Complementarity in the use of nitrogen forms in a temperate broad-leaved mixed forest

Background: The complementary use of different forms of soil nitrogen (N) might lead to a higher productivity of mixed forests than monocultures, but convincing evidence for temperate mixed forests is scarce. Aims: We searched for species differences in N uptake rates and the preference for NH4+, NO3- or glycine among five temperate broad-leaved tree species (Acer pseudoplatanus, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Tilia cordata) in a mature mixed stand. Methods: N-15 tracer was added to the soil and its accumulation in fine root biomass was analysed after 10 min, 1 h and 1 d. Results: The estimated root uptake rates of the species were in the range of 5-46 mu g N g(-1) root h(-1) for NH4+, 6-86 mu g N g(-1) h(-1) for NO3- and 4-29 mu g N g(-1) h(-1) for glycine during the first hour after tracer application. Carpinus, Tilia and Acer tended to prefer NH4+ over NO3-, while Fraxinus showed equal preference for both N forms and Fagus seemed to prefer NO3-. Conclusions: The five co-existing tree species differed in uptake rates and partly in their N form preference, but complementarity in the use of different N forms seems to be of minor importance in this forest because tree species appear to be rather flexible in their N form use.