Sulfur and nitrogen nutrition for sustainable rainfed lowland rice

Results of the greenhouse experiments showed that at the lowest S level (0.6 mg/l), application of high N (180 mg/l) increased the total N concentration in the straw but not in the grains. Increasing S from 0.6 to 5 mg/l decreased straw N concentration, but further increase in S concentration had no further effect on straw N. At 0.6 mg S/l, adding high N (120 or 180 mg/l) did not increase N uptake or had no adverse effect. Sulfur at 5 mg/l significantly increased total N uptake and leaf CO2 assimilation when 60 or 120 mg N/l was applied. A high N rate (180 mg/l) increased N uptake when S supply was increased to 10 mg/l. Sulfur and N had a significant interaction effect on grain yield of IR42. The critical level of S was 5 mg/l at 60 and 120 mg N/l, and 10 mg/l at 180 mg N/l. The root-shoot ratio [R:S] was about 0.43 at 0.6 mg S/l during mid-tillering (54 DAT [days after transplanting]), indicating more root growth than shoot. The R:S declined to about 0.2 at plant maturity, indicating that root senesced. Field study showed that grain yield of BPI Ri10 showed a quadratic relationship between N rate and grain yield and a nonlinear relationship between N uptake and grain yield. With the estimated N response function, maximum grain yield was obtained with 145 kg N/ha. Urea N application increased NO3 from 1.97 to 3.42 mg/l, while S application increased SO4 to the negative second power concentration from 7.75 to 13.4 mg/l in free water. Groundwater, however, was found to have high concentrations of NO3 to the negative first power (0.32 to 5.72 mg/l) and SO4 to the negative second power (4.6 to 29.4 mg/l). Indigo significantly increased rice grain yield by 1 t/ha and almost tripled the yield of intercrop Sesbania rostrata during the wet season rice. Because Aeschynomene intercrop did not survive, the rice crop escaped shading and yielded more than did the Sesbania - intercropped rice. Pre-rice Indigo provided a longer lasting benefit to direct-seeded rice than did Sesbania