Competitionon Soil Waterand Nitrate Among Different Vegetation Types

Below ground interaction for water among neighboring vegetation were studied in the water and wind erosion crisscross region of the Loess Plateau by field survey. The experiment included four land use types(abandoned land, Caragana land, alfalfa land and farmland) and three replications. The purpose of this study is to investigate water competition among vegetation types by contrast analysis of soil moisture and aboveground biological characteristic. The effects of tree belts on soil nitrate and soil water content of adjacent crop land were also evaluated under field conditions at the oasis-desert ecotone in the Heihe River Basin, China. In the poplar-wheat system, soil nitrate content was measured to a depth of 3m at distance of 2 m(D2), 14 m(D14), 29 m(D29) and 42 m(D42) from the tree row during the cropping season. The results showed that:Both soil water content and the crop yield were increased with distance from the tree row. The grain yields from C2(2 m away from treebelt) to C10(10 m away from treebelt) increased by 51% as compared with 17% from C14(14 m away from treebelt) to C42(42 m away from treebelt), indicating that effects of tree belt on crop growing at least in 10 m.While the nitrate concentrations were increased with decreasing distance from tree rows and increasing with soil depths. Nitrate concentrations in the D2 and D14 where closed to the treebelt increased rapidly with depth in the four measurements. In comparison, the nitrate concentrations of D29 and D42 increased slightly during July and September 2013, while remained constant in April and May 2014.More than 70% nitrate under D2 and D14 were accumulated in the(200-300 cm) soil layer. Whereas the nitrate accumulation of D29 and D42 only represented < 40% of nitrate accumulation in 0-300 cm. In comparison, the nitrate accumulation of four distances in 60-200 cm was only represented about 18% of total nitrate accumulation in 0-300 cm and there was no significantly difference among D2, D14 and D29, D42.Therefore, there was a large increase in nitrate concentrations under D2 and D14 below 2 m, and it may enter groundwater.Soil moisture content in the abandoned land and farmland decreased close to the Caragana land and alfalfa land, Caragana affected soil moisture in adjacent abandoned land by at least6 m in the horizontal direction. Soil water storage from the fourth measured point of Caragana land to the fourth measured point of alfalfa land were all smaller than 400 mm, and soil water storage(360.3 mm) of the second measured point of alfalfa land affected by Caragana and alfalfa was the lowest among all measured points.Caragana near abandoned land and alfalfa near farmland have a higher biological index than the other measured points. This means that these plants with deep roots absorbed soil water from neighboring land. Therefore, planting Caragana and alfalfa together would lead to intensive competition for soil water. Plants with shallow roots and deep roots should be arranged together when vegetation restoration is conducted in arid or semi-arid regions in order to delay soil water depletion.