American aspens (Populus tremuloides) associate with a wide diversity of soil-inhabiting mycorrhizal, endophytic, and saprotrophic fungi which can play critical roles in the cycling of C, N and P. Furthermore, aspens occupy a wide range of habitats where biogeochemical syndromes are likely to vary. However, studies of linkages between abiotic gradients, fungal community composition, and biogeochemistry are often confounded by turnover of plant communities across those same gradients. In the arid west, aspens can form monodominant stands across multiple soil types and within a 1,000 m elevation band. Thus, aspen forests in the arid west can provide ideal habitats for addressing variation in fungal community composition and function across abiotic gradients. We conducted a field survey of four aspen stands in southern Utah to evaluate how fungal communities and nutrient cycling vary across abiotic gradients. One of our study sites was the famed “Pando” clone which is widely recognized as the most massive organism known to science, spanning 43 ha and weighing 6,000 tons. We sequenced the fungal ITS2 gene to survey fungal diversity and measured soil respiration, N mineralization, and extracellular enzyme activities as measures of microbial nutrient transformations.
Results/Conclusions
We found that aspen stands had distinct fungal communities, which were associated with elevation and soil organic matter properties. Ectomycorrhizal fungi were more dominant in higher elevation, organic-rich soils, whereas saprotrophic fungi were more dominant at lower elevation sites. Elevation was negatively associated with N mineralization and positively associated with C mineralization. This effect was possibly mediated by differences in soil water holding capacity and organic matter content that were also associated with elevation. Forthcoming data on extracellular enzyme activities will provide an estimate of the investment of microbial communities in extracellular macromolecular depolymerization, which is often a bottleneck on nutrient cycling. Our data suggest a prominent role of ectomycorrhizal fungi in regulating carbon cycling and a less apparent role in nitrogen cycling. Still, it is unclear whether microbial community composition or nutrient stocks are better predictors of nutrient fluxes. We have an ongoing microbial community transplantation experiment that will evaluate the role of fungal communities in regulating nutrient fluxes while keeping soil and climatic conditions constant. We are also conducting a separate field study that will survey fungal meta-transcriptomes of aspen stands across replicate elevation gradients to provide a more direct linkage between fungal communities and soil nutrient cycling.