Isotopic Composition Of Soil Water Research Articles

Flood‐Induced Recharge of Matrix Water in a Vertic Forest Soil

AbstractVertisols shrink and swell with changes in soil moisture, influencing hydraulic properties. Vertisols are often in floodplains, yet the importance of flooding as a source of soil moisture remains poorly understood. We used blue dye and deuterated water as tracers to determine the role of the macropore network in matrix recharge under artificial flood durations of 3 and 31 days in large soil monoliths extracted from a forested soil. Gravimetric soil moisture content increased by 47% in the first three days, then increased only 3.5% from day 3–31. Post‐flood moisture content was greatest in the organic‐rich, top 10 cm and was lower at 10–75 cm where organic matter was less. Deuterium concentration revealed that soil moisture in the top 10 cm was quickly dominated by artificial flood water, but at depth remained <80% floodwater even after 31 days. Pervasive dye staining of ped surfaces in the top 4 cm indicated connectivity to flood waters but staining at depth was less and highly variable. The isotopic composition of soil water at depth continued to shift toward flood water despite no differences in dye staining between days 3 and 31. Results indicate flooding initially but incompletely recharges matrix water via macropores and suggest the importance of flooding as a source of matrix recharge in vertic floodplain soils may depend more on flood frequency than duration. Isotopic composition of matrix water in vertic soils depends on both advective and diffusional processes, with diffusion becoming more dominant as porosity decreases.

Quantitative separation of evapotranspiration components of Platycladus orientalis ecosystem based on multiple isotope models

To fully understand the changes in the evapotranspiration components in forest ecosystem and their contribution to evapotranspiration at daily scale, we used the hypothesis theory of isotopic steady state and non-steady state combined with the water isotope analyzer system to quantitatively split and compare the evapotranspiration components of Platycladus orientalis ecosystem during the growing season. Results showed that the 18O of water from different sources during the four mea-surement days (August 5, 8, 10, 11, 2016) all showed surface soil water and oxygen isotope composition (δS) > branch water and oxygen isotope composition (δX) > atmospheric water vapor oxygen isotopes composition (δV), with obvious differences due to the isotope fractionation. Oxygen isotopes composition of soil evaporated water vapor (δE) was between -26.89‰～-59.68‰ at the daily scale, showing a pattern of first rising and then decreasing. The oxygen isotopic composition of evapotranspiration water vapor in forest ecosystem (δET) was between -15.99‰～-10.04‰. The oxygen isotopic composition of transpired water vapor under steady state(δT-ISS) was between -12.10‰～-9.51‰. The oxygen isotopic composition of transpired water vapor under non-steady state (δT-NSS) was between -13.02‰～-7.23‰. δET and δT-NSS had the same changing trend throughout the day at the daily scale, while the trend of δET, δT-ISS and δT-NSS was approximately the same during 11:00-17:00. In general, the contribution rate of plant transpiration to total evapotranspiration showed that FT-ISS was between 79.1%-98.7%, and FT-NSS was between 88.7%-93.7%. Our results suggested that water consumption through soil evaporation was far less than that of vegetation transpiration in the study area, and that vegetation transpiration dominated forest evapotranspiration.

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.

Using Soil Water Stable Isotopes to Investigate Soil Water Movement in a Water Conservation Forest in Hani Terrace

Water conservation forests significantly contribute to the stability of mountain agricultural ecosystems in Hani Terrace. In this study, we analyzed the relationship between the stable isotopic composition of soil water and precipitation to determine the mechanisms of soil water movement in the small watershed of Quanfuzhuang. We observed significant seasonal variations in soil water sources: antecedent precipitation was the dominant supply during the dry season, and current precipitation dominated during the rainy season. The recharge ratio of precipitation to soil water in the grassland was significantly higher than that in the arbor land and shrubland. The influence of water infiltration, old and new soil water mixing, and soil evaporation on the soil water stable isotopes gradually decreased from the surface (0–20 cm) to the deep (60–80 cm) soil. We observed significant seasonal variability in average soil water δ18O in the upper 0–60 cm and lower variability at 60–100 cm. The average soil water δ18O was generally higher in the dry season than in the rainy season. The mixing of old and new water is a continuous and cumulative process that is impacted by soil structure, soil texture, and precipitation events. We therefore identified a significant time delay in soil water supply with increasing soil depth. Moreover, the piston flow of soil water co-occurred with preferential flow, and the latter was the dominant supply during the rainy season.

