Nitrogen acquisition in Central European tree species is driven by counteracting species interactions and available soil N

Abstract Background and aims The interactions between trees and their species-specific properties (e.g. growth rate, nutrient demand) drive the acquisition of growth-limiting nitrogen (N). In tree communities, the outcome of multiple potentially counteracting interactions can mask the underlying effects between species. Methods Using two-species approaches we investigated the interactions among seven common temperate Central European tree species differing in their morphological and physiological properties. Seedlings were grown under controlled conditions with no, intra-, or interspecific interactions at limited or excess soil N. We measured inorganic and organic net N uptake capacity and biomass and growth traits. Results Among species, inorganic and organic N acquisition was unrelated to general physiological and morphological plant properties (i.e. more N with fast growth) but was species-specific. Species interactions affected N acquisition and growth positively, negatively, and/or not depending on the species and available soil N. Which N sources were preferred changed with neighbour and soil N: With limited N, amino acids and nitrate were taken up most whereas with excess N, N acquisition was generally increased and ammonium preferred. Conclusions The interactions with different neighbouring tree species can affect inorganic and organic N acquisition of a species positively, negatively, or not at all highlighting its plasticity in response to different neighbours. This outcome strongly depends on soil N availability as seen in the strict preferences with limited vs. excess soil N. Overall, the abiotic conditions appear to provide the framework within which the biotic interactions of a species´ lead to plasticity in its N acquisition.