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(N_c) dilution curve and N nutrition index(NNI) have been proposed as effective methods to diagnose N status of different crops. The N_c 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 N_c dilution curves for rice in Northeast China and to develop a more suitable N_c dilution curve in this region. A total of17 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 N_c dilution curves was suitable to diagnose N status of the short-season japonica rice in Northeast China. A new N_c dilution curve was developed and can be described by the equation N_c = 27.7 W^(-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.