The Effects Of Terrace Hedgerows Roots On The Erodibility Of The Root-soil Complex In The Purple Soil Area

The purple soil area is due to its fast-growing but easily eroded parent material,and the climate and geological background of rainfall and hilly areas,coupled with unreasonable land reclamation and planting patterns,resulting in serious and severe soil and water erosion problems on sloping farmland.The slope-to-terraced project has created a large number of cropland bunds,and the emergence of cropland bunds has increased the utilization rate of sloping farmland and reduced soil and water erosion.In order to stabilize the cropland bunds and cope with the water and soil conservation function of the cropland bunds,the " cropland bunds + hedgerows " governance model has found a balance point for this contradiction.In order to explore the soil and water conservation value of hedgerows on sloping farmland in purple soil,and the effects of terrace hedgerows roots on the erodibility of the root-soil complex,we planted two grass hedgerows(Dolichos lablab L.and Medicago sativa L.)in the Southwest University,Chongqing.Quantitatively study the differences in root morphology,chemical composition contents,tensile mechanical properties,soil physical and chemical properties,soil anti-erodibility characteristics and root-soil complex shear performance and anti-scourability of two terrace hedgerows.The main factors affecting the shear performance and anti-scourability of the root-soil complex were screened out through principal component analysis,the influence of root system on the cropland bunds is analyzed.The main conclusions are as follows:(1)More than 80% of the root system length is concentrated in the diameter class of 0.0<d≤2.0mm,the change law of root system length with diameter can be expressed by exponential function.The resulting characteristic diameter and the average diameter meet a linear relationship,and the characteristic diameter has a very significant negative correlation with the bifurcation number,a significant positive correlation with the root volume and wood to fiber ratio,and a significant negative correlation with the number of branches and cellulose content.Both can be fitted with an exponential function.In 2019,the number of branches,cellulose and lignin content,and tensile mechanical properties index of the D.lablab hedgerow were significantly greater than that of the M.sativa hedgerow.In 2020,the root surface area,root volume,characteristic diameter,ultimate elongation and additional cohesion of the M.sativa hedgerow are significantly larger than those of the D.lablab hedgerow.Compared with the roots of grass hedgerows,the root morphology and chemical composition contents indexes of the M.sativa and D.lablab complexes showed more significant differences,but the size relationship was basically the same.(2)M.sativa and D.lablab hedges have significant effects on the bulk density,porosity,water holding capacity,organic matter,aggregation degree of micro-aggregates,dispersion coefficient and erodibility factor of the root zone soil.In the early stage of growth,D.lablab has more significant improvement effects on soil physical and chemical properties and anti-erodibility.After one year,the improvement effect of M.sativa is more significant.The indexes affecting the physical and chemical properties of the soil in the root zone are ranked as follows: root tensile mechanics index> root morphology index> root fiber content.The prominent indexes are tensile strength,characteristic diameter,root volume and cellulose.The indexes affecting soil anti-erodibility of the root zone are in the order of correlation: root morphology index> root tensile mechanical properties index> root fiber content.The prominent indicators are root length,number of root tips,ultimate tensile strength and ultimate extension rate.(3)The root system can significantly improve the shear performance and safety factor of the cropland bunds.In the early stage of growth,the roots of D.lablab have better effects on the internal friction angle and cohesion than M.sativa,and the cohesion increase by 71.06%.After 1 year,the M.sativa root system has a better effect on improving cohesion.The five types of indexes such as root system and soil have a high correlation with cohesion.The root tensile mechanical properties are the most prominent,and they are significantly or extremely significantly related to cohesion.Only the geometric mean diameter of the micro-aggregates has a significant correlation with the angle of internal friction.Principal component analysis shows that among the above five types of indicators,the order of influence is as follows: soil moisture > ultimate elongation> root surface area density>cellulose> dispersion coefficient.In 2019,the comprehensive score of the shear resistance of the D.lablab was higher,and in 2020,the M.sativa was higher,but there was no significant difference.(4)The root system can significantly increase the anti-scourability of the root-soil complex in the early growth period,especially for the D.lablab,the maximum impact index is 2.60 times that of the control,but the improvement effect is no longer significant after 1 year.The anti-scourability resistance index and the anti-scourability resistance index change values are significantly or extremely significantly positively correlated with the scouring time,and the logarithmic function and the quadratic function respectively have the best fitting results.In addition to the physical and chemical properties of the soil,the root morphology index,fiber content,tensile mechanical properties index and soil anti-erodibility index have a high correlation with the impact index and the change rate of the impact index,and the soil anti-erodibility index is the highest.The highest is 0.74.Principal component analysis shows that among the above five types of indicators,the order of influence is organic matter>root bulk density> ultimate tensile force > dispersion coefficient > hemicellulose.In2019,the comprehensive score of the impact resistance of the D.lablab was higher,but there was no significant difference,and in 2020,the M.sativa was higher,and there was a significant difference.To sum up,in the early growth period,the root system of the D.lablab hedgerows develops fast and has more fine roots and good tensile performance,which makes the root-soil complex have strong shear performance,anti-scourability and the factor of stability of cropland bunds is significantly improved.After one year of planting,the development of the the M.sativa hedgerow’s root system has gradually improved,with thick roots,the shear performance of the root-soil complex and the factor of stability of cropland bunds have no significant difference with that of the D.lablab hedgerow.It can be seen that the short development cycle of the D.lablab hedgerow can achieve remarkable results in soil consolidation and protection of cropland bunds in a short period of time.the M.sativa hedgerow has a long development cycle,but in the long run its soil stabilization and protection of cropland bunds will become more effective.Each has its own advantages and disadvantages,and suitable grass hedgerows can be selected according to different needs.