B13B-1181
Greenhouse Gas Production From a Young Boreal Hydroelectric Reservoir (Eastern Canada): A Carbon Isotope Approach
It is now accepted that boreal hydroelectric reservoirs and lakes produce greenhouse gases (GHG) mainly in the form of CO2. Much of the research has so far focused on old (> 20 year) reservoirs. However, the problems associated with a newly flooded reservoir are different because after flooding, salts and nutrients from the flooded soils are released into the water column (i.e. the reservoir's effect). It is anticipated that the CO2 fluxes should be higher in young reservoirs than in older ones, but little is known about their magnitude and their sources. The Eastmain-1 hydroelectric reservoir is a small reservoir of 603 km2 with a mean depth of 11.5m. Flooding began in November 2005 and ended in May 2006. The flooded area was covered with approximately 65% boreal forests, 21% rivers and lakes and 14% peatlands. Here, we make use stable carbon isotopes to constrain carbon sources and cycling in this disturbed environment. Ultimately, the study aims at estimating annual CO2 fluxes at the water-air interface of the reservoir. Sampling was performed four times (June 2006, August 2006, October 2006 and June 2007) to account for seasonality of the carbon cycle. Twelve sites were visited on the reservoir as well as a natural lake near the reservoir. Three sites were also sampled along a depth gradient. At each sampling site, in situ measurements included water and air temperatures, pH, alkalinity, wind speed, conductivity and dissolved oxygen content. Samples were collected for the analysis of dissolved organic and inorganic carbon (respectively DOC and DIC) and particulate organic carbon (POC) concentrations, for the analysis of the carbon isotopic compositions of DOC, DIC, POC and air CO2 at the water-air interface and finally for the C:N of DOM and POM. DOC concentrations are highest averaging 6.86±1.40 mg*l-1, DIC concentrations average 1.51±0.76 mg*l-1 and POC concentrations are up to 2 orders of magnitude lower averaging 0.036±0.018 mg*l-1. δ13C values of DOC average -27.42±0.32‰ vs V-PDB, close to average C3 plant values and vary little throughout the year as well as throughout the reservoir. δ13C-DIC values vary slightly throughout the reservoir but show large variations from one sampling campaign to the next. Depth profiles show a small decrease in δ13C-DIC with depth, in a well mixed water column. A strong relationship is observed between δ13C-DIC and DIC concentrations. Keeling type regressions (using δ13C-DIC and DIC concentrations) suggest that dissolved CO2 in the reservoir originate from the oxidation of dissolved organic matter within the reservoir.
B13B-1182
Implications of in-canopy sources and concentration profiles for Lagrangian turbulence statistics.
One-dimensional Lagrangian dispersion models are often used to infer in-canopy source/sink distributions from concentration profiles. Such models require vertical profiles of the standard deviation of the vertical windspeed (σw) and the Lagrangian timescale (τL). While (σw) can be measured, (τL) cannot, and is usually either parameterised hypothetically or inferred from its Eulerian counterpart, and thus remains uncertain. In this work, we measure (σw) and infer (τL) from measured profiles of temperature and concentrations of H2O, CO2 and CH4, measured simultaneously by Fourier transform infrared spectroscopy. The study site is a 40-m tall temperate Eucalyptus forest in south-eastern Australia. Source/sink distributions of these quantities are derived from a multilayer Soil Vegetation Atmospheric Transfer (SVAT) model with multiple constraints: (1) day-time eddy flux measurements above the canopy of sensible heat, latent heat, and CO2 (2) in-canopy Lidar measurements of the leaf area density distribution, (3) chamber measurements of CH4 and CO2 ground fluxes. An a-priori estimate of (τL) is obtained from the Eulerian timescale. A Lagrangian dispersion model is inverted to obtain an improved estimate of (τL) which optimises agreement between modelled and measured vertical profiles of temperature and the scalar concentrations. The inferred vertical profile of ôL is compared to several theoretical profiles predicted in the literature.
B13B-1183
Utilization of Soil C and N by Microbial Groups in the Presence of Living Roots
The effects of living plant roots and N on belowground C dynamics were examined in a CA annual grassland soil (Haploxeralf) during a 2-y greenhouse study. The fate of 13C-labeled plant roots ( Avena barbata L.) and soil were followed under planted and unplanted conditions; and with and without N addition (20 kg N ha-1 season-1). The treatments were applied during 2 growing seasons and each growing season was followed by a dry, fallow period (~ 150-d long). Living roots increased the turnover rate and loss of belowground 13 C during and after 2 seasons compared with unplanted soils. After 2 seasons, planted soils had 21% less belowground 13C present than in unplanted soils. However, total soil C increased in planted soils by 4.6% compared to unplanted after 2 seasons. N additions decreased belowground 13C turnover during the first treatment season in both planted and unplanted soils, however no effect of N on soil C was observed thereafter. Planted soils had larger microbial biomass and the community structure differed compared with unplanted soils. Planted soils had higher proportions of gram (-) bacteria, while unplanted soils had higher proportions of gram (+) bacteria, actinomycetes, and fungi. New root and exudate C supplied from living roots increased the turnover of microbial assimilated 13C compared with unplanted for all microbial groups. This greater turnover of belowground 13C was especially significant for gram (-) bacteria, which were stimulated in the planted soil. In contrast, the activity among microbial groups in unplanted soils was similar to that prior to the initiation of the treatments and soil wet-up. Our findings suggest that A. barbata roots increased soil C levels over time because root and exudate C inputs are significant, however that C increase will be moderated by an overall faster C mineralization rate of belowground C. Increased N deposition may slow soil C losses, however, they appear minor and temporary at the rates applied and for the plant-soil system studied.
