In this study we show that the natural abundance of the nitrogen isotope 15, δN, of plants in heath tundra and at the tundra-forest ecocline is closely correlated with the presence and type of mycorrhizal association in the plant roots. A total of 56 vascular plant species, 7 moss species, 2 lichens and 6 species of fungi from four heath and forest tundra sites in Greenland, Siberia and Sweden were analysed for δN and N concentration. Roots of vascular plants were examined for mycorrhizal colonization, and the soil organic matter was analysed for δN, N concentration and soil inorganic, dissolved organic and microbial N. No arbuscular mycorrhizal (AM) colonizations were found although potential host plants were present in all sites. The dominant species were either ectomycorrhizal (ECM) or ericoid mycorrhizal (ERI). The δN of ECM or ERI plants was 3.5-7.7‰ lower than that of non-mycorrhizal (NON) species in three of the four sites. This corresponds to the results in our earlier study of mycorrhiza and plant δN which was limited to one heath and one fellfield in N Sweden. Hence, our data suggest that the δN pattern: NON/AM plants > ECM plants ≥ ERI plants is a general phenomenon in ecosystems with nutrient-deficient organogenic soils. In the fourth site, a␣birch forest with a lush herb/shrub understorey, the differences between functional groups were considerably smaller, and only the ERI species differed (by 1.1‰) from the NON species. Plants of all functional groups from this site had nearly twice the leaf N concentration as that found in the same species at the other three sites. It is likely that low inorganic N availability is a prerequisite for strong δN separation among functional groups. Both ECM roots and fruitbodies were N enriched compared to leaves which suggests that the difference in δN between plants with different kinds of mycorrhiza could be due to isotopic fractionation at the␣fungal-plant interface. However, differences in δN between soil N forms absorbed by the plants could also contribute to the wide differences in plant δN found in most heath and forest tundra ecosystems. We hypothesize that during microbial immobilization of soil ammonium the microbial N pool could become N-depleted and the remaining, plant-available soil ammoniumN-enriched. The latter could be a main source of N for NON/AM plants which usually have high δN. In contrast, amino acids and other soil organic N compounds presumably are N-depleted, similar to plant litter, and ECM and ERI plants with high uptake of these N forms hence have low leaf δN. Further indications come from the δN of mosses and lichens which was similar to that of ECM plants. Tundra cryptogams (and ECM and ERI plants) have previously been shown to have higher uptake of amino acid than ammonium N; their low δN might therefore reflect the δN of free amino acids in the soil. The concentration of dissolved organic N was 3-16 times higher than that of inorganic N in the sites. Organic nitrogen could be an important N source for ECM and, in particular, ERI plant...
The natural abundance of the nitrogen isotope 15, δN, 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 δN was between-8.8 and-5.5‰ at the heath and between-6.1 and -3.3‰ at the fellfield. The leaf δN 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 δN 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 δN (fellfield) and N concentration than leaves of the ericoids, and Betula nana, Dryas octopetala and Salix spp. also showed NO reductase activity. These species may depend more on soil inorganic N than the ericoids. The δN of non- or arbuscular mycorrhizal species indicates that the δN 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 δN of NH -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 , NO and organic N from the soil, atmospheric N, 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 δN in organic soils.
Microbial immobilization may decrease the inorganic nutrient concentrations of the soil to the extent of affecting plant nutrient uptake and growth. We have hypothesized that graminoids with opportunistic nutrient-acquisition strategies are strongly influenced by nutrient limitation imposed by microbes, whereas growth forms such as dwarf shrubs are less affected by the mobilization-immobilization cycles in microbes. By adding NPK fertilizer, labile C (sugar) and fungicide (benomyl) over a 5 yr period in a fully factorial design, we aimed to manipulate the sink-source potential for nutrients in a non-acidic heath tundra soil. After 2 yr, N and P accumulated in the microbial biomass after fertilization with no change in microbial C, which suggests that nutrients did not limit microbial biomass growth. After 5 yr, microbial C was enhanced by 60 % in plots with addition of labile C, which points to C-limitation of the microbial biomass. Microbial biomass N and P tended to increase following addition of labile C, by 10 and 25 %, respectively. This caused decreased availability of NH % + and P, showing close microbial control of nutrient availability. The most common graminoid, Festuca ovina, responded to fertilizer addition with a strong increase, and to labile C addition with a strong decrease in cover, providing the first direct field evidence that nutrient limitation imposed by immobilizing microbes can affect the growth of tundra plants. Also in support of our hypothesis, following addition of labile C the concentrations of N and K in leaves and that of N in roots of F. ovina decreased, whilst the demand of roots for P increased. In contrast, the most common dwarf shrub, Vaccinium uliginosum, was only slightly sensitive to changes in resource availability, showing no cover change after 4 yr addition of labile C and fertilizer, and little change in leaf nutrient concentrations. We suggest that the differential responses of the two growth forms are due to differences in storage and nutrient uptake pathways, with the dwarf shrub having large nutrient storage capacity and access to organic forms of N through its mycorrhizal association. While the fungicide had no effect on ericoid mycorrhizal colonization of roots or symbiotic function inferred from plant "&N natural abundance, it decreased microbial biomass C and N after 2 yr. Throughout the fifth season, the availability of soil NO $ − and inorganic P was decreased with no change in microbial biomass C, N or P, suggesting a negative impact of benomyl on N and P mineralization.
SUMMARYThe rootless submerged carnivorous hydrophyte Utricularia vulgaris L. is a potentially useful subject for tbe quantitatii'e study of foliar nutrient uptake and u.se, because known quantities of nutrients can be supplied precisely to individual leaves. We report here resuits obtained by band-feeding prey labelled witb ^^K and ^^P to leaves of known age in plants growing in near-natural conditions.Prey-derived ^*N was rapidly taken up and translocated: in plants fed via 3-d-old leaves, about 30 "" of tbe preŷ^N appeared in tbe immature parts of the plant \vithin 2 d. Almost a)l parts of the plant that were immature at the time of feeding received and retained prey ^'N throughout the 20 d experiment. Some backward translocation of ^-^N was observed, but only up to the second day after feeding.^P was also taken up and translocated rapidly, but was not retained by young tissues after they had become mature. Backward translocation of ^^P was observed into side-shoot meristems and flowers arising on parts of the plant older tban the fed leaves. This is in contrast to '^N uptake, where side-sboot buds and flowers received labelled nitrogen only if they arose on parts of the plant younger than the fed leaves.
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