Tree habit interacts with mycorrhizal type to affect soil fungal communities in US northeastern forests

Both tree habits, which includes leaf chemistry, and mycorrhizal nutrient acquisition strategies can affect fungal communities and microbial functions related to decomposition. While conifers associating with ectomycorrhizal (EcM) or arbuscular mycorrhizal (AM) fungi should promote similar saprotrophic communities to decompose lower quality litter, EcM fungi can influence the free-living saprotrophs by directly competing for nutrients. Consequently, while mycorrhizal type is a determinant of fungal community composition, we hypothesized that AM and EcM-associating conifers would have similar fungal communities and enzymatic capabilities relative to deciduous trees. Alternatively, deciduous tree habits of opposing mycorrhizal type could support opportunistic fungi with quickly decomposing litter. To test these hypotheses, we collected soils in northern hardwood forests from beneath two coniferous (Tsuga canadensis and Thuja occidentalis) and two deciduous (Betula alleghensis and Fraxinus americana) species, EcM and AM associating, respectively. We sampled the organic horizon and 10 cm of mineral soil from beneath plots with >80% of the target species within a 5m radius. We characterized the fungal community by ITS metabarcoding and evaluated community dissimilarity and the percentage of fungal guilds (saprotrophs, EcM, and pathotrophs) associated with each tree habit and mycorrhizal type. We evaluated microbial function by measuring the potential of five hydrolytic and two oxidative extracellular enzyme activities (EEA). Overall, mycorrhizal association was a more important determinant of fungal community composition, contributing to 10% of dissimilarity relative to tree habits 5%. Second, deciduous trees had similar fungal communities relative to coniferous trees, where AM conifers had fewer saprotrophic fungi. Lastly, deciduous trees had higher carbon-targeting EEA than conifers, but only in AM soils. Phosphorus-targeting EEA increased with saprotroph percentage, while oxidative EEA increased with the percentage of saprotrophs only in AM soils. Our results indicate that predicting mycorrhizal effects on soil processes and communities is often complicated by tree habit, and that saprotrophic fungi may play a different role in enzyme production between mycorrhizal types.