Effects Of Fluctuation Of Shallow Groundwater Level On Soil Nitrogen Transport In Sandy Land

Mu Us Sandy Land is located in the arid and semi-arid area in northwest China,where the groundwater is relatively shallow,and agricultural water needs depend mainly on the pumping of groundwater.Pollution caused by the application of ammonia fertilizer and the extraction of large quantities of shallow groundwater are the main problems facing local agriculture.Nitrogen transport in soil is one of the major agricultural concerns today.The adsorption and desorption of ammonium nitrogen in soil largely controlled the transport of nitrogen in soil profile.The depth of groundwater level is one of the main factors of solute transport.The change of groundwater level directly affects the saturated and unsaturated soil layers,causes the change of moisture and environmental factors,and thus affects the solute transport.Therefore,it is of theoretical significance to study the effect of dynamic change of groundwater level on nitrogen transport for rational irrigation of ammonia and fertilization and reducing groundwater pollution.According to the fertilization situation in the study area,the nitrogen transport in the study area was studied by means of laboratory static test,one-dimensional soil column infiltration test and theoretical analysis.The adsorption/desorption process of ammonium nitrogen in different soil layers was studied by static experiments.One-dimensional infiltration test of vertical soil column was carried out to simulate nitrogen migration under different depth of groundwater level.In view of the environmental change of soil column caused by the fluctuation of water level,nitrogen transport under the alternating leaching of soil column was studied.Through the test results and analysis,the following conclusions are drawn:(1)The adsorption capacity of ammonium nitrogen in each layer of soil is significantly higher than the desorption capacity.The relationship between adsorption and desorption is consistent with the characteristics of "fast adsorption and slow desorption".Adsorption process is relatively rapid,almost at the moment of contact.The analytic process is lagging behind the adsorption process.(2)According to the experiments of nitrogen migration under the dynamic change of groundwater level,it was found that the ammonia-nitrogen solution after irrigation was mainly trapped in the soil surface by adsorption,which hindered its downward migration.The effect of the change of water level depth on ammonium nitrogen is mainly to change the nitrification degree in the upper soil column and the equilibrium state of adsorption/desorption of ammonium nitrogen in the surface soil by affecting the oxidation-deoxidation environment in the soil column.The change of buried depth of water level can significantly affect the longitudinal migration of nitrate nitrogen in soil profile,and the capillarity caused by it can also affect the distribution of nitrogen,making nitrogen accumulate into groundwater,which is easy to cause the pollution of underground water body,especially nitrate pollution.(3)The alternation of wet and dry environment can promote the formation of aerobic environment in the upper layer of soil column and accelerate the conversion of ammonium nitrogen to nitrate nitrogen.Rehydration after dry period can leach nitrate nitrogen produced by nitrification and cause potential nitrate pollution to groundwater.