Plasticity in nitrogen form uptake and preference in response to long-term nitrogen fertilization.

Background and aims: Niche complementarity arising from divergence in resource use is an important mechanism underlying species coexistence. We hypothesized fertilization with different N forms would generate plastic divergence among species with regard to their N form uptake and preference. Methods: In the eighth year of a long-term N fertilization experiment in an alpine meadow on the Tibetan plateau, we labeled 11 common plant species with ammonium-N or nitate-N in subplots without fertilization (control) or fertilized with 7.5 g N m yr in the form of ammonium, nitrate, or ammonium nitrate to trace N form uptake. Results: Depending on species, fertilization with nitrate or ammonium nitrate had positive, negative or neutral effects on NO-N uptake rate, although ammonium fertilization showed little impact. By contrast, fertilization with any N form had little impact on NH-N uptake rate. Consequently, effects of nitrate fertilization and ammonium nitrate fertilization on relative N form preference diverged among the species and the functional groups (grasses, sedges, legumes and forbs). Conclusions: Alpine plant species can diverge in N form uptake and preference in response to long-term N fertilization, and such divergence may contribute to species coexistence after long-term fertilization. [ABSTRACT FROM AUTHOR]