Spatial Pattern Of Vegetation And Soil Nutrients And Soil Erosion Of Degraded Alpine Meadow And Shrub Of Hillside In The Headwaters Region Of Three-river On Tibetan Plateau

Under the influence of global climate change and human interference,the extremely degraded grassland of the“bare land”in the source region of the Three Rivers on the Qinghai-Tibet Plateau intensified its is expanding and spreading to the hillside,forming the degraded hillside“bare land of mountain”,which leads to more serious soil erosion,and the ecological management of the“bare land of mountain”is more difficult.Therefore,it is necessary to conduct in-depth research on the degraded hillside and reveal its ecological process and driving mechanism.The study of degraded vegetation and soil changes under alpine mountain slopes has important reference value for understanding the ecological mechanism of the formation of the“bare land of mountain”type degraded mountain in the head waters region of three-Rivers on Tibetan Platean,and for proposing effective ecological restoration measures.In this study,the degradation of hillsides was investigated by selecting the range of small watersheds,and the typical hillside alpine meadows and shrubs were selected as the research objects.Based on the RUSLE model,combined with the compound radionuclide tracing method,extract vegetation,slope and soil erosion information in Wosai Township,Dari County,Qinghai Province,divide grassland degradation grades,slope grades,and soil erosion intensity grades,and determine soil nutrient content.At the same time,the spatial distribution and change of soil nutrients,vegetation and soil erosion modulus were studied according to the characteristics of vegetation community.The main results are as follows:（1）The degradation of the lower slope is more serious than that of the upper slope in Wosai Township,dari County,Qinghai Province.The vegetation coverage on the upper slope is significantly greater than that on the lower slope（P<0.05）;The coverage of bare patches and stones on the upper slope was significantly lower than that on the lower slope（P<0.05）.There was significant negative correlation between vegetation coverage and bare patch coverage（r=-0.686,P<0.05）,and extremely significant negative correlation with stone coverage（r=-0.635,P<0.01）.（2）The area of moderate degradation（MD）grassland in Wosai township is the largest,accounting for 64.6%of the total grassland area,about 343.66km².Lightly degraded（LD）and severely degraded（SD）grassland accounted for 22%and 12.65%,respectively;non-degraded（ND）and extremely severe degraded（ED）grassland accounted for 0.21%and 0.34%.The Regional Grassland Degradation Index（GDI）is1.73,which is close to the middle level of degradation.The GDI in the southwest and northeast regions is 2.91,which is more severely degraded;the northern and southern regions degrade slightly.（3）NMDS showed that the species composition of the moderately degraded meadows and shrub slopes differed greatly,and the species composition of the meadow slopes and the shrub slopes were similar.Plant community diversity showed a unimodal distribution pattern with slope length,that is,as slope length increased,Shannon-wiener index,Simpson index,Pielou index and Margalef index increased first and then decreased.The plant community richness on shrub hillside was significantly higher than that on meadow hillside（P<0.05）,and the evenness index and richness index of plant community on heavily degraded meadow hillside were significantly lower than other hillsides（P<0.05）.（4）The above-ground biomass of meadow hillside first increased and then decreased with the increase of slope length,showing a single peak curve（P<0.05）.The aboveground biomass and slope length of lightly degraded shrub slopes were extremely significant negative correlation（P<0.01）;the aboveground biomass of moderately degraded shrub slopes showed a single-peak curve with slope length（P<0.05）.The underground biomass at the bottom of the mountain was significantly lower than other slopes（P<0.05）.The underground biomass gradually decreases with the increase of soil depth,and gradually increases with the increase of slope length.（5）The content of OM,TN,and NH₄⁺-N on the meadow hillside gradually decreases from the top to the bottom of the mountain,and the content of TP and NO₃^--N is evenly distributed.The contents of OM,TN,NH₄⁺-N and TP on the shrub slopes gradually decrease from the top to the bottom of the mountain and the distribution of NO₃^--N is uniform.The soil nutrient content at the top of the degraded hillside was significantly higher than that at the bottom（P<0.05）.The optimal models for the spatial distribution of soil nutrients are Gaussian,spherical and linear models.The structure ratio C0/（C0+C）is less than 75%,which has strong spatial autocorrelation.It has strong spatial autocorrelation,less random variation,and low overall spatial heterogeneity.The spatial interpolation of soil nutrients shows that its spatial distribution is heterogeneous.（6）The soil nutrient contents of meadows and shrub slopes with different degradation degrees were significantly different（P<0.05）.The TP content of lightly degraded hillsides was significantly higher than that of other degraded hillsides（P<0.05）.The contents of TN,OM,NH₄⁺-N,and NO₃^--N on the shrub slopes were significantly higher than those on the meadow slopes（P<0.05）;and the OM content of the lightly degraded shrub slopes was significantly greater than that of moderately degraded shrub slopes（P<0.05）.The OM content of heavily degraded meadow slopes was significantly greater than that of moderately degraded meadow slopes（P<0.05）.The TN content of lightly and moderately degraded meadow hillsides was significantly greater than that of severely degraded hillsides（P<0.05）.The NH₄⁺-N of the slightly degraded meadow slope was significantly smaller than that of the moderately degraded and severely degraded slopes（P<0.05）.（7）The soil erosion rate of ¹³⁷Cs and ²¹⁰Pbex has a very significant correlation（P<0.01）.The average erosion modulus of ¹³⁷Cs and ²¹⁰Pbex on the meadow hillside is4165.7 t·km^-2·a^-1 and 3056 tkm^-2a^-1,respectively,indicating moderate erosion.The average erosion modulus of ¹³⁷Cs and ²¹⁰Pbex on the shrub slope is 1809.8 t·km^-2·a^-1and 1743.2 t·km^-2·a^-1,which are mild erosion.The average erosion modulus of ¹³⁷Cs and ²¹⁰Pbex in the study area is 3195.6 t·km^-2·a^-1 and 2578.4 t·km^-2·a^-1.Average erosion modulus:meadow>shrub,¹³⁷Cs erosion modulus>²¹⁰Pbex erosion modulus.Affected by the slope type,the“straight-concave”slope type has a lower erosion modulus than the“concave-straight,concave-convex”and compound slope types.（8）Wosai Township is dominated by slight erosion,accounting for 75.6%of the total erosion area,which is 389.4 km².Erosion mainly occurs on slopes of 5-25°,accounting for 77%of the total erosion area;and severe erosion mainly occurs on slopes of 25-35°and>35°.The area of moderately degraded grassland accounted for 64.6%of the total grassland area.The area of moderately degraded grassland with a slope of8-25°accounted for 69%of the total area of moderately degraded grassland;non-degraded grassland tended to occur in areas with a slope greater than 35°micro-erosion;Different degraded grasslands have micro-erosion,accounting for 75.6%of the total erosion area.In summary,the vegetation and soil on the alpine slopes of the“bare land of mountain”in the Three River Source Region of the Qinghai-Tibet Plateau are seriously degraded.Topography,slope,rainfall and human disturbance are the main factors that cause the large-scale“bare land of mountain”to appear in Wosai Township.On the habitat gradient,the alpine hillside soil nutrients,plant communities and erosion intensity in the small watershed of Wosai Township are heterogeneously distributed and have certain correlations.The vegetation and soil degradation and erosion intensity of shrub hillsides are weaker than that of alpine meadows,thanks to shrub vegetation intercepting precipitation,slowing runoff,and inhibiting soil erosion.Therefore,in the future restoration and management process,it is possible to consider increasing the vegetation coverage,adopting measures such as shrubland and closure to fundamentally control soil erosion.