1. Global warming may accelerate nitrogen (N) transformations in the soil, with potentially large effects in N-poor high-elevation ecosystems. To gain insight into the partitioning of inorganic and organic N inputs within the plant–soil system and how warming influences these patterns, we applied a ¹⁵N label (¹⁵NH₄Cl or ¹⁵N-glycine) shortly after snowmelt during the sixth year of experimental soil warming (+4 °C) at treeline in the Swiss Alps.
2. Seven weeks after labelling, approximately 60% of the applied label remained in the soil organic layer to 10 cm depth, whereas label recovery summed over all measured plant pools was <10% of the added label. Soil warming led to a weaker Δ¹⁵N signal in plants and no change in the amount of added label recovered in plants. This ¹⁵N dilution resulted from a greater N pool size of some plant species in warmed plots as well as enhanced availability of (unlabelled) N under warming. Temporal dynamics of foliar Δ¹⁵N in dominant dwarf shrub species suggested that these plants primarily take up N early in the season. In a subset of plots labelled with ¹³C-enriched glycine (U-¹³C₂-¹⁵N-glycine), the labelled glycine was mineralized rapidly, with approximately 50% of the applied ¹³C respired as CO₂ during the first 99 h, suggesting that effects of warmer soils on N dynamics in this treeline system are only slightly modulated by the preferences of different plant species for inorganic and organic N forms.
3. Synthesis. Plants growing in warmer soils acquired more unlabelled, soil-derived N in the sixth year of treatment, implying a sustained increase in N mineralization and availability in alpine treeline ecosystems with higher soil temperatures predicted for the future. Wide variation in the ability of plant species and functional groups to compete for early-summer N inputs means that there is a feedback between plant community shifts and N dynamics under environmental change at the treeline.