Study On Water Sources Of Plant Species Based On Stable Oxygen And Hydrogen Isotopes In The Northern And Southern Mountains Of The Lanzhou City

The water source of plant is an important aspect of the research on the relationship between plant water in arid and semi-arid areas,and it is significant to understand the adaptation of plants to seasonal arid environments in different areas.The northern and southern mountains greening area of the Lanzhou City?referred to as the northern and southern mountains of the Lanzhou City?is an important ecological barrier in Lanzhou City.Precipitation in this area is unevenly distributed throughout the year and has significant seasonal differences,with dry and wet seasons alternating.In the context of global climate change,changes in regional precipitation patterns are bound to affect plant growth and development,making the relationship between plants and water more complicated.At present,there are few studies on vegetation water sources of the northern and southern mountains of the Lanzhou City.Therefore,it is necessary to study the water use patterns and sources of plants in different seasons in this area.In this study,the typical dominant tree species Platycladus orientalis,Caragana korshinkii,Reaumuria soongorica,Ulmus pumila and Agropyron cristatum were selected as the research objects.By measuring the composition of stable isotopes(?²H and?¹⁸O)of precipitation,irrigation water,soil water,groundwater and plant xylem water in this area,the direct comparison method,multivariate linear mixed model?IsoSource?and Bayesian mixed model?MixSIR,MixSIAR and SIAR?to quantify the contribution rate of soil water at different depths and groundwater,and then compare the performance of different models to obtain a more reliable water source for each plant.The results show that:?1?The local meteoric water line?LMWL?obtained from the collected precipitation samples is:?²H=7.04?¹⁸O+3.47.Compared with the global meteoric water line?GMWL?,the slope and intercept of LMWL are low,indicating that local precipitation is affected by evaporation.Comparing the northern mountain and southern mountain of the Lanzhou City,the slope and intercept of the LMWL at the three samples of the northern mountain are slightly smaller than those of southern mountain,indicating that the evaporation of precipitation in northern mountain is higher than that of southern mountain.From different sampling points,the hydrogen and oxygen isotope values of groundwater,soil water and plant xylem water in the growing season almost all fall near or below the local meteoric water line.The hydrogen and oxygen isotope values of plant xylem water are between soil water and groundwater,indicating that all plants mainly use soil water with different depths and groundwater.?2?The direct comparison method can only qualitatively determine the main water absorption position of plants,and it is difficult to quantify the contribution rate of potential water sources to plants.The IsoSource model can calculate the range of contribution of multiple potential water sources to plant water,but the model has a single parameter setting,and does not consider the effects of isotope fractionation and the spatial difference of the isotopic composition of potential water sources on the calculation results.Compared with the IsoSource model,the MixSIR model considers the spatial difference of the isotopic composition of each water source,but the stability of the model is easily affected by the spatial difference value,which affects the reliability of the model calculation results.Through the judgment of the four evaluation indicators,the MixSIAR model is the optimal model,probably because the MixSIAR model gathers a priori information,and considers the effects of classification and continuous covariates.?3?Combining the four methods?direct comparison method,IsoSource,MixSIAR and SIAR?and the optimal model?MixSIAR?,it is concluded that the representative tree species,Platycladus orientalis and Ulmus pumila,have relatively stable water sources,mainly using 90-120 cm soil The proportion of water and some groundwater in the wet season to the use of medium and deep soil water has increased.The seasonal variation trends of water sources of Caragana korshinkii and Reaumuria soongorica at each sampling point in Beishan are basically the same,and there is a clear water competition relationship between the two.During the whole growing season,the Agropyron cristatum mainly absorbs and uses shallow soil water,and the ratio of using0-10 cm soil water in the wet season is much larger than that in the dry season.Before and after a rainfall event,Platycladus orientalis changed from the use of groundwater and deep soil water to shallow soil water,and its sensitivity to precipitation was greater than that of Caragana korshinkii and Reaumuria soongorica.