Seasonal Sources of Plant Water Uptake and Soil Evaporation Dynamics Across a Boreal Forest‐Shrub Taiga Gradient

Plant leaf surface moisture is a frequent meteorological phenomenon that has complicated sources. As such, the determination of whether surface moisture is the input water or only the redistribution of water in the soil–plant–atmosphere ecosystem is of great importance. In this study, δ18O and δD characteristic values of dew, guttation, and soil waters in Buxus sinica var. parvifolia M. Cheng were monitored during the frost-free period (June–September 2017) in Changchun, China, to differentiate the hydraulic relationship among atmospheric vapor, rainwater, soil, dew, and guttation waters and quantitatively distinguish the leaf surface moisture on the canopy and bottom of plants. The water vapor sources of the leaf surface moisture on plants’ canopy and bottom were quantitatively verified in accordance with isotope fractionation and mass conservation principles. Results demonstrated that leaf surface moisture, atmospheric vapor, soil water, and dew were closely related. Leaf surface moisture was mainly the condensation of dew. The sources of canopy and bottom leaf surface moisture were basically the same. The proportions of canopy moisture from plant guttation, atmospheric vapor, and soil water were 2.4%–2.5%, 79.8%–92.4%, and 5.1%–17.8%, respectively. By comparison, the proportions of bottom leaf surface moisture were 0.6%–1.4%, 80.0%–93.0%, and 6.4%–18.6%, respectively. Leaf surface moisture is an important water input in urban systems. Moreover, the characteristic values of stable hydrogen and oxygen isotopes of urban dew are supplemented, and the transformation of atmospheric vapor, rainwater, and soil and dew waters is revealed.

Classical tree-ring parameters like tree-ring width, maximum latewood density and cell wall thickness record summer air temperature at high latitudes well and allow for the reconstruction of summer temperature over centuries and millennia. However, information about other climatic factors (e.g. moisture and sunshine duration) is limited in these proxies, especially for temperature-limited environments in subarctic forests growing on permafrost soils. The application of stable hydrogen and oxygen isotopes can provide complementary information about moisture changes and solar irradiation.Solar irradiation and its seasonal distribution play an important role for trees in terms of photosynthesis and carbohydrate production, which can influence deuterium variations in organic matter in addition to hydrological processes. We found that hydrogen and oxygen isotopes in larch tree-ring cellulose from Siberia show a strong link with summer sunshine duration during the growth season. Negative correlations with sunshine duration during autumn of the previous year and winter of the current year can be related to the lack of light during polar nights at subarctic sites (> 60º N) and lack of needles in larch trees to produce photosynthates in a cold environment. Indirect effects might also arise as a high amount of winter sunshine means persistent high-pressure systems with low precipitation, therefore negatively affecting snow accumulation, which is an important water source after snow melt.Uptake of winter precipitation is possible in the warming spring and summer months after snowmelt and thawing of the active soil layer, resulting in significant correlations between tree-ring oxygen values and winter-spring air temperatures. Oxygen isotopes in organic matter are influenced by variations in the isotopic composition of source water, which is closely related to that of precipitation and soil water (though modified by evaporation at the soil surface).Furthermore, we observed that summer vapor pressure deficit is positively and significantly recorded in stable oxygen isotopes from northeastern Siberia, while continuously represented through spring-summer months in western Canada. Hydrogen tree-ring isotopes from the Canadian subarctic recorded a negative significant correlation with summer relative humidity, opposite to northeastern Siberia.The application of dual hydrogen and oxygen stable isotopes in tree rings can expand our knowledge beyond traditional summer air temperature reconstructions and will help to improve climate reconstructions over the past millennia.Acknowledgements: This work was supported by the project RSF 21-17-00006.

