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.