B13B-1184
Carbon Isotope Composition of Ecosystem Respired Carbon Dioxide in Three Boreal Forest Ecosystems: Measurements and Model Calculations
We conducted measurements of seasonal and inter-annual variation in the carbon isotope composition of ecosystem respired CO2 (δR) in aspen, black spruce and jack pine dominated ecosystems in northern Saskatchewan during 2004-2006 as part of the Fluxnet-Canada Research Network. All three sites showed relatively small variation (approximately -26 to -29 per mil) in δR values during the entire study. The measurements were strongly correlated with modeled δ13C values of ecosystem respired CO2. The model calculated leaf CO2 assimilation, stomatal conductance and chloroplast CO2 concentration separately for sunlit and shaded leaves within multiple canopy layers, and, therefore, allowed us to estimate canopy photosynthetic 13C discrimination. All three sites showed variation in canopy 13C discrimination in response to environmental conditions in a manner consistent with well-known leaf-level studies. Specifically, 13C discrimination was positively correlated with soil moisture and negatively correlated with photon flux density, air temperature and vapor pressure deficit. As a consequence a strong diurnal pattern was observed for 13C discrimination. The measured δR values also varied in response to environmental conditions in a manner consistent with well-known leaf-level studies of photosynthetic 13C discrimination, but with a dampened response caused by the contribution of heterotrophic respiration, which had a constant δ13C value. These results indicate that the stable isotope composition of respired CO2 is a useful ecosystem-scale tool to study constraints to photosynthesis and acclimation of ecosystems to environmental stress.
B13B-1185
Using Headspace Equilibration to Measure the d13C of Soil-Respired CO2
Soil respiration is an important component of the global carbon cycle and can account for as much as 70% of ecosystem respiration. Soil gas flux measurements have been combined with stable isotope analysis to examine ecosystem properties and processes such as water-use efficiency and the role of above ground weather in controlling soil respiration. However, current methods of measuring the δ13C of soil-respired CO2 are either inherently inaccurate or time-consuming and tedious. An alternative method of obtaining this value offers a potential solution to these problems. In this method, plastic chambers are fitted with rubber septa to allow for sample collection, then inverted and partially buried in soil. The chamber headspace is allowed to come to equilibrium with soil air. In this study we tested the viability of this method by examining whether frequent resampling of respiration chambers affected δ13C measurements, whether headspace CO2 concentration and δ13C values approached equilibrium asymptotically, and whether simulated and actual diel temperature cycles affected estimates of δ13C. All experiments were conducted on respiration chambers inverted in potting soil and placed in a Conviron growth chamber, with the exception of one field test that was conducted on respiration chambers installed in a Northern Idaho experimental forest. Samples were collected with a syringe and stored in glass vials for analysis by a ratioing mass spectrometer. We found that resampling respiration chambers as frequently as every 10 minutes had no significant effect on final δ13C values, that both chamber CO2 concentrations and δ13C values exhibited an asymptotic approach to equilibrium, and that the equilibrium value was offset from the initial flux by the amount we expected, approximately 4 ‰. However, we also found that diel temperature variation affected both headspace CO2 concentration and δ13C in the lab and in the field. We concluded that if this method is used in areas with relatively slight diel temperature fluctuation, it provides a viable alternative to current methods of measuring the δ13C of soil respiration.
B13B-1186
Simultaneous Measurements of Leaf and Soil Carbon Dioxide Flux Using a Tunable Diode Laser
A portable photosynthesis system (Li-6400, Li-Cor, NE) and a through-flow soil chamber were used to continuously measure the gas exchange of leaf and below ground components in pots containing corn, Triticale and a non-planted control. Temperature was kept constant through-out the experiment, and measurements were made at 4 min intervals over a full diurnal light cycle. A tunable diode laser (TGA100A, Campbell Scientific, UT) was used to measure the concentration and stable isotopic composition (δ13C and δ18O) of the air entering and exiting both chambers. End-member isotope values were determined by short-term incubation of component parts in Tedlar bags, and the evolved gas was measured with the laser. These data were used to determine the isotopic signature of CO2 derived from root respiration, microbial respiration of plant derived exudates and soil organic matter (SOM) to allow the partitioning of the total flux into component parts. The δ13C of SOM respiration was identical when measured with the soil chamber on the control pots and when using incubated samples from the same pots. However, incubation of the potting mix in the other treatments was more enriched (corn) and more depleted (Triticale) than the control, indicating that end-member determination of the original SOM was confounded by exudates from the plants. Using a mixing model to partition the soil respiration, and the δ13C of SOM from the control pots, corn roots contributed 25% and Triticale 72% of the below-ground respiration. Incubation of soil with the roots removed allowed non-root respiration to be partitioned into contributions from pre-existing SOM and more recent plant derived exudates for corn (23% recent carbon) and Triticale (36% recent carbon).
B13B-1187
Interannual Variability and Trends of Atmospheric Radiocarbon (14CO2) Over the Tropical Western Pacific
Radiocarbon concentration in the surface atmospheric carbon dioxide (14CO2) on the Japan-Australia transect has been measured since 1994 [Kitagawa et al., 2004]. We investigate the interannual variability and trends of atmospheric 14CO2 for northern tropics (latitudes from 20N to 30N) and southern tropics (10S-20S). Time series analysis using a digital filtering and harmonic regression curve fitting [Nakazawa et al., 1997] elucidates two-year cycles of 14CO2 and little seasonal cycles for both hemispheres. The decrease trend of 14CO2 since nuclear bomb tests, mainly due to uptakes by the ocean, has been leveled off around 2000. These characteristics of 14CO2 over the tropical Pacific are quite different from those over the Europe [Levin et al., 2004], in which the strong seasonal cycle and continuous decrease trends of 14CO2 are observed. To explain the 14CO2 time series in tropics, we are developing a model, in terms of impulse response functions of biosphere [Thompson and Randerson, 1999] and atmospheric transport. Preliminary results on the 14CO2-based estimation of carbon exchange between biosphere-atmosphere in tropics will be shown.
