Ecosystem Ecology

Nitrogen isotopes (N/N ratios, expressed as dN values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against N during creation of transfer compounds within mycorrhizal fungi decreases the N/N in plants (low dN) and increases the N/N of the fungi (high dN). Analytical models of N distribution would be helpful in interpreting dN 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 dN and then decrease, if mycorrhizal colonization were an important factor influencing plant dN. As hypothesized, plants with different mycorrhizal habits initially showed similar dN 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 dN, suggesting transfer of N-depleted N from fungi to plants. The values recorded ( 8 to 11&) are among the lowest yet observed in vascular plants. In contrast, the dN 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 dN ( 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 dN from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher dN values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude:(1) large enrichments in N 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 of 8–10& during transfer from fungi to plants, (2) based on models of N 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 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.