MINERALIZATION PATTERN OF SOIL ORGANIC N OF SEVERAL SOILS IN THE TROPICS

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 large amount of extractable carbon was detected by diluted sulfate or hot water in the soils, the mineralization pattern was represented by an equation which consisted of three exponential equations. 3) The difference in the amount of inorganic N between the dried and wet soil samples increased rapidly up to 5 weeks after the start of incubation and then became almost constant. The mineralization pattern of the decomposable organic N which appeared with the changes of the soil conditions from wet to dry (DON) could also be represented by an exponential function of reference days. 4) There was no appreciable difference in the mineralization rate and apparent activation energy between the organic N of the fresh soil samples and slowly mineralizable organic N of the dried soil samples, and between DON and the rapidly mineralizable organic N of the dried soil samples.