Effects Of Geometric Configuration And Mechanical Characteristics Of Typical Herbaceous Roots Systems On Soil Detachment Process On The Loess Plateau

The Loess Plateau is one of the most serious areas of soil erosion in China,and since the implementation of the "Grain for Green" project in 1999,significant progress has been made in vegetation restoration,leading to effective control of soil erosion.In the process of vegetation restoration,whether in afforested areas or abandoned grasslands,the role of plant root systems in soil protection and the reduction of soil erosion has been widely recognized.The influence of root systems on soil erosion mechanisms during the process of vegetation restoration has become a current research focus.In this study,eight typical grasslands on the Loess Plateau were selected for research,including Artemisia capillaris Thunb.,Artemisia argyiLevl.Et Vant.,Artemisiavestita Wall.exBess.,Leymussecalinus（Georgi）T zvel.,Stipabungeana Trin.,Bothriochloa ischcemum（Linn.）Keng.,Medicagosativa L.,Astragalu sadsurgens Pall.By utilizing root tracking methods,pullout tests,direct shear tests on rootsoil composites,and flume erosion experiments with varying slopes,a systematic exploration was conducted into the root system architecture,geometric characteristics,biomechanics,shear resistance of root-soil composites and seasonal variations in dominant community species within typical grasslands.It was performed to quantitatively reveal the interplay between root system architecture,mechanical attributes,and the capacity to resist soil detachment.Additionally,a predictive equation for soil detachment based on root characteristic parameters was constructed.This equation clarifies the mechanism through which plant root systems contribute to soil consolidation,with the aim of providing a scientific basis for optimizing vegetation allocation and sustainable management on the Loess Plateau.The key findings are as follows:（1）The variations in topological indices,connection angles,connection lengths,primary branching rates,and secondary branching rates characterizing the root system topology of grassland plants on the Loess Plateau were within the ranges of 0.59 to 0.99,0.66 to 4.96,38.33° to 77.17°,2.72 to 15.56,and 5.18 to 13.00,respectively.Stipabungeana Trin.,Artemisiavestita Wall.Ex Bess.,and Bothriochloa ischcemum（Linn.）Keng.exhibited complex root structures with higher topological indices.Leymussecalinus（Georgi）T zvel.demonstrated a strong resistance to nutrient scarcity and drought,showing the highest average connection length,connection angle,primary branching rate,and secondary branching rate.Stipabungeana Trin.,Artemisiavestita Wall.Ex Bess.,and Bothriochloa ischcemum（Linn.）Keng.had larger fractal dimensions,ranging from 1.03 to 3.56 times that of other grass species.Meanwhile,Medicagosativa L.and Astragalusadsurgens Pall.possessed relatively high root density,approaching approximately 0.1.The ratio of the maximum to minimum valued for root geometric characteristics,including biomass density,diameter,root length density,surface area density,volume density,and specific root length,ranges from 10.71 to 209.53.Stipabungeana Trin.,Artemisiavestita Wall.Ex Bess.,and Bothriochloa ischcemum（Linn.）Keng,which exhibited more complex root topology,showed relatively higher biomass density,root length density,surface area density,volume density,and specific root length ratio.Leguminous plants,on the other hand,had fewer root branches,resulting in higher topological indices and lower fractal dimensions compared to plants from the Asteraceae and Poaceae families,with ratios ranging from 1.24 to 4.73 times.Members of the Poaceae family exhibited higher fractal dimensions,root biomass,root length density,surface area density,and specific root length ratio,each being 1.33 to 2.89 times greater than those in the Leguminosae and Asteraceae families.（2）Weighted shear strength is a key indicator of soil mechanical properties,primarily influenced by soil characteristics,root system geometric features,and biomechanical properties.The shear strength of root-soil composites can be characterized by topological indices,root surface area density,root tensile strength,and water-stable aggregates.In the Poaceae family,the weighted shear strength,cohesion,and internal friction angle were 1.08 to 1.99 times that of the Asteraceae and Leguminosae families,indicating a stronger capacity for enhancing soil stability.The leguminous plants exhibited the highest root cohesion,surpassing the Poaceae and Asteraceae families by factors of 3.72 and 5.18,respectively.The ranges of weighted shear strength,cohesion,internal friction angle,and root cohesion for the eight typical grasslands were 7.59 to 11.20 kPa,1.27 to 3.58 kPa,7.24° to 10.41°,and 0.06 to 0.59 kPa,respectively.Astragalusadsurgens Pall.grassland exhibited relatively lower soil shear strength,cohesion,and internal friction angle.