Soil Water δ18O Research Articles

Identifying soil water movement and water sources of subsurface flow at a hillslope using stable isotope technique

Knowledge concerning soil water movement and water sources of subsurface flow is crucial for understanding runoff generation mechanisms and nutrient-pollution migration. Although the spatiotemporal characteristics of soil water have been confirmed, water sources of subsurface flow during rainfall remain poorly understood at the hillslope scale. We measured the δ18O and δ2H in soil water before and after storm events to explore the mixing process of new water (rainwater) and old water (pre-event soil water) at the hillslopes with different farming practices (bare land, no-tillage and straw mulching). Water source partitioning of subsurface flow during three storms (April 11, April 24, June 10) were quantified using the MixSIAR model. The results showed that both preferential flow and piston flow existed in the process of soil water movement which were detected by the variation in the isotopic values of soil water after storms. The proportional contributions of new water to subsurface flow from 0 − 30 cm in no-tillage plot was the highest (46.60%) among experimental plots. The fraction of new water to subsurface flow for no-tillage, bare land, and straw mulching plot was 27.36%, 22.39%, and 16.28%, respectively, indicating that farming practices affected water source partitioning of subsurface flow. The average proportion of old water to subsurface flow of 0 − 30 cm, 30 − 60 cm and 60 − 90 cm during three storm events were 71.77%, 76.72%, and 87.48%, respectively, suggesting that subsurface flow was mainly derived from old water with soil depths specific responses. In addition, the proportional contributions of new water to subsurface flow varied dynamically with the progress of rainfall, implying that the mixing between new and old water affected water sources of subsurface flow. These findings have crucial implications for understanding the generation mechanisms of subsurface flow and the sustainable management of agro-environmental systems.

Snowmelt Water Use at Transpiration Onset: Phenology, Isotope Tracing, and Tree Water Transit Time

AbstractStudies of tree water source partitioning have primarily focused on the growing season. However, little is yet known about the source of transpiration before, during, and after snowmelt when trees rehydrate and recommence transpiration in the spring. This study investigates tree water use during spring snowmelt following tree's winter stem shrinkage. We document the source of transpiration of three boreal forest tree species—Pinus banksiana, Picea mariana, and Larix laricina—by combining observations of weekly isotopic signatures (δ18O and δ2H) of xylem, soil water, rainfall and snowmelt with measurements of soil moisture dynamics, snow depth and high‐resolution temporal measurements of stem radius changes and sap flow. Our data shows that the onset of stem rehydration and transpiration overlaps with snowmelt for evergreens. During rehydration and transpiration onset, xylem water at the canopy reflected a constant pre‐melt isotopic signature likely showing late fall conditions. As snowmelt infiltrates the soil and recharges the soil matrix, soil water shows a rapid isotopic shift to depleted‐snowmelt water values. While there was an overlap between snowmelt and transpiration timing, xylem and soil water isotopic values did not overlap during transpiration onset. Our data showed 1–2‐week delay in the shift in xylem water from pre‐melt to clear snowmelt‐depleted water signatures in evergreen species. This delay appears to be controlled by tree water transit time that was in the order of 9–18 days. Our study shows that snowmelt is a key source for stem rehydration and transpiration in the boreal forest during spring onset.

A healthier water use strategy in primitive forests contributes to stronger water conservation capabilities compared with secondary forests

