Migration Process Of Soil Water Of Grass And Shrub Ecological System In The Loess Plateau

The fragile ecological environment and the severe soil erosion in the Loess Plateau bring about the vegetation planting in this region is needed,which become the major measures of ecological restoration.However,groundwater level is relatively deep,and the scarce precipitation conspires to make the soil water in unsaturated zone turn into the main source of plant water.This paper investigates the temporal and spatial variation and the migration process of soil water in Yangjuangou River basin,a typical hilly-gully loess plateau watershed,aiming to provide a scientific basis for the vegetation construction and the soil and water conservation.This study combine the hydrogen and oxygen with the field testing to investigate the soil water migration in the unsaturated zone in the hilly and gully region of the Loess Plateau,and discuss the source of plant water use.Samples were collected from the 0~150cm unsaturated zone,including precipitation,soil water content,Spiraea salicifolia and Stipa bungeana Trin plant xylems and roots,and were analyzed.The results showed that:(1)The soil water content present a change trend which increases at first and decreases afterward with the time increasing in Yangjuangou River basin,exhibiting an obviously monthly variation which indicated that the soil water was seriously affected by the precipitation.The soil moisture variation with depth increasing comes a differently trend,but it has a tendency to reduce during the vigorous vegetation root zone reflecting the soil water is not only affected by the evaporation,but also affected by the plant use water.Beside,the isotopic composition analysis also manifests the isotopic composition of precipitation in the Yangjuangou River basin was heavily affected by evaporation,indicating isotopic fractionation phenomena.The isotopic composition of soil water distributed on both sides of the local precipitation line also show soil water content influenced by the evaporation.Therefore,the change of soil water content is the result of the combined action among precipitation,evaporation and plant water use.(2)This paper concludes that the water migration exhibited a transformation pathway from precipitation to soil water to plant water.The change of ?D and ?18O in soil water,precipitation and xylem water appear a consistent trend with the time and depth increase indicating that there are interaction exists among these three water body.Then through combining the correlation analysis,we found that precipitation and soil water are significant correlation,xylem water and soil water are also significant correlation,while precipitation and xylem water are just correlation.These verified the coactions among precipitation,soil water and xylem water,and speculated that the soil water and precipitation were taken up by shrubland.(3)By compare the ?D and ?18O in xylem water and soil water,we found that the isotopic composition of Spiraea salicifolia and Stipa bungeana Trin xylem water was close to that in soil water at a depth of 20~40cm and 20~30cm,respectively.Multiple-source massbalance mixing models of proportional contributions(%)of plant xylem water is used to evaluate the contributions of each potential water source.The results show that soil water from the precipitation only comprised 0%~22% and 0%~21% of the Spiraea salicifolia and the Stipa bungeana Trin xylem water,while soil water at surface horizons 20~40cm and 20~30cm soil depth comprised the largest portion of the Spiraea salicifolia and the Stipa bungeana Trin xylem water(ranging from 70% to 96% and 65% to 82%,respectively).It is more likely that plant xylem water is a mixture of precipitation and soil water from several soil depths.