Water isotopic constraints on the enhancement of the mid-Holocene West African monsoon

During the early to mid-Holocene, changes in orbital precession led to considerable increases in West African monsoon (WAM) rainfall compared to today and shifted its reach further north. However, climate proxies and paleoclimate model simulations disagree over fundamental aspects of the mid-Holocene (MH; 6 ka BP) enhancement of the WAM. Here, we use a water isotope-enabled Earth system model (iCESM1) to, for the first time, directly compare simulated northern African hydroclimatic change between the mid-Holocene and pre-industrial era (PI) with the hydroclimate signal inferred from leaf wax n-alkanes in order to study the WAM's past spatial change. iCESM1 simulates a northernmost WAM extent of ∼24°N, which broadly agrees with the extent inferred from pollen and dust records (23–28°N) but falls short of that from leaf wax n-alkanes (27–31°N). While the isotopic composition of rainfall (δDP) inferred from leaf wax n-alkanes is lower during the MH than the PI, simulated MH δDP is higher in northwestern Africa, especially in boreal fall. This discrepancy can be reconciled by interpreting the inferred signal of leaf wax n-alkanes as being reflective of the isotopic composition of soil water (δDS) and its subsequent influence by soil evaporation. We postulate that leaf wax n-alkanes may overestimate inferred mean annual precipitation rates in the MH by not incorporating the enrichment of precipitation shown by iCESM1. Our results have broad implications for reconstruction of past hydrologic change in northern Africa and lend further support to the northernmost WAM extents inferred from pollen and dust records.

Sampling soil water along the pF curve for δ2H and δ18O analysis

AbstractSoil water stable isotopes are widely used across disciplines (e.g., hydrology, ecology, soil science, and biogeochemistry). However, the full potential of stables isotopes as a tool for characterizing the origin, flow path, transport processes and residence times of water in different eco‐, hydro‐, and geological compartments has not yet been exploited. This is mainly due to the large variety of different methods for pore water extraction. While recent work has shown that matric potential affects the equilibrium fractionation, little work has examined how different water retention characteristics might affect the sampled water isotopic composition. Here, we present a simple laboratory experiment with two well‐studied standard soils differing in their physico‐chemical properties (e.g., clayey loam and silty sand). Samples were sieved, oven‐dried and spiked with water of known isotopic composition to full saturation. For investigating the effect of water retention characteristics on the extracted water isotopic composition, we used pressure extractors to sample isotopically labelled soil water along the pF curve. After pressure extraction, we further extracted the soil samples via cryogenic vacuum extraction. The null hypothesis guiding our work was that water held at different tensions shows the same isotopic composition. Our results showed that the sampled soil water differed isotopically from the introduced isotopic label over time and sequentially along the pF curve. Our and previous studies suggest caution in interpreting isotope results of extracted soil water and a need to better characterize processes that govern isotope fractionation with respect to soil water retention characteristics. In the future, knowledge about soil water retention characteristics with respect to soil water isotopic composition could be applied to predict soil water fractionation effects under natural and non‐stationary conditions. In this regard, isotope retention characteristics as an analog to water retention characteristics have been proposed as a way forward since matric potential affects the equilibrium fractionation between the bound water and the water vapour.

In situ measurements of soil and plant water isotopes: a review of approaches, practical considerations and a vision for the future

Abstract. The number of ecohydrological studies involving water stable isotope measurements has been increasing steadily due to technological (e.g., field-deployable laser spectroscopy and cheaper instruments) and methodological (i.e., tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water-isotope-based methods into their studies. Several isotope effects are currently not fully understood but might be essential when investigating root water uptake depths of vegetation and separating isotope processes in the soil–vegetation–atmosphere continuum. Different viewpoints exist on (i) extraction methods for soil and plant water and methodological artifacts potentially introduced by them, (ii) the pools of water (mobile vs. immobile) measured with those methods, and (iii) spatial variability and temporal dynamics of the water isotope composition of different compartments in terrestrial ecosystems. In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil–plant–atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight the potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review in order to provide a guideline for researchers with limited previous experience with in situ methods. We also include a section on opportunities for incorporating data obtained with described in situ methods into existing isotope-enabled ecohydrological models and provide examples illustrating potential benefits of doing so. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes in situ when carrying out studies at the soil–vegetation–atmosphere continuum. Several authors have shown that reliable data can be generated in the field using in situ methods for measuring the soil water isotope composition. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been paid to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods. There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty and existing methodological uncertainties. Future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision and accuracy.

