Estimation of denitrification potential in a karst aquifer using the <Superscript>15</Superscript>N and <Superscript>18</Superscript>O isotopes of NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack>

A confined aquifer in the Malm Karst of the Franconian Alb, South Germany was investigated in order to understand the role of the vadose zone in denitrifiaction processes. The concentrations of chemical tracers Sr²⁺ and Cl^– and concentrations of stable isotope ¹⁸O were measured in spring water and precipitation during storm events. Based on these measurements a conceptual model for runoff was constructed. The results indicate that pre-event water, already stored in the system at the beginning of the event, flows downslope on vertical and lateral preferential flow paths. Chemical tracers used in a mixing model for hydrograph separation have shown that the pre-event water contribution is up to 30%. Applying this information to a conceptual runoff generation model, the values of ¹⁵N and ¹⁸O in nitrate could be calculated. Field observations showed the occurence of significant microbial denitrification processes above the soil/bedrock interface before nitrate percolates through to the deeper horizon of the vadose zone. The source of nitrate could be determined and denitrification processes were calculated. Assuming that the nitrate reduction follows a Rayleigh process one could approximate a nitrate input concentration of about 170 mg/l and a residual nitrate concentration of only about 15%. The results of the chemical and isotopic tracers postulate fertilizers as nitrate source with some influence of atmospheric nitrate. The combined application of hydrograph separation and determination of isotope values in ¹⁵N and ¹⁸O of nitrate lead to an improved understanding of microbial processes (nitrification, denitrification) in dynamic systems.     

Determinants of isotopic coupling of CO2 and water vapour within a Quercus petraea forest canopy

Concentration and isotopic composition (δ¹³C and δ¹⁸O) of ambient CO₂ and water vapour were determined within a Quercus petraea canopy, Northumberland, UK. From continuous measurements made across a 36-h period from three heights within the forest canopy, we generated mixing lines (Keeling plots) for δ_a ¹³CO₂, δ_a C¹⁸O¹⁶O and δ_a H₂ ¹⁸O, to derive the isotopic composition of the signal being released from forest to atmosphere. These were compared directly with measurements of different respective pools within the forest system, i.e. δ¹³C of organic matter input for δ_a ¹³CO₂, δ¹⁸O of exchangeable water for δ_a C¹⁸O¹⁶O and transpired water vapour for δ_a H₂ ¹⁸O. [CO₂] and δ_a ¹³CO₂ showed strong coupling, where the released CO₂ was, on average, 4 per mil enriched compared to the organic matter of plant material in the system, suggesting either fractionation of organic material before eventual release as soil-respired CO₂, or temporal differences in ecosystem discrimination. δ_a C¹⁸O¹⁶O was less well coupled to [CO₂], probably due to the heterogeneity and transient nature of water pools (soil, leaf and moss) within the forest. Similarly, δ_a H₂ ¹⁸O was less coupled to [H₂O], again reflecting the transient nature of water transpired to the forest, seen as uncoupling during times of large changes in vapour pressure deficit. The δ¹⁸O of transpired water vapour, inferred from both mixing lines at the canopy scale and direct measurement at the leaf level, approximated that of source water, confirming that an isotopic steady state held for the forest integrated over the daily cycle. This demonstrates that isotopic coupling of CO₂ and water vapour within a forest canopy will depend on absolute differences in the isotopic composition of the respective pools involved in exchange and on the stability of each of these pools with time. Received: 21 March 1998 / Accepted: 10 December 1998     

Hydraulic Lift in Cork Oak Trees in a Savannah-Type Mediterranean Ecosystem and its Contribution to the Local Water Balance

The aim of this study was to identify the sources and depth of water uptake by 15-years old Quercus suber L. trees in southern Portugal under a Mediterranean climate, measuring δ¹⁸O and δD in the soil–plant-atmosphere continuum. Evidence for hydraulic lift was substantiated by the daily fluctuations observed in Ψ_s at 0.4 and 1 m depth and supported by similar δ¹⁸O values found in tree xylem sap, soil water in the rhizosphere and groundwater. From 0.25 m down to a depth of 1 m, δD trends differed according to vegetation type, showing a more depleted value in soil water collected under the evergreen trees (−47‰) than under dead grasses (−35‰). The hypothesis of a fractionation process occurring in the soil due to diffusion of water vapour in the dry soil is proposed to explain the more depleted soil δD signature observed under trees. Hydraulically lifted water was estimated to account for 17–81% of the water used during the following day by tree transpiration at the peak of the drought season, i.e., 0.1–14 L tree⁻¹ day⁻¹. Significant relationships found between xylem sap isotopic composition and leaf water potential in early September emphasized the positive impact of the redistribution of groundwater in the rhizosphere on tree water status.     

