Role of tree mycorrhizal type in shaping nutrient dynamics and soil microbial communities following 23 years in a common garden

Caylon F. Yates, Terrence H. Bell, Suzanne Fleishman, Ryan V. Trexler and Michela Centinari, Intercollege Graduate Degree Program in Ecology, The Pennsylvania State University, University Park, PA, Caylon F. Yates and Terrence H. Bell, Department of Plant Pathology and Environmental Microbiology, The Pennsylvania State University, University Park, PA, Jing Guo, Suzanne Fleishman, Hayden Bock and David M. Eissenstat, Ecosystem Science and Management, The Pennsylvania State University, University Park, PA, Suzanne Fleishman and Michela Centinari, Plant Science, The Pennsylvania State University, University Park, PA

Background/Question/Methods
Most land plants form symbioses with mycorrhizal fungi, and the specific type of symbiosis (ectomycorrhizal vs. endomycorrhizal) can help to predict certain ecological processes. How these associations shape soil microbial composition, ecosystem function, and soil chemistry is not fully understood. We may be able to link microbiome structure to nutrient dynamics (e.g. organic matter and nitrogen) in soil and leaf litter by monitoring the microbial composition and nutrient content of soil and freshly senesced leaf litter of trees with differing mycorrhizal associations. It is often challenging to separate the influence of individual tree species from environmental effects, but we used a common garden forest to dramatically reduce confounding factors. We hypothesized that trees in a 23-year-old common garden that formed one of two prominent mycorrhizal associations (i.e. ectomycorrhizal (EM) or arbuscular mycorrhizal (AM)) would form soil environments that differed both biotically and abiotically. We also hypothesized that these environmental differences would yield differences in the decomposition of leaf litter. Using a common garden in Central Pennsylvania, we selected multiple AM and EM tree species and assessed changes in soil biological and chemical properties (pH, nitrogen and SOM) as well as changes in rhizosphere bacterial composition with 16S rRNA gene sequencing.
Results/Conclusions
Overall, soil biological and chemical properties were more strongly associated with mycorrhizal association type than other functional root traits. Soil underlying AM trees increased in both pH and soil available nitrogen compared to soil underlying EM trees. Rhizoplane-associated bacteria were more strongly associated with tree mycorrhizal type than bulk soil bacteria resulting in a higher relative abundance of Proteobacteria in AM rhizoplane communities, while EM rhizoplane communities were characterized by increased relative abundance of Acidobacteria. In general, differing mycorrhizal associations between tree species create varying soil environments. AM trees were characterized by higher pH, available nutrients and bacteria associated with high nutrient soils while EM trees altered the soil environment to have lower pH and nutrient status, and increased abundance in more oligotrophic bacteria like Acidobacteria. This work includes the decomposition dynamics of leaf litter of each tree species that is currently ongoing. Mycorrhizal status may be a strong indicator for how trees will shape soil processes in response to future environmental change.