Hydrogen And Oxygen Isotope Fractionation Of Soil Water And Water Adsorption Characteristics Of Artemisia Ordosica And Salix Psammophila In Mu Us Sandy Land

As a hub connecting surface water and groundwater exchange,soil water plays an important role in material circulation and energy flow in the soil-vegetation-atmosphere continuum（SPAC）,which plays an important role in maintaining regional ecosystem function.Focusing on Mu Us Sandy Land,based on the theory of hydrogen and oxygen isotope fractionation,this paper analysed the exchange of precipitation,soil water,groundwater and plant water in 2021 through field sampling,laboratory experiment and simulate calculation and other processes.The evaporation residual ratio of surface soil water,middle soil water and deep soil water were quantitatively estimated using Lc model and C-G model.Finally,the water absorption characteristics of Artemisia ordosica and Salix psammophila were analyzed,the utilization ratio of soil water in each layer was quantified by Iso Source and Mix Siar models.The main conclusions are as follow:（1）The stable isotopic composition of three water in the study area fluctuates range as:precipitation>soil water>groundwater,the precipitation recharge soil water,soil water recharge groundwater,soil water is the source of plant root water absorption.Theδ¹⁸O values of precipitation in the sampling period range from-19.51‰to 1.15‰,showing the characteristic of enrichment in spring and summer and depletion in autumn and winter.The precipitation water vapor is simulated from Atlantic water vapor in the westerly belt,the polar water vapor and local recirculation water vapor.Theδ¹⁸O values of soil water in Artemisia ordosica and Salix psammophila land were in the range of-11.43‰to 2.19‰and-11.39‰to 4.27‰,respectively.The stable isotope distribution of groundwater is relatively stable,theδ¹⁸O value is between-9.34‰to-6.82‰.There were significant difference between spatial scale and temporal scale of soil water evaporation ratio during sampling period,Artemisia ordosica land:（0～40cm）>（40～100cm）>（100～160cm）,summer>winter>spring>autumn,Salix psammophila land:（0～40cm）>（100～160cm）>（40～100cm）,autumn>winter>summer>spring.（2）The characteristics of stable isotopes of hydrogen and oxygen in precipitation and precipitation amount in the study area jointly demonstrate that the stable isotope curve of soil profile contains a clear process of precipitation infiltration to replenish soil water.The precipitation profile from March to October this year can be preserved to March of the next year until the new rainfall events cover and update the soil water profile formed by last year’s rainfall.Meanwhile,the isotope Lc-excess values of soil water profile support the above hypothesis.The effect of precipitation isotopes this month in soil water profile is limited to the upper soil,while the lower profile isotopes retain the information of last month’s isotopes.Taking the single-day heavy rainfall of 16.8mm in May as an example,based on isotope theory and sensor measured data,15 days after precipitation,the precipitation infiltration of Artemisia ordosica land and Salix psammophila land were 75cm and 60cm,respectively.（3）The residual ratio of soil water evaporation was estimated by Lc model method and C-G model method based on Rayleigh fractionation formula.It was verified by measured sensor data of Salix psammophila field that C-G method was more accurate than Lc method,C-G method（δ²H）is the most accurate method to estimate the residual ratio of soil water evaporation in the surface and middle layers,the values range from 0.27 to 0.47 and 0.38 to 0.55,respectively.The two models are more sensitive to humidity than temperature under natural condition.（4）According to the geological radar scan results,the root distribution of Artemisia ordosica and Salix psammophila were mainly distributed 0～60cm and 0～100cm away from the soil surface.Graphical qualitative analysis showed that Salix psammophila could flexibly transform the main water-absorbing layers with the soil depth of 0～120cm,the proportion of monthly water use of Salix psammophila was quantitatively calculated by Iso Source and Mix Siar model,the results of the latter were more reliable.Salix psammophila mainly use the depth of soil water of 40～80cm、0～40cm、40～80cm、120～160cm、80～120cm、0～40cm in March、April、May、June、July and September,respectively.Based on the measured data of root water and soil water isotope,root distribution and water content of Artemisia ordosica,the characteristic of absorbing shallow soil water by lateral roots of Artemisia ordosica were predicted.