⁵N) to standardize the extent of isotopic fractionation by ectomycorrhizal fungi. An overall decline in wood δ¹⁵N of 0.9 ‰ over the years 1900–2009 was detected, but trends diverged widely among plots, including positive, negative and no trend with time. We did not detect significant correlations in detrended wood δ¹⁵N with mean annual temperature or precipitation. The contemporary patterns in stand productivity, soil N supply and wood δ¹⁵N were moderately strong, but interpreting historical patterns in δ¹⁵N was challenging because of potential variations in N uptake related to stand dynamics. The lack of wood δ¹⁵N correlations with climate may be partly due to methodological limitations, but might also reflect the relative stability in N supply due to the overriding constraints of soil organic matter quantity and quality.     

Ectomycorrhizal Communities on the Roots of Two Beech (Fagus sylvatica) Populations from Contrasting Climates Differ in Nitrogen Acquisition in a Common Environment

Beech (Fagus sylvatica), a dominant forest species in Central Europe, competes for nitrogen with soil microbes and suffers from N limitation under dry conditions. We hypothesized that ectomycorrhizal communities and the free-living rhizosphere microbes from beech trees from sites with two contrasting climatic conditions exhibit differences in N acquisition that contribute to differences in host N uptake and are related to differences in host belowground carbon allocation. To test these hypotheses, young trees from the natural regeneration of two genetically similar populations, one from dryer conditions (located in an area with a southwest exposure [SW trees]) and the other from a cooler, moist climate (located in an area with a northeast exposure [NE trees]), were transplanted into a homogeneous substrate in the same environment and labeled with ¹³CO₂ and ¹⁵NH₄⁺. Free-living rhizosphere microbes were characterized by marker genes for the N cycle, but no differences between the rhizospheres of SW or NE trees were found. Lower ¹⁵N enrichment was found in the ectomycorrhizal communities of the NE tree communities than the SW tree communities, whereas no significant differences in ¹⁵N enrichment were observed for nonmycorrhizal root tips of SW and NE trees. Neither the ectomycorrhizal communities nor the nonmycorrhizal root tips originating from NE and SW trees showed differences in ¹³C signatures. Because the level of ¹⁵N accumulation in fine roots and the amount transferred to leaves were lower in NE trees than SW trees, our data support the suggestion that the ectomycorrhizal community influences N transfer to its host and demonstrate that the fungal community from the dry condition was more efficient in N acquisition when environmental constraints were relieved. These findings highlight the importance of adapted ectomycorrhizal communities for forest nutrition in a changing climate.     

Conservation of biotrophy in Hygrophoraceae inferred from combined stable isotope and phylogenetic analyses

The nutritional modes of genera in Hygrophoraceae (Basidiomycota: Agaricales), apart from the ectomycorrhizal Hygrophorus and lichen-forming taxa, are uncertain. New δ(15)N and δ(13)C values were obtained from 15 taxa under Hygrophoraceae collected in central Massachusetts and combined with isotopic datasets from five prior studies including a further 12 species using a data standardization method to allow cross-site comparison. Based on these data, we inferred the probable nutritional modes for species of Hygrophorus, Hygrocybe, Humidicutis, Cuphophyllus and Gliophorus. A phylogeny of Hygrophoraceae was constructed by maximum likelihood analysis of nuclear ribosomal 28S and 5.8S sequences and standardized δ(15)N and δ(13)C values were used for parsimony optimization on this phylogeny. Our results supported a mode of biotrophy in Hygrocybe, Humidicutis, Cuphophyllus and Gliophorus quantitatively unlike that in more than 450 other fungal taxa sampled in the present and prior studies. Parsimony optimization of stable isotope data suggests moderate conservation of nutritional strategies in Hygrophoraceae and a single switch to a predominantly ectomycorrhizal life strategy in the lineage leading to Hygrophorus. We conclude that Hygrophoraceae of previously unknown nutritional status are unlikely to be saprotrophs and are probably in symbiosis with bryophytes or other understory plants.