Differential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community

We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado. The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C₄ shrub) and Bouteloua gracilis (a C₄ grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δ¹⁸O value of plant stem water (for comparison with potential water sources) and the δ¹³C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized by the shrubs. Based on stem water δ¹⁸O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees were exclusively using groundwater, and the consistent δ¹³C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δ¹⁸O, shrubs may have also had access to groundwater. However, their overall δ¹⁸O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently, our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition by shrubs. Received: 23 December 1997 / Accepted: 16 September 1998     

Isotope Systematics of Sulfate-oxygen and Sulfate-sulfur in Six European Peatlands

Oxygen (O) and sulfur (S) isotope systematics in bog water sulfates were determined for six Sphagnum dominated wetlands located in the British Isles and the Czech Republic, Central Europe. Comparison of a polluted and unpolluted site showed that 4 times higher atmospheric S inputs led to 3 times higher bog water sulfate concentrations and substrate S concentrations, 3 times increased ranges of substrate S concentrations, and 3 times increased ranges of δ³⁴S values. Sites with elevated atmospheric S inputs exhibited greater geochemical variability in wetland S species. Sulfate O–S isotope composition of bog pore water at a depth of 40 cm below surface differed from that of surface bog water, indicating that dissimilatory bacterial sulfate reduction, a process known to discriminate against the heavier isotopes ¹⁸O and ³⁴S, occurred in surface peat layers. While bacterial sulfate reduction remained to be one of the main isotope-selective processes for sulfate in peat, it could not fully explain the O–S isotope systematics of peat waters. The ‘residual’ sulfate was not simultaneously enriched in the heavier isotopes ¹⁸O and ³⁴S. Mixing of residual sulfate following bacterial sulfate reduction with the product of S²⁻ reoxidation, cleavage of esters, and isotope exchange reactions may have contributed to the decoupling of the δ³⁴S_so4 and δ¹⁸S_so4 values. Large within-site differences in δ¹⁸S_so4 and δ³⁴S_so4 (up to 13 and 15‰, respectively) indicated little communication between the 0 and 40 cm peat depth at some sites. Extremely high δ¹⁸S_so4 and δ³⁴S_so4 values found in several peat bog water samples from Connemara (Ireland), Thorne Moors (England) and Ocean (Czech Republic) were not seen in streams draining the wetlands. Direct runoff of atmogenic sulfate constituted a significant portion of the bog outflow. At the wetland scale, zones of dissimilatory bacterial sulfate reduction form pockets whose lateral hydrological fluxes are small.     

Isotopic characteristics of typhonic rainwater: typhoons no. 13 (1993) and no. 6 (1996)

The stable isotope ratios of hydrogen and oxygen were measured for rainwater samples from Typhoon No. 13, which struck Japan on September 1993, and Typhoon No. 6, which passed in July 1996. Rainwater was collected every hour over 2- to 3-day periods at Beppu, Japan (33°16′N, 131°29′E), which lies on or close to the typhoon routes. The deuterium excess parameters (δD – 8 ·δ¹⁸O) of the rainwaters vary over wide ranges from 19.22 to 1.52 for Typhoon No. 13, 1993, and from 6.02 to −8.10 for Typhoon No. 6, 1996, respectively. Rainwaters with higher d-values precipitated in the forward parts of the typhoons. This is ascribed to the possibility that the water vapors supplied by the bottom air currents from the front (rear) of the typhoons may be originally formed by rapid (gentle) evaporation of seawater. Symmetrical patterns of spatial δD and δ¹⁸O distributions within the typhoon precipitations, as estimated from the variations in the isotope ratios of the typhoon rainwaters, should appear from a continuous isotopic fractionation of water vapors with the bottom air currents converging toward the typhoon center. The weighted means of δD and δ¹⁸O of the typhoon precipitations are more negative than those of ordinary rainfall, suggesting that an isotopic influence of typhoon precipitation on surface waters, e.g., river, stream, lake, and spring waters, may be important. Received: January 22, 2000 / Accepted: March 24, 2000     

