Stable isotope analysis of plant water and soil water across two vegetation types in the northern Qinghai-Tibet Plateau
Jing Li 1,2#, Fawei Zhang 1,3#, Yangong Du 1,3, Yunying Wang 1,2, Yuting Lan1,3, Mengke Si 1,3, Bo Fan1,3, Huakun Zhou1,3, Bin Wang4, Guangmin Cao 1,3*, Xiaowei Guo1,3*
1 Key Laboratory of Adaptation and Evolution of Plateau Biota, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China
2 College of Resources and Environment, University of the Chinese Academy of Sciences, Beijing, China
3 Qinghai Provincial Key Laboratory of Restoration Ecology for Cold Regions, Xining, Qinghai, China
4 New South Wales Department of Primary Industries, Wagga Wagga Agricultural Institute, Wagga Wagga 2650, Australia
# These authors contribute equally.
* Correspondence: Xiaowei Guo and Guangmin Cao, Key Laboratory of Adaptation and Evolution of Plateau Biota, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining,810008, China; Email: guoxw@nwipb.cas.cn; Tel & fax: +86 09716123010
Abstract:
Studying the interrelation of soil water and plant water is essential for an in-depth understanding of eco-hydrological processes. However, water use relationships and comparative studies between shrubs and alpine grassland of the northern Qinghai-Tibet Plateau remain poorly understood. In this study, we compared δ18O and δ2H values of water from soil, plant, precipitation, and groundwater between P. fruticosa shrub and alpine grassland locations at two neighboring sites in order to better understand the interface between plant and surrounding soils of shrubs and grasslands in the northern Qinghai-Tibet Plateau. Our results showed that δ18O and δ2H of soil water, precipitation, and plant water varied significantly over time and water sources in P. fruticosashrub and alpine grassland sites. Both soil evaporation and plant transpiration at the P. fruticosa shrub site were relatively lower than they were at the alpine grassland site. Alpine grassland plant water had a stronger dynamic fractionation effect in the process of transportation and was more sensitive to environmental conditions. However, plants at the P. fruticosashrub site displayed more flexible water use patterns, shifted their water sources between shallow soil water and deep soil water. Shrubs from alpine grassland leaded to changes in grassland water use, thereby changing soil water storage. The results of this study will provide theoretical basis for improving the availability and sustainability of soil water, provide guidance for meadow management from ecohydrological processes on the northern Qinghai-Tibet Plateau.
Keyword: Ecohydrological processes, Alpine meadow water, Qinghai-Tibet Plateau, Stable water isotopes, P. fruticosa shrub water, Soil water
Introduction
Water is one of the most crucial limiting factors determining the dynamic trends of plant in arid and semiarid ecosystems (Li et al., 2013). The availability of the primary water sources (i.e., soil water, groundwater) absorbed by plants have significant changes spatially and temporally, and determine the growth status of plants, and the distribution and growth status of plants affect the ecological structure and functions of the soil/plant system (White&Smith, 2013; Wu et al., 2019). In addition, the interaction of soil and plant water is a vital component of eco-hydrological processes (Dawson&Ehleringer, 1991; Feng et al., 2016; Chang et al., 2019). Therefore, there is an growing interest in researching studies on the interface between plants and surrounding soils (Sprenger et al., 2017; Roebroek et al., 2020).
The Qinghai-Tibet Plateau (QTP) is known as the ”Chinese Water Tower” and is an essential component of China’s ”three screens and two belts” ecological security strategic pattern. QTP is particularly vulnerable to hydrological changes under climate change (He et al., 2020). Data observed from 2001 to 2018 indicated that both soil water storage and number of days of seasonal frozen soil declined over that period. Shrub meadows (Potentilla fruticosa ) and alpine grasslands are the dominant vegetation types in the QTP. They have important water conservation functions (Dai et al., 2021), and play an important role in maintaining the water-heat balance and ecological barrier function of the QTP (Dai et al., 2019a). Previous studies of soil and plant between grasslands and shrubs only focused on exploiting different hydrological niches of plant and soil (Walker et al., 1981), soil moisture–vegetation feedbacks and their possible effects (D’Odorico et al., 2007), and functional differences in soil and plant (Ryel et al., 2008) by modeling. Additionally, physiological and physical characteristics (Volkmann et al., 2016), such as precipitation patterns (D’Odorico, et al., 2007), soil water availability (Gow et al., 2018) and distribution of fine roots (Lanning et al., 2020; Wang et al., 2021) affect the plant water use patterns, the plants influence soil water availability of different soil layers (Fu et al., 2017), with shallow soil water being affected by precipitation, and deep soil water being affected by groundwater (Feng, et al., 2016). However, on the one hand, water-use patterns of plants on the QTP only conducted on theAchnatherum splendens  grassland (Jiang et al., 2021) and alpine riparian plants (Huawu et al., 2019), which both have an environment along the river and located in the west of QTP, on the other hand, many mechanisms and significance between shrubs and grasslands are not well understood, and little research on water-use patterns and relationships between shrubs and grasslands has been conducted on the north of QTP.
The isotopic variation of both plant water and soil water can provide valuable information on the interactions between plant water and soil water (Vargas et al., 2017; Che et al., 2019), providing an effective and powerful method for revealing and partitioning the different potential water sources used by plants (Dawson et al., 2002; Rothfuss&Javaux, 2017). During water absorption by roots and transportation along shoots prior to transpiration, no isotopic fractionation of water occurs in terrestrial plants (Ehleringer&Dawson, 1992; Dawson, et al., 2002), except for the halophytes concluding coastal wetland species and woody xerophytes, that H (but not O) fractionate due to symplastic movement of water during uptake (Ellsworth&Williams, 2007) (Brum et al., 2019).
In this study, we compared δ18O and δ2H from different water sources in a pair of neighboring sites, and distinguished water use sources of P. fruticosa shrubs and alpine grassland plants on different seasons in order to in-depth understanding of the interaction of soil water and plant water for P. fruticosa shrubs and alpine grasslands on the QTP. The results are expected to provide a theoretical basis for the management of the alpine grassland ecosystem, and sustainable use of the soil water in the northern QTP.
2. Materials and methods