Abstract
Anthropogenic release of biologically available N has increased atmospheric N deposition in forest ecosystems, which may slow decomposition by reducing the lignolytic activity of white-rot fungi. We investigated the potential for atmospheric N deposition to reduce the abundance and alter the composition of lignolytic basidiomycetes in a regional network of four northern hardwood forest stands receiving experimental NO₃⁻ deposition (30 kg NO₃⁻-N ha⁻¹ year⁻¹) for a decade. To estimate the abundance of basidiomycetes with lignolytic potential, we used PCR primers targeting lacease (polyphenol oxidase) and quantitative fluorescence PCR to estimate gene copy number. Natural variation in lacease gene size permitted use of length heterogeneity PCR to profile basidiomycete community composition across two sampling dates in forest floor and mineral soil. Although past work has identified significant and consistent negative effects of NO₃⁻ deposition on lignolytic enzyme activity, microbial biomass, soil respiration, and decomposition rate, we found no consistent effect of NO₃⁻ deposition on basidiomycete lacease gene abundance or community profile. Rather, lacease abundance under NO₃⁻ deposition was lower (-52%), higher (+223%), or unchanged, depending on stand. Only a single stand exhibited a significant change in basidiomycete lacease gene profile. Basidiomycete lacease genes occurring in mineral soil were a subset of the genes observed in the forest floor. Moreover, significant effects on lacease abundance were confined to the forest floor, suggesting that species composition plays some role in determining how lignolytic basidiomycetes are affected by N deposition. Community profiles differed between July and October sampling dates, and basidiomycete communities sampled in October had lower lacease gene abundance in the forest floor, but higher lacease abundance in mineral soil. Although experimental N deposition significantly suppresses lignolytic activity in these forests, this change is not related to the abundance or community composition of basidiomycete fungi with lacease genes. Understanding the expression of laceases and other lignolytic enzymes by basidiomycete fungi and other lignin-decaying organisms appears to hold promise for explaining the consistent decline in lignolytic activity elicited by experimental N deposition.