<p>Recent studies applying stable isotopes of hydrogen and oxygen showed that different methods for extracting water from plant tissues can return different isotopic composition. One of the most used methods to extract plant water is the cryogenic vacuum distillation (CVD), which tends to extract total plant water. Conversely, the Scholander pressure chamber (SPC), which is commonly used by tree physiologists to measure water potential in plant tissues, likely accesses only the mobile plant water (i.e., xylem and inter-cellular water). However, only few studies reported the application of SPC to extract plant water for isotopic analyses, and therefore, an inter-method comparison between SPC and CVD is needed.</p><p>In this study, we analyzed the variability in the isotopic signature of plant water extracted by SPC and CVD. Furthermore, we considered the potential variability in the isotopic composition of the plant water extracted from various tissues by CVD (i.e., leaves, twig without bark, twig with bark, twig close to the trunk of the tree, and wood core), and from different tree species (i.e., alder, apple, chestnut and beech) located in three different study areas in northern Italy.</p><p>Results indicate that plant waters extracted by SPC and CVD were significantly different, likely due to the extraction of different plant water domains. The difference in the isotopic composition obtained by the two extraction methods was smaller in the beech samples compared to alder, apple and chestnut samples. The signature of alder, apple and chestnut plant water extracted by SPC was more enriched in heavy isotopes than the samples obtained by CVD (except for the leaf water obtained by CVD, which also had a marked evaporative signature). We conclude that plant water extraction by SPC does not represent an alternative for CVD, as SPC likely extracts mostly the mobile plant water, whereas CVD tends to retrieve all water stored in the sampled tissues. However, studies aiming to quantify the relative contribution of the water sources to transpiration should rely more on the isotopic composition of xylem water transpiring during the sampling day (theoretically sampled by SPC), than the isotopic composition of total plant water (sampled by CVD), which also contains a fraction of water that could be stored in plant tissues for a long time.</p><p> </p><p> </p><p>Keywords: stable isotopes of hydrogen and oxygen; cryogenic vacuum distillation; Scholander pressure chamber; plant water; xylem water.</p>

The movement of water through soil is preferential and heterogeneous. Subsurface interactions between mobile flows and the soil matrix are not uniform and are therefore difficult to predict through time and space. The use of stable isotopes of hydrogen (2H) and oxygen (18O) as conservative tracers of water movement is improving understanding of soil hydrological processes, yet field-scale observations of isotopic variability remain scarce despite implications for identifying dominant hydrologic processes. We sampled two adjacent soils at a ridge-swale topography floodplain forest to determine soil water isotopic variability at a 20 cm depth resolution in soils of differing textures and structures. Soil water isotopic composition measured through direct vapor equilibration varied more in time and space than any mobile source water sampled including throughfall, groundwater, ponded water within the swale, and free water from boreholes. Repeated, replicate soil borings within a rolling 1 m2 area indicated that soil water isotopic variance by depth was greater in coarser textured ridge soil than in the heavily structured, cohesive clay soil in the swale. Soil water isotopic variance among samples of the same depth and date was not apparently related to soil texture, organic content, or water content. Instead, the timing of soil water isotopic variance seemed to be related to soil structural change as it relates to seasonal hydrologic processes and site topography. Three isotopic mixing models using isotopic composition of throughfall and pre-event soil water were tested to conceptualize the soil water recharge regime at each site. These models represented ideal scenarios of complete bypass flow in which there was no exchange between event water and the soil matrix (test of Two Water Worlds hypothesis), complete replacement by event water (test of complete translatory flow) and conservative mixing between antecedent soil water and event water. At the profile scale, swale soil water was best modeled as rapid replacement by event water and ridge soil water was best modeled as conservative mixing between event water and antecedent soil water. However, variance within the profile of the ridge site showed that conservative mixing was a poor physical model and isotopic variance of soil water was much higher than could be predicted using mean soil water and throughfall isotopic composition alone. These results illustrate that temporal frequency and spatial scale of sampling affect inferences as to what factors control isotopic variability within the critical zone.

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.

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.

