Soil Water δ18O Research Articles

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.

Climatologic and hydrologic influences on the oxygen isotope ratio of tree cellulose in coastal southern California during the late 20th century

The oxygen isotope ratio (δ18O) of precipitation in continental, midlatitude regions is a complex measure of atmospheric dynamics and regional climate variability and can be preserved in geologic archives. However, continuous modern observations of precipitation δ18O in many midlatitude regions, particularly in the coastal western United States, are sparse. Here, tree‐ring cellulose δ18O from southern California is used to assess the potential of this proxy as an indicator of long‐term hydroclimate and atmospheric variability. From 1954 to 2004, we observed that cellulose δ18O was well replicated within a single stand of blue oak (Quercus douglasii) on interannual and decadal time scales. By using a forward mechanistic model, we demonstrate that cellulose δ18O is not driven solely by the oxygen isotope composition of precipitation at the site nor any other single hydroclimate variable. Instead, the interannual variability in cellulose δ18O prior to 1979 is positively correlated with growing season soil water δ18O and after 1979 is negatively correlated with relative humidity. In addition, 2 years (1983 and 1998) of anomalously low cellulose δ18O coincided with the wettest years in California and the strongest El Niño events of the late 20th century. For these years, decreased near surface evaporation and/or increased upper‐level condensation could account for the more depleted cellulose δ18O values. While blue oak cellulose δ18O is sensitive to atmospheric and hydroclimate variability, the varying temporal correlation between the cellulose δ18O and different environmental variables complicates any attempt to use the cellulose oxygen isotopes for reconstructions of climate variability beyond the calibration period.

Hydrologic control of the oxygen isotope ratio of ecosystem respiration in a semi-arid woodland

Abstract. We conducted high frequency measurements of the δ18O value of atmospheric CO2 from a juniper (Juniperus monosperma) woodland in New Mexico, USA, over a four-year period to investigate climatic and physiological regulation of the δ18O value of ecosystem respiration (δR). Rain pulses reset δR with the dominant water source isotope composition, followed by progressive enrichment of δR. Transpiration (ET) was significantly related to post-pulse δR enrichment because the leaf water δ18O value showed strong enrichment with increasing vapor pressure deficit that occurs following rain. Post-pulse δR enrichment was correlated with both ET and the ratio of ET to soil evaporation (ET/ES). In contrast, the soil water δ18O value was relatively stable and δR enrichment was not correlated with ES. Model simulations captured the large post-pulse δR enrichments only when the offset between xylem and leaf water δ18O value was modeled explicitly and when a gross flux model for CO2 retro-diffusion was included. Drought impacts δR through the balance between evaporative demand, which enriches δR, and low soil moisture availability, which attenuates δR enrichment through reduced ET. The net result, observed throughout all four years of our study, was a negative correlation of post-precipitation δR enrichment with increasing drought.

Earthworm-produced calcite granules: A new terrestrial palaeothermometer?

In this paper we show for the first time that calcite granules, produced by the earthworm Lumbricus terrestris, and commonly recorded at sites of archaeological interest, accurately reflect temperature and soil water δ18O values. Earthworms were cultivated in an orthogonal combination of two different (granule-free) soils moistened by three types of mineral water and kept at three temperatures (10, 16 and 20°C) for an acclimatisation period of three weeks followed by transfer to identical treatments and cultivation for a further four weeks. Earthworm-secreted calcite granules were collected from the second set of soils. δ18O values were determined on individual calcite granules (δ18Oc) and the soil solution (δ18Ow). The δ18Oc values reflect soil solution δ18Ow values and temperature, but are consistently enriched by 1.51 (±0.12)‰ in comparison to equilibrium in synthetic carbonates. The data fit the equation 1000lnα=[20.21±0.92] (103T−1)−[38.58±3.18] (R2=0.95; n=96; p<0.0005). As the granules are abundant in modern soils, buried soils and archaeological contexts, and can be dated using U–Th disequilibria, the developed palaeotemperature relationship has enormous potential for application to Holocene and Pleistocene time intervals.