B13B-1188
Temporal variation of carbon-13 signature of soil respiration in a beech forest ecosystem measured with a tunable diode laser spectrophotometer
Under temperate latitudes, soil respiration is responsible for the reemission of almost half of the carbon assimilated by the forest vegetation. Soil respiration is well-known to be very sensitive to environmental factors such as temperature and moisture and has been shown to widely vary during the growing season. The main difficulty when studying the impact of environment on this process is to differentiate the sources of CO2 in the soil and their specific response to environmental factors. With regard to this drawback, high frequency measurements of 13C in the respiratory flux of the different compartments would help in differentiating emission compartments with short residence time (i.e. using photosynthates as respiratory carbon sources) from compartments with longer residence time (i.e. using soil organic matter as respiratory carbon sources). A tunable diode laser spectrophotometer (TDLS) was installed in the Hesse forest (northeast of France) early during the 2007 growing season in order to determine the seasonal variability in the composition in 13C of the CO2 efflux released by a forest soil. This innovative method, based on the measurement of the absorption of an infrared laser at the specific wave lengths of the 13CO2 and 12CO2, allows the monitoring of the two isotopologues at a very high frequency. The concentrations of the two isotopologues in the soil respiration flux were continuously monitored from June to October 2007 using both chamber measurements and below canopy Keeling plots. Continuous TDLS measurements and punctual Keeling plots gave very similar values of 13C composition of soil respired CO2, showing the reliability of the TDLS system in this context. Results were analysed with regard to seasonal changes in climatic and edaphic variables, in the 13C signatures of the potentially respiratory sources (i.e. root carbohydrates), and in water use efficiency of the canopy as inferred from eddy flux measurements of gross photosynthesis and transpiration monitored on a tower installed at the site.
B13B-1189
Seasonal Patterns of Precipitation Isotopic Composition in the Luquillo Mountains, Puerto Rico
Precipitation isotopic signatures can help determine the relative importance of different climate patterns to the hydrologic balance and water supply of a region. A set of rain isotope collectors spanning the Luquillo Mountains in Puerto Rico was sampled monthly for two years and precipitation was analyzed for δ18O and δ2H. Rain isotopic composition varied with elevation and with season. In Puerto Rico there is little seasonal variation in temperature, but a seasonal cycle in rainfall isotopic composition was apparent. Precipitation enriched in 18O and 2H occurred during the period December-May, transitioning to precipitation depleted in 18O and 2H during the period June-November. To better understand the relationship between isotopic composition of rainfall and weather patterns, detailed analyses of rain events were matched with the isotopic compositions for the sample periods. The isotopically enriched precipitation is associated with a weather pattern of trade wind showers and orographic uplift. The isotopically depleted precipitation during summer and fall is correlated with convective rainfall associated with tropical waves embedded in the prevailing easterly airflow. Isotopic values that are distinctly depleted in 18O and 2H compared to both the aforementioned weather patterns occurred during months that the area received rainfall from large low pressure systems. An estimate of cloud height and rain condensation temperature for the different weather patterns was obtained using NEXRAD radar echo tops, which are a measure of the maximum altitude of rainfall within the clouds. Echo tops were analyzed for the four largest rain events during each monthly sample period to determine the average over the study area during each event. Average echo top altitude was correlated with the rain isotope record, even for the limited number of events analyzed in the monthly sample. A preliminary analysis indicates that 27% of rain input to the Luquillo Mountains was associated with the trade wind orographic rainfall weather pattern and 66% of rainfall occurred during the tropical wave season. The remaining 7% of rainfall was from low pressure systems with distinctly depleted isotopic signatures.
B13B-1190
Partitioning of Evapotranspiration Into Soil Evaporation and Plant Transpiration Using Isotopes of Water in Controlled Conditions
Rainfall recycling by evapotranspiration from continental surfaces is certainly the most unknown component of the global water cycle. This is due to the large variability of rainfall as well as the heterogeneity of these continental surfaces, both in time and space. Traditional measuring methods such as sap flow, micro lysimeter, water and energy balance estimation (Bowen ratio, eddy correlation) have been used since the 70s for a monitoring of real evapotranspiration fluxes over crops and others plant covers. A complementary method consists in using isotopic biogeochemistry. When making specific hypothesis, it is possible to identify and quantify the different sources of the atmospheric water vapour (vegetation and soil at different scales). Analysis of the heavy stable isotopic ratios of water in both liquid and vapour phases: 18O and 2H can allow determining the history of the water in the soil since the last rainfall event (infiltration, re-evaporation) or the root extraction depths. Field campaigns measurements (plants and soils), interpreted using the Keeling Plot method allowed some progress in the partition between evaporation and transpiration understanding. But the experimental design is not sufficient to mechanistically describe the water processes involved. The study of all the interactions is difficult due to the large number of controlling variables describing climate, vegetation and soil characteristics. A monolith experiment (including soil and growing plant) was carried out in a reactor called RUBIC (Reactor Used for Continental Isotopic Biogeochemistry, Bariac et al., Geochim. Cosmochim. Acta., 1991). Controlled conditions allowed a monitoring and regulation of climatic parameters (net radiation, air temperature, vapour pressure deficit, CO2 partial pressure, and wind speed). It was also necessary to fix soil (structure, texture, and water content) and vegetation (specie and seeding density) parameters. The collected data allow us to improve our understanding of the partition of evapotranspiration into soil evaporation and plant transpiration and to assess the hypothesis (often made in isotopic biochemistry) of a stationary state reached in the two reservoirs (soil and plant). These data also allow the evaluation of the hypothesis included in a transfer module of heavy stable isotopes of water within the bare soil and the plant (Braud et al., Journ. of Hydrol., 2005). The latter is coupled to a SVAT model (Soil-Plant-Atmosphere Transfer) called SiSPAT (Simple Soil Plant Atmosphere Transfer model, Braud et al., Journ. of Hydrol., 1995) and was extended to take into account isotopes transfer within the vegetation (root extraction and transpiration). The experimental design of RUBIC as well as the first modelling results will be presented.