Temperature versus species-specific influences on the stable oxygen isotope ratio of tree rings

Stable isotopic ratios integrate ecosystem variability while reflecting change in both environmental and biological processes. At sites, where climate does not strongly limit tree growth, co-occurring trees may display large discrepancies in stable oxygen isotopic ratios (δ¹⁸O) due to the interplay between biological processes (competition for light and nutrients, individual tree physiology, etc.) and climate. For a better quantification of the isotope variability within and among trees, the climatic and/or individual tree effects on seasonal δ¹⁸O variations in precipitation, soil water, leaf water and leaf organic material (whole leaf, cellulose and starch) and annual δ¹⁸O variations in tree-ring cellulose for Fagus sylvatica (Fs), Quercus robur (Qr), Carpinus betulus (Cb) and Pinus sylvestris (Ps) were studied in a mature temperate forest in Switzerland, using a mixed linear regression model technique. Furthermore, the influence of environmental factors on δ¹⁸O was assessed by means of three common isotope fractionation models. Our statistical analysis showed that except for Ps, a greater portion of δ¹⁸O variance in leaf compounds can be explained by individual tree effects, compared to temperature. Concerning tree-ring cellulose, only Fs and Ps show a significant temperature signal (maximum 12% of the variance explained), while the individual tree effect significantly explains δ¹⁸O for all species for a period of 38 years. Large species differences resulted in a limited ability of the isotope fractionation models to predict measured values. Overall, we conclude that in a diverse mixed forest stand, individual tree responses reduce the potential extraction of a temperature signal from δ¹⁸O.     

Linking water uptake with rooting patterns in grassland species

Water availability strongly governs grassland primary productivity, yet this resource varies dramatically in time (seasonally) and space (with soil depth and topography). It has long been assumed that co-occurring species differ in their partitioning of water use by depth, but direct evidence is lacking. We report data from two growing seasons (2004–2005) in which we measured the isotopic signature of plant xylem water from seven species (including C₃ forbs and shrubs and C₄ grasses) growing along a topographic gradient at the Konza Prairie Biological Station. Plant xylem stable oxygen isotope ratio (δ¹⁸O) values were compared to soil water δ¹⁸O profiles, recent rainfall events, and groundwater. Species varied in both their temporal patterns of water use and their responses to seasonal droughts in both years. During wet periods, species differences in water use were minimal, with common dependency on recent rainfall events stored in the upper soil layers. However, during dry periods, most C₃ species used proportionally more water from deeper portions of the soil profile relative to the C₄ grasses. Plants in uplands used more shallow soil water compared to those in lowlands, with the greatest differences across the topographic gradient occurring during dry periods. While the documented vertical root distribution varies by species and growth form in this grassland, each of the species we measured appeared to compete for the same surface layer soil moisture when water was not limiting. Thus, our results suggest that variation in precipitation history and landscape positions are greater determinants of water-use patterns than would be expected based on absolute rooting depth.     