Variation of the stable isotopes of water in the soil-plant-atmosphere continuum of a Cinnamomum camphora woodland in the East Asian monsoon region

The hydrogen and oxygen stable isotopic (2H and 18O) tracer method has become an important means of studying water transfer between the atmosphere, rivers and lakes, soils, plants, and aquifers. We conducted a study on the variations of the stable isotopes of water, and the seasonal variations in water use of Cinnamomum camphora, in the soil-plant-atmosphere continuum in a C. camphora woodland in Changsha, central-southern China, which lies within the monsoon region of East Asia. Stable isotopes were monitored in precipitation, soil water (0–130-cm depth), groundwater, twig xylem water of C. camphora, and at the same time relevant environmental variables (soil water content, and meteorological variables) were measured, from March 2017 to August 2019. The monitoring data were collated and analyzed to determine the variations of stable isotopes in different water pools and mutual exchanges. The results indicate that daily precipitation (P) greater than 3.7 mm (regarded as effective precipitation) contributed to soil water replenishment. Isotopic composition of soil water and twig xylem water fell below the LMWLP>3.7 (the local meteoric water line eliminated rainfall events with P ≤ 3.7 mm). Isotopic composition of shallow soil water (0–60 cm) and twig xylem water was positively correlated with that of cumulative antecedent precipitation within two months. These patterns of precipitation isotopic signal propagation delineate a clear pathway of water transfer in this soil and plant continuum. The slope of the soil water line in the study area was higher than that of the LMWLP>3.7 because of the situation that precipitation in warm season with a larger evaporative demand is more depleted with heavy isotopes (i.e., more negative delta values) than that in cold season when the evaporative demand is low. Therefore, it was not feasible to determine the strength of isotopic fractionation based on the isotopic water line method. According to the MixSIR model, C. camphora water uptake mainly sourced from shallow soil water during the observation period. However, in July-September when soil was dry, deep soil water (60–130 cm) contributed to C. camphora water uptake for ~50%. These findings improve our understanding of the water cycle of the soil-plant-atmosphere continuum in the East Asian monsoon region.

Investigation of soil water hydrological process in the permafrost active layer using stable isotopes

AbstractHere, we studied the isotope characteristics and source contributions of soil water in the permafrost active layer by collecting soil samples in July 2018 in Yangtze River basin. Soil moisture and temperature showed decreasing trends from 0–80 cm, and an increasing trend from 80–100 cm. The value of δ18O and δD first increased and then decreased in the soil profile of 0–100 cm; however, d‐excess increased from 0–100 cm. δ18O values became gradually positive from the southwest to northeast of the study area, while d‐excess gradually increased from southeast to northwest. The evaporation water line (EL) was δD = 7.56 δ18O + 1.50 (R2 = 0.90, p < 0.01, n = 96). Due to intense solar radiation and evaporation on the Tibetan Plateau, the elevation did not impact the surface soil. The altitude effect of the soil depths of 0–20 cm was not obvious, but the other soil layers had a significant altitude effect. Soil moisture and temperature were closely related to the stable isotopic composition of soil water. The contribution of precipitation to soil water on the sunny slope was 86%, while the contribution of the shady slope was 84%. However, the contribution of ground ice to soil water on sunny slope was 14% and the shady slope was 16%. The contribution of ground ice to soil water increased with increasing altitude on the sunny slope, but the contribution of ground ice to soil water had no obvious trend on the shady slope.

Coffee and shade trees show complementary use of soil water in a traditional agroforestry ecosystem

