Depth distribution of soil water sourced by plants at the global scale: A new direct inference approach

Abstract. We investigated plant water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in south-western France using stable water isotopes. It is generally assumed that no isotopic fractionation occurs during root water uptake, so that the isotopic composition of xylem water effectively reflects that of source water. However, this assumption has been called into question by recent studies that found that, at least at some dates during the growing season, plant water did not reflect any mixture of the potential water sources. In this context, highly resolved datasets covering a range of environmental conditions could shed light on possible plant–soil fractionation processes responsible for this phenomenon. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured fortnightly over an entire growing season. Using a Bayesian isotope mixing model (MixSIAR), we then quantified the relative contribution of water sources for F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using deeper soil water than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than the considered water sources. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected relatively depleted δ2H of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant water sources using mixing models was strongly affected by this δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

Biodiversity enhances a variety of ecosystem processes, and yet the underlying mechanisms through which these relationships occur remain a critical knowledge gap. Here, we used the natural abundance of stable isotopes to measure depth of water uptake in five common grassland species ( Asclepias tuberosa , Lespedeza capitata , Liatris aspera , Schizachyrium scoparium and Sorghastrum nutans ) growing across an experimental grassland diversity gradient. Using this approach, we addressed the following questions: 1) does the depth‐specific provenance of water uptake differ among species and/or do interspecific differences in water source manifest with increasing community diversity? 2) Does the isotopic niche space occupied by plants change with increasing diversity? 3) Is plasticity in water uptake depth across a diversity gradient associated with functional plant responses? We found that the depth of soil water used by plants was inherently different among species when grown in monocultures. All species used less shallow soil water and more intermediate‐depth soil water in mixed assemblages than in monocultures, resulting in similar interspecific differences in water source across the diversity gradient. However, plasticity in the locations of water used were positively associated with increases in plant growth in higher diversity treatments. These results indicate that plasticity in water‐use may contribute to positive biodiversity–productivity relationships commonly observed in temperate grasslands.

Located along the northeastern edge of the Qinghai‐Tibet Plateau in China, the Qilian Mountains are an important ecological barrier in Northwest China. Therefore, it is of great significance to study the plant water sources in subalpine habitats for understanding the ecological and hydrological processes in the Qilian Mountains. Here, based on the samples of precipitation, xylem water, soil water, river water, and spring water collected during May–October 2019 from subalpine habitats on the northern slope of the Qilian Mountains and the measured hydrogen and oxygen stable isotope values (δ2H and δ18O, respectively), the present study gained further insight into plant water sources in this region using the IsoSource model. Water absorption characteristics of shrubs were similar in subalpine habitats. Specifically, plants absorbed water primarily from the topsoil layer (0–30 cm) and rarely from the deep soil layers. Nevertheless, in the dry and growing seasons with high water demand, subalpine shrubs competed for water resources at similar depths. As a result of this competition, the utilization rate of deep soil water improved in some shrubs, expanding the differences in water sources of subalpine shrubs within the same habitat. In addition, in the dry and growing seasons, the water sources of the same subalpine shrub varied across different habitats. Compared with other subalpine shrubs, Salix cupularis Rehder, Salix oritrepha Schneid., Potentilla fruticosa L., Salix sclerophylla Anderss., Rhododendron anthopogonoides Maxim., and Rhododendron przewalskii Maxim. could alter their water use strategies with variations in water conditions, exhibiting a greater drought tolerance. These findings provide valuable information for understanding the subalpine region's eco‐hydrological processes and contribute to the selection of suitable species for the restoration of the subalpine ecological environment under the background of global change.

