| Application of nitrogen fertilizer to soil will lead to soil acidification,causing the loss of salt-based cations and the decline of soil fertility.The nitrate produced in nitrification will leach into the surface and underground environment without being absorbed by soil,which eventually contaminates the water.Because the nitrogen transformation involves complex chemical reactions and is affected by soil environment,it is still a challenge to accurately characterize the soil nitrogen cycle and its associated environmental evolution.Numerical model simulation has become an important method to study soil nitrogen cycle.Based on the soil column test,the nitrogen cycle of different forms of nitrogen after entering the soil was studied.Besides the analysis of the transformation of nitrogen between different forms,other reactions related to nitrogen cycle such as respiration,soil surface complexation reaction were also considered.Subsequently,a geochemical model was developed to numerically simulate the above processes,and a new sensitivity analysis method was introduced to identify the key parameters and important processes in nitrogen cycle.The specific research contents and conclusions of this dissertation are as follows:(1)The soil column test with different nitrogen source inputs was carried out.The nitrogen cycle process and the change of other solutes in column in each treatment was analyzed.It was found that the input of ammonia nitrogen causes changes in the p H and cations in soil column.This is because most of the ammonia nitrogen is adsorbed to the soil by ion exchange or surface complexation with the rest transformed to nitrate.However,the input of nitrate leads a weak impact on soil column.The experiment results indicate that the main process of the nitrogen cycle occurs in the unsaturated zone and around the water table.(2)In order to describe the nitrogen cycle process occurring in the soil column test from a more mechanistic view,a new three-dimensional model of water flow,solute transport and geochemical reaction was established.The nitrogen cycle can be better simulated in consideration of soil surface complexation reaction,and the simulation results quantify the contribution of nitrification in the process of ammonia reduction.In comparison to traditional nitrogen models,the geochemical model considers external reactions associated with the nitrogen cycle and can better describe the nitrogen conversion process from some aspects of the mechanism.(3)In consideration of the difference between the mechanism model and the empirical model,the multi-model assemble of nitrogen cycle was established with the description of the sub-process of nitrogen cycle from different views,and a new variance-based sensitivity analysis method is introduced.The sensitivity analysis method under multi-model averaging helps to reduce the bias introduced by singlemodel sensitivity analysis.The application of this method also helps to understand the nitrogen cycle and related processes: nitrification in the sub-surface during the nitrogen cycle greatly affects the nitrate distribution in the lower layer.Adsorption is also important for the nitrogen cycle,which would even exceed the effect of denitrification on nitrate.(4)The results of experiments and numerical simulations indicate the importance of nitrogen cycle process occurred in unsaturated zone.Numerical simulation of the nitrogen cycle in the three-dimensional region requires a fine spatio-temporal discrete in the unsaturated zone,which leads to significant cost in calculation.Based on efficient and accurate simulation requirements,the three-dimensional solute transport process was decomposed into one-dimensional vertical motion in unsaturated zone and threedimensional motion in saturated zone,so as to establish a simplified-3D saturatedunsaturated geochemical model.In comparison with the output of fully threedimensional model,the simplified-3D geochemical model overestimates the reactant concentration leached from upper unsaturated zone as a result of neglecting the lateral convection and dispersion in the unsaturated zone.However,the accuracy of simulation in reaction product was based on its reaction type.For example,ammonia was overestimated,while dinitrogen gas was underestimated.At the same time,it is also noted that in the region with high permeability,the two models are very close to each other;the simplified-3D model reduced computational cost at the expense of a small precision loss,thus showing its advantages.The research in this dissertation is more in-depth to research the chemical reaction process in the nitrogen cycle,providing an effective numerical tool for simulating these processes.It also addresses the complexity of the nitrogen process and the nonuniqueness of the model.The research results provide a new method for assessing the utilization of nitrogen and environmental evolution. |
