Interrelation of soil water and plant water revealed by hydrogen-oxygen isotopes across alpine shrub and grassland in northern Qinghai-Tibet Plateau

Jing Li ^1,2, Fawei Zhang ^1,3, Yunying Wang ^1,2, Yangong Du ^1,3, Huakun Zhou^1,3, Bin Wang⁴, Guangmin Cao^1,3*, Xiaowei Guo^1,3*

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

* 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

The alpine grassland shrubbization of the northern Qinghai-Tibet Plateau on the background of global change and overgrazing, is a prominent and serious problem. However, the water competition ability of shrubs and alpine grasslands is rarely reported. Here, we tracked the δ¹⁸O and δ²H of soil water, plant water, precipitation, and groundwater, analysed sources of water use in shrub and grassland by Mix SIAR model. Our results showed that both δ¹⁸O and δ²H in soil, precipitation, and plant varied significantly over time, groundwater remained relatively stable inP. fruticosa shrub and alpine grassland sites during observation. Considering groundwater, precipitation, soil water, and plant water, a progressive enrichment in δ¹⁸O or δ²H existed from groundwater and precipitation to soil water to plant water for each month. Alpine grassland was more susceptible to drought stress, had a stronger partitioning effect in dynamic transport than shrub. The P. fruticosa shrub displayed more flexible water utilisations, and was more competitive for water than grasslands. Furthermore, the plants in alpine shrub and grassland reached water use balance in August. Shrubs degraded from alpine grassland changed water use pattern of grassland, thereby changing soil water storage. These results contribute to in-depth understanding the alpine grassland shrubbization from water use patterns of grassland and shrub plants on the northern Qinghai-Tibet Plateau.

Keyword: Stable isotopes, Alpine meadow water, P. fruticosa shrub water, Soil water, Mix SIAR model, Northern Qinghai-Tibet Plateau

Water is one of the most crucial limiting factors determining the community dynamic trends and productivity of plant in arid and semiarid ecosystems (Li et al., 2013). The primary water sources, such as soil water and groundwater, absorbed by plants determine the growth status of plants, 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). Additionally, the interaction of soil and plant water is a vital component of eco-hydrological processes (Dawson&Ehleringer, 1991; Chang et al., 2019).

Previous studies about interrelation of soil and plant mostly focused on exploiting different hydrological niches (Walker et al., 1981), soil moisture–vegetation feedbacks and their possible effects (D’Odorico et al., 2007), and functional differences (Ryel et al., 2008) by modeling. Precipitation patterns (D’Odorico, et al., 2007), soil water utilization (Gow et al., 2018; Lanning et al., 2020) and roots distribution (Wang et al., 2021a) affect the plant water use patterns. They were all based on traditional situ observations. Subsequently, some scholars found that isotopic variation of both plant water and soil water provide an effective and powerful way to reveal and partition the different potential water sources used by plants (Vargas et al., 2017; Che et al., 2019; Rothfuss&Javaux, 2017).

The Qinghai-Tibet Plateau (QTP) is known as ”Chinese Water Tower” and is an essential component of ”three screens and two belts” ecological security strategic pattern (Li et al., 2022), that both show the important role of QTP in the construction of ecological civilization. However, QTP is particularly vulnerable under overgrazing and climate change (He et al., 2020). Data observed from 2001 to 2018 indicated that both soil water storage declined over that period. Shrub (Potentilla fruticosa ) meadows 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., 2019). However, on the background of global change and overgrazing, the problems of grassland shrubs and the decline of water conservation functions caused by grassland degradation, are threatening the national strategic position of water resources in the Qinghai-Tibet Plateau (Guo et al., 2020). Grassland degradation changes carbon accumulation rate (Sun et al., 2020), soil nutrients (Wu et al., 2020), evapotranspiration (Ji et al., 2022), and plant water use efficiency (Wang et al., 2021b), but the interrelation of water competition from shrubs and alpine grasslands are not well understood, and little research on water-use patterns and relationships between P. fruticosa shrubs and alpine grasslands has been conducted on the northern QTP.

To fill the gaps, we compared δ¹⁸O and δ²H from different water sources in a pair of neighboring sites, combined with field observations, and distinguished water use sources of P. fruticosa shrubs and alpine grassland plants on different seasons through Mix SIAR model to in-depth understand the interaction of soil water and plant water for P. fruticosa shrubs and alpine grasslands. Our results provided a theoretical basis for the alpine grassland shrubbization, and made contribution to restoration of the alpine grassland in the northern QTP under global climate change.