| The Loess Plateau,as the largest apple product region in China,low nitrogen use efficiency seriously restricts the development of apple orchards in this area.With the excessive nitrogen fertilizer applicati It is difficult to accurately monitor nitrate accumulation and leaching processes by traditional methods in drylands of Loess Plateau,becasue the nitrate usually accumulates in soil and only leaches with heavy rainfall occasionally.Therefore,this study combined with field monitoring and model simulation,to 1)investigate the characteristics of nitrogen accumulation and leaching in apple orchards on the Loess Plateau;2)analyze the effects of planting years and N fertilizer appilication rates on soil nitrate accumulation and leaching;and 3)quantify the main N losses.This stduy was design to provide theoretical basis for rational application of nitrogen fertilizer and reduction of nitrate deep accumulation in apple orchards on the Loess Plateau.The results showed as follows:1.The mineral nitrogen in the soil is mainly nitrate nitrogen,and the ammonia nitrogen content is very low in the 0-600 cm soil profile of apple orchards with different planting years.With the increase of planting years,the nitrate nitrogen concentrations increased and its peaks accurred at much deeper soil depth.The nitrate nitrogen accumunated up on 6000 kg ha-1 at 0-600 cm soil profile for the apple archards with 25 planting years.The longer the planting years,the more nitrate nitrogen accumulated in deep soil(200-600 cm),implying the nitrate leaching enhanced.There was a linear relationship between nitrate accumulation and historical fertilizer application rates in apple orchards with different planting years.The accumulation of nitrate nitrogen in 0-200 cm soil profile increased significantly in N800 and HY50 treatments for plots experiment,which was likely related to N application rate and soil mineralization.2.The soil water contents of apple orchard were mainly affected by rainfall,evaporation,root uptake and leakage.The soil water in the surface layer(0-20 cm and 20-60 cm)changed greatly.However,the deep soil water content varied only in the months with heavy rainfall such as in July,August and October.The simulation results showed that the Hydrus-1D model could simulate the soil water dynamic well in apple orchard(R2: 0.46-0.81),except for the deep soil layer(140-200 cm,R2=0.18).The water balance analysis showed that the transpiration(root uptake),evapolation and leakage accunted for 52,15,and 27% of annual rainfall during the study period.3.The changes of nitrate nitrogen and ammonium nitrogen contents in apple orchard soil profile were mainly affected by the N application rates,soil mineralization,plant uptake,N leaching and gas loesses.The simulation results show that the Hydrus-1D could simulate the changes of nitrate nitrogen(R2: 0.31-0.59)and ammonium nitrogen(R2: 0.57-0.97)contents well in apple orchard.The plant uptake of N was about 180 kg·ha-1 with most of nitrate form,and mainly occurred during April to Auguest.The N leaching at 200 cm soil layer was mainly nitrate leaching and occurred during base fertilizer application in October and second fertilizer application in July.With the increase of N application rates,the nitrate nitrogen accumulated in soil profile significantly.Nitrate leaching was the main N loss in apple orchard of Loess Plateau.The scenarios with different N application rates(0,400,800 kg·ha-1)showed that the nitrate leaching all were upto 800 kg·ha-1,even there was no N application.The reason was ralated to the high initial nitrate concentrations in soil profile.The results suggested that in the soil profile with large amount of N accumulation,the N leaching would be a serious problem for a long time even the N application rates would have reduced.Therefore,the N accumulation and leaching needs to concern in the apple orchard of Loess Plateau. |
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