Study On Topsoil Desiccation And Revegetation On Loess Roadcuts In The Semi-arid Loess Region

Exposed,steep earthy roadcuts are prone to soil erosion in the semi-arid loess region of China and are difficult to revegetate.Therefore,Wuqi County located in the semi-arid loess region of China was selected as the typical study area,and exposed,steep earthy roadcuts(>45 degrees)in Wuqi County were selected as the research object in this study.Combined with trend-free pre-whitening Mann-Kendall test,Spearman's rho test,etc.the trend of dry wet change and erosive rainfall in the study area were analyzed;the distribution characteristics and erosion status of the steep roadcuts in the study area were investigated,this confirmed the necessity of vegetation restoration for steep earthy roadcuts.In addition,the topsoil desiccation phenomenon which is a limiting factor was explored based on the soil water characteristic curve.Finally,on the basis of a series of studies,two methods(spray seeding using erosion-resistant fibers and deep planting of container seedlings)of revegetation suitable for the steep earthy roadcuts were proposed.This study can provide the theoretical basis and method support for the revegetation on steep earthy roadcuts in the semi-arid loess region.The results show that:(1)The annual precipitation in Wuqi County form 1960 to 2016 had a 10.82 mm/10a reduction.Almost no change in the frequency of erosive rainfall,but the total amount of erosive rainfall increased by 2.86 mm/10a,so the proportion of erosive rainfall in annual precipitation increased,and the erosive force of single rainfall has been enhanced that during the study period.Using standardized precipitation evapotranspiration index(SPEI)as index of drought evaluation,this region is currently associated with a drought trend,and the drought degree will likely continue to increase.(2)The erosion degree of typical roadcuts with the formation time from 1 to 25 years were extremely serious.Among them,the annual average erosion modulus of the roadcuts with the formation time of 6 years was the highest,reaching 1.53 × 105 t/km2·a,and the total erosion modulus of the other roadcuts were between 5-6 × 104 t/km2·a.Among 3 erosion units of the earthy road(roadcut,roadfill,road surface),expect the highest erosion intensity of road surface in the year of completion,the erosion intensity of the roadcut was the highest in the rest of the year.(3)Topsoil desiccation occurs on roadcuts and persists after the rainy season.The thickness of the topsoil desiccation layer was influenced by slope aspect(solar radiation),and gradually increase with increasing formation time and slope gradient of the roadcuts,but after the formation time of the slope reaches a certain level,the thickness of the topsoil desiccation layer on the slope may approach to a certain thickness,and the thickness is no more significantly increasing.This thickness is probably less than 40 cm.(4)The erosion-resistant fibers(HP-FGM)used for the spray seeding can effectively reduce slope erosion and overcome the limitation associated with topsoil desiccation layer of roadcuts on the growth of plantat at a dosage of 400 g/m2,when combined with a slope surface roughening treatment,it can effectively promote the vegetation restoration on the steep roadcuts over 50 degrees or about 60 degrees.Using the method of deep planting container seedlings(the depth of the planting hole was set to approximately 40 cm),plant roots can completely penetrate the topsoil desiccation layer.Above 45 degrees,the selected native species(Caragana korshinskii Kom.and Tamarix chinensis Lour.)and introduced species(Caryopteris clandonensis 'Worcester Gold')all survived,both the survival rate and the preservation rate maintained high levels(greater than 93%).The erosion resistant fiber spraying method is simple and efficient in construction,and the cost is mainly to purchase spraying machine and spray sowing matrix,which is less suitable for roadcuts above 60 degrees.The main cost of the method of deep planting container seedlings is human consumption and seedling purchase,which is less than erosion resistant fiber spraying method,it has strong adaptability to slope grade.It can achieve good revegetation effect on roadcuts of 45-80 degrees.