Tree species rather than type of mycorrhizal association drive inorganic and organic nitrogen acquisition in tree-tree interactions

Mycorrhizal fungi play an important role for the nitrogen (N) supply of trees. The influence of different mycorrhizal types on N acquisition in tree-tree interactions is, however, not well understood, particularly with regard to the competition for growth-limiting N. We studied the effect of competition between temperate forest tree species on their inorganic and organic N acquisition in relation to their mycorrhizal type (i.e., arbuscular mycorrhiza or ectomycorrhiza). In a field experiment, we quantified net N uptake capacity from inorganic and organic N sources using 15N/13C stable isotopes for arbuscular mycorrhizal tree species (i.e., Acer pseudoplatanus L., Fraxinus excelsior L., and Prunus avium L.) as well as ectomycorrhizal tree species (i.e., Carpinus betulus L., Fagus sylvatica L., and Tilia platyphyllos Scop.). All species were grown in intra- and interspecific competition (i.e., monoculture or mixture). Our results showed that N sources were not used complementarily depending on a species' mycorrhizal association, but their uptake rather depended on the competitor, indicating species-specific effects. Generally, ammonium was preferred over glutamine and glutamine over nitrate. In conclusion, our findings suggest that the inorganic and organic N acquisition of the studied temperate tree species is less regulated by mycorrhizal association but rather by the availability of specific N sources in the soil as well as the competitive environment of different tree species.