Abstract. Globally, coffee has become one of the most sensitive commercial crops, being affected by climate change. Arabica coffee (Coffea arabica) grows in traditionally shaded agroforestry systems in tropical regions and accounts for ∼70 % of coffee production worldwide. Nevertheless, the interaction between plant and soil water sources in these coffee plantations remains poorly understood. To investigate the functional response of dominant shade tree species and coffee (C. arabica var. typica) plants to different soil water availability conditions, we conducted a study during near-normal and more pronounced dry seasons (2014 and 2017, respectively) and a wet season (2017) in a traditional coffee plantation in central Veracruz, Mexico. For the different periods, we specifically investigated the variations in water sources and root water uptake via MixSIAR mixing models that use δ18O and δ2H stable isotope composition of rainfall, plant xylem and soil water. To further increase our mechanistic understanding of root activity, the distribution of below-ground biomass and soil macronutrients was also examined and considered in the model as prior information. Results showed that, over the course of the two investigated dry seasons, all shade tree species (Lonchocarpus guatemalensis, Inga vera and Trema micrantha) relied, on average, on water sources from intermediate (>15 to 30 cm depth: 58± 18 % SD) and deep soil layers (>30 to 120 cm depth: 34±21 %), while coffee plants used much shallower water sources (<5 cm depth: 42±37 % and 5–15 cm depth: 52±35 %). In addition, in these same periods, coffee water uptake was influenced by antecedent precipitation, whereas trees showed little sensitiveness to antecedent wetness. Our findings also showed that during the wet season coffee plants substantially increased the use of near-surface water (+56 % from <5 cm depth), while shade trees extended the water acquisition to much shallower soil layers (+19 % from <15 cm depth) in comparison to drier periods. Despite the plasticity in root water uptake observed between canopy trees and coffee plants, a complementary use of soil water prevailed during the dry and wet seasons investigated. However, more variability in plant water sources was observed among species in the rainy season when higher soil moisture conditions were present and water stress was largely absent.

Variation in the access to deep soil water pools explains tree-to-tree differences in drought-triggered dieback of Mediterranean oaks

Individual differences in the access to deep soil water pools may explain the differential damage among coexisting, conspecific trees as a consequence of drought-induced dieback. We addressed this issue by comparing the responses to a severe drought of three Mediterranean oak species with different drought tolerance, Quercus pubescens L. and Quercus frainetto Ten., mainly thriving at xeric and mesic sites, respectively, and Quercus cerris L., which dominates at intermediate sites. For each species, we compared coexisting declining (D) and non-declining (ND) trees. The stable isotope composition (δ2H, δ18O) of xylem and soil water was used to infer a differential use of soil water sources. We also measured tree size and radial growth to quantify the long-term divergence of wood production between D and ND trees and non-structural carbohydrates (NSCs) in sapwood to evaluate if D trees presented lower NSC values. The ND trees had access to deeper soil water than D trees except in Q. frainetto, as indicated by significantly more depleted xylem water values. However, a strong δ2H offset between soil and xylem water isotopes observed in peak summer could suggest that both tree types were not physiologically active under extreme drought conditions. Alternative processes causing deuterium fractionation, however, could not be ruled out. Tree height and recent (last 15-25years) growth rates in all species studied were lower in D than in ND trees by 22 and 44%, respectively. Lastly, there was not a consistent pattern of NSC sapwood concentration; in Q. pubescens, it was higher in ND trees while in Q. frainetto, the D trees were the ones exhibiting the higher NSC concentration. We conclude that the vulnerability to drought among conspecific Mediterranean oaks depends on the differential access to deep soil water pools, which may be related to differences in rooting depth, tree size and growth rate.

Oxygen and hydrogen stable isotope compositions of soil water in deep loess profile under different land use types of northern Shaanxi, China

Investigation of stable isotope composition under different land use types is helpful for understanding soil water movement and hydrological effects of land use change. We collected soil samples in profiles ＞ 15 m deep under four land use types (i.e. farmland, grassland, Salix cheilophila and Populus sp.) in the loess deposits of northern Shaanxi. We measured hydrogen and oxygen stable isotope composition of soil water to explore the mechanism of soil water movement and the impacts of land use types. The isotope compositions of soil water under four land use types were significantly different. The δD values of soil water under farmland, grassland, S. cheilophila and Populus sp. were -81.1‰--60.1‰, -91.2‰--61.0‰, -87.4‰--63.6‰ and -73.5‰--62.2‰, while the δ18O values were -11.2‰--7.6‰, -12.6‰--8.2‰, -11.5‰--8.1‰ and -9.9‰--7.7‰, respectively. The soil water stable isotopes fluctuated across the profiles. The soil water isotope compositions in the layers of 0-3 m changed sharply, with the δD values being -80.2‰--61.8‰, -75.9‰--65.5‰, -76.0‰--63.6‰ and -73.5‰--62.2‰, respectively. In the layers of 3-12 m, the isotope profiles of farmland and grassland were parabolic, whereas those of S. cheilophila and Populus sp. were relatively stable. Soil water isotope compositions in the layers deeper than 12 m were generally stable with the δD values of -80.8‰--71.5‰, -83.0‰--67.5‰, -87.4‰--76.0‰ and -67.5‰--64.3‰, respectively. Across the four land use types, soil water stable isotope compositions were not significantly different either in the shallow layers or in the deep soil layers, but their differences in the layers of 3-12 m were significant. Soil moisture was mainly recharged from precipitation with piston flow as the main form of soil water movement. Soil water under four land use types might be recharged by wet events of different intensities. Soil water under farmland and grassland could be recharged by wet events of small intensity, but that under S. cheilophila and Populus sp. may be mainly recharged by the rainstorm in summer and autumn.