Long-term increases in snow pack elevate leaf N and photosynthesis in Salix arctica: responses to a snow fence experiment in the High Arctic of NW Greenland

We examine the influence of altered winter precipitation on a High Arctic landscape with continuous permafrost. Gas exchange, leaf tissue element and isotopic composition (N, δ13C, δ15N), and plant water sources derived from stem and soil water δ18O were examined in Salix arctica (arctic willow) following a decade of snow-fence-enhanced snow pack in NW Greenland. Study plots in ambient and +snow conditions were sampled in summer 2012. Plants experiencing enhanced snow conditions for 10 years had higher leaf [N], photosynthetic rate, and more enriched leaf δ15N. Enhanced snow did not influence stomatal conductance or depth of plant water use. We attribute the higher photosynthetic rate in S. arctica exposed to deeper snow pack to altered biogeochemical cycles which yielded higher leaf [N] rather than to enhanced water availability. These data demonstrate the complexity of High Arctic plant responses to changes in winter conditions. Furthermore, our data depict the intricate linkages between winter and summer conditions as they regulate processes such as leaf gas exchange that may control water vapor and CO2 feedbacks between arctic tundra and the surrounding atmosphere.

Hot or not? Impact of seasonally variable soil carbonate formation on paleotemperature and O-isotope records from clumped isotope thermometry

Quantifying the timing and temperature of soil carbonate formation is important for interpreting isotopic records of Earth-surface temperature and soil water composition from paleosols. Pedogenic carbonates typically have been assumed to form at mean annual temperature, but recent work suggests warm-season bias in carbonate formation could impact the interpretation of δ18O values and carbonate formation temperatures for paleosol carbonates. We investigate the relationship between seasonally variable soil temperatures and carbonate formation temperatures estimated using clumped isotope thermometry (T(Δ47)). Holocene soil carbonates were collected along the eastern flank of the Andes (33°S) over 2km of relief and a nearly 15°C range of mean annual air temperature (MAAT). This area receives both westerly, winter season precipitation and easterly, summer season precipitation, leading to a reversal of soil moisture regimes that occurs at ∼2km elevation. In instrumented pits sampled to 1m depth, we do not observe systematic variation in T(Δ47) with depth or with elevation-dependent MAAT. Average T(Δ47) values for soil carbonates collected above 2km elevation reflect summer soil temperatures. In contrast, T(Δ47) values below ∼2km track mean annual soil temperature. These results may reflect the dominance of summer precipitation below ∼2km, which likely delays soil drying and carbonate growth until fall. This seasonal variability in carbonate formation greatly impacts carbonate formation temperature, with important implications for estimating soil water δ18O values. Calculating soil water δ18O values using T(Δ47) produces a soil water δ18O lapse rate of −4.6‰/km, remarkably similar to that of modern surface waters (−4.8‰/km), and significantly improving on previous soil water estimates assuming MAAT for carbonate formation temperatures. Although seasonal bias can prevent the straightforward translation of soil carbonate T(Δ47) into meaningful surface air temperatures, direct constraints on temperature from clumped isotopes can both provide a window into soil processes and aid in the interpretation of conventional stable isotopic data to reconstruct surface conditions in the past.

MAIDENiso: a multiproxy biophysical model of tree-ring width and oxygen and carbon isotopes

