Preliminary Study Of Vegetation Influence On Paddy Field Dissolved Nitrogen Transport Into Surface Runoff Water

Non-point source pollution?NPSP?is the main sources of water pollution,and it involves a wide range of uncertainties and is very difficult to control.The agricultural non-point source pollution?ANPSP?is the main part of NPSP,especially the loss of nitrogen and phosphorus in ANPSP is more serious.In this thesis,after planting vegetations on the paddy field,the artificial rainfall was used to collect the nitrogen concentration data in the surface runoff water,and the effects of vegetation on the nitrogen transport into surface runoff water under rainfall conditions were studied.The modified hydrus-1D model was used to simulate the paddy field nitrogen transport to surface runoff water under short-term rainfall conditions with vegetation on soil surface.And the process of nitrogen transport was analyzed to provide effective suggestions for agricultural non-point source pollution control.The four sets of experimental data?events 0723,0726,0729,0801?showed that the vegetation had great influence on the initial content of soil nitrate,runoff start time,steady runoff start time,ponding water depth at steady runoff start time and steady runoff rate.It is found that,the initial content of nitrate nitrogen in the soil with vegetation is lower than that in the bare ground.Within a certain range of rain intensity,the reduction ratio of runoff by the vegetation can reach 41.7%,indicating that the vegetation has increased the surface infiltration rate.Through the analysis of pollutants concentrations in the runoff,it is found that the vegetation has significantly reduced dissolved nitrogen in runoff water,the reduction ratio for the sum of ammonia nitrogen and nitrate nitrogen can reach 40.16%.The reduction effect on nitrate nitrogen is very obvious,and the reduction ratio can reach 80.3%.But the ammonium nitrogen concentration in runoff water with vegetation is even higher than that without vegetation.The reason for this phenomenon is that the hypoxia of plant roots and plant root exudates affect the physical and chemical properties of the surface soil and inhibit nitrification.In addition,the rapid loss of surface solute in bare land has begun before the runoff has stabilized.However,in the field covered by the vegetation,the rapid solute loss is delayed,which is consistent with the change trend of runoff,indicating that the vegetation has a lag impact on the paddy field nitrogen transport into surface runoff water.The fitting curve function was used to describe the solute transport process in the late runoff time.It is found that the ammonia nitrogen loss into surface runoff water with time was better fitted by the exponential function,and the nitrate nitrogen loss was better fitted by the power function.The 0729 and 0801 events were simulated by the modified Hydrus-1D model.The model parameters were identified by the 0729 event,and these identified parameters were used to simulate the 0801 event.The simulation results were good.The R² in 0729 event model was 0.85 or higher,which was 0.9 or higher in 0801 event.And the simulation results for NO₃^--N in No.2 field is significantly better than that in No.1 field.These study results show that the modified hydrus-1D model can be used to simulate the paddy field nitrogen transport into the surface runoff water,under short-term rainfall conditions with vegetation on soil surface.The experimental results provide a theoretical basis for the study of the paddy field nitrogen loss into the surface runoff water.According to the experimental results,it is possible to reduce the surface area pollution caused by the increase of nitrogen loss in the farmland by planting plants along the river channel.