Contrasting N Form Uptake Strategies of Alpine Plant Functional Groups Under N and P Enrichment

Abstract

Understanding how plant functional groups respond to nutrient enrichment is essential for predicting alpine ecosystem dynamics under global change. Although biomass responses to nitrogen (N) and phosphorus (P) additions have been widely investigated, shifts in plant N uptake strategies among functional groups under nutrient enrichment remain poorly understood. We conducted a 7‐year field experiment in an alpine grassland on the Qinghai‐Tibetan Plateau using factorial N (0, 7.5, 15 g m ⁻²  yr ⁻¹ ; N0, N1, N2) and P (0, 7.5, 15 g m ⁻²  yr ⁻¹ ; P0, P1, P2) additions. To evaluate N‐form uptake, we performed an in situ ¹⁵ N labeling experiment using ammonium (NH ₄ ⁺ ), nitrate (NO ₃ ⁻ ) and glycine, and collected plant and soil samples 4 h after label application. NH ₄ ⁺ was the dominant N source taken up by the plant community, with community‐level uptake enhanced under moderate N (N1) but showing no further increase under higher nutrient inputs, indicating a non‐linear threshold response. Functional groups exhibited contrasting strategies: sedges primarily preferred NH ₄ ⁺ and responded most strongly to N1, grasses showed plastic uptake across treatments, and forbs increased NO ₃ ⁻ and glycine‐derived N uptake under N2P2, suggesting niche differentiation. Plant N uptake was weakly related to soil physicochemical variables but strongly associated with plant N:P stoichiometry, underscoring the predominance of intrinsic plant traits over soil conditions in driving functional group N uptake. Our results suggest that long‐term nutrient enrichment alters N‐form uptake partitioning among functional groups, with potential consequences for niche complementarity and community composition in alpine grasslands.