Ectomycorrhizal fungal aboveground community change over an atmospheric nitrogen deposition gradient

Atmospheric nitrogen deposition has been hypothesized as one of the causal factors in the decline of ectomycorrhizal fungal (EMF) sporocarps. We asses sed the effects of N deposition on EMF of forests dominated by white spruce (Picea glauca (Moench) Voss) over a short, steep N deposition gradient in Alaska, USA. The study area had received high NH3 inputs from an industrial ammonia production facility for almost 30 yr. Current N inputs varied 20-f old over the gradient. High N inputs have increased soil N availability, wh ich has led to decreased soil pH and base cation availability; increased fo liar N and decreased foliar P, Mg, and K; increased tree growth; and replac ement of mosses by grasses. At the six lowest N sites, 144 species were enc ountered, whereas only 14 species were encountered at the six highest N sit es. With increasing mineral N availability in the organic horizon, there we re declines in both total species richness (1994 and 1995) and total sporoc arp abundance (1994 only). Correlation analysis identified two groups of ta xa that responded differently to the N inputs. One group ("nitrophobic" tax a: Cortinarius, Russula, Tricholoma, Lactarius, Hebeloma) declined in speci es richness or abundance with increasing organic horizon mineral N. In the second group ("nitrophilic". taxa: Lactarius theiogalus, Laccaria, Paxillus involutus, and Hygrophorus olivaceoalbus), sporocarp abundance was either not correlated or slightly positively correlated with organic horizon N ava ilability. Redundancy analysis identified two axes of environmental variati on that explained 62% and 7% of the variation in the EMF community and clea rly distinguished the two groups of taxa. The environmental variables inclu ded in the final model were mineral soil base cations, organic horizon mine ral N, and organic horizon net mineralization. These results support the hy pothesis that long-term N inputs alone may lead to loss of EMF sporocarp di versity and indicate that EMF should be considered in the establishment of critical loads of N.