Nontreatment effects on water x nitrogen experiment in aerobic rice

A new technology for rice under water-scarce conditions is the "aerobic rice system", in which rice is grown in nonpuddled, nonflooded aerobic soils, with supplementary irrigation in the dry season (DS). To optimize the system, insight is needed into water and nitrogen management. The authors present first-year results of a field experiment or water and nitrogen carried out in the 2004 DS at IRRI and at Dapdap, Tarlac [Philippines]. The soil was light clay at IRRI and sandy loam at Dapdap. At both sites, variety Apo was grown in a 4-time-replicated split-plot design, with water as the main plot and nitrogen as the subplot. The water treatments were irrigation twice a week (W1), once a week (W2), and once every two weeks (W3). The total water input was 1,150 mm in W1, 770 mm in W2, and 630 mm in W3 at Dapdap; and 1,340 mm in W1, 860 mm in W2, and 720 mm in W3 at IRRI. The nitrogen treatments were 0, 60, 120, 160, and 200 kg/ha. At both sites, yields were depressed by nonexperimental factors and no significant treatment effects were found. At IRRI, 2 of the 4 replicates had satisfactorily yields of 1.2-3.1 t/ha, but the other 2 replicates yielded only 0.37-0.42 t/ha. At Dapdap, all 4 replicates failed to produce any yield, although total biomass and leaf area developed satisfactorily in time. Total biomass at harvest was 5.4-8.3 t/ha, and maximum leaf area index at flowering was 1.0-1.5. The failed yields were caused by extremely low yield components. At both sites, plant iron concentrations at flowering were toxic (350-670 ppm), and those of zinc were deficient (20-30 ppm). At Dapdap, nematode concentrations in the roots at harvest were 2,600-5,000 per gram fresh root weight. The presence of micronutrient disorders and nematodes seems to be a localized problem. At the same site and in the same season in Dapdap, an experiment on rowspacing yielded 4.5-4.9 t/ha. Follow-up research aims to understand the causes of site-specific yield depression and to propose remedial measures.