Soil Water δD Research Articles

Insights into stable isotope characterization to monitor the signification of soil water sampling for environmental studies dealing with soil water dynamics through the unsaturated zone

Porous cup samplers and drainage samplers are two of the broadly used techniques to monitor soil water for agronomical studies. This study provides further insight into the sample signification of these two sampling techniques. For that purpose, temporal variations of soil water δD and δ18O values collected by these two techniques have been monitored for an experimental field studied by INRA. The stable isotope data acquired provide further evidence that soil water samples collected by these two techniques are not equivalent and correspond to different water dynamics in soils: 1) quick infiltration along preferential flow paths for drainage (short residence time) and 2) water with longer residence time for porous cups. This implies that stable isotopic tools could be useful to provide additional information to “classical” monitoring of soil water. This could be of particular interest to estimate the residence time of soil water and could be relevant to follow the effectiveness of agricultural pressure reduction programs on natural water ecosystems.

Stable Isotopic Analysis on Water Utilization of Two Xerophytic Shrubs in a Revegetated Desert Area: Tengger Desert, China

Stable isotope studies on stable isotope ratios of hydrogen and oxygen in water within plants provide new information on water sources and water use patterns under natural conditions. In this study, the sources of water uptake for two typical xerophytic shrubs, Caragana korshinskii and Artemisia ordosica, were determined at four different-aged revegetated sites (1956, 1964, 1981, and 1987) in the Tengger Desert, a revegetated desert area in China. Samples from precipitation, soil water at different soil layers, and xylem water from each species were collected in 2013. The proportion of plant water sources derived from different potential sources was determined using oxygen (δ18O) and hydrogen (δD) stable isotope analysis combined with a multiple-source linear mixing model. Results showed that the local meteoric water line (LMWL) at Shapotou was as follows: δD = 7.39δ18O + 3.91 (R2 = 0.93; n = 26). The vertical distribution of soil water content in older vegetation areas (1956a and 1964a) was much lower than that in relatively younger vegetation areas (1981a and 1987a). Mean soil water δD and δ18O values varied with depth, and the variation decreased as the age of the revegetated site increased. In general, C. korshinskii and A. ordosica mainly tapped water from the upper soil layer (10–100 cm) during the wet seasons. With increasing sand stabilization age, the proportion of water sources from shallow soil water decreased, whereas deep soil moisture utilization increased. During the dry season, C. korshinskii and A. ordosica showed evident hierarchical utilization of soil water in different soil layers. Small rainfall events did not significantly affect the water source of C. korshinskii and A. ordosica. However, large rainfall events not only complemented the deep soil moisture, but also recharged the shallow soil water after a few days, and the proportion of soil water source from deep soil layer increased from 2% ± 0.7% to 10% ± 1.4% for both plants.

Analysis of Water Movement through an Unsaturated Soil Zone in Jinan Southern Mountain Using Stable Oxygen and Hydrogen Isotopes

The stable isotope compositions of soil water from the profiles at Jinan southern mountain reveal information about the mechanism of soil movement. The stable isotopes in soil water have a wide range from-8.92 to-5.95 forδ18O and-50.01 to-79.44 forδD . The mean isotope values is-7.75 forδ18O and 65.84 forδD. The low-δD of shallow (<5cm) soil water is due to the precipitation infiltration. And deep soil water (>5cm) is enrichment, which indicates the soil water is evaporated before recharging. The stable oxygen and hydrogen isotope are plotted near the meteoric water line. The trend line for the relationship between δ18O and δD of soil water is given.