Using Water Stable Isotopes to Trace Water Sources of Three Typical Japanese Tree Species under Heavy Rainfall Conditions

Stable isotopes of xylem water (18O and D) have been successfully used to determine sources of soil water for plant transpiration, but mainly in drought-prone environments. The water uptake strategies of three representative tree species in Japan, namely cedar (Cryptomeria japonica), larch (Larix kaempferi) and beech (Fagus crenata), were investigated using δ18O and δD of water (precipitation, soil and xylem), together with wood α-cellulose δ13C and δ18O, along one growing season. The study was carried out in the research forest of Yamagata University (Shonai region), a high precipitation area in Japan, which exceeds 3000 mm per year. Precipitation water δ18O and δD increased along the summer growing season, but oxygen and hydrogen isotopic composition of soil water remained essentially unchanged. In general, xylem water isotopes of cedar and larch followed the local meteoric water line, but beech xylem water was decoupled from soil and precipitation values in July and August. For this tree species, the xylem water isotopic records were more depleted than cedar and larch xylem water isotopic values and the precipitation water isotopic records, indicating that beech used more water from soil layers during July-August than the other two species, which mainly used newly-fallen precipitation. Wood δ18O showed an opposite seasonal trend to the one found for xylem water, likely because of leaf water isotope enrichment, which was in turn controlled by seasonal transpiration rate. The higher δ13C values of cedar during summer suggested that this species had enhanced water-use efficiency during the growing season compared with the deciduous species larch and beech. Our results highlight different water use strategies among forest tree species even in areas where the annual water balance is far from limiting plant performance.

Importance of rainfall partitioning in a northern mixed forest canopy for soil water isotopic signatures in ecohydrological studies

AbstractThe forest canopy can play a significant role in modifying the amount and isotopic composition of water during its passage throughout the near‐surface critical zone. Here, partitioning of gross rainfall into interception, throughfall, and stemflow and its implications for the amount and isotopic composition of soil water was studied for red oak, eastern white pine, and eastern hemlock trees in a northern hardwood‐conifer forest in south central Ontario, Canada. Stemflow production was greatest for red oak as a result of its upward‐projecting branches and least for eastern white pine due to its horizontal branches and rougher bark. These stemflow contributions to the near‐bole soil surface failed to produce consistently wetter soils relative to distal locations from the bole for all tree species. There was also no consistent evidence of isotopic enrichment of throughfall and stemflow relative to gross rainfall or of stemflow relative to throughfall for red oak or eastern hemlock. However, there was isotopic enrichment of both throughfall and stemflow for eastern white pine with increasing maximum atmospheric vapour pressure deficit, which may reflect the potential for evaporative fractionation as a result of retention and detention of water moving through the canopy by the rougher bark of this species. Dry soil conditions limited sampling of mobile soil water during the study, and there was no consistent evidence that either throughfall or stemflow fluxes controlled temporal changes in the isotopic signature of soil water beneath the tree. Thus, the potential for throughfall and stemflow fluxes in northern hardwood‐conifer forests to modify the isotopic composition of water taken up by the tree via transpiration remains an open question.

