Ectomycorrhizal fungal communities and enzymatic activities vary across an ecotone between a forest and field
In the context of global nutrient cycles, extracellular enzymes that deconstruct plant and microbial cell walls and reduce macromolecules to soluble substrates for microbial assimilation are important for carbon (C) and nitrogen (N) acquisition. One source of extracellular enzymes are ectomycorrhizal fungi (EMF) which produce extracellular enzymes that help them obtain nutrients such as N and P by degrading complex molecules in soil organic matter (SOM). Due to environmental conditions, EMF may vary from strict phytotrophs (obtaining fixed C only from host plants) to semi-saprotrophs (obtaining some fixed C from the host plant and some from SOM) based on their extracellular enzyme production. The process of forest succession represents a unique arena for the exploration of differing environments on EMF community structure and their enzyme activities because the maturity of trees, environmental variables, and EMF communities may all be changing simultaneously. We studied the community structure and enzymatic activity of EMF associated with loblolly pine (Pinus taeda), across an ecotone between a forest where P. taeda is a member of the canopy (late succession) to an old field where loblolly pine saplings have been growing since the cessation of field management only a few years ago (early succession).
The forest and field sites differed significantly in several environmental factors. There was higher percent silt (P = 0.0259), percent clay (P = 0.0431), and soil C (P<0.0001) in the forest compared to the field; however, there was higher percent sand (P<0.0001) and soil water content (P<0.0001) in the field compared to the forest. There was no significant difference in the amount of total soil N (P = 0.0709). To assess the sources of variation in the mycorrhizal community, we used permutational manova. Both the linear discriminant encompassing environmental factors (% Sand, % Silt, % Clay, C, N, and soil water content) and tree age were weak structuring factors (p=0.08591 and p=0.0819), explaining ~9% of community matrix variance. Phenol oxidase and peroxidase did not vary significantly between the field and the forest; however, β –glucosidase (P = 0.0284) and phosphatase (P = 0.0657) activity were higher in the forest compared to the field. These results indicate that EMF may have different foraging niches in the soil, corresponding to differences in their ability break down micromaterials. Furthermore, they emphasize the importance of understanding the factors that dictate the distribution of EMF and activity of their enzymes across a habitat boundary.