Water conservation is an important ecological function of forest ecosystems, plant water use strategy is a key factor in regulating forest ecosystem water balance. However, there are still insufficient studies on the water conservation capacity and water use strategies of different forest types, especially in climate-sensitive areas. In this study, we determined the stable isotope values (δD, δ18O and d-excess) of plant water, soil water and precipitation from two typical stand types (primary forest and secondary forest) on Changbai Mountain to reveal plant water use and evaluated the water conservation capacity. The results indicated that rainwater infiltrated into the soil combined with piston flow and preferential flow in the primary forest, and preferential flow was the only form of flow in the secondary forest. The main tree species in the primary forest formed a relatively stable water use niche. Among them, the water use pattern of Quercus mongolica Fisch. ex Ledeb (Qm.) was transformed between shallow and deep soil layers with strong ecological plasticity. The dominant species in secondary forest derived water from similar soil layers with intense interspecific competition. By comparing the water use patterns, the secondary forest conformed to the hypothesis of “two water worlds”, while the primary forest conformed to the hypothesis of one reservoir. The primary forest ecosystem had stronger water conservation capacity than secondary forest ecosystem due to the regulable water use strategies of plants and the stable water conservation capacity of the soil. These results will provide theoretical support and a reference for plan future forest management strategies in the climate-sensitive areas.

Transpiration Induced Changes in Atmospheric Water Vapor δ18O via Isotopic Non-Steady-State Effects on a Subtropical Forest Plantation

Accurate simulation of oxygen isotopic composition (δ18OT) of transpiration (T) and its contribution via isotopic non-steady-state (NSS) to atmospheric water vapor δ18O (δ18Ov) still faces great challenges. High-frequency in-situ measurements of δ18Ov and evapotranspiration (ET) δ18O were conducted for two summer days on a subtropical forest plantation. δ18O of xylem, leaf, and soil water at 3 or 4-h intervals was analyzed. Leaf water δ18O and δ18OT were estimated using the Craig and Gordon (CG), Dongmann and Farquhar–Cernusak models, and evaporation (E) δ18O using the CG model. To quantify the effects of δ18OT, δ18OE, and δ18OET on δ18Ov, T, E, and ET isoforcing was calculated as the product of T, E, and ET fluxes, and the deviation of their δ18O from δ18Ov. Results showed that isotopic steady-state assumption (SS) was satisfied between 12:00 and 15:00. NSS was significant, and δ18OT was underestimated by SS before 12:00 and after 18:00. The Péclet effect was less important to δ18OT simulation than NSS at the canopy level. Due to decreasing atmospheric vertical mixing and the appearance of the inversion layer, contribution from positive T isoforcing increased δ18Ov in the morning and at night. During the daytime, the contribution from positive T isoforcing increased first and then decreased due to strong vertical mixing and variability in T rate.

Water Uptake Tradeoffs of Dominant Shrub Species in the Coastal Wetlands of the Yellow River Delta, China.

Tamarix chinensis and Ziziphus jujuba are two dominant shrub species on Chenier Island in the Yellow River Delta, China. Water is a restrictive factor determining the plant growth, vegetation composition, and community succession in this coastal zone. We investigated how water uptake tradeoffs of the two shrub species responded to soil water fluctuations caused by seasonal variations of precipitation. The soil water content, salinity and δ18O values of potential water sources (soil water in 0–20, 20–40, 40–60, and 60–100 cm soil layers, and groundwater) and plant xylem water were measured in wet (July 2013) and dry (July 2014) seasons. The IsoSource model was employed to calculate the contributions of different water sources to plant xylem water. The results showed that δ18O values of soil water decreased significantly with soil depth in the dry season, while increased significantly with soil depth in the wet season. In the wet season, when the soil water was abundant, Z. jujuba mostly used the soil water from the 60–100 cm layer, while T. chinensis took up a mixture of groundwater and soil water from the 60–100 cm layer. In the dry season, when the soil water was depleted because of low precipitation, Z. jujuba mainly took up a mixture of the soil water from 20 to 100 cm soil layers, while T. chinensis mainly used groundwater. T. chinensis and Z. jujuba showed different ecological amplitudes of water sources during dry and wet seasons. The niche differentiation of major water sources for T. chinensis and Z. jujuba demonstrated their adaptabilities to the fluctuations of soil moisture in water-limited ecosystems. Water niche differentiations of coexisting shrub species were expected to minimize their competition for limited water sources, contributing to successful coexistence and increasing the resilience of the coastal wetland ecosystem to drought.

