| As the second largest karst distribution area in the world,under the double pressure of ecological fragility and human destruction,the karst area in Southwest China has intensified soil erosion,rapidly developed rocky desertification and gradually deteriorated ecological environment,and the way of soil and water loss in this area is obviously different from that in other areas.As a plant measure of soil and water conservation,Hedgerow has significant effect on reducing soil and water loss.The terrace hedgerow is a kind of hedgerow placed on the ridge of a plot,it can lower the velocity and reduce the amount of sand on the ground,the huge underground root system reinforced the soil of the ridge,and has obvious effect of soil and water conservation.In this paper,Orychophragmus violaceus,ViciavillosaRothvar andOphiopogon bodinierithree species of grasses were used to construct the terracegrass hedgerow(hereinafter referred to as the terrace hedgerow),and the root soil composite samples were taken for direct shear test,to analyze the differences of shear strength,maximum shear stress,cohesion and internal friction angle of the composite,to explore the law of change,and to determine the morphological index and chemical composition content of the root system in the composite,and then theterrace hedgerow was taken The basic physical and chemical properties of the root zone soil were determined.Finally,the effects ofterrace hedgerow the root index and the root zone soil index on the shear resistance of the root soil composite were studied by correlation analysis and principal component analysis.The main conclusions are as follows:(1)The maximum shear stress and shear strength of the composite increase with the increase of vertical load,and the shear performance index of Ophiopogon bodinieri composite is the best.At the same slope position,Ophiopogon bodinieri is significantly higher than Orychophragmus violaceus andViciavillosaRothvar of the maximum shear stress and the shear strength,andOphiopogon bodinieri andViciavillosaRothvar were significantly higher than Orychophragmus violaceus in cohesion of terrace hedgerow complex and internal friction angle of middle slope complex,the cohesion ofOphiopogon bodinieri andViciavillosaRothvar in the middle slope is 2.80 times and 3.44 times of Orychophragmus violaceusrespectively.The same terrace hedgerow,Maximum shear stress,cohesion and internal friction angle of Ophiopogon bodinieri composite,the maximum shear stress of Orychophragmus violaceus complex and the cohesion of ViciavillosaRothvar complex show middle slope was significantly higher than up anddown slope,But the shear strength of the composite is only Ophiopogon bodinieri show middle slope > up slope >down slop,Orychophragmus violaceus andViciavillosaRothvarhave no significant difference.(2)It is found by analyzing the soil in the terrace hedgerows root area that the same slope position,three kinds of terrace hedgerowcan improve the content of soil water stable aggregates and promote the formation of large-scale aggregates,the Ophiopogon bodinieri in the middle anddownhill slopes significantly increased the micro aggregates by 1-0.25 mm,with a maximum increase of 143.29%,andOphiopogon bodinieri andViciavillosaRothvar are better than Orychophragmus violaceus in improving the basic physical and chemical properties of soil.The same terrace hedgerow,ViciavillosaRothvar can significantly improve the soil bulk density,capillary porosity,and the aggregation status anddegree of soil micro aggregates in the middle slope,Ophiopogon bodinieri can significantly improve the natural water content of downhill and the content of water stable aggregates and the degree of aggregation of soil micro aggregates in the middle slope root area which are more than 5mm and 5-2mm.In addition,the three terrace hedgerows can significantly improve the soil organic matter in the middle slope root area,among them,Ophiopogon bodinieri is the most significant,and the content of soil organic matter in its root area is 2.7 times of that in bare land.(3)The root system in the complex was found,the same slope,the cellulose content of Ophiopogon bodinieri andViciavillosaRothvar in the upper slope root system was significantly higher than that of Orychophragmus violaceus,the lignin in the middle slope and the cellulose in the lower slope are the largest in ViciavillosaRothvar,The lignin in the middle slope and cellulose in the lower slope are the largest in ViciavillosaRothvar,In addition,the root surface area and root volume of the three terrace hedgerows were Ophiopogon bodinieri,which were dominant in each slope,the total root surface area of Ophiopogon bodinieri is 2.33 times and 1.70 times of Orychophragmus violaceus andViciavillosaRothvar respectively;root morphology of different diameter grades of each slope is shown as follows,the proportion of ViciavillosaRothvar is the largest in the range of 0 <D ≤ 1mm,andOphiopogon bodinieri andOrychophragmus violaceus are the largest in the range of 1 <D ≤ 2mm andd> 2mm.Same terrace hedgerow,the total root length,total root volume and lignin inViciavillosaRothvar complex were the largest in the middle slope,The total root surface area,total root volume,total root tip number,root surface area within 0 <D ≤ 1mm and root length within d> 2mm of Ophiopogon bodinieri were also the largest in the middle slope,However,the cellulose content of root system in Orychophragmus violaceus complex was significantly higher in the middle slope anddownhill than in the uphill,1.40 times and 1.37 times of the uphill respectively,the ratio of hemicellulose to wood fiber and the root volume and root surface area in the range of 1 <D ≤ 2 mm were the largest in the uphill,the root surface area in the range of 0 <D ≤ 1mm is the largest downhill.(4)According to Pearson correlation analysis and principal component analysis,the shear resistance of three kinds of terrace hedgerows complex is closely related to root indexes(total root length,total root surface area,total root volume,cellulose,hemicellulose),the highest correlation coefficient is 0.99,Only a small amount of soil indicators(soil organic matter,soil aggregation degree and aggregation status)had an impact on it,and the root system of terrace hedgerow mainly affects the shear resistance by affecting the cohesion of the complex,among them,the shear resistance of Orychophragmus violaceus is closely related to root morphology indexes(total root length,total root surface area,0 < SA ≤ 1,1 < SA ≤ 2,1 < V ≤ 2)and soil indexes(bulk density,organic matter),the shear resistance of ViciavillosaRothvar is closely related to root morphology index(cellulose,hemicellulose,1 < V ≤ 2)and soil index(organic matter,degree of agglomeration,agglomeration status),the shear resistance ofOphiopogon bodinieri is closely related to root morphology index(total root surface area,2 < L,0 < SA ≤ 1,cellulose,1 < SA ≤ 2)and soil index(PAD,natural moisture content,capillary porosity)... |
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