Water Saving and Nitrogen Economy of Lowland Rice.
Paul Belder, Matapos Research Station, ICRISAT, P.O.Box 776, Bulawayo, Zimbabwe, Hubert J. Spiertz, Plant Sciences- Crop and Weed Ecology, Wageningen University, Haarweg 333, Wageningen, 6709 RZ, Netherlands, and Bas A.M. Bouman, Crop, Soil, and Water Sciences Division, IRRI, DAPO Box 7777, Metro Manila, Philippines.
Increasing demand for rice and decreasing water diversions to agriculture, urge for higher water productivity in rice production systems. One way to deal with this challenge is using water-saving regimes on field scale. The main objective of this study was to quantify the effects of water-saving regimes on water productivity, nitrogen use efficiency, and yield by a combined experimental and modeling approach. The role of subsurface hydrology was studied to assess the effects of water saving on the water balance. Field experiments were conducted at three locations: Muņoz and Los Baņos in the Philippines and Tuanlin in China. In experiments comparing alternate submerged-nonsubmerged (SNS) regimes - soils remained dry for several days before re-irrigation - with continuous submergence (CS), apparent nitrogen recovery and yield were at par and 15-18% of irrigation water could be saved thus leading to higher water productivity. Nitrogen supply plays a key role in enhancing water productivity because nitrogen promotes leaf area growth, biomass growth, and yield and reduces evaporative losses. In most of our experiments, the groundwater table depth was shallow (<35 cm) and soil water tensions in the root zone were at most 10 kPa indicating that there was no water deficit in SNS. When soil water tension in the root zone in water-saving regimes increased to 30-50 kPa, apparent nitrogen recoveries and yields were reduced. In areas with acute water scarcity, the aerobic rice system is an option. In this system, rice is more or less grown as an upland crop. Irrigation water savings in experiments comparing aerobic rice with CS were nearly 40% but yields dropped with around 25%. In general, time and site-specific water management requires a detailed knowledge on crop growth and development, soil hydrological processes, and N cycling in the soil-plant system.