REACTION RATE CONSTANTS FOR UREA TRANSFORMATION UNDER FLOODED CONDITIONS

摘要

The main form of fertilizer applied to lowland rice is urea, but very few researches are conducted on its movement and transformations under flooded soil conditions. An in-situ study was conducted to measure movement and transformation of urea in flooded soils. Urea transformation is difficult to model in flooded soil systems due to the complicated transformations it undergoes. An essential step in analytical modelling is the determination of model parameters such as reaction rate constants. In order to estimate the reaction rate constants, an experiment was conducted in a soil spiked with urea in glass containers and the soil moisture and temperature were maintained in the field condition. In this study only ammonium volatilized is captured and analysed periodically. The nitrate content in the glass container was found out at the end of the experiment. The reaction rate constants were estimated by solving the partial differential equations representing urea reactions in a two step process using risk solver platform in the MS-Excel environment. The method was found to have minimum number of observations without estimating the urea, ammonium and nitrate in soils periodically. The estimated reaction rate constant for ammonia volatilization was found to give a good estimate of volatilized ammonia with that of the observed volatilized ammonia. The estimated reaction rate constant of urea hydrolysis, distribution coefficient, volatilization, nitrification and denitrification are 0.0022 h~(-1), 0.9, 0.0081 h~(-1), 0.0010 h~(-1) and 0.0436 h~(-1) respectively. The reaction rate constants estimated in this study are useful for Nitrogen balance studies in flooded soils.