Determination of a Critical Nitrogen Dilution Curve for Spring Wheat

Plant‐based diagnostic tools of N deficiency can be based on the concept of critical N dilution curves describing whole‐plant critical N concentration (N c ; g kg −1 of dry matter DM) as a function of shoot biomass ( W ; Mg DM ha −1 ). This has been tested for several crops, including winter wheat ( Triticum aestivum L.) but has not been tested for spring wheat. Our objectives were to determine a critical N dilution curve specific to spring wheat, to compare this curve with existing critical N dilution curves for winter wheat, and to assess the plausibility of using it to estimate the level of N nutrition. The study was conducted at six site‐years (2004–2006) in Québec, Canada, with four to eight N fertilization rates (0–200 kg N ha −1 ). Shoot biomass and N concentration were determined on five to eight sampling dates during the growing season, and grain yield was measured at harvest. A critical N dilution curve (N c = 38.5 W −0.57 ) was determined for spring wheat and was different from those reported for winter wheat. The N nutrition index (NNI = N observed /N c ) calculated from this spring wheat critical N dilution curve was significantly related ( R 2 = 0.70; P < 0.001) to relative grain yield. This critical N dilution curve and the resulting NNI adequately identified situations of limiting and nonlimiting N nutrition and could be used to establish the N nutrition status.