Isotopic and hydrochemical data to restrict the origin of the groundwater in the Badain Jaran Desert, Northern China

Despite its extreme aridity, the Badain Jaran Desert is rich in groundwater. In the southeastern part of the desert, it is characterized by coexistence of high megadunes and a great number of lakes. Deuterium and oxygen 18 isotope compositions as well as hydrochemistry of groundwater, lake water, soil water and river water were investigated in detail to gain an insight into their relationships and the origin of the ground-water. The results show that the groundwater and the lake water are genetically related, but unrelated to local precipitation and the leakage of Heine River at northern slope of Qilian mountain. δD and δ18O values of deep soil water (lower than 40 cm) and groundwater plot on the same evaporation line E11, which shows that they have the same recharge source. The point of intersection between E11 and LMWL suggests that the groundwater originates from the water resource which has a weighted mean value that is lighter by some 6‰ δ18O than the local precipitation in Badain Jaran Desert. 3H data of water samples shows that the groundwater in Badain Jaran Desert originates from the water recharged after the nuclear test. The deep fault zone underground maybe the water circulation channel based on Helium analysis of groundwater. The result has guiding significance to rational exploitation and utilization of the local groundwater.

Effects of rainfall on soil moisture and water movement in a subalpine dark coniferous forest in southwestern China

AbstractWater content and movement in soil profile and hydrogen isotope composition (δD) of soil water, rainwater, and groundwater were examined in a subalpine dark coniferous forest in the Wolong National Nature Reserve in Sichuan, China, following rainfall events in 2003–2004. Light rainfall increased water content in the litter and at soil depth of 0–80 cm, but the increased soil water was lost in several days. Heavy rainfall increased soil water content up to 85% at depths of 0–40 cm. Following the light rainfall in early spring, the δD of water from the litter, humus, illuvial, and material layers decreased first and then gradually reached the pre‐rainfall level. In summer, light rainfall reached the litter humus, and illuvial layer, but did not hit the material layer. Heavy rainfall affected δD of water in all layers. The δD of soil interflow slightly fluctuated with rainfall events. The δD of shallow groundwater did not differ significantly among all rainfall events. Light rainfall altered the shape of δD profile curve of water in the upper layer of soil, whereas heavy rainfall greatly affected the shape of δD profile curve of water in all soil layers. Following the heavy rainfall, preferential flow initially occurred through macropores, decayed plant roots, and rocks at different depths of soil profile. With continuing rainfall, the litter and surface soil were nearly saturated or fully saturated, and infiltration became homogeneous and plug‐like. Forest soil water, particularly in deeper soil profile, was slightly affected by rainfall and, thus, can be a source of water supply for regional needs, particularly during dry seasons. Copyright © 2011 John Wiley & Sons, Ltd.

Effect of leaf litter degradation and seasonality on D/H isotope ratios of n-alkane biomarkers

Abstract During the last decade, compound-specific hydrogen isotope analysis of plant leaf-wax and sedimentary n -alkyl lipids has become a promising tool for paleohydrological reconstructions. However, with the exception of several previous studies, there is a lack of knowledge regarding possible effects of early diagenesis on the δD values of n -alkanes. We therefore investigated the n -alkane patterns and δD values of long-chain n -alkanes from three different C3 higher plant species ( Acer pseudoplatanus L., Fagus sylvatica L. and Sorbus aucuparia L.) that have been degraded in a field leaf litterbag experiment for 27 months. We found that after an initial increase of long-chain n -alkane masses (up to ∼50%), decomposition took place with mean turnover times of 11.7 months. Intermittently, the masses of mid-chain n -alkanes increased significantly during periods of highest total mass losses. Furthermore, initially high odd-over-even predominances (OEP) declined and long-chain n -alkane ratios like n -C 31 /C 27 and n -C 31 /C 29 started to converge to the value of 1. While bulk leaf litter became systematically D-enriched especially during summer seasons (by ∼8‰ on average over 27 months), the δD values of long-chain n -alkanes reveal no systematic overall shifts, but seasonal variations of up to 25‰ ( Fagus , n -C 27 , average ∼13‰). Although a partly contribution by leaf-wax n -alkanes by throughfall cannot be excluded, these findings suggest that a microbial n- alkane pool sensitive to seasonal variations of soil water δD rapidly builds up. We propose a conceptual model based on an isotope mass balance calculation that accounts for the decomposition of plant-derived n -alkanes and the build-up of microbial n -alkanes. Model results are in good agreement with measured n -alkane δD results. Since microbial ‘contamination’ is not necessarily discernible from n -alkane concentration patterns alone, care may have to be taken not to over-interpret δD values of sedimentary n -alkanes. Furthermore, since leaf-water is generally D-enriched compared to soil and lake waters, soil and water microbial n -alkane pools may help explain why soil and sediment n -alkanes are D-depleted compared to leaves.