Identification of the water source and groundwater recharge in a paddy field using stable hydrogen and oxygen isotopes

AbstractThe 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.

Vegetation and soil succession determine the water-use strategy ofCaragana korshinskiiin a chronosequence of revegetated areas in Ningxia, Northwest China

AbstractPlant and soil properties and stable isotope data from soil and xylem samples of Caragana korshinskii from four different-aged revegetated sites (1976, 1987, 1996, and 2005) were studied in a desertified steppe ecosystem of Northwest China. Results showed that the revegetation of C. korshinskii had a positive effect on the local habitat restoration. The δ18O values of soil water at the four study sites varied between different months and exhibited a monotonic decline from the soil surface to deep soil layers. The variation of soil water δ18O values in the older revegetated sites was lower than that in the younger revegetated sites. C. korshinskii mainly tapped water from secondary (60–120 cm) and relatively stable (120–250 cm) soil water in the older revegetated site, and they had relatively slight monthly fluctuations. However, in the younger revegetated site, the contribution of active (0–60 cm) soil water increased, and they exhibited a clear shift in the water-use pattern. In the initial stage of vegetation establishment, soil water content played a major role in the plant water-use strategy, but in the middle and later stages of revegetation, plant biological characteristics and soil physical properties were the main impact factors.

Interspecific variation in the timing and magnitude of hydraulic redistribution in a forest with distinct water sources

AimsTrees regulate water availability among their rooting strata through a nocturnal, passive transference of water known as hydraulic redistribution (HR). This study investigates differences in HR and groundwater use among common canopy species in longleaf pine (Pinus palustris Mill., Pinaceae) woodlands and explores environmental factors influencing HR.MethodsHR was estimated by sap flux of lateral roots and main stems of three mature canopy species (P. palustris, Quercus laevis Walter., Fagaceae and Quercus margarettae Ashe., Fagaceae). We used δ18O and δD of xylem water, soil water, and groundwater to determine water source. Finally, we related HR to environmental factors (Temperature, VWC, VPD) to better understand controls of HR dynamics.ResultsPinus palustris had higher water use than either Quercus species, and also redistributed significantly more water as a nocturnal subsidy. HR fluxes were inversely related with mean nightly temperature and independent of shallow soil moisture. Stable isotope mixing models, based on δ18O and δD, indicated that all species have access to groundwater, but utilized shallow soil water in differing amounts when available.ConclusionsIn systems with strong water potential gradients among soil strata, any species with access to a groundwater source is likely capable of HR; however, the magnitude of HR varies significantly by species, even among closely related taxa.

Water Use Characteristics of Two Dominant Species in the Mega-Dunes of the Badain Jaran Desert

The sparse natural vegetation develops special water use characteristics to adapt to inhospitable desert areas. The water use characteristics of such plants in desert areas are not yet completely understood. In this study, we compare the differences in water use characteristics between two dominant species of the Badain Jaran Desert mega-dunes—Zygophyllum xanthoxylum and Artemisia ordosica—by investigating δ2H and δ18O in plant xylem (the organization that transports water and inorganic salts in plant stems) and soil water, and δ13C in plant leaves. The results indicate that Z. xanthoxylum absorbed 86.5% of its water from soil layers below 90 cm during growing seasons, while A. ordosica derived 79.90% of its water from the 0–120 cm soil layers during growing seasons. Furthermore, the long-term leaf-level water use efficiency of A. ordosica (123.17 ± 2.13 μmol/mol) was higher than that of Z. xanthoxylum (97.36 ± 1.16 μmol/mol). The differences in water use between the two studied species were mainly found to relate to their root distribution characteristics. A better understanding of the water use characteristics of plants in desert habitats can provide a theoretical basis to assist in the selection of species for artificial vegetation restoration in arid areas.

