Study On The Root Tensile Properties And The Soil Shear Properties Of Typical Tree Species In North China

Plant root system is one of the determining factors of the shear properties of root-soil matrix which is the foundation of the research of soil fixation and slope stabilization by vegetation. In order to further and better understand the mechanisms of the tensile properties of root system and the shear properties of root-soil matrix, taking Betula platyphylla, Quercus mongolica, Pinus tabulaeformis and Larix gmelinii-which are the most prevalent species used for soil and water conservation- in the mountainous area of Northern China, this thesis aims at exploring how root moisture content, gauge length and strain rate influence the tensile properties of root system, and the relationship among root chemical composition, tensile properties and diameter. In addition, the soil shear properties, physical and chemical characteristics and root distribution in the different soil layers were investigated to explore the mechanisms by which these three factors influence each other. The main results of this thesis are as below:(1) Root moisture content influenced the tensile properties significantly; a certain amount of moisture loss could improve root tensile strength while too much moisture loss could reduce root tensile strength sharply; the little difference in root moisture content within one month could be ignored.(2) In general, the effect of strain rate on root tensile strength was insignificant, but strain rate could reduce the effect of root diameter and gauge length on the tensile strength, namely, the effect of root architecture on slope stabilization would decrease as landslide rate increasing. Root gauge length was negatively linearly correlated to the tensile strength at the strain rate of 10 mm/min, while the degree of correlation was not strong.(3) Root diameter was positively correlated to the root tensile resistance following a power function, while the influence of root diameter on the tensile strength was not stable (negative correlation or insignificant influence).(4) For Betula platyphylla and Quercus mongolica, the percent contents of cellulose and hemicelluloses significantly decreased, while the percent content of lignin and lignin/cellulose ratio significantly increased as root diameter increasing; root tensile resistance decreased with the increase of the percent contents of cellulose and hemicelluloses, and increased with the percent content of lignin and lignin/cellulose ratio; root tensile strength was positively correlated to the percent contents of cellulose and hemicellulose and negatively correlated to the percent content of lignin and lignin/cellulose ratio. For Pinus tabulaeformis and Larix gmelinii, the responding relationship among root diameter, tensile resistance and chemical compositions were opposite to those of Betula platyphylla and Quercus mongolica, while the effect of the designated chemical compositions on the tensile strength was little.(5) The differences of the chemical compositions and the tensile properties among different species were significant; the descending order of root tensile strength and the percent content of cellulose was: Betula platyphylla, Quercus mongolica, Larix gmelinii and Pinus tabulaeformis; the percent content of cellulose and tensile strength of the broadleaved species were much greater compared to the coniferous species. The tensile resistance of all four species and the tensile strength of Betula platyphylla and Quercus mongolica could be well explained by the percent content of cellulose linearly.(6) As soil deepening, the root dry matter decreased, soil unit weight increased, air and water permeability decreased, and the respective contents of organic matter, total nitrogen and total phosphorus decreased; the descending order of the shear strength of each soil layer was:20-40 cm,0-20 cm and 40-60 cm; while for plain soil, the shear properties improved as soil deepening; soil physical and chemical characteristics and shear properties were better in the soil of Larix gmelinii forest compared to plain soil; soil physical and chemical characteristics and root development and distribution influenced each other and influenced soil shear properties in each soil layer.