Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops.: Determination of critical N concentration

The concept of critical nitrogen concentration (%N-c) has been proposed as the minimum %N in shoots required to produce the maximum aerial biomass at a given time. Several authors have shown that %N-c declines as a function of aerial biomass accumulation (W) and the %N-c - W relationship has been proposed as a diagnostic tool of N status in different crops, excluding maize. From data obtained in five nitrogen fertilisation experiments in irrigated maize crops, 26 critical data-points were selected with a precise statistical procedure. An allometric relation was fitted and a critical %N-W relationship model is proposed in maize as: If W < 1 t ha(-1) %N-c = 3.40 If 1 t ha(-)1 less than or equal to W less than or equal to 22 t ha(-1) %N-c = 3.40 (W)(-0.37) The model is applicable to maize crop development between emergence and silking + 25 days. The model was tested and validated with data obtained in a network of 17 N fertilisation experiments conducted in France under contrasting pedoclimatic conditions. In only nine out of 280 data-points (3.2%), the plant N status was mispredicted when +/- 5% error around %N-c was allowed. A critical N uptake model (Nu(c), kg N ha(-1)) is proposed as Nu(c) = 34 (W)(0.63) A comparison between Nu(c) and N uptake observed in N treatments giving the maximal grain yields has shown that maize crops assimilate at least 30 kg N ha(-1) in a storage N pool at the silking stage. The significance of the critical %N-W and Nu-W relationships is discussed in relation to theoretical models proposed in whole plant ecophysiology. Different relationships calculated between leaf area index and aerial biomass accumulation, and between N uptake and leaf area were consistent with previous results for other crops. This strengthens the interest of the critical %N-W relationship for use as diagnostic tool of nitrogen status in maize crops.