Soil N:P ratio influences species coexistence via nitrogen‐form niche differentiation in alpine grasslands

Abstract Plant species coexistence in nutrient‐limited ecosystems is often maintained by niche differentiation in nitrogen (N) uptake. However, how this partitioning responds to changes in nutrient stoichiometry is still poorly understood. We conducted an in situ 15 N labelling experiment in an alpine grassland on the Qinghai–Tibetan Plateau, using NH 4 + , NO 3 − , and glycine tracers under a 6‐year factorial N and phosphorus (P) additions (0, 7.5, 15 g N m −2 year −1 ; 0, 7.5, 15 g P 2 O 5 m −2 year −1 ), thereby generating a gradient of soil available N:P ratios ranging from 1.2 to 9.0. Six coexisting plant species were assessed, including dominants ( Kobresia pygmaea , Kobresia humilis , Stipa aliena ) and subdominants ( Poa annua , Potentilla bifurca , Potentilla nivea ). All species preferred NH 4 + , but dominant species intensified inorganic N uptake, while subdominants increased glycine use under nutrient additions. Specifically, in dominant species, NO 3 − uptake increased to ~2.5–5.9 times that in CK, whereas glycine uptake decreased by 75%–78% under high N and P addition levels (N2P2). In contrast, subdominants increased glycine uptake to ~5.8–9.6 times that in CK. This divergence enhanced N‐form niche differentiation, which was positively correlated with species evenness in the alpine grassland ecosystem. Niche differentiation was most pronounced when soil available N:P ratios were below ~1.6, corresponding to the half‐saturation point of the N‐form differentiation curve. Our findings highlight that changes in soil N:P stoichiometry induced by N and P addition drive below‐ground N partitioning and promote coexistence through resource complementarity. These results provide a mechanistic basis for biodiversity maintenance in alpine grasslands under nutrient limitation. Read the free Plain Language Summary for this article on the Journal blog.