MAIDENiso is a new version of the process-based biogeochemical model MAIDEN of tree growth. Isotopic modules have been implemented to simulate stable oxygen and carbon isotopes in tree-ring cellulose (TRC). In addition to annual increment biomass, this new model version estimates δ18O and δ13C associated with the daily amount of carbon allocated to the stem. MAIDENiso only requires daily input data: minimal and maximal air temperatures, amount of precipitation, CO2 atmospheric concentration, and δ13C in CO2. MAIDENiso simulates tree-ring width, δ18O in precipitation (δ18OP), δ18O in soil water (δ18OSW), δ18O in xylem water (δ18OXW), δ18O in cellulose (δ18OTRC), and δ13C in cellulose (δ13CTRC). The model has been calibrated and validated with tree-ring series sampled in the Fontainebleau Forest (France) from 1953 to 2000. We first calibrated several parameters for the 1977–2000 period and then validated it for the independent 1953–1976 period. Over the complete interval (1953–2000), we obtained correlations between observations and simulations above 0.5 for both isotopic series and above 0.65 for tree growth series. An important feature of the model is its ability to simulate not only extreme values of tree growth, such as the effect of 1976 drought, but also its persistence over several years.

Use of the isotope flux ratio approach to investigate the C&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O&amp;lt;sup&amp;gt;16&amp;lt;/sup&amp;gt;O and &amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; exchange near the floor of a temperate deciduous forest

Abstract. Stable isotopologues of CO2, such as 13CO2 and C18OO, have been used to study the CO2 exchange between land and atmosphere. The advent of new measuring techniques has allowed near-continuous measurements of stable isotopes in the air. These measurements can be used with micrometeorological techniques, providing new tools to investigate the isotope exchange in ecosystems. The objectives of this study were to evaluate the use of the isotope flux ratio method (IFR) near the forest floor of a temperate deciduous forest and to study the temporal dynamics of δ18O of CO2 flux near the forest floor by comparing IFR estimates with estimates of δ18O of net soil CO2 flux provided by an analytical model. Mixing ratios of 12C16O2, 13CO2 and C16O18O were measured within and above a temperate deciduous forest, using the tunable diode laser spectroscopy technique. The half-hourly compositions of the CO2 flux near the forest floor (δ13CF and δ18OF) were calculated by IFR and compared with estimates provided by a modified Keeling plot technique (mKP) and by a Lagrangian dispersion analysis (WT analysis). The mKP and IFR δ18OF estimates showed good agreement (slope = 1.03 and correlation, R2 = 0.80). The δ13CF estimates from the two methods varied in a narrow range of −32.7 and −23‰; the mean (± SE) mKP and IFR δ13CF values were −27.5‰ (±0.2) and −27.3‰ (±0.1), respectively, and were statistically identical (p&gt;0.05). WT analysis and IFR δ18OF estimates showed better correlation (R2 = 0.37) when only turbulent periods (u&amp;amp;ast;&gt;0.6 m s−1) were included in the analysis. The large amount of data captured (~95 % of half-hour periods evaluated) for the IFR in comparison with mKP (27 %) shows that the former provides new opportunities for studying δ18OF dynamics within forest canopies. Values of δ18OF showed large temporal variation, with values ranging from −31.4‰ (DOY 208) to −11.2‰ (DOY 221). Precipitation events caused substantial variation (~8‰) in δ18OF over a period of approximately 24 h. A diel trend of δ18OF was observed, with more depleted values present during the daytime. Model simulations indicate that the activity of the carbonic anhydrase enzyme was quite variable during the evaluated period. These simulations indicate that more frequent sampling of δ18O of soil water could improve the estimates of δ18O of net soil CO2 flux.

Quantitative palaeoclimate reconstruction as an inverse problem: A Bayesian inference of late-Holocene climate on the eastern Tibetan Plateau from a peat cellulose δ18O record

