The Impact Of Returning Farmland On Main Service Functions Of Ecosystem In The Western Region Of Farming-pastoral Ecotone Of Northern China

The western region of Farming-pastoral Ecotone of Northern China(FPENC)is located in the Loess Plateau with the natural ecosystem is seriously degraded.As a typical ecologically fragile area,the western region of FPENC is the first experimental area and the key project implementation area in ecological projects such as Grain for Green Project(GGP)in China.The implementation process and ecological effects of ecological restoration have attracted extensive attention of scholars.Under the influence of large-scale human activities such as GGP,regional land use structure and ecosystem service functions will produce a series of changes.Determining the impacts of returning farmland on the main service functions of ecosystems in the western region of FPENC,it will help to comprehensively evaluate the effects of ecological restoration in the region and provide a basis for the planning and construction of regional ecological restoration projects.Based on ecological models and RS&GIS spatial analysis methods,this study assessed the impacts of returning farmland on soil conservation function,water supply function and water retention function in the western region of FPENC during 2000-2015.The main results are as follows:(1)The largest intensity of returning farmland was happend in 2000-2005,when is the key implementation period of GGP in the western region of FPENC.The conversion of farmland to forest and grassland resulted in the farmland reduction by 439.2km~2 and 940.83km~2,accounting for 27.24%and 58.33%of the total reduced area of farmland,which is the dominant way to reduce farmland.In 2005-2010,the total reduction area of farmland was the least in the three periods,but the reduced farmland still mainly converted to forest and grassland.The total area of returning farmland to forest accounted for 69.53%of the total reduction area from farmland during this period.The occupation of farmland by urban expansion is the dominant factor for the reduction of farmland in 2010-2015,accounting for 72.88%of the total area of farmland reduction during this period.The proportion of returning farmland to forest and grass is small,This period is not within the scope of ecological returning farmland.In the past 15 years,the area of returning farmland has gradually migrated from the northeast and southwest to the east and central regions.(2)The amount of soil conservation in the returning farmland showed an increasing trend in2000-2005 and 2005-2010,the amount of soil conservation respectively increasing by 21.51×10~4t and 31.23×10~4 t,accounting for 17.5%and 57.08%of the total soil conservation before returning farmland.The increased soil conservation is mainly from returning farmland to forests and grasslands.In 2010-2015,most of the farmland was converted into construction land,resulting in soil conservation reduced by 0.8×10~4 t in the cultivated area.The soil conservation benefits brought by returning farmland to forest and grass are different in different slope grades.The increase of soil conservation is the most in the range of slope less than 5 degrees,and the increase rate is the highest.The increase of soil conservation is the least in the gradient area of20-25 degrees.However,the steep increase of soil in the steep slope above 25°is not high,but the growth rate is high,and the area of steep slope above 25°is the key policy support area for the implementation of the new round of ecological engineering.(3)The supply of water in the returning farmland increased first and then decreased in2000-2015.Except for the reduction of water supply caused by the conversion of farmland to construction land,the other flows of farmland conversion increased the water supply.In2000-2005 and 2005-2010,the total water supply in the returning farmland respectively increased by 5.66×106 m~3 and 0.29×106 m~3,accounting for 9.1%and 1.32%of the total water supply before returning farmland in the returning farmland.The increased water supply is mainly from returning farmland to forests and grasslands.In 2010-2015,due to the large-scale expansion of construction land,the total supply of water was decreased.The total water supply in the returning farmland decreased by 15.39×106 m~3,accounting for 55.86%of the total water supply in the returning farmland in 2010.Hohhot city of Inner Mongolia Autonomous Region,is the most obvious area for water supply decline.(4)The total water retention in the western region of FPENC has been increasing from2000 to 2015.Except for the reduction of water conservation caused by occupation of cultivated land by construction land,the other flows of farmland conversion have increased the water conservation.From 2000 to 2005,the net increase of water retention in the area of returning farmland was 105.29×105 m~3,accounting for 58.8%of the total water retention in the returning farmland in 2000.The increase in water retention caused by returning farmland was much higher than that in the latter two periods.In 2005-2010,although the increase of water conservation was far less than that of the previous period,the formation of the optimal returning farmland made the growth rate of water retention was the highest in the three periods,and the increase rate of water conservation was as high as 81.07%.Although the total amount of water conservation in has slightly increased during 2010-2015,the conversion of farmland to construction land has led to a significant reduction in water conservation,and the areas with significant reduction have been concentrated in Hohhot and Liangcheng counties in Inner Mongolia Autonomous Region.