B13B-1191
Diurnal patterns of CO2 concentrations and stable isotopes in subalpine forest soils
Respiration of carbon dioxide (CO2) from soils contributes significantly to the carbon balance of ecosystems. Measurement of the stable isotopic composition (δ13C) of soil and soil-respired CO2 can provide insight into the physical and biological mechanisms controlling respiration. Very few studies examine diurnal variations in the isotopic composition of CO2 in soils because of the difficulty in making such measurements. We measured [CO2] and δ13C in and above soils in a subalpine forest near Niwot Ridge Colorado. Profile measurements were made every two hours of CO2 in air below the soil surface (-3 cm), on the soil surface, and up to 40 cm above the soil surface. Over the course of 20 days in July of 2007, the δ13C of respired CO2 (δR-soil) measured using these vertical profiles varied from -25.42 to -21.10‰ with a mean of -23.48 ± 1.11‰. Measurements in the air near the ground (δR-ground, 0.1 to 2 m) varied from -26.71 to -23.96‰ with a mean of - 25.49 ± 0.63‰. Values of δR-soil were enriched relative to δR-ground,. Diurnal patterns in δ13C of respired CO2 were not apparent even though soil temperatures varied by as much as 15°C over a 24 hour period. Soil moisture is also likely important as it affects the diffusion rate of CO2 soil and may stimulate microbial activity. Improved knowledge of the controls on soil respiration have the potential to increase process-level understanding of ecosystem respiratory fluxes and may also help better constrain terrestrial carbon cycling models.
B13B-1192
Temporal Dynamics and Environmental Controls on Carbon Isotope Discrimination at the Canopy Scale
Much is currently known about 13C isotopic discrimination by C3 plants at the leaf scale. Multidisciplinary techniques from micrometeorology and the stable isotope community have exploited this knowledge to better understand the dynamic processes and environmental controls on atmosphere/biosphere exchange. Unfortunately, there remains a dearth of measurements relating carbon isotope discrimination at the canopy scale (Δcanopy) with the net carbon ecosystem flux. Our goals here are to evaluate temporal fluctuations in Δcanopy as a result of variable environmental conditions and to critically assess the efficacy of leaf-level assumptions applied at the canopy scale. At the University of Minnesota's Rosemount Research and Outreach Center (RROC), the exchange of 12CO2 and 13CO2 isotopologues are continuously measured using tunable diode laser (TDL) and micrometeorological techniques (eddy covariance-TDL and gradient-TDL methods). We utilize these data in conjunction with eddy flux and ancillary meteorological measurements to estimate Δcanopy, a key parameter for understanding ecosystem carbon source/sink behavior. Traditionally, Δcanopy is estimated using stomatal conductance models and leaf level isotopic discrimination parameters. In this study, we similarly calculated Δcanopy (Big-Leaf approach), where stomatal conductance was obtained through inversion of the Penman-Monteith equation. Additionally, given the high resolution of eddy flux and isoflux measurements at the RROC site, we were able to calculate Δcanopy using an inverse flux approach. For this approach, we partitioned the net ecosystem flux using eddy covariance measurements and a nighttime temperature regression method, and then calculated Δcanopy from the isoflux mass balance. Both calculations of Δcanopy emphasized the diurnal, daily and seasonal variability of this important parameter. In particular, atypically hot weather strongly influenced canopy isotope discrimination. Trends in the two Δcanopy calculations were often similar; however, the Big-Leaf approach showed a strong dependence on the stomatal conductance calculation and was highly sensitive to the leaf-level discrimination parameters. Although the inverse flux approach also has limitations, especially regarding estimation of the daytime respiration flux, this method avoids uncertainties associated with canopy conductance and leaf-to-canopy discrimination assumptions inherent in the Big-Leaf approach. Moreover, validation of daytime respiration estimates through, for example, chamber measurements, should further improve the Δcanopy calculation. The inverse flux approach is, therefore, a simple and robust constraint on traditional Δcanopy estimates, and may be easily applied to isotope and flux data collected at worldwide flux network sites, such as AmeriFlux locations. This added spatial and temporal resolution in Δcanopy made available to the modeling community may serve to improve global carbon cycle budgets.
B13B-1193
Effects of Water Vapor on the Data Quality of the Stable Oxygen Isotopic Ratio of Atmospheric Carbon Dioxide
The stable oxygen isotopic ratio of carbon dioxide can potentially track fundamental indicators of environmental change such as the balance between photosynthesis and respiration on regional to global scales. The Stable Isotope Laboratory (SIL) at the Institute of Arctic and Alpine Research (INSTAAR), University of Colorado at Boulder, has measured the stable isotopes of atmospheric carbon dioxide from more than 60 NOAA/Earth System Research Laboratory (ESRL) air flask-sampling sites since the early 1990s. If air is sampled without drying, oxygen can exchange between carbon dioxide and water in the flasks, entirely masking the desired signal. An attempt to investigate how water vapor is affecting the δ18O signal is accomplished by comparing the SIL measurements with specific humidity, calculated from the National Climatic Data Center (NCDC) global integrated surface hourly temperature and dew point database, at the time of sampling. Analysis of sites where samples have been collected initially without drying, and subsequently with a drying kit, in conjunction with the humidity data, has led to several conclusions. Samples that initially appear isotopically unaltered, in that their δ18O values are within the expected range, are being subtly influenced by the water vapor in the air. At Bermuda and other tropical to semi-tropical sites, the ‘wet' sampling values have a seasonal cycle that is strongly anti-correlated to the specific humidity, while the ‘dry' values have a seasonal cycle that is shifted earlier than the specific humidity cycle by 1-2 months. The latter phasing is expected given the seasonal phasing between climate over the ocean and land, while the former is consistent with a small, but measurable isotope exchange in the flasks. In addition, we note that there is a strong (r > 0.96) correlation between the average specific humidity and the percent of rejected samples for ‘wet' sampling. This presents an opportunity for determining a threshold of specific humidity, below which air flask samples can be trusted. This approach may allow segregation of suspect and trusted data, and thus provide an improved record of oxygen isotopic ratios of carbon dioxide over the past two decades.
