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

Nitrogen isotopes (N-15/N-14 ratios, expressed as delta N-15 values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against N-15 during creation of transfer compounds within mycorrhizal fungi decreases the N-15/N-14 in plants blow delta N-15) and increases the N-15/N-14 of the fungi (high delta N-15). Analytical models of N-15 distribution would be helpful in interpreting delta N-15 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 nonglaciated 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 delta N-15 and then decrease, if mycorrhizal colonization were an important factor influencing plant delta N-15. As hypothesized, plants with different mycorrhizal habits initially showed similar delta N-15 values (-4 to -6%. relative to the standard of atmospheric N-2 at 0 parts per thousand), 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 parts per thousand in delta N-15, suggesting transfer of N-15-depleted N from fungi to plants. The values recorded (-8 to -11 parts per thousand) are among the lowest yet observed in vascular plants. In contrast, the delta N-15 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 delta N-15 (-1 to -3 parts per thousand), but the host-specific ectomycorrhizal fungus Cortinarius tenebricus had values of up to 7 parts per thousand. Plants, fungi and soil were at least 4 parts per thousand higher in N-15 from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher delta N-15 values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude:(1) large enrichments in N-15 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 N-15 of 8-10 parts per thousand during transfer from fungi to plants, (2) based on models of N-15 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 N-15 by an additional 3 parts per thousand 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.