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

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.