Soil Water δD Research Articles

Variability in the Hydrological Processes of Six Typical Woodlands Based on Stable Isotopes in Subtropical Regions in Central China

Changes in woodland characteristics induced by plants and soil greatly affect soil hydrological processes. Stable isotope technology and indoor soil moisture characteristic experiments were conducted at three rainfall levels (3.6, 23.5, and 49.8 mm) to investigate the hydrological processes under six woodlands (two pure forests and four mixed forests). The main influencing factors contributing to these changes were identified in a low mountain and hilly region in central China. The soil waterline equation in this area was soil water δD = 5.626 δ18O − 16.791 (R2 = 0.798). The slope and intercept in the soil waterline equation were smaller than the atmospheric waterline equation. From a temporal perspective, the soil moisture content varied in the same trend under different rainfall events, with the maximum and minimum values on the first day after rainfall and the day before rainfall, respectively. However, an overall trend that first increased and then decreased was observed. From a spatial perspective, the soil moisture content increased with soil depth, and the increase rate was in the order of 0–20 cm and 20–40 cm in different soil layers. The soil moisture content in mixed conifer broadleaved woodlands was high. The soil water δD and δ18O in mixed conifer broadleaved woodlands and underground soil were relatively depleted. The effects of soil water-holding capacity, particle size composition, slope, canopy closure, and other factors on soil hydraulic parameters were comprehensively analyzed. The results showed that the extremely coarse sand (1–2 mm) particle content was the main parameter affecting soil-saturated hydraulic conductivity Ks, whereas the slope was the main factor affecting soil water δD and δ18O. In needle-leaved forests, the soil water infiltration form was a rainwater and soil water mixture downward diffusion, whereas the rainwater replaced the original soil water in the needle and mixed conifer broadleaved forests.

Seasonal water source patterns in a northern Arizona pine forest

The relationships among stand density and seasonal plant water source patterns in ponderosa pine (Pinus ponderosa) forests are important for informed management decisions in a bimodal climate with increasing variability in winter and monsoon precipitation inputs. Winter precipitation recharges soil moisture, yet it has declined over the past 20 years in the southwestern United States, and monsoon precipitation is becoming more variable in both magnitude and timing. Near Flagstaff, Arizona in August 2013 (monsoon), October 2013 (post-monsoon), and May 2014 (post winter snow melt), we measured soil moisture, soil water δD at five depths, and xylem water δD in Muhlenbergia montana (a perennial C4 grass), Festuca arizonica (a perennial C3 grass), and P. ponderosa seedlings (< 2 years old), saplings (2–5 cm basal diameter), and mature trees (> 60 cm diameter at breast height) in treated (thinned and burned) and untreated (no thinning, no burning) stands. We found that soil moisture was higher at all soil depths in treated stands in May, after snow melt, and this pattern persisted through August in the deepest soil (60 cm). We also found that, in all sampling months, δD in xylem water of grasses and pine seedlings indicated use of shallower soil water than for pine saplings and mature trees, presumably due to differences in rooting depths. Additionally, in August, δD in xylem water of pine saplings and mature trees indicated greater reliance on a deeper water source in untreated stands than in treated stands, likely due to greater competition for shallow water in untreated stands. Our isotopic data indicate that grasses and seedlings used predominantly monsoon water in August and October, while pine saplings and mature trees used predominantly winter water during all sampling months. Importantly, our data indicate that regenerating trees (seedlings and saplings) used both winter and monsoon seasonal water sources, suggesting an increasingly important role for monsoon precipitation if winter precipitation inputs continue to decline. Collectively, these findings demonstrate that management actions can benefit forests via increased soil moisture, that overstory trees rely predominantly on winter precipitation, and that monsoon precipitation is important for herbaceous species and younger, regenerating overstory trees.

