Developing a nitrogen fertilizer management model for wheat in calcareous soils using the critical nitrogen dilution curve

The critical nitrogen (N) dilution curve, which expresses whole-plant critical N concentration as a function of shoot biomass, can be used as a N management diagnostic tool for cereals. The objectives of this research were to develop a critical N dilution curve for wheat grown in calcareous soils and to formulate a model for estimating N fertilizer requirement of wheat crop at different growth stages. Six N fertilization rates (0–250 kg N ha −1 ) were used to induce variability in plant growth throughout six site-years (three locations at West Delta of Egypt and two seasons [2020/21-2021/22]). Aboveground shoot biomass ( W ; Mg DM [dry matter] ha −1 ) and N concentration ( N c ; g kg −1 DM) were determined on five sampling dates during the growing season. A critical N dilution curve was developed as: Nc=50.141 W-0.424 Nc=50.141W-0.424 . The N c dilution curve was then used to develop a N fertilizer topdressing strategy. The study relied on N nutrition index inferred from the N c based on N uptake, and instead of relying on a single N recovery efficiency coefficient, a variable N recovery efficiency was developed. This approach increased the hypothetical N requirements at low N application rates while decreased requirements at high N application rates, implying that the N c dilution curve can be used successfully to estimate the rates of supplemental N application. The developed strategy will provide a solid basis for precisely managing N fertilizer, though challenge ahead at the farm level will be in determining the actual shoot biomass and N concentration.