The Mechanical Properties And Its Main Influencing Factors Of Herb Hedgerow Roots Under Karst Slope Environment

It's difficult to maintain water and soil in karst area,due to the unique geological and climatic environment and the complicated mechanism of water and soil loss.At present,the acdemic field generally agreed that vegetation is one of the key factors for soil and water conservation and ecological restoration in karst valley areas.And hedgerow which is a sort of biological measure commonly used in control water and soil loss is espaecially suitable for karst slopes.The ridge hedgerow is a special kind of hedgerow,which is setting at the ridge of plots.In addition to the function of slowing down runoff and blocking sediment,the firm and tenacious root system can also strengthen soil,and its roots and stems contribute to strenthening soil,preventing soil leakage and reducing flow and sediment,respectively.However,on account of lacking basic research on hedgerows in karst areas of China,they cannot be scientifically and widely promoted by now.The research was carried out in a karst slope in Longtan valley at Youyang County,Chongqing.Three species were choosed,Ophiopogon bodinieri,and two leguminous hedgerows Vicia villosa,and Dolichos lablab L.They were lay out at up-,mid-,and down-slopes.Root morphology,and fiber content were determined,root tensile force properties and shear strength of root-soil matrix were tested,then analyzised the major factors affecting tensile mechanical properties of root and shear strength of root-soil matrix.The dominant factors affect shear strength of root-soil matrix were known through principal component analysis,so the shear strength of root-soil matrix for different species could be evaluate.Results showed that:(1)Most of the root morphology indexes values of the whole root and root in root-soil matrixs were highest in O.bodinieri.Root morphology parameters of mid-and down-slope were significantly higher than those in upslope,but for roots in root-soil matrixs of D.lablab L.parameters in upslope were significantly higher than those in mid-and down-slope.The dominant diameter class of the three hedgerow root systems was 0.0-1.0 mm.Root surface area and root volmute were especially high in O.bodinieri,and root length performed best in D.lablab L.,root tips,root forks and root cross have significant difference interspecies in part of slope protions.The lignin content(L)of whole root for all species were ranked as: V.villosa> D.lablab L.> O.bodinieri.The hemicellulose content(H)for O.bodinieri was significantly higher than that of V.villosa or D.lablab L.,and wood to fiber ratio(WFR)was significantly lower than that of leguminous hedgerows.(2)The success rate of tensile test was 51.29%.After the test,it was known that ultimate tensile force(F)and ultimate elongation(U)of O.bodinieri were the highest among three hedgerows.For D.lablab L.,the tensile strength(T)and Young's modulus(E)were highest,approximately three times to that of O.bodinieri.Among slope positions,F,T and U of O.bodinieri were all better performed at mid-and downslope,for V.villosa T and E were significantly larger at up-and mid-slopes,respectively.The second-order parabolic function can well reflect the characteristics of the measured stress-strain curve of the three roots.And for roots which diameter under 1 mm,the strain increased largely along with the increased stress.(3)The correlation analysize indicated that T was negatively correlated with root diameter,F and U were positively correlated with root diameter.Multiple regression analysis identified negative correlations for the T,E and H contents of O.bodinieri and the T and H contents of V.villosa.For V.villosa roots,U decreased with the increase of the C and L contents but increased with the increase of the WFR.(4)There were significant differences between species in shear strength,while the root-soil matrix under the vertical load of 100 kPa,200 kPa,and 300 kPa,the two leguminous hedgerows perfumed better than that of O.bodinieri,and D.lablab L.had the highest value in maximum shear stress.The shear strength and maximum shear stress were better performed at mid-and downslope for all species.The range of internal friction angle(?) and cohesive force(c)of O.bodinieri and V.villosa were significantly lower(proximately 3 to 9 times lower)than D.lablab L.For the same species,the difference of ? and c between slope portions were ruleless.(5)The correlation analysis showed that shear strength of root-soil matrix was positively affected by RLD,RSAD and RVD,C and WFR.And principle component analysis determined the main contributing indicators were RLD,RSAD,T,U,and E.The comprehensive score indicate D.lablab L.was the highest and the factor of root tensile properties were dominant.O.bodinieri ranked second in comprehensive score,and its root morphological parameters were prominent.As the aspect of slope position,the comprehensive scores of O.bodinieri and D.lablab L.were higher in the middle slope position.