Forest Plant Water Utilization and the Eco-Hydrological Regulation in the Karst Desertification Control Drainage Area

Subtropical forests in southwestern karst areas are the top priority for ecosystem restoration, as studying the water absorption strategies of the major plants in these regions is crucial to determining the species distribution and coexistences within these seasonal subtropical forests, which will help us to cope with the forest ecosystem crisis under future climate change. We used the stable isotope ratios (δD and δ18O) of tree xylem and soil water to assess the seasonal changes in the water use patterns and hydrological niche separations of four dominant tree species in seasonal subtropical forests in southwestern karst areas. The results showed that the soil water’s isotopic composition varied gradiently in the vertical direction and that the variation of the soil water’s isotopic composition was greater in the shallow layer than in its depths. Juglans regia (HT) mainly depended on soil water at a depth of 30–60 cm (41.8 ± 6.86%) and fissure water (32.5 ± 4.21%), while Zanthoxylum bungeanum Maxim (HJ) and Eriobotrya japonica Lindl (PP) had the same water use pattern. In the dry season, HT competed with HJ and PP for water resources, and in the rainy season, HJ and PP competed with Lonicera japonica (JYH), while HJ competed with PP all the time. JYH and HT were in a separate state of hydrologic niche and they did not pose a threat to each other. Coexisting trees are largely separated along a single hydrological niche axis that is defined by their differences in root depth, which are closely related to tree size. Our results support the theory of hydrological niche isolation and its potential responses in relation to drought resistance. This study provides a method for determining more efficient plant combinations within karst forest vegetation habitats and its results will have important implications for ecosystem vegetation restoration.

Water competition among the coexisting Platycladus orientalis, Prunus davidiana and Medicago sativa in a semi-arid agroforestry system

Agroforestry has been widely used for the ecological restoration construction of semi-arid regions of the Chinese Loess Plateau. Investigating plant water sources is crucial for understanding the key ecohydrological processes of plant root water uptake and the water relations between coexisting plants. Clarifying the plant water use strategies can help to assess the sustainability of the vegetation restoration in agroforestry systems and provide references for the vegetation management. Thus, we investigated the water use characteristics of plants in an agroforestry system consisting of Platycladus orientalis, Prunus davidiana and Medicago sativa. The δD and δ18O of plant stem water and soil water in the 0–300 cm layer were measured. The MixSIAR model and proportional similarity (PS) were used to quantify the proportion of soil water absorbed by the plants in each layer and the hydrological niche overlap of plants. The results showed that the combined effects of rainfall recharge and evapotranspiration (ET) loss resulted in a decrease in soil water content (SWC) for the agroforestry system with increasing soil depth, and the SWC of the deep layer (80–300 cm) had minor variability (6.1–7.1%) throughout the study period. During the study period, due to the deep soil desiccation, P. orientalis, P. davidiana and M. sativa mainly utilized water from shallow (0–80 cm) soil layers, with a contribution rate of 65.4 ± 8.3%, 64.9 ± 7.7% and 73.0 ± 0.2%, respectively. The three coexisting plants relied on the unstable shallow soil water (recharged by rainfall) while the water use proportion of the deep soil layer was consistently low. This implies that the three plants were less resistant to drought stress, thus posing a threat to the stability and sustainability of the agroforestry system. Moreover, even in July and September with the high SWC in agroforestry, the partitioning of soil water among the three coexisting plants did not increase with increased ET loss. The three coexisting plants showed strong competition for soil water use, as indicated by the high values (0.69–0.97) of PS. Overall, the hydrological niche overlap of the coexisting plants in the agroforestry system resulted in low ecosystem stability. Density reduction or selection of suitable species is required to achieve sustainable vegetation restoration.

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.