Exotic plant communities shift water-use timing in a shrub-steppe ecosystem

Semiarid areas in the US have realized extensive and persistent exotic plant invasions. Exotics may succeed in arid regions by extracting soil water at different times or from different depths than native plants, but little data is available to test this hypothesis. Using estimates of root mass, gravimetric soil water, soil-water potential, and stable isotope ratios in soil and plant tissues, we determined water-use patterns of exotic and native plant species in exotic- and native-dominated communities in Washington State, USA. Exotic and native communities both extracted 12 ± 2 cm of water from the top 120 cm of soil during the growing season. Exotic communities, however, shifted the timing of water use by extracting surface (0–15 cm) soil water early in the growing season (i.e., April to May) before native plants were active, and by extracting deep (0–120 cm) soil water late in the growing season (i.e., June to July) after natives had undergone seasonal senescence. We found that δ ¹⁸O values of water in exotic annuals (e.g., −11.8 ± 0.4 ‰ for Bromus tectorum L.) were similar to δ ¹⁸O values of surface soil water (e.g., −13.3 ± 1.4 ‰ at −15 cm) suggesting that transpiration by these species explained early season, surface water use in exotic communities. We also found that δ ¹⁸O values of water in taprooted exotics (e.g., −17.4 ± 0.3 ‰ for Centaurea diffusa Lam.) were similar to δ ¹⁸O values of deep soil water (e.g., −18.4 ± 0.1 ‰ at −120 cm) suggesting that transpiration by these species explained late season, deep water use. The combination of early-season, shallow water-use by exotic winter-actives and late-season, deep water-use by taprooted perennials potentially explains how exotic communities resist establishment of native species that largely extracted soil water only in the middle of the growing season (i.e., May to June). Early season irrigation or the planting of natives with established root systems may allow native plant restoration.     

Seasonal courses of key parameters of nitrogen,carbon and water balance in European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) grown on four different study sites along a European North–South climate gradient during the 2003 drought

During the growing season of the exceptionally dry and warm year 2003, we assessed seasonal changes in nitrogen, carbon and water balance related parameters of mature naturally grown European beech (Fagus sylvatica L.) along a North–South transect in Europe that included a beech forest stand in central Germany, two in southern Germany and one in southern France. Indicators for N balance assessed at all four sites were foliar N contents and total soluble non-protein nitrogen compounds (TSNN) in xylem sap, leaves and phloem exudates; C and water balance related parameters determined were foliar C contents, δ¹³C and δ¹⁸O signatures. Tissue sampling was performed in May, July and September. The N related parameters displayed seasonal courses with highest concentrations during N remobilization in May. Decreased total foliar N contents as well as higher C/N ratios in the stands in central Germany and southern France compared to the other study sites point to an impaired N nutrition status due to lower soil N contents and precipitation perception. TSNN concentrations in leaves and phloem exudates of all study sites were in ranges previously reported, but xylem sap content of amino compounds in July was lower at all study sites when compared to literature data (c. 1 μmol N mL⁻¹). In September, TSNN concentrations increased again at the two study sites in southern Germany after a rain event, whereas they remained constant at sites in central Germany and southern France which hardly perceived precipitation during that time. Thus, TSNN concentrations in the xylem sap might be indicative for water balance related N supply in the beech trees. TSNN profiles at all study sites, however, did not indicate drought stress. Foliar δ¹³C, but not foliar C and δ¹⁸O followed a seasonal trend at all study sites with highest values in May. Differences in foliar δ¹³C and δ¹⁸O did not reflect climatic differences between the sites, and are attributed to differences in altitude, photosynthesis and δ¹⁸O signatures of the water sources. Except of low TSNN concentrations in the xylem sap, no physiological indications of drought stress were detected in the trees analysed. We suppose that the other parameters assessed might not have been sensitive to the drought events because of efficient regulation mechanisms that provide a suitable physiological setting even under conditions of prolonged water limitation. The uniform performance of the trees from southern France and central Germany under comparably dry climate conditions denotes that the metabolic plasticity of mature beech from the different sites studied might be similar.     

Tracing Sources of Streamwater Sulfate During Snowmelt Using S and O Isotope Ratios of Sulfate and <Superscript>35</Superscript>S Activity

The biogeochemical cycling of sulfur (S) was studied during the 2000 snowmelt at Sleepers River Research Watershed in northeastern Vermont, USA using a hydrochemical and multi-isotope approach. The snowpack and 10 streams of varying size and land use were sampled for analysis of anions, dissolved organic carbon (DOC), ³⁵S activity, and δ³⁴S and δ¹⁸O values of sulfate. At one of the streams, δ¹⁸O values of water also were measured. Apportionment of sulfur derived from atmospheric and mineral sources based on their distinct δ³⁴S values was possible for 7 of the 10 streams. Although mineral S generally dominated, atmospheric-derived S contributions exceeded 50% in several of the streams at peak snowmelt and averaged 41% overall. However, most of this atmospheric sulfur was not from the melting snowpack; the direct contribution of atmospheric sulfate to streamwater sulfate was constrained by ³⁵S mass balance to a maximum of 7%. Rather, the main source of atmospheric sulfur in streamwater was atmospheric sulfate deposited months to years earlier that had microbially cycled through the soil organic sulfur pool. This atmospheric/pedospheric sulfate (pedogenic sulfate formed from atmospheric sulfate) source is revealed by δ¹⁸O values of streamwater sulfate that remained constant and significantly lower than those of atmospheric sulfate throughout the melt period, as well as streamwater ³⁵S ages of hundreds of days. Our results indicate that the response of streamwater sulfate to changes in atmospheric deposition will be mediated by sulfate retention in the soil.     

