Fate and Transport of Ammonia Nitrogen in Andisol under Unsaturated Water Flow Conditions with Different Temperatures

Abstract

Nitrification in Andisol containing ammonium sulfate was observed at 22, 24 and 26℃ (batch tests). The decrease in ammonium nitrogen (NH₄-N) and increase in nitrate nitrogen (NO₃-N) in the batch tests were simulated well by a first-order transformation. The first-order rate constant decreased dramatically with decreasing temperature. Simultaneously, column experiments, in which ammonium sulfate solution was dripped for 9 d and then pure water was dripped for 20 d, were performed at the same temperature for the batch tests. NH₄-N remained in the upper soil layer due to adsorption, and NO₃-N produced by nitrification leached downward. The NO₃-N production decreased as temperature decreased. Then, using Richard's equation and an advection-dispersion model with Gapon convention for cation exchange and first-order transformation for nitrification, the column experiments were calculated with the rate constant obtained by the batch tests. When dissolved and absorbed NH₄-N were assumed to have the same rate constant, the calculation overestimated the NO₃-N profile in the column experiments. However, the calculation could reproduce the column experiment under the assumption that the absorbed NH₄-N had a smaller rate constant than the dissolved NH₄-N and started to nitrify when the concentration of the dissolved NH₄-N fell below 4 mmol L^-1.