Belowground Ectomycorrhizal Fungal Community Change Over a Nitrogen Deposition Gradient in Alaska

Nitrogen availability may be a major factor structuring ectomycorrhizal fungal communities. Atmospheric nitrogen (N) deposition has been implicated in the decline of ectomycorrhizal fungal (EMF) sporocarp diversity. We previously characterized the pattern of decreased sporocarp species richness over an anthropogenic N deposition gradient in Alaska (USA). To determine whether this change in sporocarp community structure was paralleled below ground, we used molecular and morphological techniques to characterize the ectomycorrhizal community of white spruce (Picea glauca) over this gradient. We then related patterns of richness and relative abundance of taxa to various N-affected environmental parameters. Species richness of EMF declined dramatically with increasing N inputs. Over 30 taxa were identified at the low-N sites, compared with nine at the high-N sites. Low-N site dominants (Piloderma spp., Amphinema byssoides, Cortinarius spp., and various dark-mantled Tomentella spp.) disappeared completely at the high-N sites, where they were replaced by Lactarius theiogalus, Paxillus involutus, Tylospora fibrillosa, Tomentella sublilacina, Thelephora terrestris, and an unidentified species. Lactarius theiogalus accounted for 44–68% of the root tips at the high-N sites, compared with 7–20% of tips at the low-N sites. Organic horizon mineral N and foliar nutrient ratios (N:P, P:Al) were excellent predictors of taxonomic richness (r2 > 0.93). Organic horizon NO3− availability was the best predictor of abundance of many taxa. These patterns suggest that long-term N deposition can lead to decline in EMF species richness, and dramatic changes in EMF community structure. The consequences of these changes for plant nutrition and ecosystem function depend on how EMF community function changes as community structure changes. We speculate that as N inputs increase, the EMF community shifts from taxa specialized for N uptake under low-N conditions (e.g., Cortinarius, Piloderma), toward taxa specialized for high overall nutrient availability (e.g., Tomentella sublilacina, Thelephora terrestris) and finally toward taxa specialized for P uptake under high-N, low-P, acidified conditions (e.g., Paxillus involutus, Lactarius theiogalus).