ABSTRACT.- Nitrogen (N) and phosphorus (P) deficiencies commonly limit plant growth in agroecosystems. While critical N concentration dilution curves (Nc) are widely used to assess N status in crops and pastures, it remains unclear whether P deficiency alters Nc values. This study aimed to evaluate the effect of P deficiency on Nc. For this, a factorial combination of several N and P fertilization rates was applied in eight field experiments, carried out during winter and spring, across Uruguay and Argentina, on annual ryegrass (Lolium multiflorum L.) and perennial tall fescue (L. arundinaceum (Schreb.) Darbysh.) pastures. This produced a dataset that included plots (i) with and without P and N deficiencies and (ii) confirmed luxury N uptake across a wide range of environmental conditions. Nc curves were estimated using both classical and Bayesian approaches. Across sites, P deficiency did not affect Nc values, irrespective of whether Nc matched or fell below reference Nc curves, and thus reduced N uptake under P deficiency was solely attributed to lower biomass accumulation. The Bayesian approach provided more precise parameter estimates but did not substantially alter fitted curve shapes. Understanding whether P deficiency alters Nc is fundamental for refining field crops nutrition diagnostics. Our findings support the robustness of Nc dilution curves across a range of soil and climatic conditions and reinforce their use as a reliable diagnostic tool for N nutrition in C3 forage grasses, even under varying soil P availability.