Modeling spatial distributions of hydrogen and oxygen stable isotopes in surface soils of the upper reaches of Minjiang River based on machine languages

To study the feasibility of simulating the spatial distribution of hydrogen and oxygen stable isotopes composition (δ2H and δ18O) in the surface soil based on the machine learning method and to investigate large-scale distribution of δ2H and δ18O in the upper reaches of Minjiang River, 183 soil samples were collected from the 0-10 cm soil layer. After variable selection, back propagation (BP) neural network, random forests (RF) and support vector machine (SVM) were used to model the δ2H and δ18O of the study area, with the accuracies being evaluated. The structural equation model (SEM) was used to reveal the mechanism between the auxiliary variables and the δ2H and δ18O of soil water. The results showed that the RF model had the highest prediction accuracy, and could explain 75.0% and 64.0% of the variations of δ2H and δ18O in the surface soil, respectively. In this model, soil water content was the most important auxiliary variable, contributing 48.9% and 37.4% to δ2H and δ18O. Vegetation factors had stronger influence on δ2H and δ18O in the surface soil than climate factors, and the influence of climate factors on δ2H and δ18O was media-ted by vegetation factors. Among all the auxiliary variables, hydrogen/oxygen isotope of precipitation had the lowest effect on δ2H and δ18O due to the fractionation. The δ2H and δ18O in the surface soil of the upper reaches of the Minjiang River changed significantly across different months during the growing season. The increases of δ2H and δ18O in the early growing season and the decreases in the late growing season were mainly affected by vegetation, while climate change led to a small fluctuation in the middle growing season.

Soil Water Stable Isotopes Reveal Surface Soil Evaporation Loss Dynamics in a Subtropical Forest Plantation

Line-conditioned excess (lc-excess), the deviation of the relationship between δD and δ18O in soil water from that of precipitation, is often used to indicate soil evaporation loss, but the conditions of using lc-excess under the influences of precipitation infiltration or percolation had not been identified. The interaction effects of climate, soil and vegetation on soil evaporation in forests are not well known. We collected soil water at 0–5, 15–20 and 40–45 cm depths and event-based precipitation from 2011 to 2015 in a subtropical forest plantation and calculated the lc-excess. Precipitation on the sampling day and percolation of upper soil water with low lc-excess affected the capacity of the lc-excess to indicate the soil evaporation fractionation signals. Lc-excess of soil water at 0–5 cm depth indicated a reliable soil evaporation loss estimate over 30 days prior to the sampling day. Soil evaporation loss was dominated by the cumulative soil temperature (Tss) during drought periods and was dominated by the relative soil water content (RSWC) during non-drought periods. High Tss decreased soil evaporation loss by increasing transpiration and relative humidity. Our results emphasize the importance of sampling the upper-most soil layer when there is no rain and vegetation during drought periods in forests when studying soil evaporation loss dynamics.

Phenology determines water use strategies of three economic tree species in the semi-arid Loess Plateau of China

Economic tree species growing in the semi-arid Loess Plateau of China are important for maintaining sound economic viability and protecting the fragile hydro-ecological environment. However, their water use strategies and ecological adaptation to dryland environments remain poorly understood. In this study, we measured variations in soil water content, root distribution, water isotopic signature for hydrogen (δ2H) and oxygen (δ18O) in xylem and soil water of peach (Prunus persica), walnut (Juglans regia L.), and apple (Malus pumila) trees, as well as carbon isotopic signature (δ13C) in their leaves during two consecutive growing seasons—2018 (normal year) and 2019 (wet year). The results showed all species consumed less deep soil water in 2019 than in 2018 due to the increased precipitation. Peach trees used more shallow soil water compared to walnut and apple trees that used more deep soil water during high water-demanding periods. Walnut trees depended more on deep soil water throughout the growing season, contributing 46.2% in 2018, compared to 26.0% for peach trees and 24.0% for apple trees. Moreover, walnut trees had lower deep soil water and leaf δ13C than peach and apple trees. The findings suggest differences in phenology periods of the three rainfed economic tree species to affect the water source used. The three tree species differed in their water use strategies: walnut trees had ecological plasticity for reducing water stress by absorbing deep soil water, and peach and apple trees had higher water use efficiencies. This study has significance for evaluating water use characteristics and drought stress responses of economic trees in water-limited environments.

