VASCULAR PLANT N-15 NATURAL-ABUNDANCE IN HEATH AND FOREST TUNDRA ECOSYSTEMS IS CLOSELY CORRELATED WITH PRESENCE AND TYPE OF MYCORRHIZAL FUNGI IN ROOTS

In this study we show that the natural abundance of the nitrogen isoto pe 15, delta(15)N, Of plants in heath tundra and at the tundra-forest ecocline is closely correlated with the presence and type of mycorrhiz al association in the plant roots. A total of 56 vascular plant specie s, 7 moss species, 2 lichens and 6 species of fungi from four heath an d forest tundra sites in Greenland, Siberia and Sweden were analysed f or delta(15)N and N concentration. Roots of vascular plants were exami ned for mycorrhizal colonization, and the soil organic matter was anal ysed for delta(15)N,N concentration and soil inorganic, dissolved orga nic and microbial N. No arbuscular mycorrhizal (AM) colonizations were found although potential host plants were present in all sites. The d ominant species were either ectomycorrhizal (ECM) or ericoid mycorrhiz al (ERI). The delta(15)N of ECM or ERI plants was 3.5-7.7%, lower than that of non-mycorrhizal (NON) species in three of the four sites. Thi s corresponds to the results in our earlier study of mycorrhiza and pl ant delta(15)N which was limited to one heath and one fellfieId in N S weden. Hence, our data suggest that the delta(15)N pattern: NON/AM pla nts > ECM plants greater than or equal to ERI plants is a general phen omenon in ecosystems with nutrient-deficient organogenic soils. In the fourth site, a birch forest with a lush herb/shrub understorey, the d ifferences between functional groups were considerably smaller, and on ly 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 conce ntration as that found in the same species at the other three sites. I t is likely that low inorganic N availability is a prerequisite for st rong delta(15)N separation among functional groups. Both ECM roots and fruitbodies were N-15 enriched compared to leaves which suggests that the difference in delta(15)N between plants with different kinds of m ycorrhiza could be due to isotopic fractionation at the fungal-plant i nterface. However, differences in delta(15)N between soil N forms abso rbed by the plants could also contribute to the wide differences in pl ant delta(15)N found in most heath and forest tundra ecosystems. We hy pothesize that during microbial immobilization of soil ammonium the mi crobial N pool could become N-15- depleted and the remaining, plant-av ailable soil ammonium N-15-enriched. The latter could be a main source of N for NON/AM plants which usually have high delta(15)N. In contras t, amino acids and other soil organic N compounds presumably are N-15- depleted, similar to plant litter, and ECM and ERI plants with high up take of these N forms hence have low leaf delta(15)N. Further indicati ons come from the delta(15)N of mosses and lichens which was similar t o that of ECM plants. Tundra cryptogams (and ECM and ERI plants) have previously been shown to have higher uptake of amino acid than ammoniu m N-, their low delta(15)N might therefore reflect the delta(15)N of f ree 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 n itrogen could be an important N source for ECM and, in particular, ERI plants in heath and forest tundra ecosystems with low release rate of inorganic N from the soil organic matter.