Studies Of Ammonia Nitrogen In The Vadose Zone By Equilibrium And Nonequilibrium Models

In recent decades, water pollution is increasingly serious in our country. The use of agricultural fertilizer is one of the main cause of water pollution. According to the survey, in Fangshan District, fertilizer nitrogen input significantly exceeded the most suitable amount of fertilizer. And water pollution in Fangshan is increasingly serious. Considering of the real fertigation situation in Xinzhen Fangshan, with ammonia nitrogen as the research object, this paper includes indoor test and simulation by Hydrus-1D, to study the migration process of ammonia nitrogen in the vadose zone and make sensitivity analysis. During the indoor test, two soil columns- one for normal(soil column 1) and the other for preferential flow(soil column 2)- were filled to simulate unsaturated flow of vadose zone in the study area. The studies show :(1) The adsorption of ammonia nitrogen is obvious while the nitrification effect can be neglected. Preferential flow has a great influence on ammonia nitrogen migration. Soil-water distribution coefficient and retardation factor were calculated according to the BTCs. The results show the strongest adsorption of ammonia nitrogen on the soil at the location of 30~45 cm.(2) Hydrus-1D is used to simulate the ammonia nitrogen migration process under the same conditions of experiment in laboratory. Comparing the measured values in laboratory with the simulation results of ammonia nitrogen in the soil columns, it is found that the simulation results have good consistency on the trend with lab measured values in column 1, and the fitting effect in column 2 is bad.(3) Comparing the simulation results of the three models, it is found that One-site Model and Mobile/Immobile Model fit better than Equilibrium Model. For the upper part of column 1 One-site Model fits better, while Mobile/Immobile Model fits better for the lower part.(4) The parameter sensitivity from higher to lower is soil-water distribution coefficient > longitudinal dispersivity in Equilibrium Model, soil-water distribution coefficient > first order rate coefficient > longitudinal dispersivity in One-site Model, and soil-water distribution coefficient > fraction of adsorption sites in contact with mobile water > mass transfer coefficient > longitudinal dispersivity > immobile water content. Different from the general understanding, the sensitivity of immobile water content is very low. Among the parameters, soil-water distribution coefficient contributes to the adsorption quantity, while the others contribute to the adsorption rate.