Variability of δ2H and δ18O in Soil Water and Its Linkage to Precipitation in an East Asian Monsoon Subtropical Forest Plantation

The linkage between δ2H and δ18O of soil water and precipitation provides a way of understanding precipitation infiltration, residence time, and soil water source. Soil water at 0–5, 15–20, and 40–45 cm depths and event-based precipitation were collected in a subtropical forest plantation. Correlations between the δ18O of soil water and precipitation on the same day were used to determine the critical threshold of precipitation infiltration. Residence time of precipitation in soil was determined with correlations between the δ18O of soil water and cumulative precipitation before sampling. Soil water source was determined by the intersection points of Soil Water Evaporation Lines (SEL) and local meteoric water lines. The results showed that precipitation >5–6 mm could pass through canopy and litter, and infiltrate into soil. Residence times varied from a few days to several months, and increased with soil depth. The model-based approach for SEL estimation were more robust than the regression-based approach due to the inverse variability in the δ2H and δ18O of soil water source and soil evaporative fractionation. Soil water at a 0–5 cm depth originated mainly from precipitation in the current season, while those at 15–20 and 40–45 cm depths originated mainly from precipitation in the previous season.

Technical note: Evaporating water is different from bulk soil water in <i>δ</i><sup>2</sup>H and <i>δ</i><sup>18</sup>O and has implications for evaporation calculation

Abstract. Soil evaporation is a key process in the water cycle and can be conveniently quantified using δ2H and δ18O in bulk surface soil water (BW). However, recent research shows that soil water in larger pores evaporates first and differs from water in smaller pores in δ2H and δ18O, which disqualifies the quantification of evaporation from BW δ2H and δ18O. We hypothesized that BW had different isotopic compositions from evaporating water (EW). Therefore, our objectives were to test this hypothesis first and then evaluate whether the isotopic difference alters the calculated evaporative water loss. We measured the isotopic composition of soil water during two continuous evaporation periods in a summer maize field. Period I had a duration of 32 d, following a natural precipitation event, and period II lasted 24 d, following an irrigation event with a 2H-enriched water. BW was obtained by cryogenically extracting water from samples of 0–5 cm soil taken every 3 d; EW was derived from condensation water collected every 2 d on a plastic film placed on the soil surface. The results showed that when event water was heavier than pre-event BW, δ2H of BW in period II decreased, with an increase in evaporation time, indicating heavy water evaporation. When event water was lighter than the pre-event BW, δ2H and δ18O of BW in period I and δ18O of BW in period II increased with increasing evaporation time, suggesting light water evaporation. Moreover, relative to BW, EW had significantly smaller δ2H and δ18O in period I and significantly smaller δ18O in period II (p<0.05). These observations suggest that the evaporating water was close to the event water, both of which differed from the bulk soil water. Furthermore, the event water might be in larger pores from which evaporation takes precedence. The soil evaporative water losses derived from EW isotopes were compared with those from BW. With a small isotopic difference between EW and BW, the evaporative water losses in the soil did not differ significantly (p>0.05). Our results have important implications for quantifying evaporation processes using water stable isotopes. Future studies are needed to investigate how soil water isotopes partition differently between pores in soils with different pore size distributions and how this might affect soil evaporation estimation.