Characterizing the spatiotemporal dynamics of shallow soil water stable isotopic compositions on a karst hillslope in Southwestern China

• Soil water stable isotopic compositions (SWSIC) exhibited large spatiotemporal variations. • SWSIC showed less temporal stability than soil water content (SWC). • SWSIC variations were less explained by environmental controls compared to SWC variations. • Temporal stability analysis method can be used to select representative sites for SWSIC. Knowledge of the spatiotemporal dynamics of soil water content (SWC) and soil water stable isotopic compositions (SWSIC) is critical for understanding water exchanges across the atmosphere-land interface. However, compared with those of SWC, studies on the spatiotemporal characteristics of SWSIC are still scarce, which limits their use for more accurately characterizing terrestrial ecohydrological processes. To examine whether SWSIC and SWC share similar spatiotemporal features, their spatiotemporal patterns along with relevant controls were investigated at shallow soils (0–10 cm) on a karst hillslope in Southwest China, based on seven sampling campaigns conducted over a two-year period. The results revealed considerable spatiotemporal variations in SWSIC. Compared to SWC, δD values exhibited smaller spatial variations, while line-conditioned excess (lc-excess) values exhibited considerably larger ones, suggesting that soil evaporation heterogeneity might not be the primary reason for spatial variations in δD of soil water in this study. The spatial structures of SWSIC were less temporally stable than those of SWC due to the combined impact of land surface processes and temporal variations in isotopic compositions of input water on SWSIC. Moreover, the spatiotemporal patterns of SWSIC were less explained by environmental variables that were also differed from those of SWC, suggesting that the knowledge gained from SWC studies might not be directly transferable to understand SWSIC spatiotemporal characteristics. Nevertheless, it should be highlighted that the temporal stability analysis method initially proposed for studying SWC could be extended to select representative sites for monitoring areal average SWSIC, due to the temporal persistence of the SWSIC spatial structures. In addition, a negative correlation between the spatial mean and standard deviation of lc-excess was found, indicating that the associated spatial variability increased with areal mean kinetic fractionation signals caused by soil evaporation. These findings have important implications for designing SWSIC monitoring schemes, which in turn can improve the interpretation of SWSIC data for various application purposes.

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.

Seasonal variability in temperature trends and atmospheric circulation systems during the Eemian (Last Interglacial) based on n-alkanes hydrogen isotopes from Northern Finland

The Last Interglacial warm period, the Eemian (ca. 130–116 thousand years ago), serves as a reference for projected future climate in a warmer world. However, there is a limited understanding of the seasonal characteristics of interglacial climate dynamics, especially in high latitude regions. In this study, we aim to provide new insights into seasonal trends in temperature and moisture source location, linked to shifts in atmospheric circulation patterns, for northern Fennoscandia during the Eemian. Our study is based on the distribution and stable hydrogen isotope composition (δD) of n-alkanes in a lake sediment sequence from the Sokli paleolake in NE Finland, placed in a multi-proxy framework. The δD values of predominantly macrophyte-derived mid-chain n-alkanes are interpreted to reflect lake water δD variability influenced by winter precipitation δD (δDprec), ice cover duration and deuterium (D)-depleted meltwater. The δD values of terrestrial plant-derived long-chain n-alkanes primarily reflect soil water δD variability modulated by summer δDprec and by the evaporative enrichment of soil and leaf water. The δDprec variability in our study area is mostly attributed to the temperature effect and the moisture source location linked to the relative dominance between D-depleted continental and polar air masses and D-enriched North Atlantic air masses. The biomarker signal further corroborates earlier diatom-based studies and pollen-inferred January and July temperature reconstructions from the same sediment sequence. Three phases of climatic changes can be identified that generally follow the secular variations in seasonal insolation: (i) an early warming trend succeeded by a period of strong seasonality (ii) a mid-optimum phase with gradually decreased seasonality and cooler summers, and (iii) a late climatic instability with a cooling trend. Superimposed on this trend, two abrupt cooling events occur in the early and late Eemian. The Sokli δD variability is generally in good agreement with other North Atlantic and Siberian records, reflecting major changes in the atmospheric circulation patterns during the Eemian as a response to orbital and oceanic forcings.

