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.     

Insights into nitrogen and carbon dynamics of ectomycorrhizal and saprotrophic fungi from isotopic evidence

The successful use of natural abundances of carbon (C) and nitrogen (N) isotopes in the study of ecosystem dynamics suggests that isotopic measurements could yield new insights into the role of fungi in nitrogen and carbon cycling. Sporocarps of mycorrhizal and saprotrophic fungi, vegetation, and soils were collected in young, deciduous-dominated sites and older, coniferous-dominated sites along a successional sequence at Glacier Bay National Park, Alaska. Mycorrhizal fungi had consistently higher δ¹⁵N and lower δ¹³C values than saprotrophic fungi. Foliar δ¹³C values were always isotopically depleted relative to both fungal types. Foliar δ¹⁵N values were usually, but not always, more depleted than those in saprotrophic fungi, and were consistently more depleted than in mycorrhizal fungi. We hypothesize that an apparent isotopic fractionation by mycorrhizal fungi during the transfer of nitrogen to plants may be attributed to enzymatic reactions within the fungi producing isotopically depleted amino acids, which are subsequently passed on to plant symbionts. An increasing difference between soil mineral nitrogen δ¹⁵N and foliar δ¹⁵N in later succession might therefore be a consequence of greater reliance on mycorrhizal symbionts for nitrogen supply under nitrogen-limited conditions. Carbon signatures of mycorrhizal fungi may be more enriched than those of foliage because the fungi use isotopically enriched photosynthate such as simple sugars, in contrast to the mixture of compounds present in leaves. In addition, some ¹³C fractionation may occur during transport processes from leaves to roots, and during fungal chitin biosynthesis. Stable isotopes have the potential to help clarify the role of fungi in ecosystem processes. Received: 7 January 1998 / Accepted: 9 November 1998     

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.     

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.     

Trophic niche differentiation in millipede communities of forests of the temperate and tropical belts

The degree of trophic differentiation of millipedes in taxocenes of temperate and tropical forests was estimated by isotopic analysis. The isotope ratio variation among species was notably wider in a tropical forest (23 millipede species, δ¹⁵N ranges within 10‰) than in a temperate broad-leaved forest (9 species, δ¹⁵N range less than 3‰). It is suggested that partitioning of food resources among species contributes significantly to the maintenance of high diversity of soil saprotrophic animals in tropical ecosystems.     

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ¹⁵N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ¹⁵N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ¹⁵N varied widely, between 0 and –2‰ for nitrogen-fixing species, between 0 and –7‰ for deciduous non-fixing species, and between 0 and –11‰ for coniferous species. Relatively constant δ¹⁵N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ¹⁵N differences among species at individual sites. In fact, the largest variation among plant δ¹⁵N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ¹⁵N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ¹⁵N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T _r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ¹⁵N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ¹⁵N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T _r. Received: 14 December 1998 / Accepted: 16 August 1999     

Comparative study of nutritional mode and mycorrhizal fungi in green and albino variants of Goodyera velutina,an orchid mainly utilizing saprotrophic rhizoctonia

The majority of chlorophyllous orchids form mycorrhizal associations with so‐called rhizoctonia fungi, a phylogenetically heterogeneous assemblage of predominantly saprotrophic fungi in Ceratobasidiaceae, Tulasnellaceae, and Serendipitaceae. It is still a matter of debate whether adult orchids mainly associated with rhizoctonia species are partially mycoheterotrophic. Here, we investigated the nutritional modes of green and albino variants of Goodyera velutina, an orchid species considered to be mainly associated with Ceratobasidium spp., by measuring their ¹³C and ¹⁵N abundances, and by molecular barcoding of their mycorrhizal fungi. Molecular analysis revealed that both green and albino variants of G. velutina harbored a similar range of mycobionts, mainly saprotrophic Ceratobasidium spp., Tulasnella spp., and ectomycorrhizal Russula spp. In addition, stable isotope analysis revealed that albino variants were significantly enriched in ¹³C but not so greatly in ¹⁵N, suggesting that saprotrophic Ceratobasidium spp. and Tulasnella spp. are their main carbon source. However, in green variants, ¹³C levels were depleted and those of ¹⁵N were indistinguishable from the co‐occurring autotrophic plants. Therefore, we concluded that the albino G. velutina variants are fully mycoheterotrophic plants whose C derives mainly from saprotrophic rhizoctonia, while the green G. velutina variants are mainly autotrophic plants, at least at our study site, in spite of their additional associations with ectomycorrhizal fungi. This is the first report demonstrating that adult nonphotosynthetic albino variants can obtain their nutrition mainly from nonectomycorrhizal rhizoctonia.     

