Chlorophyll meter-based nitrogen management of rice grown under alternate wetting and drying irrigation

Farmers have adopted alternate wetting and drying (AWD) irrigation to cope with water scarcity in rice production. This practice shifts rice land away from being continuously anaerobic to being partly aerobic, thus affecting nutrient availability to the rice plant, and requiring some adjustment in nutrient management. The use of a chlorophyll meter (also known as a SPAD meter) has been proven effective in increasing nitrogen-use efficiency (NUE) in continuously flooded (CF) rice, but its use has not been investigated under AWD irrigation. This study aimed at testing the hypotheses that (i) SPAD-based N management can be applied to AWD in the same way it is used in CF rice, and (ii) combining chlorophyll meter-based nitrogen management and AWD can enhance NUE, save water, and maintain high rice yield. Experiments were conducted in a split-plot design with four replications in the 2004 and 2005 dry seasons (DS) at IRRI. The main plots were three water treatments: CF, AWD that involved irrigation application when the soil dried to soil water potential at 15-cm depth of −20 kPa (AWD−20) and −80 kPa (AWD−80) in 2004, and AWD−10 and AWD−50 were used in 2005. The subplots were five N management treatments: zero N (N0), 180 kg N ha−1 in four splits (N180), and three SPAD-based N-management treatments in which N was applied when the SPAD reading of the youngest fully extended leaf was less than or equaled 35 (NSPAD35), 38 (NSPAD38), and 41 (NSPAD41). In 2005, NSPAD32 was tested instead of NSPAD41. A good correlation between leaf N content per unit leaf area and the SPAD reading was observed for all water treatments, suggesting that the SPAD reading can be used to estimate leaf N of rice grown under AWD in a way similar to that under CF. SPAD readings and leaf color chart (LCC) values also showed a good correlation. There were no water × nitrogen interactive effects on rice yield, water input, water productivity, and N-use efficiency. Rice yield in AWD−10 was similar to those of CF; yields of other AWD treatments were significantly lower than those of CF. AWD−10 reduced irrigation water input by 20% and significantly increased water productivity compared with CF. The apparent nitrogen recovery and agronomic N-use efficiency (ANUE) of AWD−10 and AWD−20 were similar to those of CF. The ANUE of NSPAD38 and NSPAD35 was consistently higher than that of N180 in all water treatments. NSPAD38 consistently gave yield similar to that of N180 in all water treatments, while yield of NSPAD35 about 90% of that of CF. We conclude that a combination of AWD−10 and SPAD-based N management, using critical value 38, can save irrigation water and N fertilizer while maintaining high yield as in CF conditions with fixed time and rate of nitrogen application of 180 kg ha−1. Treatments AWD−20 and NSPAD35 may be accepted by farmers when water and N fertilizer are scarce and costly. The findings also suggested LCC can also be a practical tool for N-fertilizer management of rice grown under AWD, but this needs further field validation.

Research highlights▶ A good correlation between leaf N content per unit leaf area versus SPAD reading and SPAD readings versus LCC readings was observed for continuously flooded and AWD treatments. ▶ A combination of AWD−10 and SPAD-based N management, using critical value 38, can save irrigation water and N fertilizer while maintaining high yield as in CF conditions with fixed time and rate of nitrogen application of 180 kg ha−1. ▶ The findings show that SPAD can be used under AWD conditions and that LCC can also be a practical tool for N-fertilizer management of rice grown under AWD, but this needs further field validation.