Study On Soil Characteristics And Their Relationships With Topographic Factors Of Alpine Desertified Grassland In Hongyuan

Hongyuan alpine wetland is located in the eastern edge of the Qinghai Tibet Plateau,which is a typical representative of the global alpine wetland ecosystem,and has extremely important directive significance for China and even global climate change.Under the comprehensive influence of the special geographical environment of the plateau,regional climate warming and human disturbance,alpine wetlands and meadows gradually degraded,and were succeeding in the direction of"alpine wetlands-alpine meadows-degraded meadows-sandy grassland".Based on the obvious control effect of micro geomorphic change on habitat heterogeneity in grassland area,the typical desertification grassland patches in Wache township of Hongyuan county were selected as the research object,and the research method of space series instead of time series was used to study the mechanism of micro geomorphic change on soil structure characteristics,soil nutrients and soil microbial community of alpine desertification grassland,to clarify the internal relationship between desertification and micro topography of alpine grassland,deepen the understanding of the law of alpine grassland degradation succession development,and provide scientific basis for ecological restoration and ecological service function improvement of alpine grassland.The main results are as follows:(1)The distribution characteristics of surface soil aggregates and soil nutrients in the desertification process of Alpine grassland in arid grassland and wetland direction on both sides of the desertification patch were analyzed,and the indicator factors of soil structure in the desertification process of alpine grassland were revealed.With the decrease of desertification,the average weight diameter(MWD),geometric mean diameter(GMD)and>0.25 mm aggregates content(R_0.25)of soil aggregates in arid grassland and wetland increased gradually,while the fractal dimension(D)and<0.25 mm aggregates content decreased gradually,and the stability of soil structure increased gradually,in addition,MWD,GMD and R_0.25 in the direction of arid grassland were greater than those in the direction of wetland,and the reduction degree of soil fractal dimension in the direction of arid grassland was greater than that in the direction of wetland.The stability of soil structure in the direction of arid grassland was stronger than that in the direction of wetland,and the degradation rate of grassland desertification in the direction of wetland was faster.There was a significant negative correlation between soil fractal dimension and organic matter and available nitrogen(P<0.01);There was no significant correlation between fractal dimension and soil nutrients.The results showed that>1 mm aggregates and 1?2 mm aggregates could be used as indicators to evaluate soil desertification in arid grassland and wetland respectively,that is,the higher the particle size aggregate content was,the weaker the degree of soil desertification was.In arid grassland,>1 mm aggregates could indicate that the higher the content was,the smaller the soil bulk density was,and the higher the water content was.(2)Based on the analysis of soil physical and chemical properties,soil organic carbon,total nitrogen,total phosphorus and ecological stoichiometry of 0?40 cm soil in alpine grassland with different desertification degree,the distribution characteristics of soil aggregate organic carbon storage in alpine grassland were clarified,and it was proposed that soil nitrogen content was the limiting factor affecting soil organic matter decomposition and plant growth.The results showed that soil organic carbon storage of desertified grassland was low(0.05?5.56 Mg/ha).Soil organic carbon of non desertified grassland mainly existed in 0.5?2 mm aggregates(38%)and<0.25 mm aggregates(25%),while soil organic carbon of desertified grassland mainly existed in<0.25 mm aggregates(57%?82%).The soil C/N ratios(15.18?36.11)of non desertification,light desertification and moderate desertification grassland was higher than that of topsoil(11.9±0.1)in China.The lower the decomposition efficiency of soil organic matter,and the lower the soil total nitrogen content(0.33?1.59 g/kg),the lower the soil N/P(1.10?1.82),which reflects that the soil nitrogen content is the limiting factor affecting the decomposition of organic matter and plant growth.With the aggravation of desertification,soil organic carbon,total nitrogen,total phosphorus,available nitrogen,available phosphorus,microbial biomass carbon nitrogen,C/N and C/P decreased gradually,and were positively correlated with>0.5 mm aggregate and water content,and negatively correlated with<0.25 mm aggregate,p H and bulk density(P<0.05).(3)Based on the study of the relationship between soil characteristics and slope,aspect and position,it is clear that aspect and position have obvious control effect on soil aggregate stability and soil nutrient distribution.>1 mm aggregate content,R_0.25,MWD and GMD of 0-40 cm soil from top to bottom increased gradually,and the soil structure stability of slope bottom was significantly stronger than that of slope middle and top(P<0.05).The contents of soil organic matter,total nitrogen and total phosphorus were not significantly different in the top and middle of the slope,but significantly higher in the bottom than in the middle and bottom of the slope;soil moisture content and>0.5 mm aggregates at the bottom of shady slope were significantly higher than those of sunny slope.Soil organic matter,total nitrogen,total phosphorus,microbial biomass carbon and biomass carbon at the bottom of shady slope were higher than those at the top and middle of slope,and the bottom of shady slope was significantly higher than that at sunny slope(P<0.05),but there was no significant difference among the top,middle and sunny slopes.(4)The relationship between soil bacterial community characteristics and topographic factors in alpine desertified grassland was studied,and the structure and diversity of soil bacterial community affected by slope direction were clarified.The dominant phyla of soil bacterial community in desertified grassland on different slopes were Actinobacteria Proteobacteria,Acidobacteria,Chloroflexi,and the four dominant populations accounted for66.85%-73.59%of the total abundance.The abundance of Actinobacteria,Proteobacteria and Chloroflexi increased with the increase of slope,while Acidobacteria decreased.Actinobacteria Proteobacteria and Acidobacteria have good adaptability to the habitat conditions of desertified grassland,which was closely related to the decomposition and mineralization of organic matter.The species number of soil bacterial community in different slope positions of the shady slope and sunny slope were heterogeneous,and the number of bacteria communities in the middle of sunny slope was higher than the top and bottom of slope,while the bottom of the shady slope was larger than the top and middle of the slope.Soil structure and soil physical and chemical properties had significant effects on bacterial community.The effect of 0.25?0.5 mm,1?2 mm and 2?5 mm aggregates on the bacterial OUT level was significant(P<0.05),and the explanation for the diversity of bacterial population was 44.7%,45.0%and 36.7%,respectively;The effects of soil bulk density,organic matter,total nitrogen,microbial biomass carbon and total phosphorus on bacterial community were significant(P<0.05),and the explanations for bacterial diversity were 43.3%,42.9%,38.0%,37.7%and 36.1%,respectively.Therefore,0.25?0.5 mm,1?2 mm,2?5 mm,bulk density,organic matter,total nitrogen,microbial biomass carbon and total phosphorus were the main driving factors for the changes of soil bacterial community and functional structure.