The depth distribution of soil water contributions to plant water uptake is poorly known. Here we evaluate the main water sources used by plants at the global scale and the effect of climate and plant groups on water uptake variability and depth distribution. The global meta‐analysis is based on isotope data (δ2H and δ18O) extracted from 65 peer‐reviewed papers published between 1990 and 2017.We applied a new direct inference method to quantify the overlap between xylem water and soil water sources used by plants. The median overlap between xylem water and soil water at different depths varied between 28% and 100%, but they were generally >50%. The shallow (0‐10 cm) soil water overlap with xylem water was largest in cold regions (100% ± 0%) and lowest at tropical sites (about 28%). Conversely, the median overlap between xylem water and deep soil water was largest in the arid and the tropical zones (>75%) and much smaller in the temperate and cold zones. Our results suggest that the isotopic composition of xylem water reflects mostly the signature of shallow soil water (<30 cm) in the cold and the temperate zones, whereas in the arid and the tropical zones, plants appear to exploit water in deeper soil layers. Our novel, simple statistically‐based direct inference method performed well in determining these differences in water sources, and can be applied more widely to isotope‐based plant water uptake studies.

In arid to dry sub-humid Mediterranean climate regimes, significant pressure is exerted on soil and water resources of agricultural lands, especially during the dry season when the natural water supply is minimal and water demand for domestic use and irrigation is at its peak. Therefore, there is concern about the impacts of longer-than-normal dry spells as well as land-use/land-cover changes on the provision of ecosystem services (ESs). To address these challenges, a Critical Zone Observatory (CZO) was established in the Alento River basin, which is in Campania, an administrative region of southern Italy, and has a drainage area of approximately 411 km2. A hydraulic system of five dams store and supply water in the basin for irrigation, flood control, hydropower generation, and domestic uses. This basin belongs to the Cilento area, which is renowned worldwide for the presence of the Cilento and Vallo di Diano National Park (the largest in Italy), for the seat of the Mediterranean diet and the Velia archeological site, and is part of the UNESCO-IHP HELP (Hydrology for the Environment, Life and Policy) network. Therefore, the “Alento” CZO is an ideal “lighthouse” where the ongoing transdisciplinary research activities help provide inspiring solutions, training, and communication to enhance soil health and ecosystem services in representative Mediterranean agroecosystems. Our presentation focuses on integrated monitoring and modeling activities being conducted in two experimental catchments, which have different soil types and land uses. A leitmotif in these activities is the characterization of the hydro-geological responses of these test sites, as well as the entire basin, to the typical seasonality of the Mediterranean climate. Each catchment is equipped similarly with in-situ sensor networks and cosmic-ray neutron sensors for soil moisture monitoring. Thermo-gravimetric and time-domain reflectometry measurements were used to calibrate and validate soil moisture measurements on a seasonal basis. Environmental tracers (stable isotopes of oxygen and hydrogen) were measured fortnightly in water samples collected from rainfall, streams, water tables, xylem, and soil water during a 2-year-long field campaign. Spectral analysis and co-located soil physical, chemical, and hydraulic properties were carried out at 161 positions in a 25-m grid size. Remote sensing products from satellite and Unmanned Aerial Systems were used to estimate vegetation characteristics, soil moisture, and soil properties. The data collected thus far are being implemented in several hydrological models, including Hydrus-1D and HydroGeoSphere, to simulate water flow and solute transport in the critical zone at different spatial scales and elucidate questions about the resilience and vulnerability of current soil and water resources to global changes in these environments.

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.

Riparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ18O and δ2H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ13C values of leaf tissue.

Mixing of Rhône River water in Lake Geneva (Switzerland–France) inferred from stable hydrogen and oxygen isotope profiles

