NITROGEN TRANSFORMATIONS 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 varie tal improvement and investment in irrigation facilities. Nitrogen effi ciency in tropical rice is low. N-15 recovery rarely exceeds 30-40% in wetland rice production systems. Ammonia (NH3) volatilization and den itrification are recognized as major nitrogen loss mechanisms in such systems. Information on the relative importance of the two loss proces ses is available for few sites in Asia. The greatest losses of N are r eported to occur when the fertilizer treatment leads to a high concent ration of ammoniacal N in the floodwater. Results from the studies usi ng micrometereological technique suggest that ammonia volatilization m ay be the most important loss process in wetland rice ecosystems. Dire ctly measuring denitrification in the held proved more difficult than measuring NH3 volatilization due to difficulty in distinguishing the m ain end product of denitrification (N-2) against a large background of atmospheric N-2. However, the directly measured (N-2 + N2O) - N-15 fl ux 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 lo wer floodwater pH and lower partial pressure of NH3 (pNH(3)) 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 farm ers' desire to reduce cost of production as well as ecological conside rations such as increased methane production which contribute to globa l climate change. Other promising alternative practices for increasing fertilizer N efficiency include improved timing and application metho ds, particularly through better incorporation of basal N fertilizer wi thout standing water, deep placement, and use of coated fertilizers.