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

A field study was conducted to evaluate the effects of associated mycorrhization type on tree-tree interactions using the framework of the MyDiv tree diversity experiment established at the Bad Lauchstädt Experimental Research Station of the Helmholtz Centre for Environmental Research (UFZ) (Saxony-Anhalt, Germany). Inorganic and organic net N uptake capacity was quantified in fine roots of AM (i.e. Acer pseudoplatanus L., Fraxinus excelsior L., and Prunus avium L.) as well as EM associated tree species (i.e. Carpinus betulus L., Fagus sylvatica L., and Tilia platyphyllos Scop.) using 15N/13C isotopes. Trees grew either in monocultures or 2-species mixtures, either only ectomycorrhizal, only arbuscular mycorrhizal, or in combination. Generally, ammonium was preferred over glutamine and glutamine over nitrate. Inorganic and organic N sources were not used complementarily depending on a species´ mycorrhizal association, but their uptake rather depended on the competitor indicating species...