Effects of a funnel-shaped canopy on rainfall redistribution and plant water acquisition in a banana (Musa spp.) plantation

Most of the world’s tropical landscapes are experiencing changes in land-use and land-cover. In Yunnan Province in southwestern China, land-use changes are widespread throughout the tropics, with large areas of tropical forests being converted to banana (Musa spp.) plantations. In this study, we explored the effects of banana trees’ funnel-shaped leaves on rainfall redistribution and plant water acquisition during the 2017/2018 rainy season. Both the conventional and isotopic (δD and δ18O) methods were used to conduct rainfall partitioning, assess throughfall distribution, and predict plant water sources. We found that the mean contribution of throughfall, stemflow, and interception loss to gross rainfall (rainfall amount: 0.3–33.3 mm) were 71.8 ± 6.8 %, 17.6 ± 3.6 %, and 10.6 ± 3.8 %, respectively. The percentage of stemflow under the banana canopy was noticeably higher than previously reported for other species. The maximum amount of throughfall below the banana canopy was 1.4–4.4 times greater than the gross rainfall (rainfall amount: 14.7–70.5 mm). Soil water content and soil water δD and δ18O showed both horizontal and vertical heterogeneities within the banana plantation. Analysis of δD and δ18O indicated that banana trees absorbed 72.3 % of their water from the shallow soil stratum at 0−30 cm depth. In addition, the acquisition proportion of 0−80 cm soil water ranged from 10.2%–16.3% in the horizontal directions (0−360°). These findings indicated that banana trees’ wide and long leaves considerably altered rainfall redistribution, which in turn affected their water acquisition characteristics. As banana plantations expand in this area, there is an urgent need to further examine environmental consequences such as soil erosion and surface runoff resulting from banana cultivation.

Hydrogen isotopic compositions along a precipitation gradient of Chinese Loess Plateau: Critical roles of precipitation/evaporation and vegetation change as controls for leaf wax δD

The hydrogen isotopic compositions (δD) of long-chain plant leaf waxes can reflect changes of continental hydrology and thus have been increasingly utilized for paleoclimate reconstruction. One of the unresolved major issues is whether variations of leaf wax δD (δDwax) signals along a precipitation gradient reflect changes of precipitation δD, precipitation amount, and/or evapo-transpiration. This ambiguity limits our interpretation of δDwax in geological records as well as the quantitative reconstruction of paleohydrological variability. Here we systematically investigated δD of soil water and waxes extracted from soil and plant leaves in the Chinese Loess Plateau (CLP) and its surrounding areas along a precipitation gradient with mean annual precipitation (MAP) varying from 140 mm to 676 mm. The results showed that while the variation of modeled precipitation δD has no significant correlation with MAP, soil water δD, soil δDwax, and individual plant δDwax exhibit negative correlations with MAP. In relatively arid areas, the δD values of soil water and plant leaf waxes are significantly more positive due to much lower precipitation relative to evapo-transpiration, suggesting that effective precipitation (P:E ratio) has played a crucial role in the D-enrichment of soil and plant leaf waxes in this region. While parallel decreasing trends in δDwax are found along the increase of precipitation gradient among dominant plants (including Artemisia spp., Aster hispidus, Stipa bungeana, and Cleistogenes squarrosa) in the region with similar slopes, the large δD offsets between plant groups suggest that plant type is an important factor in controlling plant hydrogen isotope fractionations. Our results indicate that δDwax preserved in paleosols can be used to infer past conditions of water availability in arid and semi-arid inland regions, but with a mechanism that is different from the influence of “amount effect” in humid areas. Moreover, vegetation changes should be constrained by independent paleobotanical data before monsoon related paleohydrology in the CLP and its surrounding areas can be quantitatively reconstructed using the plant wax δD proxy.