Recharge and Infiltration Mechanisms of Soil Water in the Floodplain Revealed by Water-Stable Isotopes in the Upper Yellow River

The stable isotopes (δD and δ18O) in soil water allow tracing of the flow and transportation of water in the soil. However, there are few studies on the use of soil water stable isotopes to explore the soil water in the floodplain, especially in determining the soil water source and infiltration mechanism. The Bayesian mixing model (MixSIAR) was integrated with the line conditioned excess (lc-excess) of stable isotopes (δD and δ18O) in precipitation, soil water (0–150 cm), river water, and groundwater to determinate the source and recharge mechanisms of two different soil profile types in the floodplain of the upper Yellow River in Lanzhou, China. The results showed that soil water below 80 cm was affected by river water recharge, affecting soil water content and stable isotopic composition at S1 sampling points (profile parallel to river water); this effect was not observed at S2 (profile is higher than the river water) sampling points. The isotopic compositions of the soil water sources at the two sampling points (S1: δD = −77.41‰, δ18O = −11.01‰; S2: δD = −74.02‰, δ18O = −10.56‰) were depleted more than those in the long-term amount-weighted precipitation isotopes (δD = −56.30‰, δ18O = −8.17‰). The isotope signatures of soil water sources are similar to the isotope characteristics of some high-intensity precipitation events (≥30 mm/day), indicating that soil water originates from a fraction of the total precipitation. The piston flow (60%) and the preferential flow (40%) coexist, but soil moisture and rainfall intensity will affect the sequence of the two infiltration methods. This study provides insights for understanding the hydrological process of the upper Yellow River and evaluating groundwater quality and protecting the floodplain environment.

Isotopic fractionation from deep roots to tall shoots: A forensic analysis of xylem water isotope composition in mature tropical savanna trees

Studies of plant water sources generally assume that xylem water integrates the isotopic composition (δ2H and δ18O) of water sources and does not fractionate during uptake or transport along the transpiration pathway. However, woody xerophytes, halophytes, and trees in mesic environments can show isotopic fractionation from source waters. Isotopic fractionation and variation in isotope composition can affect the interpretation of tree water sources, but most studies to date have been greenhouse experiments. Here we present a field-based forensic analysis of xylem water isotope composition for 12 Eucalyptus tetrodonta and Corymbia nesophila trees. We used a 25-tonne excavator to access materials from the trees' maximum rooting depth of 3 m to their highest canopies at 38 m. Substantial within-tree variation occurred in δ2H (−91.1‰ to −35.7‰ E. tetrodonta; −88.8‰ to −24.5‰ C. nesophila) and δ18O (−12.3‰ to −5.0‰ E. tetrodonta; −10.9‰ to −0.3‰ C. nesophila), with different root-to-branch isotope patterns in each species. Soil water δ2H and δ18O dual isotope slopes (7.26 E. tetrodonta, 6.66 C. nesophila) were closest to the Local Meteoric Water Line (8.4). The dual isotope slopes of the trees decreased progressively from roots (6.45 E. tetrodonta, 6.07 C. nesophila), to stems (4.61 E. tetrodonta, 5.97 C. nesophila) and branches (4.68 E. tetrodonta, 5.67 C. nesophila), indicative of fractionation along the xylem stream. Roots of both species were more enriched in 2H and 18O than soil water at all sampled depths. Bayesian mixing model analysis showed that estimated proportions of water sourced from different depths reflected the contrasting root systems of these species. Our study adds evidence of isotopic fractionation from water uptake and along the transpiration stream in mature trees in monsoonal environments, affecting the interpretation of water sources. We discuss the findings with view of interpreting aboveground xylem water isotopic composition, incorporating knowledge of root systems.

The relationship of δD and δ18O in surface soil water and its implications for soil evaporation along grass transects of Tibet, Loess, and Inner Mongolia Plateau

