Experimental Study On The Ability Of Root System To Separate Soil Under Different Grass And Shrub Modes

Soil separation is the first stage of erosion process,which provides material basis for runoff and sediment transport.Accurate prediction of soil separation process is an important prerequisite for exploring soil anti-erosion ability.In view of the current situation of severe water erosion in the hilly and gully areas of the northern Loess Plateau,which still needs to further promote the ecological construction strategy,the typical fibrous root system herb Leymus chinensis,straight root herb Artemisia and shrub Caragana Caragana and their composite communities in the Loess Plateau are selected as the research objects respectively.The undisturbed soil samples are collected in the field,and the soil separation test of concentrated water flow is carried out in the laboratory,taking the soil separation rate as the characterization index,Exploring the root distribution characteristics,soil physical and chemical properties and the change characteristics of mechanical properties of root soil complexes under different grass shrub modes,fully understanding the soil consolidation and corrosion resistance of roots under mixed mode,clarifying the distribution mode of different roots in three-dimensional space under mixed conditions and single vegetation conditions,and exploring the corrosion resistance mechanism of different types of root soil complexes play an important role in the selection of plant species,At the same time,it can also provide a solid theoretical basis for vegetation restoration and construction on the Loess Plateau,which is of great significance for the ecological restoration of the Loess Plateau.Through systematic analysis and research,the following conclusions are obtained:(1)By analyzing the root morphology of different grass shrub models at different depths,the spatial distribution and stem level composition of roots were clarified,and the root morphological characteristics of different plant species and the influence of mixed growth on root growth were expounded.Under the condition of single vegetation,the root content of herbaceous plants is significantly higher than that of shrubs.Among them,the root volume density of Artemisia ordosica and the root length density and root surface area density of Leymus chinensis have significant advantages.The root density increases significantly after grass shrub mixed growth,especially the increase of root length density and root surface area density of Caragana + Artemisia ordosica.In addition,with the increase of depth,the root density decreases,and the change range of root decreases after mixed growth.Artemisia,Leymus chinensis and Caragana have obvious advantages in the diameter range of d >3mm,1mm< D ≤3mm and 0mm< D ≤1mm respectively.With the mixing of grass and shrub vegetation,the proportion of d>4mm is significantly lower than that of herbaceous plants,and the proportion of d>2mm is higher than that of Caragana.The difference is that the proportion of Caragana+Artemisia tenuifolia decreased in the range of very fine roots,increased in the range of medium diameter,increased in the range of low diameter,decreased in the proportion of medium diameter,and decreased in the proportion of d>1mm.(2)The change law of soil physical and chemical properties under different grass and shrub modes was clarified,and the influence mechanism of root system on different physical and chemical indexes was emphatically discussed.Under the condition of single vegetation,the organic matter content and sand content of Artemisia officinalis were the highest,the clay content,powder content,average weight diameter and > 0.25 mm aggregate content of Aries were the highest,and the soil bulk density of Caragana was the highest.The mixed growth of grass and shrub had the most obvious effect on the improvement of aggregate characteristics and shear strength characteristics.After the mixed growth of Caragana and Artemisia officinalis,the content of soil clay and silt increased significantly,the content of sand decreased significantly,and the change of soil organic matter and soil bulk density was not significant.After the mixed growth of Caragana and Aries grass,the content of organic matter increased significantly,the soil bulk density decreased significantly,and the change of mechanical composition was not obvious.The addition of roots significantly increased the shear strength of soil,the effect of shrub roots was significantly stronger than that of herbaceous plants,and the shear strength of soil was the largest under the mixed growth mode of grass and shrub.The characteristics of soil organic matter and aggregates were positively correlated with root indexes,and the soil bulk density was negatively correlated with root density.In the presence of Artemisia officinalis,the root content was negatively correlated with soil clay,while in the presence of Leymus chinensis,it was positively correlated.In addition to sand content,soil basic physical and chemical indexes and root indexes were positively correlated with weighted shear strength.Under the condition of single vegetation,the correlation between soil organic matter and root length density is the highest,and the root weight density is after grass shrub mixed growth.The two aggregate indexes of Caragana and Artemisia officinalis had the strongest correlation with root weight density,while the root length density and root surface area density of Leymus chinensis were respectively.After mixed growth,the correlation between root weight density and average weight diameter was the highest,while the correlation between root surface area density and water stable aggregate content >0.25 mm was the highest.There was no significant relationship between soil mechanical composition and root characteristics,but the correlation between soil bulk density and root density was the strongest.Soil clay content,silt content,average weight diameter and > 0.25 mm water stable aggregate content mainly increase soil shear strength by increasing soil cohesion.Soil organic matter,soil bulk density,root weight density,root length density,root surface area density and root bulk density mainly increase the shear strength of soil by increasing the occlusal friction between soil particles.(3)The variation law of hydrodynamic indexes under different flow and slope conditions is clarified,and its response mechanism to soil separation capacity is explored.Finally,the optimal hydrodynamic indexes are selected.Hydrodynamic parameters increase with the increase of flow and slope.The flow velocity,flow depth,Reynolds number,flow power and unit flow power increase significantly with the increase of flow.The changes of flow shear force,flow power and unit energy of discharge section are less significant,but the change of Froude number is not significant.Flow velocity,Froude number and flow power change significantly with the change of slope,while the changes of flow depth and flow shear force are less significant,while the changes of Reynolds number,unit flow power and unit energy of discharge section are not significant.The soil separation rate has a linear positive correlation with the water flow shear force,water flow power and unit water flow power,while it has a power function positive correlation with the unit energy of the water flow section.Among the four hydrodynamic parameters,the fitting effect between the water flow shear force and the soil separation rate is the best,which can be considered as the best hydrodynamic index to characterize the soil separation ability.(4)The influence mechanism of soil and root factors on soil separation capacity was clarified,and the estimation method of soil separation rate was put forward on the basis of hydrodynamic influence.The correlation results showed that the root indexes such as root weight density,root length density,root surface area density and root volume density were significantly negatively correlated with the overall soil separation rate.The root length density in the range of 1<d<3mm was more representative,and the relationship between them was in the form of power function.The soil separation rate has an obvious negative correlation with the soil physical and chemical properties represented by the average weight diameter,>0.25 mm water stable aggregate content,organic matter content and soil bulk density,and the three soil mechanical indexes of cohesion,internal friction angle and weighted shear strength.The correlation of soil mechanical indexes is stronger than that of soil physical and chemical properties,and the weighted shear strength is the best soil factor to describe the soil separation rate,The shear strength and soil separation rate show an obvious power function.The soil separation prediction model is established by using the analysis method of multivariate nonlinear fitting method for the known hydrodynamic conditions and screened root and soil factors.After verification,it has a certain fitting accuracy and can provide some support for the prediction of soil separation ability.