Tree diversity effects on rhizodeposition and rhizosphere priming: the role of the mycorrhizal colonization in coupling C and N cycles in diverse tree stands

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The majority of temperate tree species associates predominantly with either arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi, which provide mineral nutrients to their host in exchange for carbon. Since the two mycorrhizal types differ fundamentally in their nutrient economy they were suggested to provide an integrated index of biogeochemical transformations relevant to carbon (C) cycling and nutrient retention in forests. Yet little is known about the group-specific role of the mycorrhizal type in the tree diversity–ecosystem functioning relationship. The central aim of this research project is to determine functional group differences in rhizosphere C fluxes and soil nitrogen cycling between diverse AM and ECM tree stands. Among the key biogeochemical processes I will focus on root exudation and decomposition, which represent cause and effect of the microbial priming effect to stimulate nutrient release from soil organic matter. On the basis of the assumed organic nutrient economy of ECM stands, I hypothesize that increased root exudation is a primary mechanism by which ECM trees sustain productivity in diverse stands, while diverse AM stands mainly depend on leaf litter effects on bulk soil processes. To achieve the objectives of this project I will combine an established tree diversity experiment that manipulates mycorrhizal type abundances with innovative and beyond state-of-the-art ecological and ecophysiological methods that are complemented by 3-D laser ablation tomography, solid-state 13C-CPMAS nuclear magnetic resonance spectroscopy, and metabolomics. This project will refine a conceptual model on the role of the mycorrhizal type in tree diversity effects on soil functions for the incorporation in global simulation models.