Simulation Study On The Effect And Mechanism Of Nitrogen Leaching In Greenhouse Vegetable Field Under Integration Of Water And Fertilizer

Nitrogen leaching in farmland has caused a series of problems,such as groundwater pollution,soil fertility decreased and resource wastage,which has attracted the attention of scholars in various countries.Compared with other ecosystems,a higher risk of nitrogen leaching appeared in greenhouse vegetable（GV）field under the guidance of high yield by‘overfertilization and overirrigation’.Therefore,how to reduce nitrogen leaching while ensuring high yield is the current research hotspot in GV field.Water and fertilizer integration is considered to be an effective technology to reduce nitrogen leaching loss.So the cucumber-tomato（C-T）rotation system including four treatments:control treatment（CK）,farmers’conventional flooding fertilization（FP）,drip irrigation fertilization（FPD）and drip irrigation optimized fertilization（OPTD）,in typical GV field of Beijing suburbs was set up in this study.The soil NO₃^--N leaching loss in the GV field system was quantified using the in-situ field monitoring combined with the DNDC model.The integrated impact of water and fertilizer on soil NO₃^--N leaching was explored and the transport mechanism of NO₃^--N in the soil under integration of water and fertilizer was analyzed,which was expected to provide scientific guidance for the decision-making of the optimal management of water and fertilizer and the control of nitrogen leaching in the GV field.The main results were as follows:（1）High nitrogen leaching caused by fertilization and irrigation events of C-T rotation system was found in GV field.NO₃^--N was the main form of nitrogen leaching in GV field system.The total amount of NO₃^--N leaching in single season under FP treatment was 48.55-392.09 kg N ha^-1.Compared with FP,the total seasonal NO₃^--N leaching loss was reduced by 55.51%and 66.61%under FPD and OPTD treatments,respectively,which indicated that drip irrigation and drip irrigation optimized fertilization treatments can effectively reduce the risk of nitrogen leaching.So the decrease of chemical fertilizer and irrigation water input was the focus of nitrogen leaching reduction in GV field system.（2）The nitrogen leaching process of the GV field was simulated well by using the DNDC model.The DNDC model was verified by in-situ field data.And the verified DNDC model can be used to simulate vegetable yield,5cm soil temperature,0-20cm soil moisture and NO₃^--N leaching.The RMSE values between simulated and measured values under FP treatment were 12.85%,29.76%,32.21%and15.04%,respectively.The verified DNDC model had a good simulation effect on the process of nitrogen leaching and transportation in GV field soil.（3）Irrigation water amount and nitrogen fertilizer application rate were the main factors affecting the accumulation and migration of NO₃^--N in GV field.Under the condition of soil nitrogen surplus,irrigation amount was the key factor to determine NO₃^--N leaching.NO₃^--N was easily accumulated on the top soil layer under lower irrigation rate.Increasing the amount of irrigation significantly accelerated the downward movement of NO₃^--N in the soil.Increasing the amount of nitrogen applied not only promoted the surface accumulation of NO₃^--N,but also increased the accumulation of NO₃^--N at 20 cm in the soil.（4）Different management measures had different contributions to the reduction of NO₃^--N leaching,and the superposition effect on NO₃^--N leaching were found among various factors.The combination of different measures has great potential to reduce nitrogen leaching.Compared with conventional treatment,NO₃^--N leaching was reduced by 59.04%under 20%reduction with fertilizer and irrigation water.NO₃^--N leaching was reduced by 69.04%with the integration of water-saving,fertilizer-saving,drip irrigation and increasing soil organic carbon,which had better effect on reducing nitrogen leaching.In general,it is a better method to apply the DNDC model to evaluate and control nitrogen leaching loss in GV fields.On the basis of ensuring yield,changing the traditional flooding irrigation into drip irrigation can effectively improve water and fertilizer utilization efficiency and reduce NO₃^--N leaching.Optimizing fertilization amount,irrigation amount and fertilization irrigation mode can regulate the migration process of NO₃^--N and reduce the loss of NO₃^--N from the source and process.In addition,on the basis of controlling measures such as nitrogen reduction and water saving,combined with improving soil organic carbon content,it is also a more effective way to reduce nitrogen leaching loss in the GV field.