Replacing the Nitrogen Nutrition Index by SPAD Values and Analysis of Effect Factors for Estimating Rice Nitrogen Status

Determination of the nitrogen nutrition index (NNI) of a crop requires measurement of the amount and status of N in shoot biomass, which restricts its wide application in paddy fields under field conditions. Thus, a convenient and efficient method is required. We estimated the N concentration of leaves in the rice canopy on the basis of dry matter weight, Nw (%) and per unit area, N-a (mg cm(-2)), and measured Soil-Plant Analysis Development (SPAD) values as a potential replacement for NNI. Two rice cultivars with six N fertilization rates (0, 75, 150, 225, 300, and 375 kg N ha(-1)) were used in a field study from 2010 to 2012 in Hangzhou, China. The SPAD values, N-a, and N-w of the upper four leaves in the rice canopy were measured at three developmental stages (tillering, booting, and heading) alongside the NNI. The results showed that with increasing N rates the SPAD values, N-a, N-w, and NNI increased in all developmental stages and leaf positions each year. The SPAD values, N-a, and N-w showed linear regression with NNI, but the regression parameters varied between developmental stages. Once developmental stages were determined, the equation parameters did not differ among years, leaf positions, and cultivars. The relationships between N-a and NNI, and SPAD values and NNI were closer than those between N-w and NNI, suggesting that SPAD values and N-a may replace NNI for a quick and approximate prediction of rice plant N nutrition in the field.