Shifts in tree water sources are important for understanding the spatiotemporal dynamics of ecosystem water fluxes. However, our understanding of tree water uptake remains limited, constraining reliable predictions of local and global hydrological processes under ongoing climate change. The isotopic composition of water (δ2H and δ18O) is a powerful tracer of the Earth’s water cycle, as isotopic differences among water reservoirs, together with mixing and fractionation processes, allow water movements to be traced across the hydraulic continuum.This study aims to characterize the tree water sources of Scots pine (Pinus sylvestris L.) in a Mediterranean forest (Pyrenees, NE Spain) during the 2024 growing season. To do so, the isotopic composition of water in several ecohydrological compartments was measured. Precipitation, soil water pools at multiple depths (10, 20, 30, 40, and 60 cm), and xylem water from four individuals were sampled biweekly. Bulk soil water was extracted using cryogenic vacuum distillation, whereas xylem water was obtained using a flow-rotor centrifuge (cavitron). The cavitron enables access to mobile xylem water (e.g., sap) and is not affected by the well-known methodological artifacts associated with cryogenic extraction. Bayesian isotope mixing models were applied to quantify the relative contributions of distinct water pools to xylem water and their temporal evolution. Dynamics of total water uptake were estimated from transpiration data.Our results show that Scots pine predominantly relied on shallow soil water (10 cm) during most of the growing season, with xylem water closely reflecting the isotopic signature of recent precipitation. A decoupling between the isotopic signature of precipitation and xylem water emerged as seasonal drying progressed. Under dry conditions, tree water uptake was low, and tree water sources shifted towards deeper soil layers (40-60 cm). Overall, these patterns indicate a strong coupling between rainfall inputs and tree water use during periods of high transpiration demand, suggesting that the contribution of deeper soil water reserves represent only a very small fraction of tree total water use during a growing season. These findings underscore the ecological importance of shallow soil water and recent precipitation in sustaining forest function and highlight the role of vegetation water use in regulating atmospheric water fluxes.

Understanding vegetation water utilization can provide scientific basis for vegetation protection and rational distribution of water resources in arid desert oasis area. In this study, sapling (DBH≤10 cm), mature (10 cm<DBH≤40 cm), and overmature (DBH>40 cm) Populus euphratica were chosen as sample trees in natural oasis of Dariaboui. We measured oxygen isotopes of xylem water and potential water sources (surface water, soil water in 0-3 m soil layer and groundwater) of P. euphratica with different tree ages. The IsoSource model was used to study water sources of P. euphratica. The results showed that soil water content increased with increasing soil depth, while the δ18O value of soil water decreased firstly and then stabilized. When groundwater depth was 2 m, the δ18O value of xylem water increased with increasing tree ages. When the depths of groundwater were 4.2 and 8 m, the δ18O values of xylem water decreased with increasing tree ages. Water sources of P. euphratica varied across tree age. When the depth of underground water was 2 m, the main water source for young trees was surface water, and the contribution proportion was 64.7%. For mature and overmature trees, it was deep soil water and ground-water, with a contribution of about 30%. When the depth of groundwater was 4.2 m, the main water sources of P. euphratica of different tree ages were deep soil water and groundwater, and the contribution proportion was about 30%. The lowest contribution of P. euphratica of different tree ages was shallow soil water, accounting for about 10%.

Salinization is a major threat to the sustainability of land and water resources, especially in arid and semiarid regions. Understanding the water uptake from different soil depths for desert plants is useful for exploring salinity-tolerance mechanism in desert plants in extremely-arid and salinity-affected area. To understand water uptake from different soil depths for desert plants in Dunhuang, NW China, we used oxygen isotope composition in plant xylem water and soil water to determine the water sources in three different saline sites differing in their degree of soil electrical conductance (site 2 &amp;lt; site 1 &amp;lt; site 3). The co-existing desert plants in each saline site extracted different depth of soil water respectively: K. foliatum mainly used shallow soil water (0–20 cm); H. caspica and N. tangutorum mainly used deep soil water (40–200 cm); A. sparsifolia used water from the 120–200 cm soil layers, while T. ramosissima and E. angustifolia mainly extracted deeper soil water (&amp;gt;200 cm). Compared to that in saline site 2, Tamarix ramosissima and Alhagi sparsifolia can switch their water sources to deeper soil water when enduring more salt stress. Also, a significant and positive correlation between soil EC and soil water δ18O values was observed, indicating the evaporation would cause increase in salt concentration and isotopic enrichment in the upper soil profile. Overall, our results suggest that plants may explore deeper soil water to adapt to salt stress under severe salinity. This work may contribute to selecting salt-tolerant plants species which is vital to saline soil rehabilitation and utilization.

In this study, three typical plants capable of restoring in the Heidaigou open-pit mine dump, namely, Pinus sylvestris var. mongolica, Caragana korshinskii, and Medicago sativa, were taken as the research objects. The δ2H and δ18O values of atmospheric precipitation, soil water, stem water, and leaf water were measured using the stable isotope technique, and the distribution characteristics of the δ2H and δ18O values of different water sources were identified. The IsoSource model (version1.3.1) was used to calculate the contribution rate of different water sources to the plants, and the differences and dynamic changes in the water sources for P. sylvestris var. mongolica, C. korshinskii, and M. sativa during the rainy season were examined. Results showed that the water source of the three plants was found to be mainly soil water, and the utilization of each potential water source varied in different periods of the rainy season. In June, when SWC was sufficient, P. sylvestris var. mongolica and M. sativa primarily absorbed and utilized shallow and middle soil water, with relative utilization ratios of 55.5% and 59%, respectively, while C. korshinskii has a more balanced utilization ratio of soil water in each layer, with shallow soil water utilization at 33.7%, middle soil water at 34.2%, and deep soil water at 32.2%. In August, when SWC decreased, P. sylvestris var. mongolica, C. korshinskii, and M. sativa were all transferred to deep soil water, with utilization ratios of 75.8%, 78.8%, and 71.1%, respectively. The values showed that these three typical plants are capable of restoring can respond to external water changes through the plastic transformation of water absorption sources. Among them, C. korshinskii can flexibly use soil water in each layer, has stronger survival competitiveness in drought, and can better adapt to the fragile ecological environment of a mining dump.

