Topography Variations Alter the Nitrogen Utilization Strategies of Two Dominant Plant Species in an Alpine Steppe

Topographic factors are important potential determinants of soil nitrogen (N) availability as they influence temperature and moisture at the local scale, thereby affecting plant N use strategies. To date, the effects of slope aspect and position on N acquisition pattern for alpine species remain poorly understood. We conducted a field ¹⁵N labeling experiment (i.e.,¹⁵N-NO₃⁻, ¹⁵N-NH₄⁺ or ¹⁵N-glycine) at three different aspects and altitudes to investigate effects of terrain changes on uptake of inorganic and organic N for two common plant species (Stipa purpurea and Saussurea japonica) in a semi-arid alpine steppe on the northern Tibetan Plateau. Plant N utilization strategies were largely controlled by changes in soil nutrients along the topographical gradients. The mean value of NO₃⁻ absorption rate was the highest among the three N forms for the two species (S. purpurea: 5.49 µg N g^− 1 dry mass; S. japonica: 4.73 µg N g^− 1 dry mass), but there were differences across various types of topography. At the lower sites, the two species showed a clear absorption preference for NH₄⁺ and glycine, as these two N forms were the highest among the three slope positions. S. purpurea and S. japonica at the south-facing aspect had the highest NO₃⁻ absorption rate, which was well matched with the higher soil NO₃⁻ availability of the southern-facing aspect (1.65–3.19 mg kg^− 1). Our study reveals that plant N utilization strategies are intricately linked to the variability of soil nutrients across different topographical gradients. These results underscore the critical role of physical topography in shaping N acquisition patterns in alpine ecosystems, offering valuable insights for anticipating how changes in climate may impact plant N uptake strategies in these environments.