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

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.