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 (r 2 Ͼ 0.93). Organic horizon NO 3 Ϫ 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).

Abstract. Human activity in the last century has led to a significant increase in nitrogen (N) emissions and atmospheric deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the deposition of pollution that would be harmful to ecosystems is the determination of critical loads. A critical load is defined as the input of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge.The objectives of this project were to synthesize current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and to estimate associated empirical N critical loads. The receptors considered included freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Ecosystem impacts included: (1) biogeochemical responses and (2) individual species, population, and community responses. Biogeochemical responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root : shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species.The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1-39 kg NÁha , spanning the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, and trees.The critical load approach is an ecosystem assessment tool with great potential to simplify complex scientific information and communicate effectively with the policy community and the public. This synthesis represents the first comprehensive assessment of empirical critical loads of N for major ecoregions across the United States.

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Summary• Ectomycorrhizal fungal species vary in their response to nitrogen (N) availability and ability to use organic N. We hypothesized that taxa dominant at sites with high soil inorganic N would be less likely to use organic N than taxa dominant at low soil inorganic N. We also asked whether these taxa differed in natural abundance of N isotopes.• Pure culture N use for taxa from an N deposition gradient in Alaska was examined and N isotopes of sporocarps, soils and foliage collected over this gradient were quantified.• Taxa common in low inorganic N soils grew on protein, glutamine and serine, whereas dominant taxa in high inorganic N soils grew on glutamine, but poorly on protein and serine. Sporocarp δ 15 N was highest in protein users, and lowest in nonprotein users. With increasing soil inorganic N, sporocarps became more isotopically enriched relative to foliage.• The importance of organic N use might decline with increasing N availability, although field tests are required. The relationship between organic N use and N isotopes also merits further study. However, sporocarp isotopic enrichment may be a useful indicator of soil N availability.