B13B-1194
Physiological, anatomical and leaf hydraulic effects on leaf water δ18O enrichment in different plant species
Stable oxygen isotope ratios (δ18O) of plant and source waters are valuable tools in the analysis of water and carbon fluxes at leaf, plant, and ecosystem scales. Recent improvements in mechanistic models have significantly advanced the understanding of isotopic leaf water enrichment, which is an important source of δ18O variability in plants and ecosystems. However, the marked variability in leaf water δ18O values that have been reported for different plant species hampers efforts to interpret and then apply data on leaf water δ18O values for studies conducted at the ecosystem scale. To improve the understanding and application of δ18O values in leaf water, we tested the interplay of physiological, morphological, anatomical and leaf hydraulic properties as drivers of leaf water δ18O values across 17 Eucalyptus species growing in a common garden. We observed large differences in leaf water δ18O across the 17 species. These differences were only partly driven by physiological and leaf morphological differences across species. A sensitivity analysis using state-of-the-art leaf water enrichment models showed that the parameter ‘effective path length’ (L) is of critical importance for the variability of leaf water δ18O across different species. The data show that L can be related to a suite of leaf properties that include physiology, anatomy and hydraulics. Consequently, consideration of leaf properties will significantly improve the interpretation of δ18O values in leaf water across different plant species and will therefore help in the application of δ18O values in carbon and water cycle assessments at both the plant and the ecosystem scale.
B13B-1195
Simultaneous Flux Measurements of CO2, its Stable Isotope Ratios and Trace Gases Based on Eddy Accumulation Technique for Flux Partitioning
For the purpose of determining the CO2 uptake by terrestrial ecosystem, eddy covariance method (EC) is commonly used in the tower-flux measurements. The flux measured by this method is called enet ecosystem exchange (NEE)f. NEE has the meaning of difference between two component fluxes, photosynthetic uptake and respiratory release of CO2. Magnitude of both the component fluxes is far larger than NEE. Both the component fluxes have difference in response function against changes in environmental factors, such as temperature and water. Therefore it is important to evaluate the characteristics of variations in the comporent fluxes individually in the future prediction of CO2 uptake by terrestrial ecosystem. Separation of NEE into the componet fluxes is usually done by using an approximate temperature expression of respiratory flux. This approximate expression is based on the assumption that the NEE observed at nighttime equals to the respiratory flux. The photosynthetic uptake of CO2 is defined as difference between the observed NEE and grespirationh approximated as a temperature-function. Because of its technical simplicity, this approach has provided useful information about climatology of the gross CO2 fluxes. However, the temperature expression of respiratory flux has several limitations in its application. We are now developing a flux-partitioning method using chemical tracers (e.g. stable isotopes of CO2 and carbonyl sulfide) as additional constraints. The flux partitioning using stable isotopes of CO2 is based on the imbalance of net flux of the CO2 isotopes between grespirationh and gphotosynthesish. On the other hand, because of this similarity in the control factors for uptake ratio, the net flux of carbonyl sulfide (COS) is regarded as a possible constraint for the functioning of variations in photosynthetic CO2 uptake by terrestrial ecosystem. Field observation of fluxes of those chemical tracers by EC method is difficult due to stringent requirements for on-site measurement. Therefore, as a first step, we are planning to measure those fluxes based on an eddy accumulation technique coupled with flask sampling and high precision lab analysis. We report current progress of the development.
B13B-1196
Isotopic Fractionation in Non-Equilibrium Diffusive Environments: Implications for Chamber- Based Studies of Soil Gas Fluxes
Isotopic studies are frequently carried out in environments where diffusion is the dominant mode of transport, including the ocean, freshwaters, ice, solid earth, shallow subsurface, and within terrestrial vegetation. While researchers develop novel ways of exploiting isotopic fractionations as a way to track biochemical or physical processes, exploration of basic fractionation mechanisms associated with diffusive transport lags behind. This study examines the implications of diffusive-transport induced fractionations on the interpretation of isotopic processes in the geophysical environment using the example of gaseous δ13C-CO2 flux from soils. We use a simple iterative diffusion model to simulate isotopic transport of δ13C-CO2 from soil into a headspace chamber, followed by laboratory measurements using large and small chambers. Unlike existing models, we treat 12C-CO2 and 13C-CO2 as separate gas species that always travel at their respective theoretical diffusivities. In both model runs and laboratory measurements, the combined presence of isotopic and concentration gradients results in an observed fractionation of smaller average magnitude than the accepted theoretical diffusion fractionation for these species of 4.4 permil. The observed fractionation is not constant, but falls continuously during the headspace equilibration period, which means that isotopic data from a non-steady state diffusive environment can be misinterpreted when steady state diffusion models are applied. These results may also be extended to other gas flux environments including near- surface boundary layers where non-equilibrium diffusive environments are present due to the non-infinite nature of sources and/or sinks.
B13B-1197
Influence of Tree Height on the Carbon Isotopic Discrimination of Canopy Photosynthesis in Southeastern Pine Forest Ecosystems
Intensive investigations of carbon and water exchange in highly productive pine forests in the Southeastern US are restricted to a limited numbers of locations that are equipped with eddy covariance towers. These towers are mostly located within homogenous stands. However, the southeastern pine forests are composed of plantations of different ages/heights that are interlaced with hardwood forests. We have measured variability in photosynthetic parameters, and the 13C of ecosystem, foliage and soil respired CO2 over a 3-yr period at the Ameriflux tower site in Gainesville, FL, a slash pine ecosystem. Additionally we examined trends in canopy foliage bulk organic matter 13C, leaf wax 13C and the 13C of foliage respired CO2 as a function of tree height. Sampled tree heights ranged from 5 to 25 meters along the transect, characteristic of pine plantations within this region. A highly significant positive correlation was observed between tree height and the 13C of foliage bulk organic matter. Leaf wax 13C mirrored the trend observed in foliage respired CO2 and bulk organic matter, with approximately a –3 ‰ offset from foliage respired CO2. Point measurements of upper-crown light-saturated net photosynthesis rate were not correlated with height, but were likely confounded by water stress effects. Research in other forest ecosystems has demonstrated tree height effects on hydraulics and leaf gas exchange, but these effects have not been explored in southern pines. These data suggest that southern pine hydraulics and leaf gas exchange may be influenced by tree height, and that scaling of isotopic data in these forests will require careful consideration of age and height variation.
