The fate of ammonium nitrogen applied to flooded rice as affected by zeolite addition

High rice (Oryza sativa L.) yields are closely related to plant absorption of a large amount of nitrogen (N). However, there is little information on the fate of N applied at the middle growth stages of rice. Labeled N-15 ammonium sulfate was applied at the panicle formation stage in Experiment I, and 10 d after heading in Experiment II. Zeolite was also added at the concentration of 0, 0.01, and 0.1 kg(-1) to increase the cation exchange capacity (CEC) of the soil. The amount of N-15 fertilizer in the soil surface water decreased exponentially and the fertilizer disappeared within 2 d after application. The soil that received zeolite at 0.1 kg kg(-1) exhibited significantly less (NH4+)-N-15-N in the surface water and in the soil solution than the soil without the zeolite amendment. A significantly larger amount of exchangeable (NH4+)-N-15-N was observed in the high zeolite-treatment of soil compared to the low zeolite-treatment of soil. The amount of exchangeable (NH4+)-N-15-N increased initially, and thereafter, decreased to traces 4 d after application in Experiment I, while 6 or 9 d after application in Experiment II. The disappearance of exchangeable (NH4+)-N-15-N could be attributed mainly to the uptake by plants. The zeolite amendment or the time of N application did not significantly affect the amount of immobilized N. The rate of N adsorption was inhibited with increasing zeolite application. Moreover, zeolite application did not increase the recovery percentage of ammonium sulfate by rice plants. The total recovery of applied PIT ranged from 65 to 75%, irrespective of the zeolite treatments or the time of N application.