Study On Soil-plant-atmosphere Continuum Of Qinghai Picea Crassifolias In Eastern Qilian Mountains Based On Stable Isotope Method

Understanding the hydrogen and oxygen stable isotope composition characteristics of different water bodies in soil-plant-atmosphere continuum is of great significance for revealing regional hydrological processes and water cycle mechanisms.In this study,based on field monitoring and indoor experiments from May to October in 2018 and 2019,and combined with Iso Source model,we analyzed the stable isotopic composition,relationship and indicative significance of precipitation,soil water（0～100 cm depth）and xylem water of Qinghai Picea crassifolia forest in the eastern Qilian Mountains,and explored the circulation process among different water bodies.The main conclusions are as follows:（1）Theδ²H andδ¹⁸O in warm season are much higher than those in cold season due to the influence of temperature and precipitation events in warm season.The slope and intercept of LMWL(δ²H=7.38δ¹⁸O+7.96)are smaller than those of GMWL(δ²H=8.17δ¹⁸O+10.56),indicating that the evaporation in the study area is strong,the humidity is low,and the precipitation is affected by evaporation.The shallow soil water was greatly affected by dynamic fractionation.Among them,the surface soil（0～10 cm）was most seriously affected by evaporation fractionation,and the stable isotope value changed greatly,while the deep soil water was less affected by dynamic fractionation,and the hydrogen and oxygen stable isotope values tended to be consistent.According to the SWL,the vertical variation of the surface（0～10 cm）to deep（60～100 cm）soil water line,the slope of the soil water line gradually increased,in the 0～60 cm soil depth and evaporation fractionation showed a negative trend,with the increase of soil depth,lc-excess increased and evaporation weakened,in the 60～100 cm by evaporation had little effect.Compared with precipitation and soil water,the seasonal variation of stable isotope values in groundwater is the smallest,which is similar to that of deep soil water,indicating that groundwater is deeper and more stable than other water bodies.The seasonal variation of stable isotope value of river water is not significant.The slope and intercept of xylem water line are much lower than that of atmospheric water line and close to soil water line.The distribution characteristics ofδ²H andδ¹⁸O in xylem water indicate thatδ²H andδ¹⁸O values fall in the distribution range of soil water,indicating that Qinghai Picea crassifolia uses soil water at different depths during its growth.（2）During the whole observation period,the contribution of precipitation to soil water decreased regularly with the increase of soil depth.However,when there is a large rainfall event,the precipitation can be first transmitted to the deep soil through the preferential flow path.The monthly average contribution of river water to soil moisture was the smallest in June and July,and the largest in September.The contribution rate of groundwater to soil water at different depths fluctuated as a whole.In addition,we found that the contribution rate of groundwater to soil water at 10～20 cm depth in October was the highest in shallow soil water（0～30 cm）.The surface soil（0～10 cm）can respond positively to precipitation,and the response of the lower soil to precipitation in time is lagging.Theδ¹⁸O in soil water at 0～60 cm depth has a similar seasonal variation trend with that in precipitation,but there is a certain lag.（3）The contribution rate of shallow soil water（0～30 cm）to Qinghai Picea crassifolia was the highest in the growing season.Specifically,the contribution rate of 60～100 cm soil water to Qinghai Picea crassifolia was the highest in May,the influence of utilization of soil water in each soil layer on Qinghai Picea crassifolia more equal in June,July and September.In August,the utilization rate of 0～10 cm soil water increased,and the contribution rate of10～20 cm and 30～60 cm soil water to Qinghai Picea crassifolia was higher in October.（4）Most of the evapotranspiration of Qinghai Picea crassifolia ecosystem in the eastern Qilian Mountains comes from plant transpiration,which is an important part of the evapotranspiration of Qinghai Picea crassifolia ecosystem in the eastern Qilian Mountains.Based on the analysis of the main meteorological factors and environmental factors affecting evaporation,it shows that the relative humidity in the Qinghai Picea crassifolia ecosystem has a reverse inhibitory effect on f _T,while the temperature and soil moisture content have a positive effect on f _T,but the influence of temperature,relative humidity and soil moisture content on f _Tis not strong enough,which may indicate that f _Tis affected by many factors in addition to temperature,relative humidity and soil moisture content under natural conditions at high altitude.