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

BackgroundSeasonal variations of stable hydrogen and oxygen isotopes in river water can be used to indicate hydrological processes and the water cycle in the river basin. This study presents a high-resolution time series of stable oxygen isotopic record (δ18O) in the lower Changjiang (Yangtze) River mainstream and aims to investigate the regional water cycle concerning natural climate changes and anthropogenic activities.ResultsThe river water δ18O shows apparent seasonal variations, which is overall higher during the dry season and lower during the flood season. This basin-scale seasonal variation of δ18O in precipitation over the Changjiang catchment is calculated based on the long-term meteorological and hydrological data. The seasonal variation of δ18O in the lower Changjiang river water is largely constrained by monsoon-induced precipitation, but is more directly controlled by the mixing of waters from the upper reaches and the tributary lakes in the middle valley.ConclusionsThe relative contributions of waters from the upstream and tributary lakes to the lower Changjiang mainstream have been considerably altered by the construction of Three Gorges Dam via the regulation of river discharges. A comparison of river water isotopic records with water discharges from the lakes suggests that it takes about 2 weeks (~ 17 days) for the Changjiang river water to circumvent the mid-lower reaches of the catchment before traveling to the sea. This study suggests that the stable hydrogen and oxygen isotopes can be harnessed as sensitive indicators for water cycling within a large catchment that is driven primarily by natural forcing though subject to substantial human impacts.

The stable isotopes of oxygen-18 and deuterium were utilized to identify different water sources mixing in the soil layer that recharged to groundwater of a paddy field during the growth season in 2014. Based on the measurements of deuterium and oxygen-18 in soil water, rainwater and groundwater in the paddy field of Chianan Plain in Southwest Taiwan in the wet season was collected, and the relationship between δD and δ18O in soil water and groundwater recharge was investigated in this study. The soil water in the paddy field of Chianan plain was collected with suction lysimeters for the identification of different sources of subsurface flow. The isotopic compositions of hydrogen and oxygen in different water bodies were used to evaluate the groundwater recharge sources in the paddy field. The intensity of rainfall and evaporation influenced the saturation conditions of pore water present in the soil layer. In general, transiently intense rainfall tended to be difficult in recharging groundwater. The results show the use of stable isotopes in water bodies can be used to monitor or identify the source/sink of soil layer and their respective contributions to groundwater recharge. In this research, the isotopic compositions of hydrogen and oxygen in the soil water at different depths before and after event water were identified. The top soil layer (<30 cm depth) in the presence of the isotope depleted soil water was probably subjected to evaporation. The soil water has more depleted isotope composition that was observed in shallow soil layers. More depleted fraction of isotopes in groundwater was similar to rainwater, which suggested that the groundwater was primarily from rainfall. In addition, the isotopic compositions of hydrogen and oxygen in groundwater slightly deviated from the local meteoric water line in southern Taiwan.

Carbonate nodules are pedogenic carbonates commonly found in regions of high evapotranspiration, such as arid and semiarid areas. The stable carbon isotopes in these nodules are influenced by the type of vegetation (C3 or C4 plants), while the stable oxygen isotopes are controlled by soil water and temperature. Carbonate nodules persist in the soil, and their isotopic signatures can reflect the paleoclimatic conditions under which they formed. Carbonate nodules are distributed across the alluvial plains of southwestern Taiwan; however, the present humid conditions may not be favorable for their formation. The objective of this study was to evaluate the climatic conditions and vegetation types that influenced the formation of carbonate nodules using stable carbon and oxygen isotopes. Four pedons with different ages of soil formation were sampled in accordance with marine transgression and regression phases, corresponding to 10,000 years before present (yr BP), 8,000 yr BP, 5,000 yr BP, and 3,000 yr BP. Carbonate nodules were collected from the pedons, and their stable carbon and oxygen isotopes were analyzed using an automated carbonate preparation device. The δ13C values indicated a mixed C3/C4 vegetation, with a predominance of C4 plants (68.0 to 98%). The mean annual temperature (12.3-14.0°C), calculated using the climofunction of temperature and δ18O, was lower than the present (24.7°C). The mean annual precipitation (1036 to 1342 mm yr-1), calculated from the geochemical climofunction, was also lower than the present (1829 mm yr-1). The radiocarbon ages of the carbonate nodules ranged from 4063 yr BP to 690 yr BP, implying that climatic conditions may have been drier and cooler than present during this time frame. This may be due to a weaker East Asian Summer Monsoon, which favored calcification in the soils. These climatic conditions are consistent with the formation environment of carbonate nodules.