There has been a sustained need to quantify past climate changes from proxy records to better understand the driving mechanisms and thus to improve the prediction of the future. Transfer function is an intuitive and frequently used method in this regard. However, this method is unable to provide interpretive and predictive results from proxy records, because physical processes are not included. The inverse proxy modelling method opened up a new avenue for quantitative palaeoclimate reconstructions through the integration of proxy records with deterministic models. It is fundamentally different from the statistical approach, representing a conceptual advance in quantitative palaeoclimatology. Here we demonstrate the potential of this method by placing a mechanistic model and a 6000 year long peat cellulose δ18O record obtained from the high-cold and monsoonal eastern Tibetan Plateau ( c. 3500 m a.s.l.) in a Bayesian paradigm. In this worked example, the marginal posterior probability distributions of palaeoclimate variables such as the δ18O of soil water, temperature, and relative humidity were inferred jointly through the solution to an ill-posed inverse problem using the Markov chain-Monte Carlo method. Our results indicate that the observed variation of the peat cellulose δ18O record in this monsoonal area essentially reflects the changes in the oxygen isotopic composition of soil water, which is closely linked to that of rainfall. Compared with hydrology, temperature and humidity have little influence on the oxygen isotope fractionation of leaf water.

Oxygen isotope ratios in Holocene carbonates across a climatic gradient, eastern Washington State, USA: Evidence for seasonal effects on pedogenic mineral isotopic composition

Oxygen isotope ratios of pedogenic carbonates were analyzed along a climatic gradient in the Palouse region of eastern Washington State to evaluate changes in their isotopic composition with changing climate. A modern analog approach was used to assess the relationship between the istotope ratios in the pedogenic carbonates with those in soil water (measured in June when we anticipated carbonate formation might occur because of low soil water content), and meteoric water from two National Atmospheric Deposition Program (NADP) sites adjacent to our sequence. Mean annual meteoric water δ18O values (wrt VSMOW) from the two NADP sites varied slightly with elevation, —13.9‰ at the Starkey site (elevation 1253 m) and —13.0‰ at the Palouse site, (elevation 766 m) but showed distinct seasonal trends with weighted means ranging from —11.9‰ during autumn to —15.0‰ during the winter. Carbonate δ18O (wrt VPDB) varied significantly with MAP, ranging from —14.1‰ at the most arid site to —11.4‰ at the wettest site but displayed a trend opposite to that of soil water δ18O (—4 to —12‰ VSMOW for the 70 cm depth). Neither measured soil water δ18O values nor mean meteoric water δ18O values generated realistic temperatures of formation for the observed trend in carbonate δ18O values. When mean annual temperatures across the gradient were used for temperature of formation, calculated soil water δ18O in equilibrium with carbonate δ 18O was similar to winter precipitation δ18O at the most arid sites but became increasingly similar to mean annual meteoric δ 18O as MAP increased. We suggest that the unexpected trend in carbonate δ18O is largely due to the diminishing influence of winter precipitation/snowmelt on soil moisture as MAP increases across the climatic gradient and that seasonal effects of meteoric water may need to be considered in pedogenic mineral isotopic equilibrium in temperate regions.

Hydrologic characteristics of lake‐ and stream‐side riparian wetted margins in the McMurdo Dry Valleys, Antarctica

AbstractWater is a limiting factor for life in the McMurdo Dry Valleys (MDV), Antarctica. The active layer (seasonally thawed soil overlying permafrost) accommodates dynamic hydrological and biological processes for 10–16 weeks per year. Wetted margins (visually wetted areas with high moisture content) adjacent to lakes and streams are potential locations of great importance in the MDV because of the regular presence of liquid water, compared with the rest of the landscape where liquid water is rare. At 11 plots (four adjacent to lakes, seven adjacent to streams), soil particle size distribution, soil electrical conductivity, soil water content and isotopic signature, width of the wetted margin, and active layer thaw depth were characterised to determine how these gradients influence physicochemical properties that determine microbial habitat and biogeochemical cycling. Sediments were generally coarse‐grained in wetted margins adjacent to both lakes and streams. Wetted margins ranged from 1·04 to 11·01 m in average length and were found to be longer at lakeside sites than streamside. Average thaw depths ranged from 0·12 to 0·85 m, and were found to be deepest under lake margins. Lake margins also had much higher soil electrical conductivity, steeper topographic gradients, but more gradual soil moisture gradients than stream margins. Patterns of soil water δ18O and δD distribution indicate capillary action and evaporation from wetted margins; margin pore waters generally demonstrated isotopic enrichment with distance from the shore, indicating evaporation of soil water. Lake margin pore waters were significantly more negative in DXS (DXS = δD‐8δ18O) than streamside pore waters, indicating a longer history of evaporation there. Differences between lake and stream margins can be explained by the more consistent availability of water to lake margins than stream margins. Differences in margin characteristics between lakes and streams have important consequences for the microbial habitat of these margins and their functional role in biogeochemical cycling at these terrestrial–aquatic interfaces. Copyright © 2009 John Wiley &amp; Sons, Ltd.

