Transformations and losses of urea nitrogen after application to flooded rice

Abstract

Direct measurements of gaseous loss, 15N labelling and intensive sampling of floodwater and soil columns were used to follow the transformations and movement of fertilizer nitrogen (N) applied as urea (80 kg N ha-1) into the floodwater of a young rice crop. During the first 11 days after urea application, about 46% of the applied N was lost from the water-soil-plant system. Only 11% was volatilized as ammonia, despite very high floodwater pH values (up to pH 10) and some strong winds. Gaseous loss as nitrous oxide and leaching of N beyond a soil depth of 100 mm were both negligible. All the evidence indicates that the other 35% of the applied N was lost as dinitrogen produced by denitrification of nitrite and nitrate after nitrification of ammonium near the soil surface. No further losses from the system were detected after the first 11 days. Final recovery of the fertilizer N by the above-ground crop was only 17%. The results show that the several processes which resulted in gaseous loss of N operated concurrently, commencing almost immediately after fertilizer application. The major microbiological transformations appeared to occur near the soil-water interface (c. 0-20 mm depth).