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
Zhou1,3, Bin Wang4, Guangmin Cao1,3*, 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
* 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:
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
Introduction
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.
2. Materials and methods