Ammonia volatilization from a paddy field following applications of urea: Rice plants are both an absorber and an emitter for atmospheric ammonia

Ammonia (NH3) volatilization from a paddy field following applications of urea was measured. Two lysimeters of Gray Lowland soil with a pH (H2O) of 5.7 were used for the experiment. Urea was applied at a rate of 50 kg N ha(-1) by incorporation as the basal fertilization (BF) and at rates of 30 and 10 kg N ha(-1) by top-dressing as the first (SF1) and second (SF2) supplemental fertilizations, respectively. Two wind tunnels per lysimeter were installed just after BF; one was transplanted with rice plants (PR plot), and the other was without rice plants (NR plot). Weak volatilization was observed at the PR plots after BF. By contrast, strong volatilization was observed at the PR plots after SF1 with a maximum flux of 150 g N ha(-1) h(-1); however, almost no volatilization was observed after SF2. The NH3 volatilization loss accounted for 2.1%, 20.9%, 0.5%, and 8.2% of the applied urea at each application, BF, SF1, SF2, and the total application, respectively, for which only the net fluxes as volatilization were accumulated. The NH3 volatilization fluxes from the paddy water surface (F-vol) at the NR plots were estimated using a film model for its verification. After confirmation of good correlation, the film model was applied to estimate F-vol at the PR plots. The NH3 exchange fluxes by rice plants (F-ric) were obtained by subtracting F-vol from the observed net NH3 flux. The derived F-ric showed that the rice plants emitted NH3 remarkably just after SF1 when a relatively high rate of urea was applied, although they absorbed atmospheric NH3 in the other periods. in conclusion, rice plants are essentially an absorber of atmospheric NH3; however, they turn into an emitter of NH3 under excess nutrition of ammoniacal nitrogen. (c) 2007 Elsevier B.V. All rights reserved.