Research On Evaluation Of Salt Reduction Effect Of Plants And Their Enhancement Of Slope Stability In Loess Region Of Northeastern Qinghai-Tibet Planteau

In this study,six dominant herbs of Medicago sativa L.,Elymus nutans Griseb.,Agropyron trachycaulum Linn.Gaertn.,Achnatherum splendens(Trin.)Nevski,Poa pratensis L.,Puccinellia distanx(L.)and four dominant shrubs of Caragana korshinskii Kom.,Zygophyllum xanthoxylon Bunge Maxim.,Nitraria sphaerocarpa Maxim.,Lycium barbarum L.were selected as the test subjects in Xining Basin.Based on salt stress test using Na₂SO₄solution,the effects of salt solution on growth and physiological of herb and shrub species as well as their salt tolerance were analyzed and evaluated.The effects of Na₂SO₄stress on mechanical properties of roots were also analyzed via single tension test after Na₂SO₄stress test.The salt reduction effect of herb and shrub species and their combinations on saline soil were assessed via field plot planting test in self-established test area of Changling ditch in the Xining Basin.On the basis of field plot planting test,the relationship and interaction mechanism between salt content,moisture content,root content,root mass ratio of herb and shrub species and shear strength indexes of root-soil composite system of combination of herb and shrub species were analyzed via direct shear test and triaxial compression test of undisturbed and remolded root-soil composite system of combination of herb and shrub species respectively,and a multivariate regression model was established between salt content,moisture content,root content,root mass ratio of herb and shrub species and shear strength indexes of root-soil composite system of combination of herb and shrub species.Meanwhile,the effects of plant roots on improving slope stability under the rainfall were assessed via simulated rainfall test and numerical simulation calculation.Through the above research,the following conclusions are as follows.(1)Under the Na₂SO₄ stress,the four growth indexes of plant height,ground diameter,root length and dry weight of three herbs(E.nutans,A.trachycaulum and M.sativa)and three shrubs(C.korshinskii,Z xanthoxylon and L.barbarum)showed a decreasing trend with the increase of Na₂SO₄ level.The chlorophyll content of these plants increased first and then decreased with the increase of Na₂SO₄ level.The electrical conductivity,malondialdehyde,proline,superoxide dismutase,catalase and peroxidase of leaves of these herb and shrub species increased with the increase of Na₂SO₄ level.A.trachycaulum and L.barbarum have the relatively stronger salt tolerance than other herbs and shrubs.(2)When the growth period is 1 a,the total salt content of soil in the depth of0?20 cm below the surface is significantly reduced by the four herbs(E.nutans,A.trachycaulum,A.splendens and M.sativa)and four shrubs(C.korshinskii,Z xanthoxylon,L.barbarum and N.sphaerocarpa).M.sativa and C.korshinskii have a relatively stronger salt reduction effect than other herbs and shrubs.The total salt content of soil in the depth of 0?40 cm below the surface is significantly reduced by the combinations of herb and shrub species,combination 1(M.sativa+A.trachycaulum+A.splendens+C.korshinskii)and combination 5(M.sativa+E.nutans+P.pratensis+P.distanx+C.korshinskii)have the relatively stronger salt reduction effect than others.(3)Under the Na₂SO₄ stress,the single root tensile force and single root tensile strength of three herbs(E.nutans,A.trachycaulum and M.sativa)and three shrubs(C.korshinskii,Z.xanthoxylon and L.barbarum)decreased and increased with the increase of Na₂SO₄ level respectively.Under the Na₂SO₄ stress,the single root tensile force and single root tensile strength of three herbs(E.nutans,A.trachycaulum and M.sativa)and three shrubs(C.korshinskii,Z.xanthoxylon and L.barbarum)showed an increase following a exponential function and a decrease following a power function as the root diameter increased respectively.The cellulose content of roots of different herb and shrub species showed significant correlations with Na₂SO₄ level,root diameter and indexes of single tensile mechanical properties,followed by root lignin content.(4)Six kinds of combinations of herb and shrub species have significantly effects in increasing cohesion in the soil in the depth of 0?30 cm below the surface.According to the results of the triaxial compression test,it can be concluded that under the other three factors remaining constant,the cohesion of soil without roots and root-soil composite system samples showed a tendency of initial increase and then decrease with the increase in salt content,and their internal friction angle showed a decreasing trend.With the increase of moisture content,the cohesion and internal friction angle of soil without roots and root-soil composite system samples decrease gradually.With the increase in root content,the cohesion of root-soil composite system samples increases gradually,while their friction angle increases exhibits an initial increasing and then decreasing trend.With the increase in root mass ratio of herb and shrub species,the cohesion of root-soil composite system samples exhibits an initial increasing and then decreasing tendency,and their internal friction angle increased gradually.(5)After simulated rainfall,the moisture content of plant slope and bare slope increased significantly in the depth range of 0?50 cm and 0?20 cm respectively.With time elapsing,the moisture content of each layer of plant slope and bare slope decreased gradually.After simulated rainfall,the salt content of plant slope and bare slope decreased in the depth range of 0?50 cm and 0?20 cm respectively.With time elapsing,the salt content of plant slope and bare slope soil decreased first and then increased.(6)Before and after simulated rainfall,the bare slope have relatively significant horizontal and vertical displacement compared with the plant slope.Before the simulated rainfall,the shear strain increment zone and the shear plastic zone emerged at the shallow soil of the bare slope,but the same phenomenon did not emerge at the plant slope.After simulated rainfall,the area of zone of shear strain increment and shear plastic distribution increased.Before and after simulated rainfall,the safety factor of plant slope and bare slope exhibit an initial decreasing and then increasing tendency,and in different time,the safety factor of plant slope is relatively higher than that of bare slope.It implied that plantation of plants on slope has a significant effect on improving the stability of shallow soil.The results of this study will provide a theoretical support to further explore the salt tolerance mechanism of plant planted in Na₂SO₄ influenced circumstance and the salt reduction effect of plant as well as the effects of roots of combination of herb and shrub species enhancing soil shear strength and improving slope stability in arid semi-arid region.At the same time,it has a guiding significance and practical application value for scientific and effect in preventing and controlling of geological hazards such as land salinization,slope soil erosion and shallow landslides in the study area and areas with similar natural conditions in the area.