WaxPSM: A Forward Model of Leaf Wax Hydrogen Isotope Ratios to Bridge Proxy and Model Estimates of Past Climate

AbstractThe D/H ratio of epicuticular plant waxes (δDwax) preserved in sedimentary archives is a powerful tool for paleoclimate reconstruction, but comparisons to other proxy records or to climate model simulations requires a proxy system model (PSM) that accounts for transformations between δDprecip and δDwax. Here we present a new, publicly available PSM for plant waxes, WaxPSM. WaxPSM predicts δDwax from observational data or any isotope‐enabled modern, paleo, or future climate model experiment. δD values of the C29 n‐alkane are calculated based on precipitation or soil water δD and observed apparent fractionation values, adjusted for plant‐type differences. Using WaxPSM, we assess three key uncertainties in δDwax records: the degree to which variations in δD may reflect changes in vegetation rather than climate, structural uncertainties that arise from limited water isotopic observations, and the impacts of land cover change on climate reconstructions during the Last Glacial Maximum and the Preindustrial period. Parametric and structural uncertainties can cause δDwax variations up to 50‰, but in most cases, the differences are ∼10–30‰. The drier subtropics are additionally impacted by the incorrect structural assumption that plants' source water, δDsoil, is isotopically similar to the climate variable of interest, δDprecip. We recommend a coordinated, systematic effort to elevate observational constraints on δDprecip, δDsoil, and the δD of multiple compound classes, which would dramatically reduce parametric and structural uncertainties and allow further complexity to be built into the model.

Temporal and Spatial Distribution of the Soil Water δD and δ18O in a Typical Karst Valley: A Case Study of the Zhongliang Mountain, Chongqing City

In this study, we analyzed the stable hydrogen and oxygen isotopes of precipitation and three different land use patterns (cultivated land, grass land, and forest land) at 0-15 cm and 15-45 cm in a karst ridge-trough area (Zhongliang Mountain, Beibei District, Chongqing) in May 2017 and September 2017 to investigate the spatial and temporal variation of stable isotopes in different soil profiles using the isotope tracer technique. The results show that:① The average values of the soil water δD and δ18O are -50.0‰±33.6‰ and -7.9‰±4.3‰, respectively, and all plot around the local meteoric water line (LMWL), indicating that precipitation is the main source of the soil water supply in this area. ② The seasonal variations of δD and δ18O of the soil water are significant in different months of the rainy season, May (-19.4‰±6.8‰ and -4.1‰±1.0‰)>September (-82.2 ‰±14.0‰ and -11.9‰±2.2‰). ③ However, there is no significant difference in the soil water δD and δ18O under different land use patterns. ④ The soil water δD and δ18O change with soil depth gradients, which decrease along the depth in vertical direction for all types of soil land use in May but mainly increase/decrease in the cultivated land and woodland/grassland in September, respectively.

Evaluations of the downward velocity of soil water movement in the unsaturated zone in a groundwater recharge area using δ18 O tracer: the Kumamoto region, southern Japan

Water and substances from the surface infiltrate the unsaturated zone before reaching groundwater. Yet, little study has been done on the unsaturated zone due to the difficulty of sampling. A lot of studies have been carried out on the top soil down to a depth of one metre and on shallow aquifers because they are easily accessible for sampling. The unsaturated zone of the Kumamotoregion recharge areas is important due to concerns about groundwater pollution from agriculture. The aim of this study was to estimate the downward velocity of soil water movement through the unsaturated zone and the recharge rate using δ18O as a tracer. Five sampling sites were selected and a core was taken from each site. The cores were cut into 0.1 m pieces and soil water was extracted from each to analyze for δD and the δ18O content. Average δD and δ18O compositions of soil water were similar to the isotopic compositions of summer precipitation. Annual average recharge rate and the downward velocity of soil water in each site were estimated by fitting a vertical δ18O profile pattern to a precipitation δ18O time series as a theoretical water displacement flow model for recharge. An estimated annual average recharge rate in the recharge area ranged from 745 to 1058 mm/yr with the annual average downward velocity of 1.37 to 2.34 m/yr. Based on the estimated downward velocity, the infiltration time for soilwater to reach the aquifer was determined as ranging from 9 to 24 years, which corresponds with previous groundwater age estimations presented in an earlier published study on the same area. It was assumed that contaminants will reach aquifers in 9 to 25 years if the effects of diffusion and microbiological reaction are not taken into account.

