Experimental Study And Numerical Simulation Of Soil Nitrogen Migration And Transformation

Soil nitrogen is one of the main elements affecting crop yield and quality,and the application of nitrogen fertilizer is of great significance to agricultural production.In this study,ammonium bicarbonate fertilizer was used as experimental nitrogen fertilizer.Due to the instability of ammonium bicarbonate itself,combined with unreasonable fertilization and irrigation methods,the loss of nitrogen elements would be aggravated.Increasing the application of chemical fertilizer to increase crop yield may cause fertilizer loss,waste of resources,land and groundwater pollution and other problems.Therefore,it is of great significance to study the migration and transformation of ammonium bicarbonate in soil for the development of modern agriculture.In this paper,the effects of rainfall intensity and fertilizer application on the spatial and temporal distribution of soil moisture,ammonia nitrogen and nitrate nitrogen were studied through indoor simulated rainfall experiments and combined with the basic theory of soil moisture and nitrogen migration and transformation.Based on soil capillary water theory and solute transport theory,a physical model of soil water and solute nitrogen transport and transformation under rainfall conditions was established,and the model was solved by finite difference method.Combined with the data of water content,ammonia nitrogen and nitrate nitrogen concentration in laboratory experiments,the reliability of the model of soil water and nitrogen transport and transformation under rainfall conditions was analyzed and verified.The main conclusions of this study are as follows:1)Soil samples from Yichang area were treated and used as test soil samples.The soil bulk density was measured by ring knife method.The soil bulk density was 1.407g/cm~3 in this study.The soil-water characteristic curves of the tested soil samples were measured by centrifuge method.The hydraulic diffusivity of soil samples was measured by horizontal infiltration test of soil moisture.Finally,according to the soil-water characteristic curve and the required hydraulic diffusivity,the hydraulic conductivity of the tested soil sample is calculated.2)The effects of rainfall intensity and fertilizer application rate on spatial and temporal distribution of soil water and nitrogen were studied through laboratory experiments on soil water and nitrogen migration and transformation under rainfall conditions.The results show that in the early stage of rainfall,the wetting front movement of shallow soil is greatly affected by rainfall intensity.The higher the rainfall intensity,the higher the rate of wetting front movement,and the linear relationship between wetting front movement and infiltration time.As infiltration time goes on,the rate of wetting front migration slows down and becomes more obvious in the redistribution stage.At this time,the relationship between wetting front migration and infiltration time is exponential.During the rainfall period,the soil moisture content increases rapidly.With the increase of infiltration time,the growth rate of soil moisture decreases gradually.In the redistribution stage,the soil moisture content decreases gradually,the reduction rate decreases gradually,and the final water content reaches a stable value.Under the same fertilization rate,the higher the rainfall intensity,the lower the ammonia nitrogen concentration in the fertilizer layer and the higher the ammonia nitrogen concentration in the lower layer.Under the same rainfall condition,the higher the amount of fertilizer applied,the higher the concentration of ammonia nitrogen;with the passage of time,the concentration of ammonia nitrogen decreases gradually as a whole.Under the same rainfall condition,the greater the amount of fertilizer applied,the greater the concentration of nitrate and nitrogen.Under the same fertilization condition,the farther the rainfall intensity is,the farther the nitrate nitrogen migration distance is,the deeper the concentration peak value is;with the passage of time,the nitrate nitrogen concentration increases gradually as a whole.3)In order to better study the change of solute movement in soil,TDR was used to measure the characteristics of soil water content and conductivity simultaneously in situ,and the correlation between conductivity and ion concentration was established.The change of ion movement in soil was deduced from the change of water content and conductivity.Based on the data of No.1 soil column in the experiment of nitrogen migration and transformation,it is concluded that the change trend of soil conductivity is basically the same as that of water content.That is to say,as the wetting front moves downward,the soil moisture content of each layer increases,the conductivity also increases.After the rainfall stops,the water content of each layer decreases under the action of potential gradient,and the conductivity also decreases.When the soil moisture content is about 0.06,the soil conductivity monitored by TDR is 0.When the soil moisture content rises to 0.16,the value of the soil conductivity will change.However,in the stage of water content decline after rainfall stops,the soil conductivity also decreases sharply,and finally stabilizes at a lower level.The more intense the movement of soil water,the greater the proportion of mobile water to total water,and the greater the conductivity measured by TDR.In the vertical profile of the soil column,the concentration of soil ions is higher than that of nitrogen,because there are non-nitrogen ions in the soil.The ionic concentration at 40 cm depth is 0,which is because the measured conductivity is 0 due to the unchanged water content.Therefore,water flow is the basis of reflecting the ionic concentration in soil by conductivity.In the soil column profile with water change,the spatial distribution of ion concentration is similar to that of nitrogen concentration.4)A physical model of soil water and nitrogen transport and transformation under rainfall was constructed.Meanwhile,the basic physical parameters of soil and VG model parameters were substituted into the model,and the physical model was solved by finite difference method.The spatial and temporal distribution data of soil water content,ammonia nitrogen and nitrate nitrogen in soil water and nitrogen transfer and transformation experiments under rainfall conditions were compared with the simulation results of numerical solution,and the reliability of the model to simulate soil water,ammonia nitrogen and nitrate nitrogen concentration was analyzed and verified.Through comparative analysis,the model has a good simulation effect on soil moisture distribution under rainfall conditions.The simulation effect on soil ammonia nitrogen and nitrate nitrogen concentration is slightly worse than that of water,but the overall model can better reflect the response of nitrogen solutes to rainfall fertilization factors.Generally speaking,the model of water and nitrogen transfer and transformation is reliable,which can provide theoretical guidance for the study of the law of water and nitrogen transfer and transformation in soil.