Chronic acid deposition can reduce soil pH and phosphorus (P) availability and affects many temperate hardwood forests of the northeastern United States. In previous work, we demonstrated that experimentally reversing soil acidity altered the community structure of mycorrhizal fungi colonizing tree roots in mineral soil (A horizon). However, in our experimental plots, the organic soil horizon (O horizon) displays a higher density of fine tree roots than the mineral soil. In these root mats, changes in soil pH and P availability could have different effects on mycorrhizal communities than in mineral soil. In this study, we predicted that within root mats 1) increases in soil pH would affect the community structure of ectomycorrhizal (EcM) and arbuscular (AM) mycorrhizal fungi and 2) extracellular enzyme activity, particularly phosphomonoesterase activity, would be affected by changes in soil pH and P availability. We examined these predictions in a previously established pH and P manipulation experiment which increased pH and P availability in 12 experimental plots. The community structure of EcM and AM fungi was profiled with fragment analysis and cloning of rDNA and extracellular enzyme activities were measured from 62 root mats of three focal tree species: Acer saccharum, Fagus grandifolia, and Quercus rubra.
Experimentally raising soil pH, but not P availability, had a significant effect on both EcM and AM fungal communities. This change in community structure for both mycorrhizal fungal groups was consistent for root mats from tree species that form EcM fungal relationships (F. grandifolia and Q. rubra) and AM fungal relationships (A. saccharum). A decrease in phosphomonoesterase production was observed with pH elevation and P addition and was consistent across tree species. This reduction in phosphomonoesterase production is similar to previous results from mineral soil conducted in our experimental plots and suggests an overall reduction in the biological demand for organic P with increases in soil pH and inorganic P availability. The activity of carbon (C) acquisition enzymes, β, 1-4 glucosidase and phenol oxidase, however, did not display a similar response to the soil treatments across tree species. β, 1-4 glucosidase activity was significantly higher and phenol oxidase activity was significantly lower in plots where pH was elevated, but only for root mats from A. saccharum. This suggests that mycorrhizal fungi associated with A. saccharum are experiencing a trade off between P scavenging and C acquisition, which may not be experienced by EcM fungi in root mats.