The effects of rainfall and irrigation on cherry root water uptake under drip irrigation

Studying the effects of rainfall and irrigation water on spatiotemporal variability of root water uptake are critical for understanding water utilization processes by plants and their importance in hydrological ecosystem functions. In this study, the spatiotemporal patterns of δ18O in soil water in the soil profile and proportion of root water uptake from different soil depths and irrigation water under three irrigation treatments (T1, T2 and T3, i.e. 70%, 85% and 100% of design irrigation quota) were analyzed during the growth period by detecting the stable hydrogen and oxygen isotopes (δD and δ18O) of soil water, xylem sap, rainfall and irrigation water (local groundwater) in a cherry orchard under drip irrigation during 2015–2016. Results showed that mean δ18O value of soil water in the profile was strongly linked with seasonal variation of soil water content, and was positively correlated with irrigation quota. Rainfall and irrigation promoted significant cherry root access to soil water at shallower depths. With the growth of cherries came changes in the proportion of water sources. Initially, root water uptake was mainly from the middle soil layer (20–50cm), then topsoil (0–20cm), and finally the deep soil layer (50–100cm). The contribution of irrigation water to root water uptake decreased from the postharvest stage, and the effect of irrigation amount on improving the contribution of irrigation water to root water uptake also declined greatly. Therefore, based on comprehensive consideration of the patterns of cherry water uptake and rainfall characteristics during the cherry growth period in Beijing, the optimal water regulation treatment was 100% of design irrigation quota before the end of fruit growth stage, 85% at the postharvest stage and 70% during the end of the growth period.

Deuterium isotope characteristics of precipitation infiltrated in the West Ordos Desert of Inner Mongolia, China

Understanding the soil-profile temporal and spatial distribution of rainwater in arid and semiarid regions provides a scientific basis for the restoration and maintenance of degraded desert ecosystems in the West Ordos Desert of Inner Mongolia, China. In this study, the deuterium isotope (δD) value of rainwater, soil water, and groundwater were examined in the West Ordos Desert. The contribution of precipitation to soil water in each layer of the soil profile was calculated with two-end linear mixed model. In addition, the temporal and spatial distribution of δD of soil water in the soil profile was analyzed under different-intensity precipitation. The results showed that small rainfall events (0-10 mm) affected the soil moisture and the δD value of soil water in surface soil (0-10 cm). About 30.3% to 87.9% of rainwater was kept in surface soil for nine days following the rainfall event. Medium rainfall events (10-20 mm) influenced the soil moisture and the δD value of soil water at soil depth of 0-40 cm. About 28.2% to 80.8% of rainwater was kept in soil layer of 0-40 cm for nine days following the medium rainfall event. Large (20-30 mm) and extremely large (＞30 mm) rainfall events considerably influenced the soil moisture and δD value of soil water in each of the soil layers, except for the 100-150 cm layer. The δD value of soil water was between those δD values of rainwater and groundwater, which suggested that precipitation and groundwater were the sources of soil water in the West Ordos Desert. Under the same intensity rainfall, the δD value of surface soil water (0-10 cm) was directly affected by δD of rainwater. With increasing soil depth, the variation of soil water δD decreased, and the soil water of 100-150 cm kept stable. With increasing intensity of precipitation, the influence of precipitation on soil water δD lasted for a longer duration and occurred at a deeper soil depth.