Foliar <Superscript>15</Superscript>N natural abundance indicates phosphorus limitation of bog species

Foliar δ¹⁵N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae) raised bogs revealed marked differences in plant δ¹⁵N correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (−2.03 to −15.55‰, and −0.39 to −12.06‰, respectively) across the bogs, with foliar δ¹⁵N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where ¹⁵N depletion was least. In strong contrast, restiad species (Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had δ¹⁵N levels consistently around 0‰ (means of −0.12‰ and +0.15‰ respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean −4.97‰). The isotopic differences between heath shrubs and restiads were similar in floristically dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Leptospermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited, which was probably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of δ¹⁵N depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in δ¹⁵N of the restiad species through the N and P gradients suggests that these are not suffering from P limitation.     

Sulfur,carbon, and nitrogen stable isotope ratios in soft tissues and trophic relationships of fish from the near-shore waters of the peter the great bay in the Sea of Japan

Stable isotope ratios of sulfur (³⁴S/³²S), carbon (¹³C/¹²C), and nitrogen (¹⁵N/¹⁴N) were analyzed in the soft tissues of 12 common species of fish from the near-shore waters of the Peter the Great Bay in the Sea of Japan. The average δ¹³C values of individual species varied from −20.7‰ for planktivorous fish to −16.8‰ for benthivorous fish, reflecting the growing relative contribution of benthic primary producers to fish nutrition. The majority of the various species representatives studied can be assigned to one trophic level, as indicated by their narrow range of δ¹⁵N values (9.9 to 12.6‰). Large interspecific variations were found in the sulfur stable isotope ratios of fish (the mean δ³⁴S values ranged from 11.2 to 19.5‰). This is the result of the different contributions to fish nutrition of infaunal invertebrates that are depleted in ³⁴S due to the microbial food chain of the bottom sediments.     

Interpretation of nitrogen isotope signatures using the NIFTE model

Nitrogen cycling in forest soils has been intensively studied for many years because nitrogen is often the limiting nutrient for forest growth. Complex interactions between soil, microbes, and plants and the consequent inability to correlate δ¹⁵N changes with biologic processes have limited the use of natural abundances of nitrogen isotopes to study nitrogen (N) dynamics. During an investigation of N dynamics along the 250-year-old successional sequence in Glacier Bay, Alaska, United States, we observed several puzzling isotopic patterns, including a consistent decline in δ¹⁵N of the late successional dominant Picea at older sites, a lack of agreement between mineral N δ¹⁵N and foliar δ¹⁵N, and high isotopic signatures for mycorrhizal fungi. In order to understand the mechanisms creating these patterns, we developed a model of N dynamics and N isotopes (Nitrogen Isotope Fluxes in Terrestrial Ecosystems, NIFTE), which simulated the major transformations of the N cycle and predicted isotopic signatures of different plant species and soil pools. Comparisons with field data from five sites along the successional sequence indicated that NIFTE can duplicate observed patterns in δ¹⁵N of soil, foliage, and mineral N over time. Different scenarios that could account for the observed isotopic patterns were tested in model simulations. Possible mechanisms included increased isotopic fractionation on mineralization, fractionation during the transfer of nitrogen from mycorrhizal fungi to plants, variable fractionation on uptake by mycorrhizal fungi compared to plants, no fractionation on mycorrhizal transfer, and elimination of mycorrhizal fungi as a pool in the model. The model results suggest that fractionation during mineralization must be small (˜2‰), and that no fractionation occurs during plant or mycorrhizal uptake. A net fractionation during mycorrhizal transfer of nitrogen to vegetation provided the best fit to isotopic data on mineral N, plants, soils, and mycorrhizal fungi. The model and field results indicate that the importance of mycorrhizal fungi to N uptake is probably less under conditions of high N availability. Use of this model should encourage a more rigorous assessment of isotopic signatures in ecosystem studies and provide insights into the biologic transformations which affect those signatures. This should lead to an enhanced understanding of some of the fundamental controls on nitrogen dynamics. Received: 1 July 1998 / Accepted: 23 December 1998     

Changes in stable isotopic signatures of soil nitrogen and carbon during 40 years of forest development

