Water use characteristics of different pioneer shrubs at different ages in western Chinese Loess Plateau:Evidence from δ^(2)H offset correction

Caragana korshinskii Kom.and Tamarix ramosissima Ledeb.are pioneer shrubs for water and soil conservation,and for windbreak and sand fixation in arid and semi-arid areas.Understanding the water use characteristics of different pioneer shrubs at different ages is of great importance for their survival when extreme rainfall occurs.In recent years,the stable isotope tracing technique has been used in exploring the water use strategies of plants.However,the widespreadδ^(2)H offsets of stem water from its potential sources result in conflicting interpretations of water utilization of plants in arid and semi-arid areas.In this study,we used three sets of hydrogen and oxygen stable isotope data(δ^(2)H andδ^(18)O,correctedδ^(2)H_c1 based on SW-excess andδ^(18)O,and correctedδ^(2)H_c2 based on−8.1‰andδ^(18)O)as inputs for the MixSIAR model to explore the water use characteristics of C.korshinskii and T.ramosissima at different ages and in response to rainfall.The results showed thatδ^(2)H_c1 andδ^(18)O have the best performance,and the contribution rate of deep soil water was underestimated because ofδ^(2)H offset.During the dry periods,C.korshinskii and T.ramosissima at different ages both obtained mostly water from deeper soil layers.After rainfall,the proportions of surface(0-10 cm)and shallow(10-40 cm)soil water for C.korshinskii and T.ramosissima at different ages both increased.Nevertheless,there were different response mechanisms of these two plants for rainfall.In addition,C.korshinskii absorbed various potential water sources,while T.ramosissima only used deep water.These flexible water use characteristics of C.korshinskii and T.ramosissima might facilitate the coexistence of plants once extreme rainfall occurs.Thus,reasonable allocation of different plants may be a good vegetation restoration program in western Chinese Loess Plateau.

Comparison of isotope-based linear and Bayesian mixing models in determining moisture recycling ratio

Stable water isotopes are natural tracers quantifying the contribution of moisture recycling to local precipitation,i.e.,the moisture recycling ratio,but various isotope-based models usually lead to different results,which affects the accuracy of local moisture recycling.In this study,a total of 18 stations from four typical areas in China were selected to compare the performance of isotope-based linear and Bayesian mixing models and to determine local moisture recycling ratio.Among the three vapor sources including advection,transpiration,and surface evaporation,the advection vapor usually played a dominant role,and the contribution of surface evaporation was less than that of transpiration.When the abnormal values were ignored,the arithmetic averages of differences between isotope-based linear and the Bayesian mixing models were 0.9%for transpiration,0.2%for surface evaporation,and–1.1%for advection,respectively,and the medians were 0.5%,0.2%,and–0.8%,respectively.The importance of transpiration was slightly less for most cases when the Bayesian mixing model was applied,and the contribution of advection was relatively larger.The Bayesian mixing model was found to perform better in determining an efficient solution since linear model sometimes resulted in negative contribution ratios.Sensitivity test with two isotope scenarios indicated that the Bayesian model had a relatively low sensitivity to the changes in isotope input,and it was important to accurately estimate the isotopes in precipitation vapor.Generally,the Bayesian mixing model should be recommended instead of a linear model.The findings are useful for understanding the performance of isotope-based linear and Bayesian mixing models under various climate backgrounds.