Principles and practices of real-time nitrogen management: a case study on irrigated rice in China

Rice production consumes about 20% of the total N fertilizer used for agriculture in the world. This N fertilizer is often not effectively used by irrigated rice because of improper timing and rates of application. Excessive rates of N fertilizer can lead to losses of inorganic N from agricultural lands into surface water and groundwater, thereby threatening our environment. Traditional methods of fixed N fertilizer scheduling might be inappropriate for maximization of fertilizer N-use efficiency in current rice production systems because these practices do not consider the dynamic nature of N supply and demand in irrigated rice ecosystems. Real-time N management was developed on the basis of the physiology of rice leaf photosynthesis, tillering, and leaf area growth to optimize canopy development and maximize biomass accumulation and yield formation. A chlorophyll meter (SPAD) or leaf color chart (LCC) is used to monitor leaf N status and determine the timing of N application in real-time N management. Both SPAD and LCC measurements correlate closely with leaf area-based N content. A single critical value of SPAD and LCC can be used to determine the need for N topdressing throughout a growing season. The critical values of SPAD reading and LCC score, however, differ depending on cultivar, location, and season. Real-time N management using SPAD or LCC has been tested in several countries and several sites in China. SPAD- or LCC-based N management improved fertilizer N-use efficiency by reducing total N input in most cases and by increasing grain yield in some cases. Results of the N-response curve, real-time N management using SPAD, and site-specific N management (SSNM) indicated that farmers in China overapplied N to the rice crop by at least 50%. The overapplication of N occurred mainly during the early vegetative stage (within 10 days after transplanting). Indigenous N supply capacity at Chinese sites was 54% higher than that at IRRI, but farmers did not adjust the N rate for the high N supply from soil in China. The improper timing and rate of N application were largely responsible for poor agronomic N-use efficiency (AE) at Chinese sites. The poor AE was caused by low physiological N-use efficiency (PE). Nitrogen uptake ability was not the major limiting factor for fertilizer N-use efficiency of the rice crop in China. There is a great possibility to optimize the timing and rate of N application in irrigated rice in China through real-time N management and SSNM.