Understanding what governs patterns of soil δ¹⁵N and δ¹³C is limited by the absence of these data assembled throughout the development of individual ecosystems. These patterns are important because stable isotopes of soil organic N and C are integrative indicators of biogeochemical processing of soil organic matter. We examined δ¹⁵N of soil organic matter (δ¹⁵N_SOM) and δ¹³C_SOM of archived soil samples across four decades from four depths of an aggrading forest in southeastern USA. The site supports an old-field pine forest in which the N cycle is affected by former agricultural fertilization, massive accumulation of soil N by aggrading trees over four decades, and small to insignificant fluxes of N via NH₃ volatilization, nitrification, and denitrification. We examine isotopic data and the N and C dynamics of this ecosystem to evaluate mechanisms driving isotopic shifts over time. With forest development, δ¹³C_SOM became depth-dependent. This trend resulted from a decline of ~2‰ in the surficial 15 cm of mineral soil to −26.0‰, due to organic matter inputs from forest vegetation. Deeper layers exhibited relatively little trend in δ¹³C_SOM with time. In contrast, δ¹⁵N_SOM was most dynamic in deeper layers. During the four decades of forest development, the deepest layer (35–60 cm) reached a maximum δ¹⁵N value of 9.1‰, increasing by 7.6‰. The transfer of >800 kg ha⁻¹ of soil organic N into aggrading vegetation and the forest floor and the apparent large proportion of ectomycorrhizal (ECM) fungi in these soils suggest that fractionation via microbial transformations must be the major process changing δ¹⁵N in these soils. Accretion of isotopically enriched compounds derived from microbial cells (i.e., ECM fungi) likely promote isotopic enrichment of soils over time. The work indicates the rapid rate at which ecosystem development can impart δ¹⁵N_SOM and δ¹³C_SOM signatures associated with undisturbed soil profiles.     

Carbon nutrition of mature green orchid <Emphasis Type="Italic">Serapias strictiflora</Emphasis> and its mycorrhizal fungus <Emphasis Type="Italic">Epulorhiza</Emphasis> sp.

We studied the nutritional modes of the orchid Serapias strictiflora and its mycorrhizal fungus Epulorhiza sp. using the differences in carbon isotopic composition (δ¹³C) of C₃ orchid and C₄ maize tissues. We found that if cultivated in substrate lacking any organic compounds, the mycorrhizal extraradical mycelia (δ¹³C = −26.3 ± 0.2 ‰) developed well, despite being fully dependent on nutrition from orchid roots (δ¹³C = −28.6 ± 0.1 ‰). If the mycorrhizal fungus had additional access to and colonized decaying maize roots (δ¹³C = −14.6 ± 0.1 ‰), its isotopic composition (δ¹³C = −21.6 ± 0.4 ‰) reflected a mixture of biotrophy and saprotrophy. No statistically significant differences in δ¹³C of new storage tubers were found between Epulorhiza-associated orchids with (δ¹³C = -28.2 ± 0.1 ‰) and without access to maize roots (δ¹³C = −28.6 ± 0.2 ‰). We conclude that autotrophy is the predominant nutritional mode of mature S. strictiflora plants and that they supply their mycorrhizal fungus with substantial amount of carbon (69 ± 3 % of the fungus demand), even if the fungus feeds saprotrophically.     

Nutrient cycling relative to δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C natural abundance in a coastal wetland with long-term nutrient additions

Because nitrogen and phosphorus are primary resources for plant, algal, and microbial production, increases in nutrient inputs can markedly alter aquatic ecosystems. Coastal wetland plots at Belle W. Baruch Marine Field Laboratory (South Carolina, USA) have been amended with nitrogen and phosphorus for ~20 years to determine the effects of nutrient loading on coastal wetlands. We conducted a survey of δ¹⁵N and δ¹³C natural abundance in coastal wetland organic pools (sediment, vegetation) with long-term nutrient amendments (control, no addition; nitrogen; phosphorus; and nitrogen + phosphorus additions). Additionally, we conducted laboratory assays to quantify pore water nutrient availability and nitrification rates. Marsh vegetation (Spartina alterniflora) had enriched δ¹³C values (mean −14‰) relative to bulk sediment samples (mean −18‰). Nitrogen-amended plots (alone and in combination with phosphorus) had enriched δ¹³C values in the surface sediment (0–5 cm; mean −16.1‰) relative to control (mean −16.5‰) and phosphorus-amended plots (mean −16.8‰). Nitrogen-amended plots also had depleted δ¹⁵N in S. alterniflora leaf tissues (−3.3‰) and surface sediment samples (mean 2.1‰) relative to leaf tissues (mean 2.1‰) or sediment samples (mean 5.8‰) from control or phosphorus-only amended plots. Nitrate availability (as increased pore water concentration) was higher in N-amended plots although ammonium availability did not differ. Phosphorus availability was higher only in phosphorus-only amended plots. Overall, we found that long-term nutrient amendments to coastal wetlands significantly altered nutrient availability and uptake rates as well as natural abundance of δ¹³C and δ¹⁵N in multiple organic matter sources.     

