SPAD-based N-management of rice under alternate wetting and drying irrigation saves water and nitrogen fertilizer

The use of SPAD chlorophyll mater has been proven effective in increasing N-use efficiency (NUE) in continuously flooded (CF) rice, but its use has not been investigated under alternate wetting and drying (ADW) irrigation. Experiments were conducted in split-plot design with four replications in the 2004 and 2005 dry seasons at IRRI, to test the hypothesis that combining SPAD-based N-management and AWD can enhance NUE amd water productivity. The mainplots were three water treatments: CF, AWD that involved irrigation application when soil water potential of -20 kPa (AWD-20) and -80 kPa (AWD-80) in 2004; AWD-10 and AWD-50 were used in 2005. The subplot were five N Management treatments: zero N (NO), 180 kg N/ ha in four splits (N180) and SPAD-based N-management where N was applied when SPAD reading of the youngest fully extended leaf was less than or equaled to 35 (W-SPAD 35), 38 (NSPAD 38) and 41 (NSPAD 41). In 2005, NSPAD 32 was tested instead of NSPAD 41. Rice yields in AWD-10 were similar to these of CF, yields of other AWD treatments were lower. AWD reduced irrigation water input by 15-35% and significantly increased water productivity compared with CF. NSPAD 38 and NSPAD 35 saved 20-40% N fertilizer compared with N180. Despite lower N rates, N SPAD 38 consistently gave similar yield as N180 in all water treatments. The agronomic NUE of NSPAD 38 and NSPAD 35 was consistently higher than that of N180. A good correlation between leaf N content per unit leaf area and SPAD reading were observed for all water treatments, suggesting that SPAD reading can be used to estimate leaf N in AWD similar to CF. The authors conclude that a combination of AWD-10 and SPAD-based N management, using values of 35-38, can save water and N fertilizer while maintaining high yield as in CF conditions.