B13B-1198
The isotopic composition of urban plants as indicators of the distribution of air pollution in the Los Angeles Basin
Many isotopes in plant biomass reflect either the isotopic composition of the physical environment or the biological responses of plants to their environment. Therefore, spatial variability in the isotopic composition of urban plants has great potential to provide information about the urban environment and its effect on plants and other organisms. We intensively sampled winter annual plants around the Los Angeles Basin in 2004 and 2005 and mapped their stable and radiocarbon isotopic composition (13C, 14C, 15N, 18O). Radiocarbon (14C), which is present in naturally-derived atmospheric carbon dioxide (CO2) but absent from fossil fuel-derived CO2, was most depleted in plants collected in downtown Los Angeles and enriched at greater distances from the urban center. The spatial pattern of plant radiocarbon was similar to the spatial pattern of carbon monoxide (CO), indicating similar combustion-related pollution sources. Stable carbon isotopes showed a similar trend, although with greater spatial variability, indicative of the many factors that influence photosynthetic fractionation. Stable nitrogen isotopes, on the other hand, were highly enriched along the urban coast and became depleted with increasing elevation and distance from the coast. Maps of NOx concentrations revealed an overlap between locations of 15N enrichment and high NOx emissions. In addition, enriched marine sources near the coast likely contributed to plant enrichment. In contrast, oxygen isotopes were most enriched in the hottest inland areas as a function of greater evaporative demand. Generally speaking, spatial patterns in all four isotopes corresponded to spatial patterns of climate and/or air pollutant concentrations and gradients of urbanization. Plants appear to serve as very useful biomonitors of environmental conditions in urbanized and polluted regions.
B13B-1199
Grasland Stable Isotope Flux Measurements: Three Isotopomers of Carbon Dioxide Measured by QCL Spectroscopy
To improve our understanding of greenhouse gas dynamics of managed ecosystems such as grasslands, we not only need to investigate the effects of management (e.g., grass cuts) and weather events (e.g., rainy days) on carbon dioxide fluxes, but also need to increase the time resolution of our measurements. Thus, for the first time, we assessed respiration and assimilation fluxes with high time resolution (5Hz) stable isotope measurements at an intensively managed farmland in Switzerland (Chamau, 400m ASL). Two different methods were used to quantify fluxes of carbon dioxide and associated fluxes of stable carbon isotopes: (1) the flux gradient method, and (2) the eddy covariance method. During a week long intensive measurement campaign, we (1) measured mixing ratios of carbon dioxide isotopomers (12C16O2, 12C16O18O, 13C16O2) with a Quantum Cascade Laser (QCL, Aerodyne Inc.) spectroscope and (2) collected air samples for isotope analyses (13C/12C) and (18O/16O) of carbon dioxide by Isotope Ratio Mass Spectrometry (IRMS, Finnigan) every two hours, concurrently along a height profile (z = 0.05; 0.10; 0.31; 2.15m). In the following week, the QCL setup was used for closed-path eddy covariance flux measurement of the carbon dioxide isotopomers, with the air inlet located next to an open-path Infra Red Gas Analyzers (IRGA, LiCor 7500) used simultaneously for carbon dioxide measurements. During this second week, an area of grass inside the footprint was cut and harvested after several days. The first results of in-field continuous QCL measurements of carbon dioxide mixing ratios and their stable isotopic ratios show good agreement with IRGA measurements and isotope analysis of flask samples by IRMS. Thus, QCL spectroscopy is a very promising tool for stable isotope flux investigations.
B13B-1200
Transition of the Isotopic Composition of Leaf Water to the Isotopic Steady State in Soybean and Corn
The isotope composition of leaf water (δL) plays an important role in the isotopic water and carbon fluxes between terrestrial plants and the atmosphere. The objective of this study is to improve our understanding of environmental and biological controls on the transition of δL to steady state through laboratory experiments. Plants (soybean, Glycine max; corn, Zea mays) were grown hydroponically with water of a known isotopic content in a greenhouse. On the day of the experiment, they were first moved to ambient environment in full sunlight for at least 6 hr and then into a dark container inside the lab for up to 48 hr in which water vapor isotope ratios, temperature, and humidity were controlled. This arrangement created a step change in the forcing on the plant isotopic exchange. Leaves were sampled prior to the transfer to the dark container and 6 more times every 4 - 12 hr over the experiment. In the first set of experiments, humidity inside the container was saturated to mimic dew events in field conditions. In the second set, humidity was controlled at approximately 95%. Water from the leaf samples was extracted by a vacuum line and was analyzed for both δD and δ18O. The dataset will allow us to evaluate leaf water isotopic theories by exploring the transitions of δL in response to the step change. Specifically, we are interested in whether the stomatal opening is an effective pathway for gaseous exchange in total darkness and how the transitional behaviors of δL differ between the C3 and C4 photosynthesis pathways.
B13B-1201
Discovering the Importance of Bi-directional Water Fluxes in Leaves
The stable isotope ratio 18O/16O is used for constraining climate change models, partitioning ecosystem water fluxes and for studies of plant ecophysiology. Leaf water enrichment is an essential starting point for each of these applications. In order to obtain a complete picture of the role leaf water plays, not only the 18O values from leaf water but also the signature of transpired water must be accurately predicted for plants under varying environmental conditions. We used a novel chamber approach using highly depleted water (-330 ‰) as a vapor source to leaves of the velvet bean (Mucuna pruriens). We used a Walz gas exchange system consisting of a chamber that is controlled for humidity, light, and temperature. Water and carbon dioxide fluxes were measured by an infrared gas analyzer and chamber vapor was collected in cold traps chilled to - 60°C. Three leaves were collected after 2 hours to insure isotopic steady-state followed by leaf water extraction and isotope analysis. From this experiment we were able to measure the outward flux of soil source water and the inward flux of ambient vapor over a range of environments that varied in relative humidity (80%, 45%, 20%), light (50, 1000 μmolm-2s-1) and CO2 (50, 800 ppm). Leaf water isotopic values were below the source water values reflecting the influx of the labeled vapor. The degree to which leaf water values were depleted was strongly related to the relative humidity. The Craig-Gordon model overestimated depletion of leaf water under high relative humidity and predictions were improved with the Péclet correction. However, our initial analysis indicates that these models may not fully account for stomatal conductance in predicting leaf water isotopic values.