Stable hydrogen and oxygen isotope compositions in soil-plant-atmosphere continuum (SPAC) in rocky mountain area of Beijing, China

The stable hydrogen and oxygen isotopes are environmental isotopes, which widely exist in various kinds of water. Their relative abundance variation in water can indicate the water circulation and mechanism of water use in plant. This research selected two major kinds of greening tree species, evergreen coniferous Platycladus orientalis and deciduous broad-leaved Quercus variabilis, in Beijing mountainous area, and the water movement process in soil-plant-atmosphere continuum was investigated by the variation characteristics analysis of stable hydrogen and oxygen isotope compositions in precipitation, soil water, groundwater, plant stem water and leaf water. The results showed that the meteoric water line equation of the study area was δD=7.17δ18O+1.45 (R2=0.93), and the soil evaporation line equation was δD=3.85δ18O+1.45 (R2=0.76). A certain degree of evaporation fractionation existed in the processes of rainfall infiltration into soil water. In different seasons, the δD and δ18O values of precipitation, soil water and spring water had different variation regularity. In rainy season, the mean δD and δ18O values were in order of precipitation> spring water>soil water, with the precipitation and soil water supplied spring water together; in dry season, the order was precipitation > soil water > spring water, and the precipitation and spring water both contributed to soil water. The δD and δ18O fitting line equations of stem water of P. orienta-lis and Q. variabilis were respectively δD=5.03δ18O-30.78 and δD=3.0δ18O-48.92. The uptake water of Q. variabilis was more enriched than that of P. orientalis, and the depth of Q. variabilis water uptake in soil profile was shallower than P. orientalis. The leaf water isotopic variation of Q. varia-bilis was more sensitive to atmospheric environment, with the kinetic isotopic fractionation of Q. variabilis being more enriched than that of P. orientalis, but they had the same response to variation of environmental condition.

Water sources of dominant sand-binding plants in dry season in southern Horqin Sandy Land, China

It's important to explore the water sources of sand-binding plants and their relationship to reveal the mechanism underling species coexistence and vegetation stability. In the present study, 12 sand-binding species in two typical habitats (fixed dune and dune slack) in southern Horqin Sandy Land were selected. The δD and δ18O values of plant water, rain water, ground water and soil water were determined, and the percentages of soil water at different depths used by plants were calculated with the IsoSource model. Our results showed that the δD and δ18O values of stem water were significantly different among various life forms in both habitats except for those of trees and shrubs in dune slack. From trees to grass, the depth of soil water contributed to main water source of plant became shallower in dune slack: trees and shrubs mainly used soil water in 50-150 cm or 30-50 cm layer, subshrubs mainly used soil water in 10-30 cm layer while grass relied on soil water of 0-10 cm layer. Shrubs mainly used soil water of 0-30 cm layer and subshrubs mainly used soil water around 50 cm at fixed dune. This study indicated that in dry season plants at fixed dune are more dependent on soil water of 0-50 cm layer compared with those in dune slack. The water sources of sand-binding plants are correlated with plant life form and root distribution range, and the later might play a more important role.

Tropical circulation intensification and tectonic extension recorded by Neogene terrestrial δ18O records of the western United States

Abstract Terrestrial water isotope records preserve a history of hydrological cycling that is influenced by past climate and surface topography. δ18O and δD records from authigenic minerals of the western United States display a long-term increase during the Neogene in the vicinity of the Sierra Nevada and the central Rocky Mountains (Rockies), but a smaller increase or decrease in the northern Great Basin. Interpretations of these isotopic trends require quantitative estimates of the influence of climatic and environmental changes on δ18O and δD of soil water. Here we use a coupled atmosphere-land model with water-isotopologue tracking capabilities, ECHAM5-JSBACH-wiso, to simulate precipitation and δ18O responses to elevation-independent changes in Neogene geography, equator to pole temperature gradient (EPGRAD), grassland expansion, and tropical Pacific sea surface temperatures. Both precipitation and soil water δ18O (δ18Osw) respond strongly to Neogene strengthening of the EPGRAD, but weakly to other forcings. An increase in EPGRAD leads to significant drying and 18O enrichment (3‰–5‰) of soil water over the northern Sierra Nevada and central Rockies as a result of Hadley circulation strengthening and enhanced coastal subtropical subsidence. These large-scale circulation changes reduce inland moisture transport from the Pacific Ocean and Gulf of Mexico. Our simulated δ18Osw responses could explain 50%–100% of the proxy δ18O increases over the Sierra Nevada and central Rockies, suggesting that climate change rather than surface subsidence may have been the dominant climate signal in δ18O records in these regions. On the contrary, δ18O responses to climate changes are small in the Great Basin, indicating that the observed δ18O increase over this region was likely a direct response to surface subsidence with elevation losses of 1–1.5 km. Adding this elevation loss to current Great Basin elevations reveals the former existence of a uniformly high plateau extending from the Sierra Nevada to the central Rockies prior to Neogene extension. This revised elevation history brings Neogene δ18O and δD paleoaltimetry of the western United States in accordance with independent lines of structural evidence and early Cenozoic elevation reconstructions.

