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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 15N 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 kg-1 to increase the cation exchange capacity (CEC) of the soil. The amount of 15N 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 15NH4 +-N in the surface water and in the soil solution than the soil without the zeolite amendment. A significantly larger amount of exchangeable 15NH4 +-N was observed in the high zeolite-treatment of soil compared to the low zeolite-treatment of soil. The amount of exchangeable 15NH4 +-N increased initially, and thereaft...

The kinetics of mineralization of soil organic nitrogen (N) in samples of several air-dried and fresh soils in the tropics was studied. Four different paddy soil samples in the IRRI lowland field were used. Parameters for mineralization of soil organic N were computed by using Arrhenius' law and Michaelis-Menten's equation. The following results were obtained: 1. The mineralization pattern of organic N in the wet soil samples were represented by an exponential equation as a function of reference days, which were determined by the duration of the incubation period and apparent activation energy. Although significant differences in the mineralization rate and mineralization potential among the soils were observed, there was no appreciable difference in the value of apparent activation energy, which were relatively low compared with those of Japanese paddy soils. 2. The mineralization pattern of the dried soil samples was represented by an equation consisting of two exponential equations. When a lar...