Modeling On Farmland Subsurface Drainage And Nitrogen Losses In Alpine-cold Region

In Iowa,USA,nitrogen fertilizer is widely applied to increase crop yields.However,application of nitrogen fertilizer also causes serious nitrogen loss,where the nitrate-nitrogen has become one of the main pollution sources in the subsurface drainage.In order to maintain crop yield all while reducing drainage and nitrate-nitrogen in the subsurface drainage,the objectives of this study are to explore the appropriate crop rotation system,farmland subsurface drainage engineering system,and nitrogen fertilizer application amount.DRAINMOD-N II and Root Zone Water Quality Model2（RZWQM2）were modified based on field data from 2005～2009.We then used a 20-year（1989–2009）field experiment data in Iowa to compare the performance of DRAINMOD-N II and RZWQM2 in predicting nitrate–nitrogen（NO₃^--N）concentration and losses in a subsurface drained field at nine nitrogen application rates(0 to 252 kg N ha^-1).A modified DRAINMOD-N II model was used to simulate the effects of winter cover crop rye on subsurface drainage and NO₃^--N loss.Furthermore,simulations from DRAINMOD-N II give an optimization design for local farmers.Main conclusions in this stud are as follows:1.Two calibrated agriculture system models can be used to simulate subsurface drainage,NO₃^--N loss and concentration.The evaluation statistics（Nash–Sutcliffe efficiency,ratio of the root mean squared error to the standard deviation,and percent bias）showed that simulations from both models were within the error range,indicating that both models can be used to simulate NO₃^--N loss and concentration in subsurface drainage.Furthermore,DRAINMOD-N II performing slightly better than RZWQM2,as model evaluation results showed that the simulations of DRAINMOD-N II model are closer to the measured data and has higher simulation accuracy.In comparison to the RZWQM2 model,the DRAINMOD-N II model has friendly operation interface,requires relatively few parameters and the parameter are easier to obtain.We thus suggest that DRAINMOD-N II model is more suitable in alpine-cold region of northern Iowa.2.Subsurface drainage,NO₃^--N loss and NO₃^--N concentration simulation.The subsurface drainage simulations showed that there was a positive correlation between subsurface drainage and precipitation.The NO₃^--N loss and concentration increased with the increase of nitrogen application rates,but NO₃^--N loss and concentration still affected by precipitation and other factors.With the same amount of nitrogen fertilizer application,the nitrate concentration in the subsurface drainage showed great annual variation.The nitrate concentration is less than the Maximum Contaminant Level（MCL）,10 mg L^-1 when nitrogen fertilizer application amount is less than the recommended level of 123 kg N ha^-1.Nitrogen conversion process simulation from two models showed that the nitrogen mineralization,denitrification,and fixation increase with nitrogen application rate.With same amount of nitrogen fertilizer application,the simulated nitrogen mineralization increased with precipitation from DRAINMOD-N II model but decreased from RZWQM2,this is because these two models use different carbon and nitrogen cycle sub-modules.3.Simulations from the modified DRAINMOD-N II model showed that winter cover crop with rye sustainably reduce subsurface water drainage and NO₃^--N loss.Planting winter rye cover crop can reduce subsurface drainage and NO₃^--N loss in corn-soybean rotation.The results indicated that the calibrated model satisfactorily simulated monthly and annual subsurface drainage and NO₃^--N loss.The errors mainly showed in March and November when temperature was low and with higher wind speeds.Long-term simulations indicated that planting winter rye cover crop can reduce drainage.Five years dynamic subsurface drainage decreased by 5%at the beginning of cover crop application,and decreased by 14%as the cover crop duration time increases.Planting winter rye can also increases ET,plant nitrate uptake and increase nitrate fixation.Meanwhile planting winter rye can reduce net mineralization.The simulation from the calibrated model is well matched with the field data,indicating that DRAINMON-N II model perform well on simulation of hydrology and nitrogen movement.The DRAINMON-N II model can be used to simulate different cropland management practice’s effect on subsurface drainage and nitrogen movement,providing reference for optimizing cropland management.4.DRAINMOD-N II model could be an effective tool to optimize subsurface drainage engineering system design based on agriculture management practices.The results showed that drain spacing and drain depth are the main limit factor of drainage intensity under the same geological environment conditions,same drain pipe and drainage coefficient.Drainage intensity decreases as drain spacing increases.Water logging factor is one of the main factor affecting crop relative yield which increases as drainage intensity increase,but not affected by drain depth.Subsurface drainage and nitrate-nitrogen loss increased as drainage intensity increases.The optimize agricultural drainage design was selected based on above results,with 1 cm day^-1 drainage intensity,16 m pipe space and 1 m pipe depth helps decreasing subsurface drainage discharge and nitrogen loss,thus improving crop yield.Results from this study provided theory and data base for improving drainage system design at study area,which also provide scientific basis for building drainage system at China.