Real time N management in rice under alternate wetting and drying irrigation

Chlorophyll meter (SPAD) and leaf color chart (LCC) readings can satisfactorily estimate leaf-N status of rice under continuous flooding conditions. Under water scarcity, alternate wetting and drying (AWD) irrigation is used to save water in rice production. Can SPAD and LCC be used for real-time N management of rice under AWD irrigation? This study was carried out to determine (1) if SPAD and LCC readings can estimate leaf-N concentration in rice and (2) the effect of SPAD-based N management on grain yield and agronomic N used efficiency (ANUE) of rice grown under different water regions. The experiment was conducted in the 2004 dry season at IRRI Farm, Los Banos, Laguna , in split-plot design with four replications. The main plots were three water regime treatments: continuous flooding (CF), and irrigation when soil water potential at 15 cm soil depth reaches -20 kPa (AWD -20) and -80 kPa (AWD -80). The subplots were five N-management treatments: ZeroN (No), 180 kg N/ha (N180), N application when SPAD meter reads 35 (NSPAD35), 38 (NSPAD38) and 41 (NSPAD41). Nitrogen applied in SPAD-based treatment varied according to SPAD reading and water treatment. N applied in NPSAD35 was 90-110 kg N/ha, 130-150 kg N/ha in SPAD38 and 170-250 kg/ha in NSPAD41. Nitrogen content per unit leaf area was linearly related with SPAD and LCC readings (P less 0.001) in all water treatments. WxN interaction effects on grain yield and ANUE were not significant. CF yielded significantly more than AWD-20, which is in turn significantly yielded more than AWD-80. NSPAD41 had significantly higher grain yield than N0 and other SPAD-based treatments (P less 0.05), but it had yield similar to that of N180. Though differences among SPAD-based treatments were significant, the difference was less than 8% of the average yield. ANUE was significantly highest in NSPAD (P less 0.05) irrespective of water treatment. The results suggest that SPAD may be used in estimating leaf-N status under AWD irrigation. The SPAD reading of 35, which has been the threshold value for N application in rice under well-watered condition, can be used, as well, under AWD irrigation.