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The mechanisms for maintaining the species diversity of plant communities under conditions of resource limitation is an important subject in ecology. How interspecific relationships influence the pattern of nutrient absorption by coexisting species in N-limited ecosystems is still disputed. We investigated the effect of neighbor species on the uptake of inorganic and organic N by three common plant species using N-15 tracer techniques in a semi-arid alpine steppe on the northern Tibet. The results showed that the plant species varied in their capacity to absorb NO3 (-)-N, NH4 (+)-N, and glycine-N with or without neighbor species. Carex moorcroftii and Leontopodium nanum showed much more plasticity in resource utilization than Stipa purpurea when neighbor species were present. When C. moorcroftii and S. purpurea coexisted, they all increased their N-15 uptake for the NO3 (-)-N (C. moorcroftii 2.2-fold increase and S. purpurea 2.2-fold increase) and glycine-N treatments (C. moorcroftii 2.9-fold increase and S. purpurea 3.4-fold increase), which indicated that neighborhood had a positive effect for N absorption between the two species. However, L. nanum was a less effective competitor for N utilization than the neighbor species across almost all treatments. The dominant species appeared to have an inhibitory effect on N absorption by the accompanying species in this alpine steppe environment. Thus, interspecific neighbor pairs may result in both a mutually beneficial cooperative relationship and a competitive relationship among neighbors in resource use patterns in extreme environments. Resource use plasticity in altered neighbor species may be due to phenotypic plasticity based on the conditions of the realized niche, offering a valuable insight into niche complementarity and providing a general and important mechanism for resource partitioning in an alpine area.