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Using an 15N labelling method (15N-NH4+ and 15N-NO3−), we conducted a time series experiment of N absorption for two common plant species (Stipa purpurea and Artemisia nanschanica) in three seasons (summer, late autumn, and early spring) in a semi-arid alpine steppe ecosystem on the Tibetan Plateau. The soil NO3− content was significantly higher than the exchangeable NH4+ content in summer, whereas exchangeable NH4+ was the dominant inorganic form of N during the cold seasons (late autumn and early spring). Both S. purpurea and A. nanschanica showed a preference for NO3− in summer. The uptake rates of NO3− ranged from 1.09 to 3.94 μg 15N g−1 DW root h−1 for S. purpurea and from 2.85 to 7.82 μg 15N g−1 DW root h−1 for A. nanschanica. In contrast, both species showed a clear preference for NH4+ during the late autumn and early spring, which was strongly dependent on snowfall events. The uptake rate of NH4+ ranged from 0.33 to 4.25 μg 15N g−1 DW root h−1 for S. purpurea and from 0.67 to 9.05 μg 15N g−1 DW root h−1 for A. nanschanica. We also observed that 15N recovery was mainly retained in the roots with a smaller proportion of 15N stored in the shoots across all seasons. The seasonal shift in the pattern of N absorption may be influenced by changes in the ratios of soil exchangeable NH4+ to NO3− in the different seasons. This finding offers valuable insights into the nutrient acquisition strategies of alpine plants and provides an important mechanism for the temporal pattern of N utilisation by plants in alpine environments.