An isotope study (δ18O and δD) of water movements on the Loess Plateau of China in arid and semiarid climates

Precipitation infiltration and evaporation are the main controlling factors on soil water content (SWC) in the Chinese Loess Plateau (CLP). However, the temporal and spatial variations of soil water in the CLP are still unclear. Here, we investigate the stable isotope compositions (δ18O and δD) of soil water for five different vegetation cover types in the central CLP, to trace the dynamics and movement mechanisms of soil water. Our results show that the depth of precipitation infiltration is approximately 120cm in five different vegetation cover types under natural rainfall conditions throughout the year. The rapid 18O-enrichment of shallow (<30cm depth) soil water, which is observed in all profiles, indicates that the evaporation effect mainly occurs in the shallow layer. The δ18O isotope dynamic pattern between 30 and 120cm depth is probably controlled by the precipitation infiltration characteristics at a mean annual precipitation of 572.4mm. In contrast, deep (>120cm depth) soil water is in a steady state in our study period, which suggest that the residence times of this water can be several months or more. Although the vegetation cover types can affect the profile dynamics of δ18O, we find that variations in seasonal precipitation are the key factor that influences the profile dynamics of δ18O, which is attributed to the large differences in the climate parameters and the frequency of rainfall. We suggest that δ18O is more sensitive in tracing the precipitation infiltration depth and recharge mechanisms of soil water than the soil water content. Further observation over a much longer time scale and an combination of both the oxygen and hydrogen isotope compositions of soil water in the CLP would provide more insight into role of isotopic techniques in tracing the soil water cycle.

Spatial variability of shallow soil moisture and its stable isotope values on a karst hillslope

Soil moisture (θ) and its stable isotope values are two of the most commonly used parameters for studying hydrological processes in soils. Despite their unique ability to aid in distinguishing between soil water evaporation and plant transpiration, the spatial variability of soil water isotope values is not fully understood. In the current study, 10m×10m grids were established within a 90m×120m plot on a highly heterogeneous karst hillslope. Two sampling campaigns were conducted during the early growing season, on April 15, and during the mid-growing season, on August 18, 2011. Stratified soil samples were collected from the shallow soil layer (0–30cm) to measure θ and its stable isotope values, which were represented by soil water δD values (δDθ). Related soil properties, land cover and topography were also measured and treated as influencing factors. On both sampling dates, θ decreased with depth, while δDθ stayed constant for all soil layers and were similar to the most recent rainfall values. Additionally, the variance of δDθ was smaller than that of θ, especially on August 18 when the most recent rainfalls had similar δD values. The high contrast between θ and δDθ caused the impacts of other factors on δDθ to be masked by the impact of the recent rainfall. Soil moisture presented a moderate to strong spatial dependence, which was consistent with the spatial variability of the influencing factors that were significantly correlated with θ. Soil water δD value exhibited weak spatial dependence and random spatial patterns that were different from the other influencing factors. Moreover, significant correlations between these same influencing factors and δDθ disappeared after a partial analysis with θ as a controlled variable, which means δDθ was indirectly affected by these influencing factors through θ. This suggests that the spatial variability of δDθ was being controlled at fine scales. Our results highlight the importance of analyzing the spatial variability of δDθ, its influencing factors and θ in shallow soil layers separately.