Functional tipping points in plant–microbial N competition revealed by form-specific uptake in alpine grasslands

This dataset comes from a long-term nitrogen (N) addition experiment combined with an in situ 15N labelling study in an alpine grassland on the eastern Qinghai–Tibetan Plateau (Hongyuan, Sichuan, China; 3500 m a.s.l.). The field design includes six NH₄NO₃ addition rates (N0, N2, N4, N8, N16, N32; 0–32 g N m⁻² yr⁻¹) with five replicates per treatment (30 plots). On 6 July 2022, 10 atom % 15NH₄⁺ or 15NO₃⁻ was applied, and plants and soils were sampled 24 h after labelling from 0–15 cm depth. The dataset provides treatment-level soil variables (ammonium-N, nitrate-N, soil moisture, pH), plant traits (leaf and root N content, total/above-/below-ground biomass, root:shoot ratio), and 15N-based metrics of N uptake. These include NH₄⁺ and NO₃⁻ uptake by plants and microorganisms, total N uptake, plant:microbial N uptake ratios (PL:MO), and NH₄⁺/NO₃⁻ uptake preferences for both groups. The data support analyses of plant–microbial N competition and N-form partitioning along an N enrichment gradient in cold, N-limited grasslands.