A reappraisal of the critical nitrogen concentration of wheat and its implications on crop modeling

• Critical N concentration (Ncc) of wheat is widely used, but defined inconsistently.
• Higher Ncc in wheat leaves was found than those used in APSIM-wheat model.
• Ncc determined by developmental stage showed less variation than by biomass.
• Ncc-stage relationship helped unify Ncc-biomass curves across cultivars and regions.
• N relationships in wheat simulation models may need re-appraisal/update.

The concept of critical nitrogen (N) concentration (Ncc) has been used for both diagnostic purposes and modelling of wheat-N relations. Ncc has been derived with two contrasting approaches: one against above ground biomass (Ncc-biomass), and one against developmental stages (Ncc-stage). While the former has been claimed in diagnostic use, both approaches are adopted in wheat simulation models. This paper provides data from North China Plain (NCP) to re-exam the Ncc-stage relationships used in two widely used wheat models (APSIM and CERES) and to compare the Ncc-biomass vs. Ncc-stage relationships. The results revealed significant higher maximum and critical N concentrations in leaves of wheat in NCP than the values used in the APSIM-wheat model. Recalibration of the APSIM model with the new N concentrations led to improved simulations for wheat biomass and N uptake, particularly under low N input. Our results also show that the Ncc-stage relationship appeared to be more robust than the Ncc-biomass relationship, and it helped explain the variations in wheat Ncc-biomass curves from different regions. This likely reflects the fact that Ncc-stage curve captures the stage-driven formation of structural biomass and carbohydrate reserves of wheat, which is the main cause for N dilution. The implications of the findings on modelling of wheat–nitrogen relationships and on nitrogen management practices are also discussed.