As high latitudes warm, there is limited knowledge of how rapidly changing species composition and density, combined with shifting precipitation and thawing permafrost, will affect critical zone water fluxes across the subarctic. Here, we use stable isotopes of hydrogen and oxygen to assess the role of soil moisture, precipitation dynamics and plant species on the timing, magnitude and sources of plant water uptake at three sites along an elevational gradient in a subarctic, alpine catchment in southern Yukon, Canada. The sites ranged from a low‐elevation boreal forest to higher elevation shrub taiga with variable shrub cover. We sampled soil and xylem water approximately every 3 weeks from pre‐leaf out to post‐senescence over two hydrologically distinct years. We answer the questions: (1) What are the seasonal and interannual changes in the isotopic composition of soil and xylem water across this range of subarctic vegetation covers?, (2) How does the seasonal origin of xylem water vary in wet and dry conditions? and (3) Do different shrub species at the same location rely on different sources of water? Results showed that while δ 2 H and δ 18 O of volume weighted precipitation became more negative with elevation, the opposite was true of xylem water. Despite less snowfall at lower elevations, plant water uptake was more reflective of snow water at the forest than at the high elevation shrub sites. Near‐surface bulk soil water had lower line‐conditioned excess at the forest than at the shrub sites throughout the season and with depth, highlighting increased contributions from soil evaporation at the forest. Differences in annual precipitation and climate had a strong influence on stable isotopes of hydrogen and oxygen in the soil. These results demonstrate that vegetation type and elevation strongly mediate plant water sourcing and evaporative partitioning in subarctic catchments, underscoring the need to account for species‐specific and landscape‐scale variability when predicting blue/green water fluxes in a changing climate.

A growing number of studies show a discrepancy between the isotopic composition of xylem water and plant water sources. We tested the effect of arbuscular mycorrhizal fungi (AMF) on the isotopic composition of Acacia caven xylem water. As the most common plant-fungal association, AMF might explain this isotopic mismatch. Seedlings were grown with and without AMF and irrigated with the same water. After 120 days, stem and soil samples were collected and following cryogenic distillation, H and O isotopic composition of xylem and soil water, as well as irrigation water, was measured. Xylem water of non-mycorrhizal seedlings was significantly depleted in 2H compared to soil water (differences up to −15.6‰). When AMF were present, the depletion was significantly higher and appeared for both H and O (differences up to −24.6‰ for δ2H and − 2.9‰ for δ18O between soil and xylem water). Results suggest that isotopic fractionation occurred during water uptake in this xerophytic species. To explain this, we propose an aquaporin-driven mechanism mediating water transport via transmembrane passage. Furthermore, we show for the first time, that AMF enhance the observed discrimination against heavy isotopes, probably by enforcing water passage through aquaporins. Given their ubiquity, AMF could question the fractionation-free assumption during root water uptake.

Forest trees greatly influence both the routing of water downward into the subsurface and the re‐routing of water upward through water uptake and transpiration. To reveal how the subsurface soil water pools used by trees change across seasons, we analysed 2 years of stable isotope ratios of precipitation, soil water from different depths (using both bulk sampling and suction‐cup lysimeters), and xylem in a mixed beech and spruce forest. Precipitation as well as mobile and bulk soil waters all showed a distinct seasonal signature; the seasonal amplitude decreased with depth, and mobile soil waters fluctuated less than bulk soil waters. Xylem water signatures in both tree species were similar to the bulk soil water signatures and rather different from the mobile soil water signatures. The beech and spruce trees had different isotope ratios, suggesting the use of different water sources, and these differences were larger under dry antecedent conditions than wet antecedent conditions. Despite these differences, both species predominantly transpired waters with a winter‐precipitation isotopic signature throughout the summer, including during wet conditions when more recent precipitation was available. Over most of the sampling dates, the fraction of recent precipitation (i.e. from the preceding 30 days) in xylem water was low, despite both species typically demonstrating the use of both shallow and deeper soil waters. These results provide evidence that the soil water storages used by these trees are largely filled in winter and bypassed by recent precipitation, implying long residence times.

