B23C-0438
Investigation Into Causes for Recent Fluctuations in Atmospheric Methane Growth Rate Using Stable Isotopes and a Chemical Transport Model
Atmospheric CH4 is a major greenhouse gas. The CH4 growth rate has varied substantially over the past 2 decades. The causes for the interannual fluctuations are still under debate. The aim of our research was to improve our understanding of the fire contribution to the CH4 growth rate fluctuations during 1996 - 2007. In our analyses we used trace gas (CH4, CO), stable isotopes (13C/12C and D/H) and satellite observations to study the source and sink processes, with the help of modeling techniques. We conducted a series of perturbation simulations to test the competing hypotheses related to the positive anomaly observed during the 1998 El Nino, including changes in either wetland and/or biomass burning emissions, and potential decreases in the OH sink process from ENSO and resultant changes to the CO-OH-CH4 system. By using the stable isotope data to characterize the general nature of CH4 source and sink processes over time, we were able to link the observed combined anomalies in CH4, δ13C-CH4 and CO over the Pacific Ocean to the increasing fire emissions during the 1997 to 2001 El Nino/La Nina period. We used a chemical transport model (GOES-Chem) to quantify the contributions from biomass burning to CH4 and CO. We found that the fire emissions contributions can explain most of the CH4 and CO increases between 1997 and 1999. Since biomass burning is a major source of CO, the similar changes in CO supported the aforementioned findings that fire emissions might have played an important role in CH4 growth rate fluctuations during the ENSO periods.
B23C-0439
Effects of Aridity and Fog Deposition on C3/CAM Photosynthesis and N-cycling in Welwitschia mirabilis
Environmental controls on photosynthesis and N-cycling in Welwitschia mirabilis are evaluated through δ13C and δ15N analyses of leaf material from 26 individuals in the southermost population of this long-lived gymnosperm, which is endemic to the Namib Desert. The coastal Namib Desert in southwestern Africa is hyperarid in terms of rainfall, but receives up to 100 days of fog each year. This climate regime leads to interesting water relations in the Namib flora and fauna. Among many enigmatic characteristics, photosynthesis in W. mirabilis has puzzled researchers since the 1970's. Although it is predominantly a C3 plant, δ13C ranges from -17.5 to -23.5‰ in natural habitats, and can be as enriched as -14.4‰ under artificial growing conditions. Recently the CAM pathway has been confirmed, but the driver for CAM utilization has not been identified. In this study we incorporate new δ13C compositions for plants in the middle of the 100 km aridity gradient which spans the natural distribution of W. mirabilis. Initial results show an enriched δ13C signal (-20‰) in the more exposed individuals compared with those in a sandy drainage depression (-22‰). In addition, the documented correlation between rainfall and δ15N found in Kalahari C3 plants (Swap et al. 2004) is used to interpret the δ15N values in this W. mirabilis population. Initial results indicate that the fog deposition may significantly affect the nutrition of these unusual plants from the Namib Desert.
B23C-0440
Isotope Fractionation During N Mineralization and the N Isotope Composition of Terrestrial Ecosystem N Pools
It has been an open question for several decades whether N mineralization is a fractionating process. This question is important for N cycling in terrestrial ecosystems because even a small fractionation during N mineralization could potentially have a large influence on the N isotope composition of other ecosystem N pools. Fractionation during N mineralization should result in a difference between the N isotope composition of the soil microorganisms, that of its substrates, and products. We analyzed the N isotope composition of the soil microbial biomass in a variety of ecosystems, and found that it was 15N enriched compared to that of other soil N pools, such as soil soluble, organic and inorganic N (Dijkstra et al. 2006a,b). We observed a negative correlation between the 15N enrichment of the microorganisms and the relative C and N availability for soil from ecosystems in Hawaii and Arizona, across a broad range of climates, grasslands and forests, and more than four million years of ecosystem development. This suggests that during N dissimilation (and associated transaminations) and N export, the lighter 14N N isotope is preferentially removed in a manner similar to that proposed for animals and ectomycorrhizae. This was further confirmed by the positive correlation between microbial 15N enrichment and net N mineralization rate (Dijkstra et al. 2008, Ecology Letters 11: 389-397) and by culture experiments with Escherichia coli (Collins et al. 2008). Since mineralization is the largest flux of N in ecosystems, fractionation during N mineralization has the potential to influence the N isotope composition of other N pools, such as inorganic N, plant N and soil organic matter N. We demonstrate that the N isotope compositions of these ecosystem N pools exhibit differences that are consistent with fractionation during N mineralization. Our results show that the N isotope composition can be used as a measure to trace N mineralization and decomposition in ecosystems. Collins JG, Dijkstra P, Hart SC, Hungate BA, Flood NM and Schwartz E. 2008. Nitrogen source influences natural abundance 15N of Escherichia coli. FEMS Microbiol Lett 282: 246-250 Dijkstra, P, Ishizu A, Doucett RR, Hart SC, Schwartz E, Menyailo OV and Hungate BA 2006a. 13C and 15N natural abundances of soil microbial biomass. Soil Biol Biochem 38:3257-3266. Dijkstra, P, Menyailo OV, Doucett RR, Hart SC, Schwartz E and Hungate BA 2006b. C and N availability affects the 15N natural abundance of the soil microbial biomass across a cattle manure gradient. Eur J Soil Sci 57:468-475. Dijkstra P, LaViolette CM, Coyle JS, Doucett RR, Schwartz E, Hart SC and Hungate BA 2008. 15N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function. Ecol Lett 11: 389-397.
