The Changes In Properties Of Disturbed Soil Along Freeway And Its Effects On Vegetation Restoration

In recent years, with the transnormal development of freeway construction, the vegetation reconstruction and recovery of disturbed soil have become an urgent problem in roadside ecological environmental protection. Due to human disturbances, roadside soil and vegetation were disturbed and damaged in the construction process of freeway. Before the recovery of destroyed soil and vegetation, natural properties of soil disturbed by freeway construction should be fully understand firstly, so that the appropriate scheme of vegetation reconstruction and recovery should be formulated according to principles of "adjusting measures to local conditions" and "selection in line with the principle of trees" .Only in this way, vegetation reconstruction and recovery should play a sufficient role to keeping ecological balance, steadying the slop, preventing and controlling soil erosion, beautifying the environmentIn this article , according to the actual needs of freeway greening, the physical and chemical characters of disturbed soil in roadside region of Baoji-Niubei freeway and vegetation on its surface were analyzed ,levels of disturbed soil nutrient and effects on vegetation reconstruction and recovery were estimated.Then some specific advices put forward for soil improvement and vegetation econstruction.(1) Disturbed soil texture is directly related to the natural soil.The natural soil along Baoji-Niubei freeway is loam. So soil in different location of roadside is aslo loam, where the human disturbances are only disrupted the original soil structure, not change the soil texture, except abandon dreg field. However, there are natural soil and lots of construction waste and gravel from tunnel excavation in abandon dreg fields, so the soil texture changed fundamentally. According the investigation, the gravel content of soil in abandon dreg field of Baoji-Niubei freeway are 30%~60%.(2) Analysis of soil nutrients showed that, soil organic matter and total nitrogen of nearly most disturbed soil belonged to "very low" or "low" levels according to soil nutrient classification in the Loess Plateau. of soil organic matter, total nitrogen and inorganic nitrogen, the results indicated that there are positive correlation soil total nitrogen, inorganic nitrogen and organic matter by partial correlation analysis. Soil organic matter is the basis of soil nitrogen. It is an important role to improve soil organic matter for improving soil fertility.(3) Statistics of the number of species appeared in sample showed that, vegetation in roadside region of Baoji-Niubei freeway had certain species diversity. Natural invasive species almostly appear in different position, natural invasive species were mainly pioneer species and farmland weeds.The natural vegetation coverage of investigated abandon dreg field was low basically. One reason of natural invasive could be that Baoji-Niubei freeway is located in northern piedmont of QinLing mountain, where natural species are abundant. Natural invasion species are beneficial for forming artificially thickets layer structure, establishing stable community and improving soil conditions. However, the disturbed soil is poor; so the natural invasion is very slow and limited,which is hard to satisfy people on the required rate of vegetation reconstruction and recovery.(4) Analysis of soil physical and chemical properties and the ground vegetation biomass in abandon dreg fields showed that, the nutrient content of soil organic matter and total nitrogen was scarcity. There were larger differences in recovery status of abandon dreg field between condintion of covering natural soil and no covering natural soil . If the content of gravel in abandon dreg field soil was more than 30%, the impacts of covering natural soil on vegetion recovery were more obvious than not turn the soil. This displays not only the number of species but also the biomass were better than the condition of no covering natural soil. It is necessary to grow Leguminous plant such as Medicago sativa in abandon dreg fields for improving soil conditions and providing well growth environment (improve soil fertility)for natural invasive species. Therefore it is suggested that most when the content of gravel is more than 30% , the abandon dreg field should coveering soil, and planting legumes grass or shrubs to improve soil fertility at the same time.(5) Amorpha fruticosa is the most widely used shrubs in slope protection of Baoji-Niubei freeway. Amorpha fruticosa is survival and grows well in various protection modes. The best slope protection mode for Amorpha fruticosa growing is vaulted skeleton and geocell form. Native species have invaded in Amorpha fruticosa shrub and form living ground layer. The effect shrub and grass communities structure on soil and water conservation is better than that of simply shrubs or pure herb structure, whose community structure is more stable. Analysis of the growth of Amorpha fruticosa showed that,oversize density could affect the invasion of ground herb species, which is not conducive to the formation of living ground layer, so it is necessary to control the density of Amorpha fruticosa to promote herb invasion and make it as soon as a complex layer structure. The technical measure in the process of planting Amorpha fruticosa with is control density on purpose and sowing legume herbs such as Medicago sativa at the same time, for the purpose of rapid improvement in soil fertility and promoting the invasion of herbs to create a stable community structure. Due to the investigation results, the appropriate canopy density to control is about 0.6.