Analysis Of Changes In Soil Nutrients And Their Driving Forces Under The Transition Of Land Use In The Agro-pastoral Ecotone Of Ningxia

The agro-pastoral ecotone is a typical ecologically fragile and environmentally sensitive area in China.Due to natural conditions and human activities,the land use types in this area are diverse and frequently changing,resulting in multiple land use patterns coexisting and the soil structure,soil quality,and ecological functions exhibiting obvious variability and fragility.In order to improve the local ecological environment and achieve a balance in the structure and function of the regional ecosystem,a large number of ecological restoration policies and projects have been implemented in this area,resulting in significant changes in land use patterns.Studying the changes in soil nutrients under different land use patterns is of great practical significance for maintaining the stability of the regional soil ecosystem and achieving sustainable land use.This study took the agro-pastoral ecotone of Ningxia as the research object.Based on the use of remote sensing methods to classify the land use change patterns in the study area into eight types,including farmland,farmland to forest,farmland to grassland,farmland to fallow land,grassland,grassland to forest,grassland to farmland,and grassland to fallow land,the Kruskal-Wallis test method was used to analyze the soil nutrient characteristics under different land use change patterns and evaluate the soil nutrients.The random forest regression model was used to reveal the driving factors of soil nutrient changes,in order to propose targeted strategies and suggestions for optimizing the land use pattern in the agro-pastoral ecotone of Ningxia.The main research results were as follows:(1)The two main land use types in the agro-pastoral ecotone of Ningxia are grassland and farmland.From 2000 to 2017,the largest area of land converted was farmland,reaching 5102.51 km2,of which 99.68%was converted to grassland.The grassland area increased by 5086.22km2,of which 96.39%came from farmland,3.61%came from wasteland and forest,and the increased area was mainly concentrated in parts of Haiyuan County and Xiji County.Compared to grassland and farmland,the areas of forest and fallow land remained relatively stable.(2)From 2000 to 2017,compared with the conversion of grassland to farmland and wasteland,the change of surface soil organic carbon,surface and deep soil nitrogen from grassland to forest was significantly different(P<0.01);compared with grassland to farmland,There was a difference in the change of total potassium in the deep soil from grassland to forest(P<0.05);there was no difference in the change of total phosphorus(P>0.05).Among them,when grassland was transformed into farmland,the content of total potassium in the deep soil increased,while the contents of organic carbon,total nitrogen and total potassium decreased under the transformation of other land types.Compared with the conversion of farmland to grassland,the change of surface soil organic carbon,surface and deep soil nitrogen from farmland to forest was significantly different(P<0.01);the change of farmland to wasteland,forest or grassland,the change of soil surface total phosphorus was difference(P<0.05);there was no difference in total potassium change(P>0.05).Among them,the content of organic carbon and total nitrogen in the surface soil increased when farmland was transformed into grassland,while the contents of organic carbon,total nitrogen and total phosphorus all decreased under the transformation of other land types.(3)From 2000 to 2017,the soil in the agro-pastoral ecotone of Ningxia was relatively high in total potassium and total phosphorus,but low in organic carbon and total nitrogen.The soil nutrients changed from a moderate level to a low level.Among them,total nitrogen had the greatest impact on the comprehensive index value of soil nutrients.Over the 17-year period,soil nutrients in grasslands have changed from medium to low levels;after grasslands were transformed into forests,soil nutrients were still at medium levels,but after grasslands were transformed into farmlands and wastelands,soil nutrients showed a sharp decline to a low level.The soil nutrient level of farmland also changed from medium level to low level;after farmland was converted into forest,soil nutrient level was still at medium level,but after farmland was transformed into grassland and wasteland,soil nutrient level changed from medium level to low level.(4)The factors that significantly affect the changes in soil nutrients in the agro-pastoral ecotone of Ningxia are soil particle size,precipitation,evaporation,population density,and changes in land use,with respective feature importance values of 16.00%,16.00%,8.70%,8.70%,and 7.60%.The impact of other factors on soil nutrient changes is not significant.The random forest regression model predicts an R2 of 0.38.In summary,the main land use types in the agro-pastoral ecotone of Ningxia are grassland and farmland.Due to the influence of climate change and human activities,there is frequent bidirectional conversion between grassland and farmland,while the areas of forest and wasteland remain relatively stable with smaller changes.Over time,the soil nutrient content in both grassland and farmland has decreased.When grassland and farmland are converted into forest,there is no change in soil nutrients,but when they are converted into other land use types,soil nutrients decrease.The significant factors affecting the changes in soil nutrients in the study area are soil particle size,precipitation,evaporation,population density,and changes in land use.Therefore,when optimizing the land use pattern in the study area,factors such as soil texture,climate change,and policy environment adaptation should be considered comprehensively,and targeted measures should be taken to achieve regional soil ecosystem stability and sustainable land use.