Stable isotope composition of <Emphasis Type="Italic">Chara rudis</Emphasis> incrustation in Lake Jasne,Poland

Stable isotope composition (δ¹³C and δ¹⁸O) was analysed in mineral incrustation of Chara rudis and surrounding waters. This macroalga forms dense and extensive charophyte meadows and may significantly contribute to the calcium carbonate precipitation and deposition of marl lake sediments. The study aimed to find out if charophyte calcium carbonate was precipitated in an isotopic equilibrium with lake water and if the precipitation was related to the environmental conditions. Two apical internodes of 10 individuals of C. rudis were collected monthly between June and late October 2008 at three permanent study sites (1.0 m, 1.5 m and 2.0 m deep) in a small (15.1 ha) and shallow (mean depth: 4.3 m) mid-forest lake with extensively developed charophyte meadows (Lake Jasne, mid-Western Poland). Basic physical–chemical analyses were performed at each study site, and water samples for further laboratory determinations, including stable isotope analyses, were collected from the above searched C. rudis stands and, simultaneously, at three comparative sites in the macrophyte-free pelagial. The difference in δ¹³C between incrustation and water from above C. rudis exceeded 2‰ V-PDB at each site. In the case of δ¹⁸O, it exceeded 2‰ between July and September. Accordingly, it is postulated that calcium carbonate was not precipitated in an isotopic equilibrium with lake water. Incrustation was enriched in heavier carbon isotope, ¹³C, and water was enriched in ¹⁸O. δ¹³C of incrustation and DIC were positively correlated, whereas negative relation was found between δ¹⁸O of incrustation and water. Several dependencies were found with water chemistry above the plants. The content of mineral incrustation in Chara dry weight had negative influence on the δ¹⁸O but not on the δ¹³C. Community depth, structure and PVI had no effect. No significant differences appeared between isotope composition in the pelagic zone and Chara stands.     

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4–6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9–20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the δ¹⁸O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ¹⁸O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ¹⁸O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar δ¹³C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2–3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.     

Isotopic signature of nitrate in two contrasting watersheds of Brush Brook,Vermont, USA

We used the dual isotope method to study differences in nitrate export in two subwatersheds in Vermont, USA. Precipitation, soil water and streamwater samples were collected from two watersheds in Camels Hump State Forest, located within the Green Mountains of Vermont. These samples were analyzed for the δ¹⁵N and δ¹⁸O of NO₃⁻. The range of δ¹⁵N–NO₃⁻ values overlapped, with precipitation −4.5‰ to +2.0‰ (n = 14), soil solution −10.3‰ to +6.2‰ (n = 12) and streamwater +0.3‰ to +3.1‰ (n = 69). The δ¹⁸O of precipitation NO₃⁻ (mean 46.8 ± 11.5‰) was significantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3‰) and soil waters (mean 14.5 ± 4.2‰) even during snowmelt periods. Extracted soil solution and streamwater δ¹⁸O of NO₃⁻ were similar and within the established range of microbially produced NO₃⁻, demonstrating that NO₃⁻ was formed by microbial processes. The δ¹⁵N and δ¹⁸O of NO₃⁻ suggests that although the two tributaries have different seasonal NO₃⁻ concentrations, they have a similar NO₃⁻ source.     

Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees

The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of –125/–15‰ (δD/δ¹⁸O), –48/–6‰, and –67/–7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ¹⁸O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ¹⁸O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ¹⁸O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions. Received: 7 April 1999 / Accepted: 8 December 1999     

Diurnal variations of needle water isotopic ratios in two pine species

Diurnal fluctuations of leaf water isotope ratios (δ¹⁸O and δD) were measured for Jeffrey (Pinus jeffreyi Balf.) and lodgepole (Pinus contorta Douglas ex Louden) pine. Two trees per species were sampled every few hours on 15–16 October 2005 and 19–20 June 2006. Diurnal gas exchange was measured during the summer sampling. In fall 2005, leaf water δ¹⁸O ranged from 0.7 to 9.0‰, and leaf water δD ranged from −70 to −50‰. In summer 2006, leaf water δ¹⁸O ranged from 7.7 to 20.7‰, and leaf water δD ranged from −61 to −24‰. Diurnal variation of leaf water isotope values typically reached a maximum in early afternoon, began decreasing around midnight, and reached a minimum in mid-morning. Both periods showed a high degree of enrichment relative to source water, with leaf water–source water enrichments ranging up to 37.8‰ for δ¹⁸O, and up to 95‰ for δD. Leaf water enrichment varied by season with summer enrichment being greater than fall enrichment. A steady-state model (i.e., modified Craig–Gordon modeling) for leaf water isotope compositions did not provide a good fit to measured values of leaf water. In summer, a non-steady state model provided a better fit to the measured data than the steady-state model. Our findings demonstrate substantial leaf water enrichment above source water and diurnal variations in the isotopic composition of leaf water, which has application to understanding short-term variability of atmospheric gases (water vapor, CO₂, O₂), climate studies based on the isotopic composition of tree rings, and ecosystem water fluxes.     

Hydrogen and oxygen isotope ratios of tree-ring cellulose for riparian trees grown long-term under hydroponically controlled environments

Saplings of three riparian tree species (alder, birch and cottonwood) were grown for over 5 months in a hydroponics system that maintained the isotopic composition of source water in six treatments, ranging from –120 to +180‰δD and –15 to +10‰δ¹⁸O. The trees were grown in two greenhouses maintained at 25°C and at either 40 or 75% relative humidity, creating differences in transpiration rates and leaf water isotopic evaporative enrichment. The cellulose produced in the annual growth ring was linearly related to source water with differences in both slope and offset associated with greenhouse humidity. The slope of the isotopic composition of source water versus tree-ring cellulose was less than 1 for both δD and δ¹⁸O indicating incomplete isotopic exchange of carbohydrate substrate with xylem water during cellulose synthesis. Tests using the outer portion of the tree-ring and new roots were similar and showed that the tree-ring values were representative of the cellulose laid down under the imposed environmental conditions. The fraction of H and O in carbohydrate substrate that isotopically exchange with medium water was calculated to be 0.36 and 0.42 respectively, and biochemical mechanisms for these observed fractions are discussed. A mechanistic model of the biochemical fractionation events for both δD and δ¹⁸O leading to cellulose synthesis was robust over the wide range of cellulose stable isotope ratios. The experimental results indicate that both water source and humidity information are indeed recorded in tree-ring cellulose. These results help to resolve some of the disparate observations regarding the interpretation of stable isotope ratios in tree-rings found in the literature. Received: 4 January 1999 / Accepted: 12 August 1999     

Tracing the Sources of Exported Nitrate in the Turkey Lakes Watershed Using 15N/14N and 18O/16O isotopic ratios