（3）Herbaceous plant root systems can significantly reduce soil detachment and enhance soil resistance to erosion.Compared to bare land,grassland exhibited a reduction in soil detachment capacity and rill erodibility by 1.97%to 98.59%and 25.35%to 97.22%,respectively,while critical shear stress increased by 16.21%to 13,990.20%.There were significant differences in soil detachment capacity,rill erodibility,and critical shear stress among the eight typical herbaceous plants on the Loess Plateau,with ranges of 0.01 to 8.24 kg m^-2 s^-1,0.015 to 0.411 s m^-1,and 0.050 to 6.059 Pa,respectively.Soil detachment capacity and rill erodibility were highest for Astragalusadsurgens Pall.and lowest for Bothriochloa ischcemum（Linn.）Keng.,with the former being 69.61 times and 27.4 times the latter,respectively.Among the Poaceae species,Stipabungeana Trin.exhibited the highest critical shear force,while Medicagosativa L.grassland had the lowest.With increasing successional years,there was a decreasing trend in soil detachment capacity and rill erodibility,while the critical shear force gradually increased.Leguminous plants displayed relatively higher soil detachment capacity and rill erodibility,with values ranging from 10.61 to 13.07 times that of Asteraceae and Poaceae plants.However,leguminous plants had a lower critical shear force,reducing it by 97.45%to 97.69%compared to Poaceae and Asteraceae plants.（4）Hydrodynamics serve as the driving force behind soil detachment capacity.Soil detachment capacity decreased as a power function（p<0.01）in response to slope flow velocity,shear stress,stream power,and unit stream power.Furthermore,soil detachment capacity decreased as a power function（p<0.01）with soil cohesion,bulk density,water-stable aggregates,and silt content.The configuration,geometry,and root-soil composite shear characteristics of plant root systems significantly affected the soil detachment process.In particular,soil detachment capacity decreased as a power function（p<0.01）with fractal dimension,while it increased as a power function（p<0.01）with topological indices.Additionally,soil detachment capacity decreased as a power function（p<0.01）with root biomass density,root length density,and surface area density.It also exhibited a power-law decrease（p<0.01）with weighted shear strength.（5）Plant root systems were critical factors that influenced rill erodibility.Their root configuration and geometric features contributed significantly to the total rill erodibility,accounting for 87.1%.Among these contributions,root configuration features,geometric features,and the interaction between configuration and geometry contributed 33.6%,15.4%,and 38.1%,respectively,to rill erodibility.The topological characteristics of plant root systems had the most substantial impact on rill erodibility.Furthermore,rill erodibility decreased exponentially or in accordance with power functions（p<0.01）with soil cohesion,bulk density,water-stable aggregates content,organic matter content,soil silt content,fractal dimension,root length density,surface area density,and weighted shear strength,while it increased exponentially（p<0.01）with topological index.（6）During the vegetation growth process,there was a decreasing trend in soil detachment capacity and rill erodibility,while critical shear force exhibited an increasing trend.Different growth stages of Setaria viridis（L.）Beauv.,Artemisia capillaris Thunb.,Bothriochlo aischcemum（Linn.）Keng.and Artemisiavestita Wall.Ex Bess.showed variations in soil detachment capacity,rill erodibility,and critical shear force ranging from 0.11 to 5.01 kg m^-2 s^-1,0.018 to 0.419 m s^-1,and 3.06 to 3.83 Pa,respectively.In the early stages of vegetation growth,soil physical properties were the primary factors influencing the soil detachment process.However,as vegetation rapidly grew,especially by July,the influence of plant root systems on soil detachment significantly strengthened.In this study,quantitative equations were established to relate soil detachment capacity to water flow shear stress（τ）,water-stable aggregates（AG）,weighted shear strength（τ_w）,topological index（TP）,and root system surface area density（RSZD）(D_c=0.05 ×τ^2.14AG^-1.20τ_w^-2.19TP^0.99RSAD^-0.53:R²=0.94,NSE=0.64).Rill erodibility demonstrated power function relationships with soil aggregates（AG）,weighted shear strength（τ_w）,topological index（TP）,and surface area density（RSAD）(K_r=0.56 × AG-0.96τ_w^-0.066TP^1.01RASD^-0.05;R²=0.97,NSE=0.81),and critical shear force showed power function relationships with soil bulk density and specific root length（τc=0.03BD5.26SRL0.34）.