Ammonia volatilization losses from urea applied to flooded rice early in growth are typically 30-50%. Urea accounts for about 70% of the fertilizer nitrogen used in rice in southeast Asia and Australia, so reductions in ammonia volatilization losses could greatly improve efficiency of the N use, save energy, reduce costs, and improve the incomes of rice farmers. Urease inhibitors (UI) which slow the hydrolysis of urea into ammonium, have been shown to reduce ammonia volatilization losses from flooded rice. Field trials in Thailand, China, and the Philippines have shown that significant yield increases could be obtained if the UI is combined with algicides and nitrification inhibitors. The success of inhibitors in single or in combined applications depends on the type of inhibitor, their decomposition, conversion, and absorption, soil and floodwater properties, weather and the importance of other N loss mechanisms, particularly denitrification. In this project promising UI's will be evaluated in field trials in the Philippines and Vietnam in combination with Algicides and nitrification. The effects of these ameliorants on N recovery, on crop growth yield, ammonia volatilization and denitrification losses, and N balances on selected sites and treatments will be measured. Soil, floodwater, crop and climatic data will be collected for developing, calibrating, and validating an inhibitor sub-rountine for the CERES Rice model. It will enable the CERES Rice model to stimulate the UI effect and identify niches and optimum management strategies for UI use in lowland rice systems.