Why leaf N : P is not reliable for diagnosing nutrient limitation of productivity: answer from leaf phosphorus fractions

Summary Nitrogen (N) enrichment is expected to exacerbate plant phosphorus (P) limitation, yet this assumption has seldom been empirically tested. We investigated the effects of 7–9 yr of N and P addition on grassland aboveground net primary productivity (ANPP), leaf N : P, and P fractions. Nitrogen addition more than doubled ANPP and increased community leaf N : P from 9.3 to 17.2, a shift traditionally interpreted as indicating N‐induced P limitation under the stoichiometric framework. However, subsequent P addition following N enrichment did not increase ANPP, despite decreasing leaf N : P, directly challenging the predicted P limitation. Plants in N‐enriched communities exhibited increased nucleic acid P and residual P concentrations. Phosphorus addition following N enrichment increased metabolic P and lipid P concentrations, but not nucleic acid P or residual P. Allocation to nucleic acid P was more tightly linked to plant growth, indicating that plants in N‐enriched communities can adapt to low total P by reallocating P toward nucleic acid P. Our results provide a novel mechanism that explains the uncertainties of leaf N : P as a sole indicator of nutrient limitation for primary productivity, and highlight the critical role of leaf P fraction allocation in mediating plant adaptation to stoichiometric imbalance.