Nitrogen availability patterns in white-sand vegetations of Central Brazilian Amazon

Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions. Accordingly, we measured soil and foliar N concentration and N isotope ratios (δ¹⁵N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation types in regard to soil N concentration, C:N ratio and δ¹⁵N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios, and no significant difference between site types was observed. Foliar δ¹⁵N was depleted, varying from −9.6 to 1.6‰ in the white-sand vegetations. The legume Aldina heterophylla had the highest average δ¹⁵N values (−1.5‰) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted δ¹⁵N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar δ¹⁵N among plants, a significant and gradual ¹⁵N-enrichment in foliar isotopic signatures throughout the campina–campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in ¹⁵N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in foliar δ¹⁵N values throughout the campina–campinarana transect.     

13C natural abundance variations in carbonates and organic carbon from boreal forest wetlands

¹³C natural abundance variations were measured in peat soil and vegetation from two contrasting boreal forest wetlands: an upland watershed basin and a permanently saturated lowland mire. Evidence of methane oxidation was shown in the permanently saturated wetland with δ¹³C values as low as -97 ‰ in carbonate minerals found in floating peat mats. It is postulated that¹³C depleted CH₄ is oxidized in the mat and reacts with calcium ions to form calcite (identified through x-ray diffraction). Methane flux measurements during the summer of 1992 showed much lower fluxes in areas with floating peat mats relative to open water. Secondary carbonates in the basin peat have isotope compositions close to the δ¹³C values of the peat organic carbon (-25 ‰), indicating their origin from fermentation and possibly from sulfate-reduction. In the upland basin peat deposits, the δ¹³C_PDB values of organic C were constant with depth, while the permanently saturated mire had localities of¹³C enrichment in deeper layers of the peat. The¹³C enrichment may reflect areas of intense CH₄ production in which¹³C enriched residual substrate is left behind during the production of highly¹³C depleted CH₄.     

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.     

Elucidating the nutritional dynamics of fungi using stable isotopes

Mycorrhizal and saprotrophic (SAP) fungi are essential to terrestrial element cycling due to their uptake of mineral nutrients and decomposition of detritus. Linking these ecological roles to specific fungi is necessary to improve our understanding of global nutrient cycling, fungal ecophysiology, and forest ecology. Using discriminant analyses of nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope values from 813 fungi across 23 sites, we verified collector-based categorizations as either ectomycorrhizal (ECM) or SAP in > 91% of the fungi, and provided probabilistic assignments for an additional 27 fungi of unknown ecological role. As sites ranged from boreal tundra to tropical rainforest, we were able to show that fungal δ¹³C (26 sites) and δ¹⁵N (32 sites) values could be predicted by climate or latitude as previously shown in plant and soil analyses. Fungal δ¹³C values are likely reflecting differences in C-source between ECM and SAP fungi, whereas ¹⁵N enrichment of ECM fungi relative to SAP fungi suggests that ECM fungi are consistently delivering ¹⁵N depleted N to host trees across a range of ecosystem types.     

Influence of substratum on the variability of benthic biofilm stable isotope signatures: implications for energy flow to a primary consumer

