An optimized strategy of nitrogen-split application based on the leaf positional differences in chlorophyll meter readings

Abstract

Modern rice production faces the multiple challenges of increasing grain yields, reducing inputs of chemical fertilizer. However, the disequilibrium between nitrogen (N) supplement from the soil and the demands for N of plants is a serious obstacle to these goals. Plant-based diagnosis can help farmers with better choice of the time and amount of topdressing of N fertilizer. Our objective was to evaluate a non-destructive assessment of rice N demands based on the relative SPAD value (RSPAD) due to leaf positional differences. In this study, two field experiments were conducted, including a field experiment of different N rates (Exp. I) and an experiment to evaluate the new strategy of nitrogen-split application based on RSPAD (Exp. II). The results showed that higher N inputs significantly increased grain yield in modern high yielding super rice, but at the expense of lower nitrogen use efficiency (NUE). N nutrition index (NNI) can adequately evaluate situations of excessive, optimal, and insufficient N nutrition in rice, and the optimal N rate for modern high yielding rice is higher than conventional cultivars. The RSPAD is calculated as the SPAD value of top fully expanded leaf versus the value of third leaf, which taking into account of non-uniform N distribution within a canopy. RSPAD can be used as an indicator for higher yield and NUE, and guide for better management of N fertilizer. Furthermore, we developed a new strategy of nitrogen-split application based on RSPAD, in which N rate was saved by 18.7%, yield was increased by 1.7%, and agronomic N use efficiency was increased by 27.8%, when compared with farmers’ practices. This strategy of N fertilization showed a great potential in ensuring high yielding and improving NUE at lower N inputs.