A field-based method for simultaneous measurements of the δ&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O and δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C of soil CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; efflux

Abstract. Three approaches for determining the stable isotopic composition (δ13C and δ18O) of soil CO efflux were compared. A new technique employed mini-towers, constructed of open-topped piping, that were placed on the soil surface to collect soil-emitted CO2. Samples were collected along a vertical gradient and analyzed for CO2 concentration and isotopic composition. These data were then used to produce Keeling plots to determine the δ18O and δ13C of CO2 emitted from the soil. These results were then compared to the δ18O and δ13C of soil-respired CO2 measured with two other techniques: (1) flux chambers and (2) estimation from the application of the diffusional fractionation factor to measured values of below ground soil CO2 and to CO2 in equilibrium with soil water δ18O. Mini-tower δ18O Keeling plots were linear and highly significant (0.81&lt; r 2 &gt; 0.96), in contrast to chamber δ18O Keeling plots, which showed significant curvature, necessitating the use of a mass balance to calculate the δ18O of respired CO2. In the chambers, the values determined for the δ18O of soil respired CO2 approached the value of CO2 in equilibrium with surficial soil water, and the results were significantly δ18O enriched relative to the mini-tower results and the δ18O of soil CO2 efflux determined from soil CO2. There were close agreements between the three methods for the determination of the δ13C of soil efflux CO2. Results suggest that the mini-towers can be effectively used in the field for determining the δ18O and the δ13C of soil-respired CO2.

Carbon and oxygen isotopic variability in Neogene paleosol carbonates: constraints on the evolution of the C4-grasslands of the Great Plains, USA

Understanding the development of grassland ecosystems is critical for understanding the evolution of their associated vertebrate faunas. We focus on the central and southern portions of the Great Plains, which is the largest continuous grassland in North America and has a well-documented mammalian fossil record. Modern biomass in this region is dominated by C4 grasses, but paleontological evidence indicates the region was forested prior to the middle Miocene, implying the region was formerly dominated by C3 plants. We examine long- and short-term variations in the abundance of C4 grasses and climatic conditions in the central and southern Great Plains using the stable carbon and oxygen isotope composition of paleosol carbonates from 28 sections of 11 lithostratigraphic units in the region. The carbon isotope composition (δ13C) of paleosol carbonate reflects the proportion of C3 and C4 plants that grew in an ancient soil. The long-term pattern of δ13C values indicates that the percentage of C4 grasses in the Great Plains was moderate (ca. 20%) throughout most of the Miocene, increased from 6.4 to 4.0 Ma, and reached modern levels by 2.5 Ma. Based on the range in δ13C values within sections and the mean δ13C values for different carbonate morphologies that reflect different intervals of carbonate accumulation, the abundance of C4 biomass did not vary substantially on short time scales. The variability in C4 biomass on short time scales does not correlate with age of section, latitude, longitude, or the mean percentage of C4 biomass. The oxygen isotope composition (δ18O) of soil carbonate is controlled by soil temperature and the δ18O value of soil water. Great Plains paleosol δ18O values vary considerably in relation to time and latitude. Removing the variation due to latitude with least squares linear regression reveals a temporal pattern that is consistent with the global pattern of Neogene climate change. Quantitative interpretation of the δ18O record is difficult without independent control on temperature or the oxygen isotope composition of soil water, both of which have probably varied with long-term climate change. Comparisons of measured δ18O values for each section with predicted values based on modern temperature data and estimated modern meteoric water δ18O values suggest that (1) most Miocene sections have δ18O values consistent with warmer temperatures and more positively shifted soil water δ18O values than today, (2) three Miocene sections in western Nebraska probably had soil water compositions that were more negative than similar-aged localities in the region, and (3) the Plio-Pleistocene sections have δ18O values consistent with cooler temperatures and more negative soil water δ18O values than today. The paleosol δ13C values do not correlate strongly with the pattern of climate change recorded by the δ18O values, suggesting that long-term change in mean temperature is not the main control on the abundance of δ13C biomass in the Great Plains. We propose two testable hypotheses to account for our data and other published data that bear on the evolution of the Great Plains ecosystem.