Moisture plays a pivotal role in the establishment of vegetation in sandy areas, underscoring the need to comprehend the water utilization strategies employed by established trees for the judicious use of water resources. Despite this significance, there exists a research gap concerning the water uptake patterns and consumption disparities between the dominant trees, namely the dry willow (Salix matsudana) and small-leafed poplar (Populus simonii), in the Mu Us sandy region. Consequently, our study sought to investigate the water utilization patterns and transpiration water consumption of these two plants. This was achieved through the analysis of hydrogen and oxygen isotope compositions in xylem water, soil water, and groundwater, coupled with the assessment of stem flow rates of tree trunks. The findings reveal that both Salix matsudana and Populus simonii exhibited variations in soil water content with soil depth, characterized by an initial increase followed by a subsequent decrease. During the months of July, August, and September, both species demonstrated the ability to absorb water from multiple sources concurrently. Specifically, Salix matsudana and Populus simonii predominantly utilized middle and shallow soil water sources in July and September, respectively. However, in August, both species primarily relied on shallow soil water for absorption. Over the period from July to September, the sap flow rate of Salix matsudana surpassed that of Populus simonii by 1888.2 mL·h−1 to 2499.04 mL·h−1, representing a 1.5 to 2.2 times increase. This underscores the necessity for Salix matsudana to draw water from middle and deep soil layers to compensate for shallow water deficits. In summary, schemes for the establishment of vegetation in sandy areas should consider the dynamic nature of water uptake and evapotranspiration, emphasizing the importance of regulating these processes for efficient water conservation and utilization.

Assessing the ecohydrological separation hypothesis and seasonal variations in water use by Ginkgo biloba L. in a subtropical riparian area

Juniper expansion across the western United States has the potential to alter watershed hydrology, especially within riparian areas. Given the uncertainties in the ecohydrological response to the expansion, this study focused on examining the water use strategies between two woody species co‐occurring in a riparian area in south central Montana—Salix exigua(sandbar willows) andJuniperus scopulorum(rocky mountain junipers)—in order to address three questions: (1) Are junipers and willows using the same soil moisture pool that contributes to streamflow? (2) Are junipers transpiring more water than willows on a per tree or per sapwood area basis? (3) Are the seasonal transpiration rates between junipers and willows different? To determine the differences in water use strategies between willows and junipers, we used stable isotope analyses to trace different sources of water, water potential to determine seasonal water stress patterns and transpiration rates to quantify water loss. Our isotopic analyses suggest that junipers and willows in the riparian area were not directly using stream water but relied on different pools of soil water at different times of the year: shallow soil water in spring when soils were wet and deeper soil water in late summer. We also found that junipers transpired more than willows during the spring and late fall, but that both species had similar transpiration rates during periods of low streamflow. However, higher juniper transpiration rates in spring and late fall can potentially lead to soil moisture deficits if winter snowpack is low, suggesting that the additional water loss through transpiration by junipers may be mitigated under wet winters but exacerbated under dry winters.

Understanding the source water utilization of rice-based cropping systems helps develop improving water management strategies for paddy management. We investigated the effects of altered flooding regimes and crop diversification on plant root water uptake on a fully-replicated field trial at the International Rice Research Institute in the Philippines. All potential water pools, e.g., plant and soil extracted water, were analyzed for their water stable isotopic compositions (δ2H and δ18O). We determined the relative contributions from different water sources to root water uptake (RWU) of rice plants by applying a multi-source mixing model (Stable Isotopes Analysis in R, SIAR). The sensitivity of the model to the incorporation of prior information based on in-situ measurements of soil water content and root length density was investigated as well. The modeling results showed that wet rice plants mainly extracted surface ponded water (~56–72%) during both wet and dry seasons followed by soil surface (0–0.02 m) water (~17–19%) during growth. Dry rice extracted ~40–50% of its water from shallow soil (0–0.5 m) and ~35% from 0.1 to 0.3 m depth when the plants were matured. The mixing model results were better constrained with the additional information on soil water content and root length density. The relative contributions of the soil water sources to RWU decreased with depth and reflected the exponential shape of the root density profile. The main water source for wet rice was surface ponded water (independent of the season), whereas shallow soil water was the main source for dry rice.

Efficient cooling of cities at global scale using urban green space to mitigate urban heat island effects in different climatic regions