B13B-1202
Linking Temporal Decomposition Dynamics of Crop Residues to Soil Tillage Intensity in Manitoba, Canada
Heterotrophic respiration from agricultural soils may be differentiated as originating from microbial decomposition of recent litter inputs (crop residue carbon) and resident soil organic matter (SOM) pools of varying age and stages of decomposition. Differences in preferential C isotope discrimination between C3 and C4 photosynthetic pathways has application to determining temporal C cycling dynamics in agroecosystems. Addition of crop residues with a different δ13C than SOM facilitates determining the contribution of residue to soil emissions of CO2 and discrimination of SOM derived emissions. In the present study, a tunable diode laser trace gas analyzer was used to determine δ13C values and 12CO2 and 13CO 2 fluxes over an agricultural field in the Red River Valley of southern Manitoba, Canada. Soil emissions of δ13CO2 were also measured on select days using a static chamber method. Measurement campaigns were conducted in the fall/early winter of 2006 and spring of 2007 following harvest of a maize crop. Stable isotopic CO2 gradients were measured from the center of four 200 by 200m plots and fluxes were calculated using the aerodynamic flux gradient method. The soil in two of the experimental plots underwent intensive tillage, while the other two plots received reduced tillage. In fall/early winter of 2006 an increase in δ13C (becoming less negative) over plots indicates the contribution to respiration originating from the fresh corn residue. The influence of soil tillage on C sequestration is being evaluated from respiratory loss of residue and SOM from the reduced and intensive tillage treatments.
B13B-1203
Carbon isotope ratios of soil respiration from soil CO2 profiles and surface chamber measurements using a tunable diode laser spectrometer
A method was developed to measure δ13C of soil CO2 from soil gas samples injected directly into the sample air stream of a tunable diode laser (TDL) spectrometer. Measurement precision was ±0.2‰, and each measurement required two minutes. An open chamber design was also evaluated for use with the TDL. Comparison of Keeling plot intercepts from CO2 profiles generated in a laboratory soil medium and δ13C of surface fluxes showed a 4.4‰ diffusive enrichment, as predicted by theory. Surface CO2 flux rates were measured accurately by the TDL, but the precision of chamber isotope measurements with the TDL was lower than would be predicted from instrument uncertainty (RMS error >0.4‰). This may be due to small pressure fluctuations caused by flow through the chamber or variations in background air. Soil respiration δ13C signatures were calculated from soil CO2 profiles from garden plots with and without (trenched) tree (boxelder, Acer negundo) roots for a growing season. Respiration in plots containing roots was 2‰ enriched relative to trenched plots during the active leaf period of the deciduous trees. This trend was contrary to expectations and potentially caused by enriched respiration from roots, or a priming effect from root exudates, resulting in increased decomposition of enriched soil organic matter.
B13B-1204
Getting to the Source of the High Isotopic Values of Ethiopian Precipitation
Oxygen and deuterium isotopic values of meteoric waters from Ethiopia are unusually high when compared to waters from other high-elevation settings in Africa and worldwide. These high values are well-documented; however, the climate processes responsible the isotopic anomaly in Ethiopian waters have not been thoroughly investigated. Current explanations for the isotopic composition of Ethiopian waters invoke very different moisture transport paths. One model suggests that Ethiopian rainfall originates in the Congo Basin where transpiration returns water to the atmosphere without isotopic differentiation. Another model suggests that Ethiopian rainfall originates in the Indian Ocean and travels westward where the first condensation of ocean moisture in Ethiopia yields precipitation with an isotopic composition close to that of ocean water. Here we present additional isotopic data from Ethiopian waters, use products from the Tropical Rainfall Monitoring Mission (TRMM) and National Centers for Environmental Prediction (NCEP) reanalysis project to evaluate wind directions during precipitation events in Ethiopia, and revisit the question of where the high isotopic values of Ethiopian precipitation originate. Isotopic analyses from more than 250 waters collected in the highlands and rift valleys of Ethiopia all yield values greater than -4 per mil and -22 per mil (VSMOW) for oxygen-18 and deuterium isotopes respectively, which are consistent with results from previous studies of Ethiopian waters. The TRMM and NCEP data show that the majority of low-level winds during precipitation events in Ethiopia are westerly and suggest that the moisture source for Ethiopian rainfall is transpired water from the Congo Basin. We propose that transport of transpired moisture from the Congo Basin is responsible for the high isotopic values of Ethiopian meteoric waters and present a general model for the isotopic and climate dynamics of rainfall in the region.
B13B-1205
The Moisture Isotopes in Biosphere and Atmosphere network (MIBA): initial results from the UK.
The Moisture Isotopes in Biosphere and Atmosphere network (MIBA) is a relatively new IAEA project which was initiated in order to improve the availability of global isotope data on water cycle components, and integrate isotope applications in hydrological cycle, carbon cycle and climate research. It is designed to supplement the data available from the Global Network of Isotopes in Precipitation (GNIP). MIBA is composed of a global network of sites which encompass a diverse range of ecosystems and locations. Monthly or bi-monthly samples of atmospheric water vapour and leaf, stem and soil water from the dominant plant type are collected and their oxygen and hydrogen isotope compositions analysed. Coordination of the sampling and analyses for MIBA is the responsibility of regional sub-networks. Here we provide some background to MIBA, and present the first results from the MIBA-UK sub-network which is composed of three sites (in England, Scotland and Wales) and commenced sampling in January 2005.
B13B-1206
Variability and Coherence of Oxygen and Carbon Stable Isotope Ratios of Tree Ring Cellulose in Coast Redwood Between Distant Sites.