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 2. Short‐term dynamics of foliar and soil component fluxes in an old‐growth ponderosa pine forest

The oxygen isotope contents (δ18O) of soil, xylem, and leaf water and ecosystem respiration were studied in a ponderosa pine forest during summer 2001. Our goal was to assess whether δ18O of CO2 could be used to quantify the relative contributions of soil and foliar respiration to total nocturnal ecosystem respiration. The δ18O in leaf and soil water showed enrichment over a 2‐week sampling period as the weather became hot and dry (leaves 0.9 to 15.0‰, and soil −10.4 to −3.1‰), while δ18O of xylem water remained constant (−12.9‰). Water in the soil was enriched in 18O near the soil surface (−6.4‰ at 5 cm depth) relative to greater depths (−11.1‰ at 20 cm). The δ18O of ecosystem respiration became gradually enriched over the 2‐week sampling period (from 24.2 initially to 32.9‰ at the end, VSMOW scale). Soil respiration contributed 80 ± 12 percent to the total respiratory flux, close to estimates from scaled‐up chamber data (77% [Law et al., 2001a]). Quantitative application of the isotopic approach to determine respiratory proportions required direct measurement of δ18O of soil and xylem water, air and soil temperature, and humidity. Better estimates of the isotopic signatures of component fluxes could be achieved with additional measurements and more detailed modeling. Results demonstrate that (1) there is variability in δ18O of precipitation inputs to ecosystems, (2) immediately following a precipitation event, δ18O of ecosystem respiration may reflect δ18O of precipitation, (3) periods of hot dry weather can substantially enrich ecosystem water pools and subsequently alter the isotope content of CO2 in ecosystem respiration, and (4) stable oxygen isotopes in CO2 can be used to quantify the foliar and soil components of ecosystem respiration.

Characteristics of soil moisture in permafrost observed in East Siberian taiga with stable isotopes of water

AbstractSoil moisture and its isotopic composition were observed at Spasskaya Pad experimental forest near Yakutsk, Russia, during summer in 1998, 1999, and 2000. The amount of soil water (plus ice) was estimated from volumetric soil water content obtained with time domain reflectometry. Soil moisture and its δ18O showed large interannual variation depending on the amount of summer rainfall. The soil water δ18O decreased with soil moisture during a dry summer (1998), indicating that ice meltwater from a deeper soil layer was transported upward. On the other hand, during a wet summer (1999), the δ18O of soil water increased due to percolation of summer rain with high δ18O values.Infiltration after spring snowmelt can be traced down to 15 cm by the increase in the amount of soil water and decrease in the δ18O because of the low δ18O of deposited snow. About half of the snow water equivalent (about 50 mm) recharged the surface soil. The pulse of the snow meltwater was, however, less important than the amount of summer rainfall for intra‐annual variation of soil moisture.Excess water at the time just before soil freezing, which is controlled by the amount of summer rainfall, was stored as ice during winter. This water storage stabilizes the rate of evapotranspiration. Soil water stored in the upper part of the active layer (surface to about 120 cm) can be a water source for transpiration in the following summer. On the other hand, once water was stored in the lower part of the active layer (deeper than about 120 cm), it would not be used by plants in the following summer, because the lower part of the active layer thaws in late summer after the plant growing season is over. Copyright © 2002 John Wiley &amp; Sons, Ltd.

