OPPORTUNITIES TO MANIPULATE NUTRIENT-BY-WATER INTERACTIONS IN RAIN-FED LOWLAND RICE SYSTEMS

Water stress, accompanied by changes in soil aeration, severely limits rice productivity in rainfed systems. These factors affect nutrient a vailability. Nitrate (NO3) that accumulates in aerobic soil is rapidly lost through leaching or denitrification in flooded soil. Green manur es can act as NO3 catch crops and legumes may gain additional N from b iological N fixation. Direct seeding permits additional crops to be gr own. Roots are commonly shallow in rainfed lowlands. It is not clear t o what extent rice yields in rainfed lowlands are Limited by water, nu trients, and the interactions between them, over diverse soil types, c ultural practices and seasonal conditions. Research must determine wha t really Limits root growth, water extraction and nutrient uptake. Som e evidence suggests that manipulation of controlled-release fertilizer and root system development may be the key to optimizing nutrient rel ease and capture in fluctuating environments. The potential for using strategic application of nutrients Co buffer water Limitation and stab ilize yields must be examined. Models such as QUEFTS (Quantitative Eva luation of the Fertility of Tropical Soils) provide a potential framew ork for analyzing the effects of soil fertility and water availability on growth and yield of rice. (C) 1998 Elsevier Science B.V.