Fog water uptake is an important hydrologic input for redwood ( Sequoia sempevirens) trees and is isotopically distinct from rainfall. The utilization of this resource may depend on climatic factors such as precipitation abundance and changes in sea surface temperature. Increment cores from 3-5 redwood trees at 4 sites were cross-dated and δ18O and δ13C of α-cellulose extracted from subdivided annual rings was measured. Trees from southern sites had latewood cellulose over 4‰ more enriched in 18O than trees from northern sites. Inter-annual variation in latewood cellulose δ18O ranged between 2.3 and 3.5‰ for all sites for the 45+ years measured. Correlations of latewood δ18O variation between sites were greatest for those in close proximity to each other (as high as 0.84 for sites 40 km apart). However, some distant sites also showed substantial coherence (r = 0.43 for sites 380 km apart). In general, cellulose obtained from the center of the ring (middlewood) was more depleted in 18O than latewood and is likely to reflect the use of precipitation water in middlewood cellulose. We observed significant correlations between sites for both middlewood δ18O (r as high as 0.64) and the difference between latewood and middlewood δ18O (as high as 0.65). Significant between-site correlations were also observed for the inter-annual variation in δ13C of cellulose for both latewood and middlewood ring segments. These results indicate that inter-annual variation in tree ring δ18O and δ13C is coherent across much of the redwood forest range and that stable isotopes in these tree rings are capturing a common environmental signal and possible physiological response that may provide valuable information regarding hydrologic inputs, climate cycles and tree response for this ecosystem.ƒn
B13B-1207
Stable Isotopic Constraints on the Geographic Sources of Marijuana in Alaska
Marijuana in Alaska can have numerous sources. Confiscated plants are known to originate either from within the state (e.g., Fairbanks and the Matanuska-Susitna Valley) or from numerous areas outside the state (e.g., Latin America, Canada and the contiguous United States). Latin America reportedly supplies a large percentage of the marijuana currently distributed in the lower 48 states of the U.S.A. However, in more remote areas of the country such as Fairbanks, Alaska, the supply proportions from different geographic areas are not well known. This is due to an insufficient ability to trace source regions from which confiscated marijuana was originally grown. As such, we have analyzed multiple stable isotopes (C, N, O and H) preserved in marijuana samples to identify the likely geographic source from which the marijuana originated (Drug Enforcement Agency license # RW0324551). These samples were confiscated in Fairbanks, Alaska and supplied to us by the University of Alaska Fairbanks (UAF) Police Department. Among 36 marijuana plant samples, we found an unexpectedly large range in the stable carbon isotope compositions (‰13C = -62.2‰ to -24.4‰), with twelve of the 36 samples exhibiting exceedingly low δ13C (-36.1‰ to -62.2‰) relative to typical δ13C of other C3 plants. Interior growing conditions (e.g., hydroponics and/or greenhouses) and a variety of CO2 sources (e.g., CO2 from tanks and fermentation CO2 generators) frequently supplied to growing marijuana to improve yields may account for these exceptionally low δ13C values. Stable oxygen and hydrogen isotope compositions (δ18O and δD vs. V-SMOW) of the marijuana samples were found to range from 10.0‰ to 27.6‰ and -197.1‰ to -134.9‰ respectively. The large range of values suggests that the samples originated from multiple sources ranging from low to high latitudes. δ15N of the marijuana samples also exhibited a large range (-7.0‰ to 14.8‰). This project has implications for the improvement of forensic technology in relatively remote areas such as Alaska. Officers for the Alaska Bureau of Drug and Alcohol Enforcement spend a significant amount of time controlling the production and distribution of marijuana, while the resources allocated for law enforcement must be utilized over a wide geographic area.
B13B-1208
Geospatial modeling of plant stable isotope ratios - the development of isoscapes
Large-scale spatial variation in stable isotope ratios can yield critical insights into the spatio-temporal dynamics of biogeochemical cycles, animal movements, and shifts in climate, as well as anthropogenic activities such as commerce, resource utilization, and forensic investigation. Interpreting these signals requires that we understand and model the variation. We report progress in our development of plant stable isotope ratio landscapes (isoscapes). Our approach utilizes a GIS, gridded datasets, a range of modeling approaches, and spatially distributed observations. We synthesize findings from four studies to illustrate the general utility of the approach, its ability to represent observed spatio-temporal variability in plant stable isotope ratios, and also outline some specific areas of uncertainty. We also address two basic, but critical questions central to our ability to model plant stable isotope ratios using this approach: 1. Do the continuous precipitation isotope ratio grids represent reasonable proxies for plant source water?, and 2. Do continuous climate grids (as is or modified) represent a reasonable proxy for the climate experienced by plants? Plant components modeled include leaf water, grape water (extracted from wine), bulk leaf material ( Cannabis sativa; marijuana), and seed oil ( Ricinus communis; castor bean). Our approaches to modeling the isotope ratios of these components varied from highly sophisticated process models to simple one-step fractionation models to regression approaches. The leaf water isosocapes were produced using steady-state models of enrichment and continuous grids of annual average precipitation isotope ratios and climate. These were compared to other modeling efforts, as well as a relatively sparse, but geographically distributed dataset from the literature. The latitudinal distributions and global averages compared favorably to other modeling efforts and the observational data compared well to model predictions. These results yield confidence in the precipitation isoscapes used to represent plant source water, the modified climate grids used to represent leaf climate, and the efficacy of this approach to modeling. Further work confirmed these observations. The seed oil isoscape was produced using a simple model of lipid fractionation driven with the precipitation grid, and compared well to widely distributed observations of castor bean oil, again suggesting that the precipitation grids were reasonable proxies for plant source water. The marijuana leaf δ2H observations distributed across the continental United States were regressed against the precipitation δ2H grids and yielded a strong relationship between them, again suggesting that plant source water was reasonably well represented by the precipitation grid. Finally, the wine water δ18O isoscape was developed from regressions that related precipitation isotope ratios and climate to observations from a single vintage. Favorable comparisons between year-specific wine water isoscapes and inter-annual variations in previous vintages yielded confidence in the climate grids. Clearly significant residual variability remains to be explained in all of these cases and uncertainties vary depending on the component modeled, but we conclude from this synthesis that isoscapes are capable of representing real spatial and temporal variability in plant stable isotope ratios. http://isoscapes.org