Tracing the soil water response to autumn rainfall in different land uses at multi-day timescale in a subtropical zone

Identifying the soil water response processes to rainfall events can provide scientific guidance to sustain and enhance ecosystem services of the terrestrial ecosystem. This study presents the results of soil water content and isotopic compositions response to a typical rainfall in the subtropical zone in east China. The response of soil water (including soil water content, hydrogen and oxygen isotope ratios) to rainfall event at three soil layers (0–10, 10–20, 20–30 cm) in two croplands, two orchards (tea and peach), and two forests were analyzed. The results showed that soil water recharge processes were varied in different land uses. The variation in the isotopic composition of soil water showed a hysteresis pattern, and this pattern was affected by land use types. The extents of isotopes depletion in topsoil (0–10 cm) were along the sequence of forest (9.90%) < orchard (10.86%) < cropland (13.70%), in contrast to the reduction of soil water content (17.46% in cropland, 32.06% in forest, and 41.78% in orchard). Little depletion in isotopes indicated limited evaporation effect on soil water, and the corresponding large reduction in water content suggested large infiltration or water recharge into deep soil, these implying a good water buffer for forest soils compared with crop soils. Soil texture and antecedent soil water were the main factors influencing the variations of both soil water content and isotopic compositions. The identification of different soil water changing processes may be important for water management and plant-available water evaluation.

Spatio‐temporal heterogeneity in soil water stable isotopic composition and its ecohydrologic implications in semiarid ecosystems

AbstractSpatio‐temporal heterogeneity in soil water content is recognized as a common phenomenon, but heterogeneity in the hydrogen and oxygen isotope composition of soil water, which can reveal processes of water cycling within soils, has not been well studied. New advances are being driven by measurement approaches allowing sampling with high density in both space and time. Using in situ soil water vapour probe techniques, combined with conventional soil and plant water vacuum distillation extraction, we monitored the hydrogen and oxygen stable isotopic composition of soil and plant waters at paired sites dominated by grasses and Gambel's oak (Quercus gambelii) within a semiarid montane ecosystem over the course of a growing season. We found that sites spaced only 20 m apart had profoundly different soil water isotopic and volumetric conditions. We document patterns of depth‐ and time‐explicit variation in soil water isotopic conditions at these sites and consider mechanisms for the observed heterogeneity. We found that soil water content and isotopic variability were damped under Q. gambelii, perhaps due in part to hydraulic redistribution of deep soil water or groundwater by Q. gambelii in these soils relative to the grass‐dominated site. We also found some support for H isotope discrimination effects during water uptake by Q. gambelii. In this ecosystem, the soil water content was higher than that at the neighbouring Grass site, and thus, 25% more water was available for transpiration by Q. gambelii compared with the Grass site. This work highlights the role of plants in governing soil water variation and demonstrates that they can also strongly influence the isotope ratios of soil water. The resulting fine‐scale heterogeneity has implications for the use of isotope tracers to study soil hydrology and evaporation and transpiration fluxes to improve understanding of water cycling through the soil–plant–atmosphere continuum.

The Role of Matric Potential, Solid Interfacial Chemistry, and Wettability on Isotopic Equilibrium Fractionation

Core Ideas Matric potential controls the equilibrium fractionation factor between soil water and water vapor. Surface chemistry determined by X‐ray spectroscopy affects the equilibrium fractionation factor. A conceptual isotope retention characteristic approach is presented. Soil water stable isotopes are widely used for geo‐ and ecohydrological applications. However, the signature of the soil water isotopic composition in the environment depends on various factors. While recent work has shown matric potential effects on equilibrium fractionation, little work has examined other soil parameters concerning soil water energy status like the surface wettability, usually quantified in terms of contact angle. We simultaneously explored the role of matric potential, contact angle, and soil surface chemistry effects on the equilibrium fractionation factor during soil water evaporation. We present a simple laboratory experiment with four different soils of various textures. Subsamples of each texture class were treated with dichlorodimethylsilane to modify surface wetting properties. Additionally, we tested two natural soil samples to explore wettability effects. Samples were dried at temperatures between 40 and 550°C to produce chemically modified surface properties. All samples were spiked with water of known isotopic composition at different water contents. The isotopic signature was determined using the vapor‐bag equilibration method. The matric potential of each sample was measured with a soil water potential meter, the contact angle was determined with the sessile drop method, and the surface chemistry by X‐ray photoelectron spectroscopy. In addition to temperature and soil matric potential, the elemental composition has apparently some control on the equilibrium fractionation factor. Based on findings, we introduce a new soil water isotope retention characteristic approach to summarize how these factors (matric potential, contact angle, and soil surface chemistry) each control the equilibrium fractionation factor for 18O/16O and 2H/H. Corresponding retention curve approach parameters are promising to be applied in the future to predict soil water fractionation effects under natural and non‐stationary conditions.