Isotopic analysis of water sources of mountainous plant uptake in a karst plateau of southwest China

AbstractEcosystem in the karst region of southwest China is very fragile due to a very limited amount of water storage for plant uptake in the thin and rocky soils underlain by rock fractures. Plants in these karst regions are thought to take water from the soils and shallow fractured rock zone (subcutaneous zone) as well. However, the role of subcutaneous water in maintaining karst vegetation remains unclear, and proportions of the water sources for plant uptake in different environment conditions are unknown. In this study, five typical species of plants at two sites were selected in a karst plateau of Qingzhen, central Guizhou Province of China. Proportions of the possible water sources contributed for the plant uptake from two soil layers and subcutaneous zone were determined on the basis of δD and δ18O values of plant stem water, soil water and subcutaneous water. The analysis reveals that most plants take water from the soil layers and the subcutaneous zone as well, but proportions of these water contributions for plant uptake vary seasonally and depend on site‐specific conditions and plant species. Plant uptake of the subcutaneous water for all species averages less than 30% of the total monthly amount in June and September, compared with more than 60% in dry December. Plants tend to take a larger proportion of water from the upper soil layer at the bush site than at the forest site in June and September (63 vs 28% in July; 66 vs 54% in September for all species in average). In December, however, 98% of water is taken from the subcutaneous zone at the bush site which is much greater than 68% at the forest site. Compared to deciduous arbor, evergreen shrub takes a greater proportion of subcutaneous water in the December drought. Copyright © 2011 John Wiley & Sons, Ltd.

Leaf wax n-alkane δD values of field-grown barley reflect leaf water δD values at the time of leaf formation

Leaf wax n-alkanes from barley ( Hordeum vulgare) from a field in Switzerland exhibited changes in δD values on the order of 20‰ over a growing season, while source water (soil water) and leaf water varied by 40‰. Additionally the seasonal variability in δD values of leaf wax n-alkanes of different barley leaves can only be found across different leaf generations (i.e. leaves that were produced at different times during the growing season) while n-alkane δD values did not vary significantly within a leaf generation. Interestingly, δD values of n-alkanes correlated best with the δD values of leaf water at midday of the sampling day but showed no significant correlation with soil water (e.g. precipitation) δD values. These results provide empirical evidence that leaf wax δD values record leaf water enrichment, and therefore integrate the isotopic effects of precipitation and evapotranspiration. Our results show that leaf wax n-alkane δD values from grasses are ‘locked in’ early during leaf development and hence record the environmental drivers of leaf water enrichment, such as vapor pressure deficit (VPD). Our data have important implications for the interpretation of paleorecords of leaf wax δD. We suggest that leaf wax n-alkane δD values from sedimentary records could be used to estimate changes in the degree of leaf water enrichment and hence VPD.

Water uptake and transport in lianas and co-occurring trees of a seasonally dry tropical forest

Water uptake and transport were studied in eight liana species in a seasonally dry tropical forest on Barro Colorado Island, Panama. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and basal sap flow were measured during the 1997 and 1998 dry seasons. Sap flow of several neighboring trees was measured to assess differences between lianas and trees in magnitudes and patterns of daily sap flow. Little seasonal change in θv was observed at 90–120 cm depth in both years. Mean soil water δD during the dry season was −19‰ at 0–30 cm, −34‰ at 30–60 cm, and −50‰ at 90–120 cm. Average values of xylem δD among the liana species ranged from –28‰ to –44‰ during the middle of the dry season, suggesting that water uptake was restricted to intermediate soil layers (30–60 cm). By the end of the dry season, all species exhibited more negative xylem δD values (–41‰ to –62‰), suggesting that they shifted to deeper water sources. Maximum sap flux density in co-occurring lianas and trees were comparable at similar stem diameter (DBH). Furthermore, lianas and trees conformed to the same linear relationship between daily sap flow and DBH. Our observations that lianas tap shallow sources of soil water at the beginning of the dry season and that sap flow is similar in lianas and trees of equivalent stem diameter do not support the common assumptions that lianas rely primarily on deep soil water and that they have higher rates of sap flow than co-occurring trees of similar stem size.

