Nitrogen transformation in wetland rice ecosystems

In Asia, rice production has increased an average 2.7% annually -- -due to greater fertilizer use and crop intensification together with varietal improvement and investment in irrigation facilities. Nitrogen efficiency in tropical rice is low. 15N recovery rarely exceeds 30-40% in wetland rice production systems. Ammonia (NH3) volatilization and denitrification are recognized as major nitrogen loss mechanisms in such systems. Information on the relative importance of the two loss processes is available for few sites in Asia. The greatest losses of N are reported to occur when the fertilizer treatment leads to a high concentration of ammoniacal N in the floodwater. Results from the studies using micrometereological technique suggest that ammonia volatilization may be the most important loss process in wetland rice ecosystems. Directly measuring denitrification in the field proved more difficult than measuring NH3 volatilization due to difficulty in distinguishing the main end product of denitrification (N2) against a large background of atmospheric N2. However, the directly measured (N2 + N2O) -15N flux for rice in Indonesia, Thailand and the Philippines rice fields was less than 1% of the applied N. Green manure incorporation in wetland rice fields reduced N losses from mineral N source due to resulting lower floodwater pH and lower partial pressure of NH3 (pNH3) than that of urea applied alone. At present, the integrated use of green manure and mineral N is receiving much attention in the hope of meeting farmers' desire to reduce cost of production as well as ecological considerations such as increased methane production which contribute to global climate change. Other promising alternative practices for increasing fertilizer N efficiency include improved timing and application methods, particularly through better incorporation of basal N fertilizer without standing water, deep placement, and use of coated fertilizers.