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 assessed 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-fold over the gradient. High N inputs have increased soil N availability, which has led to decreased soil pH and base cation availability; increased foliar N and decreased foliar P, Mg, and K; increased tree growth; and replacement of mosses by grasses. At the six lowest N sites, 144 species were encountered, whereas only 14 species were encountered at the six highest N sites. With increasing mineral N availability in the organic horizon, there were declines in both total species richness (1994 and 1995) and total sporocarp abundance (1994 only). Correlation analysis identified two groups of taxa that responded differently to the N inputs. One group (“nitrophobic” taxa: Cortinarius, Russula, Tricholoma, Lactarius, Hebeloma) declined in species 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 availability. Redundancy analysis identified two axes of environmental variation that explained 62% and 7% of the variation in the EMF community and clearly distinguished the two groups of taxa. The environmental variables included in the final model were mineral soil base cations, organic horizon mineral N, and organic horizon net mineralization. These results support the hypothesis that long-term N inputs alone may lead to loss of EMF sporocarp diversity and indicate that EMF should be considered in the establishment of critical loads of N.