Spatial Pattern And Variaion Of Bald Patches And Soil Nutrition In 'bare Land' Degraded Apine Grassland In Three-river Source Region On The Tibetan Plateau

The degradation trend of alpine grasslands on the Tibetan Plateau has seriously affected the important service functions of ecological in grassland ecosystems such as water conservation,climate regulation,and biodiversity maintenance.Among them,the appearance and expansion of large-scale‘bare land'degraded grassland in the Three River Source Region is the most serious,and the appearance of bald patches is an important feature of the formation of‘bare land'degraded grassland.The generation and expansion of bald patches leads to the formation of‘bare land'.Therefore,the study of bald patches has an important scientific reference for understanding the formation mechanism of‘bare land'.In this study,the bald patches on typical beaches(MQ1,MQ2,MQ3)and slopes(DR1,DR2,DR3)were selected as the research objects in the Three Rivers Source Region.The landscape ecology,geostatistics,fractal geometry,GIS and other methods were used to analyze the spatial distribution pattern of bald patches in degraded grasslands and the soil spatial variability of physical and chemical indicators on a local scale.Combined with the community characteristics of aboveground vegetation,the changes and mechanisms of the bald patch-soil-vegetation system in the formation process of degraded grassland in‘bare land'on beach and slope land are discussed.The main results are as follows:(1)The orthophoto maps of bald patches distribution was obtained by Unmanned Aerial Vehicle.304,352 and 677 patches were extracted from MQ1,MQ2 and MQ3plots respectively.MQ1 plot was dominated by large patches(>5 m~2),MQ 2 was medium patches(1-5 m~2),while MQ3 was small patches(<1 m~2).(2)In beach lands,the coverages bare spots in the MQ1,MQ2,and MQ3 are low,and there is no obvious change tendency between plots.In sloping fields,with the decline of the slope position,the coverage of bare spots has a significant tendency to increase.Overall,the coverage of bare spots on slopes is higher than that on beach lands.(3)The average area of bald patches in MQ1 was 6.0 m~2 and significantly higher than MQ2 and MQ3(p<0.05).The average circumference of bald patches in MQ2 was6.87 m,and significantly higher than MQ1 and MQ3.The order of shape index was MQ3>MQ2>MQ1,and had the significant differences(p<0.05).The fractal dimensions of the bald patch shape in MQ1,MQ2 and MQ3 were 1.26,1.22,1.33respectively.(4)With the decline of the slope position,the average height of edible forage grass decreased,the average height of legumes and shrubs had an increase trend.The average height of weeds gradually increased,and the average height of the weeds in lower positions(DR3)was significantly higher than the upper and middle position of the slopes.(5)Soil texture of flat field was sandy loam.Soil texture of upper position and middle slope is silty loam and soil texture of lower position slope is loam.The volum fractal dimension of sown grassland soil particle-size was 2.42 and larger than degraded grassland.The volum fractal dimension in middle position of slope grassland soil particle-size was 2.57 and higher than upper and middle position of slopes.(6)The soil total nitrogen,available nitrogen and soil organic carbon in flat field(MQ1,MQ2,MQ3)were significantly higher than sown grassland(p<0.05).The soil pH,total phosphorus,ammonium nitrogen,nitrate nitrogen in flat field was significantly lower than sown grassland(p<0.05).The soil bulk density(1.02 g/cm~3)of grassland in lower position of slope was significantly higher than the higher and middle slopes(p<0.05).Soil organic carbon,total nitrogen,available phosphorus,ammonium nitrogen,nitrate nitrogen in middle position of slope was significantly higher than lower position in slope(p<0.05),and there was no significant difference in the soil nutrients between higher and middle position of slope.(7)By the semivariogram model,the results showed that soil bulk densityand total nitrogen of flat degraded grassland and sown grassland had strong spatial correlation and were mainly influenced by structural factors,such as soil types and soil parent material.Soil pH,water content,available nitrogen,and available phosphorus were mainly affected by stochastic factors,which showed weak spatial correlation.The soil nutrients characteristics of slope field with spatial correlation showed different patterns in slope positions and influenced by structural factors and chaotic factors.Spatial interpolation method showed that physical and nutrients were uniformly distributed at flat fields,while spatial variation was greater in the slope field.To summarize,the distribution pattern of soil total nutrients in bald patch plots of beach land is affected by structural factors,and the available nutrients are mainly affected by stochastic factors;the distribution pattern of soil nutrients in bald patch plots of different slope positions is affected by structural factors and random factors.During the expansion process of the bald patches on the‘bare land',the stability of the soil system gradually decreased,prompting the small bald patches to easily develop into regular large patches,and eventually formed a large area of‘bare land'.