Nitrate produced by bacterially mediated nitrification in soils is isotopically distinct from atmospheric nitrate in precipitation. ¹⁵N/¹⁴N and ¹⁸O/¹⁶O isotopic ratios of nitrate can therefore be used to distinguish between these two sources of nitrate in surface waters and groundwaters. Two forested catchments in the Turkey Lakes Watershed (TLW) near Sault Ste. Marie, Ontario, Canada were studied to determine the relative contributions of atmospheric and microbial nitrate to nitrate export. The TLW is reasonably undisturbed and receives a moderate amount of inorganic nitrogen bulk deposition (8.7 kg N · ha⁻¹· yr⁻¹) yet it exhibits unusually low inorganic nitrogen retention (average = 65% of deposition). The measured isotopic ratios for nitrate in precipitation ranged from +35 to +59‰ (VSMOW) for δ¹⁸O and −4 to +0.8‰ (AIR) for δ¹⁵N. Nitrate produced from nitrification at the TLW is expected to have an average isotope value of approximately −1.0‰ for δ¹⁸O and a value of about 0 to +6‰ for δ¹⁵N, thus, the isotopic separation between atmospheric and soil sources of nitrate is substantial. Nitrate produced by nitrification of ammonium appears to be the dominant source of the nitrate exported in both catchments, even during the snowmelt period. These whole catchment results are consistent with the results of small but intensive plot scale studies that have shown that the majority of the nitrate leached from these catchments is microbial in origin. The isotopic composition of stream nitrate provides information about N-cycling in the forested upland and riparian zones on a whole catchment basis. Received 5 October 1999; accepted 18 August 2000     

A 200-year coral stable oxygen isotope record from a high-latitude reef off Western Australia

 A core from a coral colony of Porites lutea was analysed for stable oxygen isotopic composition^*. A 200-year proxy record of sea surface temperatures from the Houtman Abrolhos Islands off west Australia was obtained from coral δ¹⁸O. At 29′S, the Houtman Abrolhos are the southernmost major reef complex of the Indian Ocean. They are located on the path of the Leeuwin Current, a southward flow of warm, tropical water, which is coupled to Indonesian throughflow. Coral δ¹⁸O primarily reflects local oceanographic and climatic variability, which is largely determined by spatial variability of the Leeuwin Current. However, coherence between coral δ¹⁸O and the current strength itself is relatively weak. Evolutionary spectral and singular spectrum analyses of coral δ¹⁸O demonstrate a high variability in spectral composition through time. Oscillations in the 5–7-y, 14–15-y, and quasi-biennial bands reflect teleconnections of local sea surface temperature (SST) to tropical Pacific climate variability. Deviations between local (coral-based) and regional (instrument) SST contain a cyclic component with a period of 15 y. Coral δ¹⁸O suggests a rise in SST by 0.6 ′C since AD 1944, consistent with available instrumental SST records. A long-term warming by 1.4 ′C since AD 1795 is inferred from the coral record. Accepted: 3 July 1998     

Nitrogen dynamics of storm runoff in the riparian zone of a forested watershed

The influence of storm runoff processes on stream nitrogen dynamics was investigated in a headwater riparian swamp on the Oak Ridges moraine in southern Ontario. Hydrologic data were combined with analysis of an isotopic tracer (¹⁸0) and nitrogen (NH ₄ ⁺ , NO ₃ ^– ) concentrations in saturation overland flow and stream discharge. Storm runoff was separated into its event and pre-event components using¹⁸O in order to examine the effect of water source on nitrogen chemistry. Laboratory experiments were also used to study nitrogen transformation associated with storm runoff-surface substrate interactions in the swamp. In most storms N0₃-N and NH₄-N concentrations in the initial 3–4 mm throughfall increment were 10–20x and 20–100x higher respectively than stream base flow concentrations. Maximum stream N0₃-N concentrations were < 2x to 6x higher than base flow concentrations and preceded or coincided with peak stream discharge. Storm-to-storm variations in stream N0₃-N behaviour also occurred during the hydrograph recession phase. NH₄-N concentrations attained an initial peak on the rising hydrograph limb, or at peak stream discharge. A second NH₄-N increase occurred during the late recession phase 3–5 h after maximum stream discharge. Inorganic-N concentrations in surface runoff were similar to peak streamflow.The close agreement between observed N0₃-N concentrations and values predicted from a chemical mixing model indicate that stream N0₃-N variations were controlled mainly by the mixture of throughfall and groundwater in surface stormflow from the swamp. Laboratory experiments also indicated that N0₃-N in surface runoff behaved conservatively when mixed with swamp substrates. With the exception of the late hydrograph recession phase, observed stream NH₄-N concentrations were much lower than concentrations predicted by the chemical mixing model. The rapid loss of NH₄-N from mixtures of surface stormflow and swamp substrates in laboratory experiments and the absence of uptake in sterilized substrates indicated that NH₄-N retention in surface storm runoff was due to biotic processes.