Nitrogen transport and transformation in packed soil columns from paddy fields

Abstract

Our understanding of nitrogen transformation in paddy fields or wetlands is limited due to the complex interactions between soil, water, and biomass. Therefore, we studied transformation patterns resulting from the oxic level in the soil, and studied saturated (anoxic) and unsaturated (oxic) flow conditions. We present a model designed to predict concentrations of nitrate and ammonium at several soil depths resulting from the processes of nitrification, denitrification, and ammonification. Model equations were obtained that describe NO3-N and NH4-N concentrations in terms of position, rate constant, and average flow velocity. Although many parameters were included in the model equations, some were determined from the literature and others were derived from experiments. A sensitivity analysis of the rate coefficients for NO3-N and NH4-N revealed that they are extremely sensitive to denitrification and ammonification respectively. Experimental results show that there were large differences in the transformations of NO3-N and NH4-N, the water pressure distributions, and the oxygen reduction potentials (ORP) between saturated and unsaturated pore water flow conditions. The performance of the model for sequential transformations during the transport of NO3-N is well documented under both saturated and unsaturated flow conditions.