Isotopic evidence for the role of plant development on transpiration in deciduous forests of southern United States

We evaluated D/H ratios of soil‐ and plant‐extracted water during the 1997 growing season to assess the influence of temperature, humidity, and rainfall on water distribution in deciduous forests. Three state parks (Chicot in Arkansas; Natchez in Mississippi, and St. Bernard in Louisiana) were identified along a 13.5‐cm precipitation gradient established during the studied growing season within the Mississippi River basin. Samples were collected for isotopic determinations from five to six species at each site early (March) and late (June) in the growing season. To capture the isotopic variability in water sources, samples of rainwater, groundwater, and soil water were collected. Isotopic results for rainwater showed an average increase of 4‰ from March to June. This increase did not transfer to soil water: soil water δD values throughout the growing season showed values close to those measured for March rainwater. In contrast, leaf water showed δD values that were 15‰ to 20‰ higher in March compared to June δD values. Elevated March δD values in leaf water were observed in virtually all species at the three sites. Change in leaf water δD value during the growing season was not correlated with precipitation rate, temperature, humidity, or changes in atmospheric water vapor isotopic composition. We propose that this widespread March isotopic enrichment resulted from enhanced evaporative demand induced by accelerated plant growth early in the growing season. This suggestion implies a decoupling of environmental factors and plant response, pointing to the important role of plant developmental timing in ecosystem functioning.

Seasonal changes in water source of four families of loblolly pine ( Pinus taeda L.)

Source water utilized by four families of loblolly pine (Pinustaeda L.) was assessed by comparing the H isotope composition (δD) of xylem sap and of soil water from four depths (0–20 cm, 20–40 cm, 1.2 m, and 2.1 m) across 1 year. Soil water δD values varied with soil moisture content in the well-drained, sandy site and at each of the four soil depths. In September and November 1997 and May through November 1998, xylem sap δD values closely matched the soil water δD values of the upper soil horizons (0–20 and 0–40 cm) and, therefore, reflected significant water uptake from upper regions of the soil profile. In March 1998, xylem sap δD values closely matched the soil water δD values of the 1.2 m soil depth and, therefore, indicated that trees during this portion of the growing season were obtaining their water from deep in the soil profile. Analysis of source water use with a two-ended mixing model in the 3 months of collection that exhibited a range of soil water δD values across the soil profile confirmed that trees utilized different sources of water depending upon season of the year. In September 1997 and November 1998, source water uptake was primarily from the upper soil profile while in March 1998, source water uptake was from deep in the soil profile. With few exceptions, we did not find striking differences in source water use between drought-hardy families and those that were locally adapted.

Temporal and spatial partitioning of water resources among eight woody species in a Hawaiian dry forest.

