A New Critical Nitrogen Dilution Curve for Rice Nitrogen Status Diagnosis in Northeast China

In-season diagnosis of crop nitrogen (N) status is crucial for precision N management. Critical N (Nc) dilution curve and N nutrition index (NNI) have been proposed as effective methods to diagnose N status of different crops. The Nc dilution curves have been developed for indica rice in the tropical and temperate zones and japonica rice in the subtropical-temperate zone, but they have not been evaluated for short-season japonica rice in Northeast China. The objectives of this study were to evaluate the previously developed Nc dilution curves for rice in Northeast China and to develop a more suitable Nc dilution curve in this region. A total of 17 N rate experiments were conducted in Sanjiang Plain, Heilongjiang Province in Northeast China from 2008 to 2013. The results indicated that none of the two previously developed Nc dilution curves was suitable to diagnose N status of the short-season japonica rice in Northeast China. A new Nc dilution curve was developed and can be described by the equation N_c = 27.7W^{− 0.34} if W ≥ 1 Mg dry matter (DM) ha⁻¹ or N_c = 27.7 g kg⁻¹ DM if W < 1 Mg DM ha⁻¹, where W is the aboveground biomass. This new curve was lower than the previous curves. It was validated using a separate dataset, and it could discriminate non-N-limiting and N-limiting nutritional conditions. Additional studies are needed to further evaluate it for diagnosing N status of different rice cultivars in Northeast China and develop efficient non-destructive methods to estimate NNI for practical applications.