Soil evaporation plays an important role in non-productive water loss and local climate regulation. The deviation of the relationship between δD and δ18O in surface soil water from that in precipitation is used to indicate the integrative results of the soil evaporation process, but the distinction between the slope of the soil water evaporation line (SEL) and line-conditioned excess (lc-excess) as individual indicators has not been identified. Furthermore, spatial variability and interaction effects of climate, soil, and vegetation on soil evaporation in arid and semi-arid grasslands remain poorly understood. We established 3 west-to-east sampling transects along with increasing precipitation and decreasing aridity index gradients in grasslands of the Tibetan (TP, 1500 km), Loess (LP, 600 km), and Inner Mongolia (MP, 1200 km) Plateaus. Slope of SEL and weighted mean lc-excess were calculated according to the δD and δ18O in soil water, which was extracted from soils collected at 10 cm intervals to a depth of 1 m along with soil profiles during the vigorous growing season. We found that lc-excess appeared relatively more robust than slope in representing soil evaporation process because lc-excess was determined by evaporation fractionation factors and soil residual water storage, while slope was regulated only by evaporation fractionation factors. Soil evaporation, as indicated by lc-excess, exhibited decreasing trends from west to east, and was controlled by relative humidity in TP, and by vegetation coverage in LP and MP. Our results highlighted that the soil evaporation process may be more sensitive to climate change in TP, and vegetation restoration in LP and MP.

Variability in tree water uptake determined with stable water isotopes in an African tropical montane forest

AbstractEcohydrological processes in tropical rainforests are insufficiently understood, and existing studies yield contradictory results. We investigated relative contributions of different soil depths to tree water uptake of 83 trees and possible species‐specific differences in a 50 × 50 m forest plot at four dates in a tropical montane forest in Kenya using stable water isotopes and the Bayesian mixing model framework MixSIAR. We found distinct individual tree differences (e.g. Drypetes gerrardii taking 75% of its water from <0.5 m, or a rather large shift in uptake patterns based on the climatic conditions, that is the fourth sampling date), but no consistent species‐specific or small‐scale spatiotemporal patterns in water uptake and depth contributions. Soil water δ18O showed a lateral variation of up to 6‰, which was accounted for by a spatial interpolation of soil water isotopes and enabled us to improve allocations of water uptake sources to individual trees. Our results show that ignoring the lateral variability of water isotope signatures in soils complicates the applicability of a mixing model in this context and might be a widespread constraint reducing the validity and comparability of mixing model results. Further research on underlying processes of water fluxes in forest ecosystems is urgently needed and we point out the need for considering large individual differences in water uptake patterns and small‐scale variability of soil water isotopic composition despite homogeneous soil characteristics.

Age-related water use characteristics of Robinia pseudoacacia on the Loess Plateau

Understanding water use characteristics of revegetation species is crucial for evaluating plant adaptability and guiding the sustainability of vegetation restoration in semiarid regions. Ecological restoration projects have been implemented for decades in degraded ecosystems, achieving significant changes in vegetation cover. However, water use characteristics of the main tree species at different ages remain poorly understood in such systems. We investigated water use characteristics of Robinia pseudoacacia in plantations of different stand-age (18 and 30 years). The species is the most widely planted tree in revegetation efforts on the Loess Plateau. The δ2H and δ18O of xylem and soil water within 500 cm of the soil surface and the δ13C values of plant leaves were measured during two consecutive hydrological years. The results showed that that water uptake proportions from across the soil columns changed in 18-yr R. pseudoacacia between a drier (2016) and wetter year (2017). In contrast, shallow soil water was largely comparable in a stand of 30-yr R. pseudoacacia in 2016 and 2017, and similarly the pattern of water uptake by roots from the middle and deep soil column was comparable. However, leaf-level water use efficiency (WUEi) of trees in the older plantation was higher during the wetter year, thereby partly alleviating a low infiltration to precipitation ratio. These findings suggest that different stand-age plantation trees have distinct water use characteristics and display different responses to variations in precipitation. Older plantation trees respond to increased water availability by increasing WUEi instead of switching water sources. This means that stand-age is an essential factor to be considered in ecological restoration management, which can enhance the effectiveness of vegetation restoration strategies. The study indicates useful input from research to management throughout the continuity of restoration effort.

Water Uptake from Different Soil Depths for Desert Plants in Saline Lands of Dunhuang, NW China

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 < site 1 < 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 (>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.