The oxygen isotope composition of nitrate generated by nitrification in acid forest floors

The oxygen isotope composition of nitrate is used increasingly for identifying the origin of nitrate in terrestrial and aquatic ecosystems. This novel isotope tracer technique is based on the fact that nitrate in atmospheric deposition, in fertilizers, and nitrate generated by nitrification in soils appear to have distinct oxygen isotope ratios. While the typical ranges of δ18O values of nitrate in atmospheric deposition and fertilizers are comparatively well known, few experimental data exist for the oxygen isotope composition of nitrate generated by nitrification in soils. The objective of this study was to determine δ18O values of nitrate formed by microbial nitrification in acid forest floors.Evidence from laboratory incubation experiments and field studies suggests that during microbial nitrification in acid forest floor horizons, up to two of the three oxygen atoms in newly formed nitrate are derived from water, particularly if ammonium is abundant and nitrification rates are high. It was, however, also observed that in ammonium-limited systems with low nitrification rates, significantly less than two thirds of the oxygen in newly formed nitrate can be derived from water oxygen, presumably as a result of heterotrophic nitrification. It can be concluded from the presented data that the δ18O values of nitrate formed by microbial nitrification in acid forest floors typically range between +2 and +14‰, assuming that soil water δ18O values vary between −15 and −5‰. Hence, oxygen isotope ratios of nitrate formed by nitrification in forest floors are usually distinct from those of other nitrate sources such as atmospheric deposition and synthetic fertilizers and, therefore, constitute a valuable qualitative tracer for distinguishing among these sources of nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of δ18O values, particularly for atmospheric nitrate deposition and for nitrate from microbial nitrification.

Isotope hydrology of the Allt a' Mharcaidh catchment, Cairngorms, Scotland: implications for hydrological pathways and residence times

The hydrology of oxygen-18 (18O) isotopes was monitored between 1995 and 1998 in the Allt a' Mharcaidh catchment in the Cairngorm Mountains, Scotland. Precipitation (mean δ18O=−7·69‰) exhibited strong seasonal variation in δ18O values over the study period, ranging from −2·47‰ in the summer to −20·93‰ in the winter months. As expected, such variation was substantially damped in stream waters, which had a mean and range of δ18O of −9·56‰ and −8·45 to −10·44‰, respectively. Despite this, oxygen-18 proved a useful tracer and streamwater δ18O variations could be explained in terms of a two-component mixing model, involving a seasonally variable δ18O signature in storm runoff, mixing with groundwater characterized by relatively stable δ18O levels. Variations in soil water δ18O implied the routing of depleted spring snowmelt and enriched summer rainfall into streamwaters, probably by near-surface hydrological pathways in peaty soils. The relatively stable isotope composition of baseflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in δ18O levels in various catchment waters provided a first approximation of mean residence times in the major hydrological stores. Preliminary estimates are 0·2–0·8 years for near-surface soil water that contributes to storm runoff and 2 and >5 years for shallow and deeper groundwater, respectively. These 18O data sets provide further evidence that the influence of groundwater on the hydrology and hydrochemistry of upland catchments has been underestimated. Copyright © 2000 John Wiley & Sons, Ltd.

Evaluation of evaporation rate from forested soil surface using stable isotopic composition of soil water in a headwater basin

The stable isotopic composition of rain water and soil water was analysed and the soil surface evaporation rate was evaluated using the isotope value of soil water taken from a forested headwater basin in central Japan. Rainfall, throughfall, soil water, groundwater and discharge water were sampled at intervals of one month to determine the isotopic compositions of deuterium (D) and oxygen-18 (18O) therein. In addition, tensiometric observation of soil water was performed continuously. The stable isotopic ratio of soil water was homogenized between the soil surface and a depth of 100 cm by means of active soil water movement above 100 cm in depth. The mean δ18O in deep soil water below a depth of 100 cm was 0·24‰ higher than the volume weighted mean δ18O in throughfall. Consequently, the mean evaporation rate from the soil surface was calculated as 5% of the evapotranspiration rate using the difference between the δ18O in throughfall and in deep soil water, based on the Rayleigh equation under equilibrium condition. Copyright © 1998 John Wiley & Sons, Ltd.

