Mechanisms of nitrogen limitation affecting maize growth: a comparison of different modelling hypotheses
Two hypothetical mechanisms exist for quantifying crop nitrogen (N) demand and N-deficit effects on crop growth. The Critical N mechanism uses a critical N concentration, while the Leaf N mechanism distinguishes active N in leaves from the N elsewhere in shoots. These two mechanisms were implemented in parallel in a maize model (Amaize) to evaluate their adequacy in predicting crop growth and development. In the Leaf N mechanism, two approaches for quantifying N-deficit effects, by reducing green leaf area (GAI) or diluting specific leaf nitrogen (SLN), were also examined. The model-predicted plant biomass, grain yield, and N uptake were compared with measurements from 47 maize crops grown on 16 sites receiving different N fertiliser treatments. The results showed that model-predicted plant biomass, grain yield and N uptake were insensitive to the approaches used for quantifying N-deficit effects in the Leaf N mechanism. The model-predicted plant biomass, grain yield and N uptake using either N approach were significantly related to measurements (P < 0.01) but had considerable deviations (r2 = 0.66–0.69 for biomass, 0.50–0.54 for grain yield: 0.17–0.33 for N uptake). The linear fits of the predicted against measured values showed no significant difference (P > 0.1) among the three N approaches, with the Leaf N mechanism predicting smaller deviation than the Critical N mechanism. However, the Critical N mechanism was better in simulating plant growth dynamics in early plant growth stages. The Leaf N mechanism distinguished functional from structural N pools in plants, having a sound physiological base. The simulation using the Leaf N mechanism with both SLN dilution and GAI reduction for quantifying N-deficit effects was the best in predicting crop growth and yield.