Lowland dry forests are unique in Hawaii for their high diversity of tree species compared with wet forests. We characterized spatial and temporal partitioning of soil water resources among seven indigenous and one invasive dry forest species to determine whether the degree of partitioning was consistent with the relatively high species richness in these forests. Patterns of water utilization were inferred from stable hydrogen isotope ratios (δD) of soil and xylem water, zones of soil water depletion, plant water status, leaf phenology, and spatial patterns of species distribution. Soil water δD values ranged from -20‰ near the surface to -48‰ at 130 cm depth. Metrosideros polymorpha, an evergreen species, and Reynoldsia sandwicensis, a drought-deciduous species, had xylem sap δD values of about -52‰, and appeared to obtain their water largely from deeper soil layers. The remaining six species had xylem δD values ranging from -33 to -42‰, and apparently obtained water from shallower soil layers. Xylem water δD values were negatively correlated with minimum annual leaf water potential and positively correlated with leaf solute content, an integrated measure of leaf water deficit. Seasonal patterns of leaf production ranged from dry season deciduous at one extreme to evergreen with near constant leaf expansion rates at the other. Species tapping water more actively from deeper soil layers tended to exhibit larger seasonality of leaf production than species relying on shallower soil water sources. Individuals of Myoporum sandwicense were more spatially isolated than would be expected by chance. Even though this species apparently extracted water primarily from shallow soil layers, as indicated by its xylem δD values, its nearly constant growth rates across all seasons may have been the result of a larger volume of soil water available per individual. The two dominant species, Diospyros sandwicensis and Nestegis sandwicensis, exhibited low leaf water potentials during the dry season and apparently drew water mostly from the upper portion of the soil profile, which may have allowed them to exploit light precipitation events more effectively than the more deeply rooted species. Character displacement in spatial and temporal patterns of soil water uptake was consistent with the relatively high diversity of woody species in Hawaiian dry forests.

Partitioning of soil water among canopy trees in a seasonally dry tropical forest.

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about -15‰ at 0.1 m to -50 to -55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of -60‰. θv increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θv dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θv remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season.

Partitioning of water resources among plants of a lowland tropical forest.

Source water used by plants of several species in a semi-evergreen lowland tropical forest on Barro Colorado Island, Panama, was assessed by comparing the relative abundance of deuterium, D, versus hydrogen, H (stable hydrogen isotope composition, δD) in xylem sap and in soil water at different depths, during the dry season of 1992. Ecological correlates of source water were examined by comparing xylem water δD values with leaf phenology, leaf water status determined with a pressure chamber, and rates of water use determined as mass flow of sap using the stem heat balance method. Soil water δD values decreased sharply to 30 cm, then remained relatively constant with increasing depth. Average δD values were-13‰, for 0-30 cm depth and-36.7‰ for 30-100 cm depth. Soil water δD values were negatively associated with soil water content and soil water potential. Concurrent analyses of xylem water revealed a high degree of partitioning of water resources among species of this tropical forest. Xylem water δD of deciduous trees (average=-25.3±1.4‰) was higher than that of evergreen trees (average=-36.3±3.5‰), indicating that evergreen species had access to the more abundant soil water at greater depth than deciduous species. In evergreen shade-tolerant and high-light requiring shrubs and small trees, δD of xylem water was negatively correlated with transpiration rate and leaf water potential indicating that species using deeper, more abundant water resources had both higher rates of water use and more favorable leaf water status.

CH 4 production via CO 2 reduction in a temperate bog: A source of 13C-depIeted CH 4

The flux and δ 13 C of CH 4 released from an acidic peat bog ( pH = 3.5), located in the foothills of the Cascade Range in Washington state, USA, were measured over two annual cycles. The CH 4 flux ranged from 1–380 mg CH 4-C m −2 d −1, with an annual average of 73-mg CH 4-C m −2 d −1, and correlated with soil temperature. Ebullition accounted for 89% of the methane flux. The average δ 13 C of the CH 4 flux was −74 ± 5%., with no relationship between the δ 13 C and flux of CH 4. The δ 13 C of CH 4 and CO 2 in bubbles disturbed from the peat soil was −73 ± 4 and −2 ± 3%., respectively. The δD of CH 4 in bubbles was −308 ± 35%, and the δD of soil water was −65 ± 3%. Measurements of the rate of aceticlastic methanogenesis and CO 2 reduction in peat soil, using 14Clabeled acetate and sodium bicarbonate, show that acetate was not an important CH 4 precursor and that CO 2 reduction could account for all of the CH 4 production. The in situ kinetic isotope effect for CO 2 reduction ( α r ), calculated using the δ 13 C of soil water CO 2 and the CH 4 flux, was 0.932 ± 0.007. The δ 13C of CH 4 and CO 2 coexisting in soil waters provides an isotopic tracer of CH 4 production via CO 2 reduction.