Long-Term Simulated Nitrogen Deposition Alters the Composition of Fungi Active in the Forest Floor

Abstract

Global increases in the rate of atmospheric nitrogen deposition have the potential to alter the composition and function of soil microbial communities. In a long-term field study simulating future rates of atmospheric N deposition, plant litter decay has slowed and soil organic matter has accumulated in conjunction with a decline in both lignolytic enzyme activity and expression of fungal lignolytic genes. Here, I tested the hypothesis that simulated atmospheric N deposition would alter the composition of basidiomycete and ascomycete fungal communities, which may underlie the previously observed biogeochemical responses. The actively metabolizing forest floor fungal community was characterized from cDNA clone libraries constructed from 28S fungal rRNA extracted from the forest floor of two northern hardwoods stands in the lower peninsula of Michigan, USA. The active basidiomycete communities under ambient and simulated atmospheric N deposition differed significantly in terms of membership and the dispersion of members over a phylogenetic tree. Furthermore, suggestive, albeit nonsignificant, differences in the fraction of unique phylogenetic branch length (the UniFrac metric) between simulated and ambient atmospheric N deposition were observed for basidiomycetes. In contrast, the active ascomycete communities under ambient and simulated atmospheric N deposition did not exhibit significant differences in these same metrics. Collectively, these results indicate that chronic N deposition has altered both the composition and function of litter decaying fungi and that these changes have ecosystem-level implications for the cycling and storage of C in forest ecosystems.