Benthic biofilms have been identified using stable isotope analysis (SIA) as an important resource supporting many freshwater food webs. However, biofilm δ¹³C signatures are highly variable in freshwaters, which may hamper our understanding of energy flow through food webs in these systems. There has been little consideration of the influence that substratum may have on biofilm δ¹³C signature variability and energy flows to primary consumers. We investigated the effect of organic and inorganic substrata on biofilm dynamics by examining: (1) temporal variability of biofilm stable isotope (δ¹³C, δ¹⁵N) signatures on allochthonous leaf-litter (Eucalyptus camaldulensis) and cobble substrata over 12 months in a lowland river in south-eastern Australia; and (2) the effect of substrata on biofilm energy flows to a grazer snail, Physa acuta (Gastropoda: Physidae), using SIA and ecological stoichiometry in a laboratory experiment. The temporal study indicated that cobble biofilm varied significantly in δ¹³C signature during the 12 months (up to 11‰), whereas the δ¹³C signature of leaf biofilm was less variable (less than 2‰). In contrast, biofilm δ¹⁵N signatures varied temporally on both cobble (2.6‰) and leaf (1‰) substrata. This suggests that leaf biofilm was more reliant on leaf tissue for carbon and therefore less limited by carbon supply than cobble biofilm whereas for nitrogen biofilm on both substrata was reliant on external sources. In the laboratory experiment, snails fed leaf biofilm reflected more of an allochthonous δ¹³C signature than cobble biofilm fed snails, suggesting assimilation of leaf carbon via the heterotrophic microbial community within the biofilm. Snails grew largest on cobble biofilm, which had lower C:N ratios than leaf biofilm. Our results demonstrate that the type of substratum can influence the temporal variability of biofilm δ¹³C signatures and energy flow to primary consumers.     

Nitrogen and carbon isotope responses of Chinese cabbage and chrysanthemum to the application of liquid pig manure

The effects of the liquid pig manure (LM) used in organic farming on the natural abundance of ¹⁵N and ¹³C signatures in plant tissues have not been studied. We hypothesized that application of LM will (1) increase δ¹⁵N of plant tissues due to the high δ¹⁵N of N in LM as compared with soil N or inorganic fertilizer N, and (2) increase δ¹³C of plant tissues as a result of high salt concentration in LM that decreases stomatal conductance of plants. To test these hypotheses, variations in the δ¹⁵N and δ¹³C of Chinese cabbage (Brassica campestris L.) and chrysanthemum (Chrysanthemum morifolium Ramatuelle) with two different LMs (with δ¹⁵N of +15.6 and +18.2‰) applied at two rates (323 and 646 kg N ha^-1 for cabbage and 150 and 300 kg N ha^-1 for chrysanthemum), or urea (δ¹⁵N = -2.7‰) applied at the lower rate above for the respective species, in addition to the control (no N input) were investigated through a 60-day pot experiment. Application of LM significantly increased plant tissue δ¹⁵N (range +9.4 to +14.9‰) over the urea (+3.2 to +3.3‰) or control (+6.8 to 7.7‰) treatments regardless of plant species, strongly reflecting the δ¹⁵N of the N source. Plant tissue δ¹³C were not affected by the treatments for cabbage (range −30.8 to −30.2‰) or chrysanthemum (−27.3 to −26.8‰). However, cabbage dry matter production decreased while its δ¹³C increased with increasing rate of LM application or increasing soil salinity (P < 0.05), suggesting that salinity stress caused by high rate of LM application likely decreased stomatal conductance and limited growth of cabbage. Our study expanded the use of the δ¹⁵N technique in N source (organic vs. synthetic fertilizer) identification and suggested that plant tissue δ¹³C maybe a sensitive indicator of plant response to salinity stress caused by high LM application rates.     

Subantarctic Macquarie Island – a model ecosystem for studying animal-derived nitrogen sources using 15N natural abundance

Plants collected from diverse sites on subantarctic Macquarie Island varied by up to 30‰ in their leaf δ¹⁵N values. ¹⁵N natural abundance of plants, soils, animal excrement and atmospheric ammonia suggest that the majority of nitrogen utilised by plants growing in the vicinity of animal colonies or burrows is animal-derived. Plants growing near scavengers and animal higher in the food chain had highly enriched δ¹⁵N values (mean = 12.9‰), reflecting the highly enriched signature of these animals' excrement, while plants growing near nesting penguins and albatross, which have an intermediate food chain position, had less enriched δ¹⁵N values (>6‰). Vegetation in areas affected by rabbits had lower δ¹⁵N values (mean = 1.2‰), while the highly depleted δ¹⁵N values (below −5‰) of plants at upland plateau sites inland of penguin colonies, suggested that a portion of their nitrogen is derived from ammonia (mean ¹⁵N =−10‰) lost during the degradation of penguin guano. Vegetation in a remote area had δ¹⁵N values near −2‰. These results contrast with arctic and subarctic studies that attribute large variations in plant ¹⁵N values to nitrogen partitioning in nitrogen-limited environments. Here, plant ¹⁵N reflects the ¹⁵N of the likely nitrogen sources utilised by plants. Received: 18 December 1997 / Accepted: 13 June 1998