Ecosystem Geography. Robert G. Bailey. Springer-Verlag, New York, NY, 1996, 204 pp., illus., folded maps, US $34.50, (paperback) ISBN 0-387-94586-5, US $69.95 (hardcover), ISBN 0-387-94354-4.

Seasonal drought will become more intense and frequent in some regions due to global climate change, which may lead to significant changes in the competitive interactions and uptake depths of plant water sources. Based on δ18O and δD in xylem and soil water, we investigated the impacts of seasonal drought on plant water sources from July 2011 to October 2013 for a subtropical coniferous plantation in southeastern China. Our results indicated that the studied tree species of Pinus massoniana, Pinus elliottii and Cunninghamia lanceolata appeared to have inter-specific competition for water resources from similar depths. There was a switch of the major water source from shallow soil during the non-drought periods (July to October) to deep soil during the drought periods (November–June). Similar seasonal trend of water uptake was estimated by δ18O and δD. However, relative to the results of δD, water sources predicted by δ18O showed that trees seemed to derive more deep soil water during the drought periods and more shallow soil water during the non-drought periods. The differences of plant water source partitioning based on δ18O and δD were significant during both the drought and non-drought periods, which probably resulted from the artifacts of the cryogenic vacuum distillation and spectral contamination correction. These findings will have important implications for further studies when just one of the dual stable isotopes of δ18O and δD is applied.

Water use by woody plants on contrasting soils in a savanna parkland: Assessment with δ2H and δ18O

In savanna parklands of southern Texas, patches of grassland and ‘discrete clusters’ of small trees and shrubs occur on sandy loam surface soils underlain by an argillic horizon (claypan) at 40 cm. Large trees and shrubs in ‘groves’ occur on deep (2 m) sandy loam soils without an argillic horizon. δ2H and δ18O of rainfall, groundwater, and soil and plant water were measured to: (1) determine if coexistence in woody patches occurs via vertical stratification of soil water uptake; (2) document differences in plant water acquisition on contrasting soil types; and (3) evaluate recharge and evaporative losses of soil moisture from grassland vs. wooded landscape elements. Groundwater was isotopically similar to weighted rainfall, suggesting local recharge at this site. Linear regressions of soil water δ2H on δ18O yielded slopes less than the meteoric water line, indicating significant evaporative losses of soil moisture in all landscape elements. Interspecific differences in root density distribution were significant; some woody species had roots well below 1.6 m, while others had few roots below 0.8 m. δ2H and δ18O values of stem water from all plants in groves were lower than those of soil water in the upper 1.5 m of the profile, suggesting all species obtained their water from depths >1.5 m. Deep roots of trees and shrubs at this savanna parkland site thus appeared to have a functional significance that was not revealed by biomass or density determinations. Root densities of species in discrete clusters (claypan present) were typically greater than those of the same species in groves (claypan absent), especially in the upper 80 cm of the soil profile. Consistent with rooting profiles, δ2H and δ18O values of plant water indicated that trees and shrubs in discrete clusters with fine- textured subsoils obtained most of their water at depths <1.5 m. As with groves, there was no indication of water resource partitioning between species. In summary, we saw no isotopic evidence that co- occurring woody plants at this savanna parkland site were partitioning soil moisture vertically during late summer/early fall, despite marked differences in their root density distributions. This supports other lines of evidence which indicate that species interactions in tree/shrub clumps are competitive, and that species composition is therefore unstable in those landscape elements.