Effect of Different Cutting Frequencies on Biomass Accumulation and Root Vertical Distribution of Atriplex canescens(Pursh) Nutt. Response of Root Spatial Pattern and Growth Characteristics of Switchgrass to Soil Saline Type Difference of Fine Root Vertical Distribution of Robinia pseudoacacia under the Different Climate Regions in the Loess Plateau Regulative Effect of Optimized Cultivation Practice to the Root Vertical Distribution and Activity in Winter Wheat