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

Abstract

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. 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-15 or nitate-N-15 in subplots without fertilization (control) or fertilized with 7.5 g N m(-2) yr(-1) in the form of ammonium, nitrate, or ammonium nitrate to trace N form uptake. Depending on species, fertilization with nitrate or ammonium nitrate had positive, negative or neutral effects on NO3-N uptake rate, although ammonium fertilization showed little impact. By contrast, fertilization with any N form had little impact on NH4-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). 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.