Changes Of Soil Water,Carbon And Nitrogen In Deep Soil Of Weibei Upland After Apple Orchards Were Planted With Food Crops

In the past 20 years since the implementation of the project of returning farmland to forest(grassland)on the Loess Plateau,long-term large-scale apple planting has caused a series of ecological problems such as soil dry layer,nitrate nitrogen accumulation and carbon storage reduction,which seriously restricts the green and sustainable development of regional agriculture.However,in Weibei dry tableland,due to the explosion of apple planting area,single variety structure,shortage of rural labor force,unstable food market and other factors,apple price is low and unmarketable,and input is not proportional to output.In order to ensure national food security and high-quality development of orchards,it has become a common phenomenon that large areas of orchards on the Loess Plateau have been planted as food crops.At present,the research on the change characteristics of soil water,nitrate nitrogen and organic carbon in deep soil after apple orchards were planted with food crops is relatively weak.Therefore,Weibei dry tableland in this study was took as the research area,selected apple orchards aged 10 years,15 years,20 years and 30 years as well as the corresponding apple orchards aged 2 years,5 years and 6 years as the research objects.By comparing and analyzing the differences of soil moisture,nitrate and organic carbon in 0-10 m profiles of different places,and based on the spatio-for-time method,the effects of changing apple orchards of different tree ages to grain crops on water recovery in deep soil dry layer,accumulation and transport of soil nitrate and changes in organic carbon storage were studied.It is of great significance for soil quality improvement and agricultural sustainable development in Loess Plateau.The main achievements of this study are as follows:(1)The dynamic change characteristics of soil moisture,nitrate nitrogen and organic carbon in apple orchards of different stands were clarified.Fruit trees mainly consumed 0-3 m soil layer moisture at the early stage of growth.As the planting time of apple orchards gradually increased,the soil moisture in 2-10 m soil layer of apple orchards decreased significantly compared with farmland.As for soil nitrate nitrogen,the accumulation of soil nitrate nitrogen increased continuously,and more than 90% of the nitrate nitrogen accumulated in the 0-5 m soil profile,but the accumulation depth of its cumulative peak value continuously migrated downward.As for soil organic carbon,soil organic carbon storage showed a trend of first decreasing and then increasing,but there was no significant difference between different stages of soil organic carbon storage.Therefore,it is suggested that orchards in the study area should be planted reasonably,and daily management and fertilization should be strengthened to achieve high quality development of orchards.(2)The dynamic changes of soil moisture,nitrate nitrogen and organic carbon contents and reserves in different orchards of different forest ages were expounded.After apple orchards of different age were planted with food crops,on the one hand,soil desiccation was alleviated,and soil moisture recovered rapidly,which could be restored to a depth of about 7m within 2 years.The recharge depth did not increase significantly within 2 to 6 years after planting,but the soil moisture content in each layer increased continuously within 3 to 7 m.On the other hand,with the recovery of soil moisture,nitrate nitrogen is leaching rapidly,which poses a huge potential threat to groundwater.As for soil organic carbon,soil organic carbon storage decreased significantly after planting food crops for 2 years in orchards of different forest ages,but gradually recovered after planting food crops for 6 years.It is necessary to consider the problems of soil moisture recovery and soil quality degradation caused by rapid leaching of nitrate nitrogen when planting.This study can provide theoretical basis for optimizing land use structure and improving soil quality.(3)The influence mechanism of soil water condition,soil depth,planting years and other factors on the Interaction mechanism of soil water,nitrogen,water and carbon after planting was revealed.The results show that soil water storage after planting modification has the most significant direct impact on soil nitrate nitrogen accumulation after planting modification(r(?)0.47),and the planting years of apple orchards before planting modification play a decisive role in soil nitrate nitrogen accumulation after planting modification(r(?) 0.204).Leaching of soil cumulative nitrate nitrogen lags behind the downward movement of soil water.There is no significant relationship between soil water storage and organic carbon storage after cropping,and soil depth plays a decisive role in determining soil organic carbon storage after cropping(r(?)-0.663).At the same time,the restoration years of soil water in orchards of different age were analyzed.It took 2 years,23 years and 34 years for the water storage of 3-10 m soil profile in orchards of initial fruit stage,full fruit stage and aging stage to recover to the stable soil water storage of many years of planting food crops.Therefore,attention should be paid to the management and development of local water resources,woodland and agricultural land should be more reasonable planning,to protect the ecological environment.