Processes of nitrogen loss from fertilizers applied to flooded rice fields on a calcareous soil in north-central China

Abstract

Losses of nitrogen were investigated after applications of ammonium bicarbonate and urea to flooded rice at transplanting. Ammonia (NH₃) volatilization was determined by direct micrometeorological methods, and total loss of fertilizer nitrogen (N) was measured by¹⁵N balance. All the loss appeared to be in gaseous forms, since there was no evidence of leaching and runoff was prevented. The difference between N loss and NH₃ loss was thus assumed to be denitrification loss.

Both NH₃ volatilization and denitrification losses were large, being 39% and 33%, respectively, of the ammonium bicarbonate N, and 30% and 33%, respectively, of the urea N applied by farmers' methods.

Ammonia fluxes from the field fertilized with ammonium bicarbonate were very high for two days, and then declined rapidly as the NH₃ source in the floodwater diminished. Moderate fluxes from the field fertilized with urea continued over 6 days, but calculations showed that NH₃ transfer from floodwater to atmosphere was retarded during the middle period of the experiment, particularly on day 2 when a thick algal scum appeared on the water surface. The results indicate that this algal mass obstructed the transport of NH₃ across the water-air interface until the scum was dispersed by wind action. Nevertheless, the prolonged NH₃ losses on the urea treatment were due primarily to high floodwater pH values promoted by the strong algal growth during the daylight hours.

Nitrogen-15 balance studies showed that incorporation of fertilizer into drained soil substantially increased recoveries of fertilizer N in rice plants and soil compared with incorporation of fertilizer in the presence of standing floodwater. Ammonia loss measurements on these treatments when urea was applied suggested that the improvement in fertilizer N efficiency was due mainly to reductions in NH₃ loss.