In-situ monitoring of soil water isotopic composition for partitioning of evapotranspiration during one growing season of sugar beet (Beta vulgaris)

Field-based quantitative observations of hydrological feedbacks of terrestrial vegetation to the atmosphere are crucial for improving land-surface model parametrizations. This is especially true in the specific context of partitioning of evapotranspiration (ET) into soil evaporation (E) and plant transpiration (T): land surface models are able to compute E and T separately while observed transpiration fractions (T/ET) are still sparse.In this study, we present the application of an on-line non-destructive method based on gas-permeable tubing for the in-situ collection of soil water vapor. This allowed for monitoring of the hydrogen and oxygen isotopic compositions (δ2H and δ18O) of soil water during a field campaign where ET of sugar beet (Beta vulgaris) was partitioned. T/ET estimates obtained with the non-destructive method were compared with the commonly used destructive sampling of soil and subsequent cryogenic extraction of soil water under vacuum. Finally, isotope-based T/ET estimates were compared to those obtained from a combination of micro-lysimeter and eddy covariance (EC) measurements. Significant discrepancies between the values of isotopic composition of evaporation derived destructively and non-destructively from those of soil water using a well-known transfer resistance model led in turn to significant differences in T/ET. This is in line with recent findings on the systematic offsets of soil water isotopic composition values in relation to the water sampling and extraction measurement techniques and calls for further investigation of these isotopic offsets for accurate separation of E from T in the field. These discrepancies were, however, smaller than those observed between δ2H- or δ18O-based T/ET estimates, and more than three times smaller than those between isotope-based and lysimeter-based estimates.

Diurnal variations in the content and oxygen isotope composition of phosphate pools in a subtropical agriculture soil

Phosphorus (P) is an essential nutrient that affects crop yields, soil ecosystems and environmental pollution. The oxygen isotope composition of phosphate (δ18OP) in soil is mainly controlled by P source, temperature, the δ18OW of soil water, and biological processes and has been used as a unique tracer for the sources and interconversion of P pools within soil. Understanding if and how the diurnal variation of the environmental factors affects the δ18OP of soil P pools is thus indispensable to elucidating the biogeochemical cycling of different P pools and to a better use of soil δ18OP as a tracer. Soil samples from the same location with different moisture contents were collected every 3 or 12 h for 7 days for the measurement of the content and oxygen isotope composition of soil water and P pools. The results show that soil temperature, moisture and the δ18OW of soil water had clear diurnal variations, with δ18OW decreasing with increasing soil moisture. However, both the content and δ18OP of different soil P pools did not show a significant diurnal variation. These characteristics indicate that short-term diurnal changes in environmental conditions and biological activities have little effect on the content and δ18OP of soil P pools. Therefore, the diurnal sampling time has no effect on the content and δ18OP of soil P pools. However, the equilibrium δ18OP value represents the complete exchange of oxygen isotopes between phosphate and water. Comparisons of the measured values with the equilibrium value could decipher the degree of P recycling by organisms. This study shows that the mean, not the measured (at the sampling time), temperature and δ18OW of soil water should be used to calculate the representative equilibrium δ18OP value of soil to avoid bias.

Prolonged warming and drought modify belowground interactions for water among coexisting plants.

Understanding how climate alters plant-soil water dynamics, and its impact on physiological functions, is critical to improved predictions of vegetation responses to climate change. Here we analyzed how belowground interactions for water shift under warming and drought, and associated impacts on plant functions. In a semi-arid woodland, adult trees (piñon and juniper) and perennial grasses (blue grama) were exposed to warming and precipitation reduction. After 6 years of continuous treatment exposure, soil and plant water isotopic composition was measured to assess plant water uptake depths and community-level water source partitioning. Warming and drought modified plant water uptake depths. Under warming, contrasting changes in water sources between grasses and trees reduced belowground water source partitioning, resulting in higher interspecific competition for water. Under drought, shifts in trees and grass water sources to deeper soil layers resulted in the maintenance of the naturally occurring water source partitioning among species. Trees showed higher water stress, and reduced water use and photosynthesis in response to warming and drought. This case study demonstrates that neighboring plants shift their competitive interactions for water under prolonged warming and drought, but regardless of whether changes in moisture sources will result in increased competition among species or maintained partitioning of water resources, these competitive adaptations may easily be overridden by climate extremes.