B23C-0441
Modeling Non-Steady Isotopic Effects Caused by Biological Kinetic Transient Complexation During Denitrification in Soils
The composition and location of 15N atoms on N2O molecules has been used to characterize soil biological N cycling and N2O surface emissions. Besides the complexity of the processes related to N2O transformations and movements (e.g., chain-like denitrification reaction, soil moisture and temperature dynamics, aqueous and gaseous advection and diffusion) which make interpretation of the isotopic N2O composition very difficult, a theoretical aspect has been overlooked. The theoretical formulation of biological kinetic reactions in isotopic applications makes common use of first-order and quasi steady-state assumptions, according to which the rates of change of the concentration of intermediate complexes can be neglected. When isotopically-labeled reactants are used, these assumptions are not necessarily accurate since isotopic effects during complexation occur at orders of magnitude that compare with the truncation used under first-order and quasi steady-state conditions. Both assumptions, in fact, always lead to a constant fractionation factor and may therefore yield incorrect estimates of the isotopic effect and a misleading interpretation of the reaction signature. We have analyzed the isotopic signature of denitrification in biogeochemical soil systems reported by Menyailo and Hungate (2006), where high 15N2O enrichment during N2O production and inverse isotope fractionation during N2O consumption could not be explained with first-order kinetics and the Rayleigh equation, or with quasi steady-state Monod kinetics. When the quasi steady-state assumption was relaxed, transient Monod kinetics accounting for isotopic effect occurring at the complexes accurately reproduced the observations and aided in interpretation of experimental isotopic signatures. These results may imply a substantial revision in using the Rayleigh equation for interpretation and in modeling biological kinetic isotope fractionation with first-order kinetics or quasi steady state Monod kinetics.
B23C-0442
Tropospheric N2O Isotopic Composition: Instrumentation Development and Initial Data for Reducing N2O Source and Sink Uncertainties
Measurements of nitrous oxide isotopic composition in the troposphere provide a means for minimizing much of the uncertainty in the regional and global budgets of this important atmospheric species which arises predominately from biological sources in soils and oceans. Continuous atmospheric surface measurements of the concentration of N2O have provided an important resource in analysis of the budget through inversion studies. The utilization of these concentration data, however, has reached the limit of information that can be extracted about the N2O budget and leaves large uncertainty remaining. Combined with the isotopic signatures of N2O source and sink process end-members, isotopic N2O measurements in the troposphere will place added constraints on the budget. This research pursues N2O isotopomer analysis of tropospheric samples to trace the origin and fate of N2O in the atmosphere. Impediments regarding instrumentation have prevented pursuit of this line of research in the scientific community. Instrumentation developed by our group yields initial data for air samples from Boston, MA, which demonstrate the ability of N2O isotopic analysis to distinguish biospheric controls on N2O from stratosphere-troposphere exchange. Data from Boston are explored and applied alongside theoretical analysis of N2O isotopic composition in the troposphere to examine the ability of isotope data to lower uncertainty in particular related to the role of stratosphere-troposphere exchange, which is largely unknown in global budget estimates and forces uncertainty into current estimates of biosphere- atmosphere exchange.
B23C-0443
Isotopic Constraints on the Global Budget of Atmospheric Nitrous Oxide
Using the Caltech/JPL two-dimensional model of the terrestrial atmosphere, we develop a simple model for nitrous oxide (N2O) that is based on laboratory kinetics measurements and constrained to reproduce the age of air in the stratosphere. We study three types of models. The Baseline Model assumes that the primary sources of N2O are the land, the ocean and agriculture, and the primary sink is destruction in the stratosphere. The Standard Model includes additional N2O sources from rivers, estuaries and coastal zones as well as fossil fuel combustion and industrial processes, as recommended by IPCC [2007]. Extended Models explores the consequences of a climate-related acceleration of the Brewer-Dobson circulation that transports N2O from the troposphere to the stratosphere and projections of future concentrations of N2O. The model includes all the commonly studied isotopologues and isotopomers of N2O and can account for most of known observations. These observations include the abundances and trends of the isotopologues and isotopomers of N2O since the Pre-Industrial Era. The data suggest that the negative trends in the isotopic fractionations appear to slow down in recent decades, a result that can be explained by the Standard Model but not the Baseline Model. We also discuss more speculative results on future projections of N2O.
B23C-0444
Portable Cavity Ringdown Spectrometer for Methane Isotope Ratio Measurements
Close to 45% (244 Tg/yr) of the methange (CH4) in the atmosphere is produced in anaerobic soil conditions (wetlands and rice paddies). Under aerobic soil conditions, bacteria oxidize CH4 to produce CO2 and H2O. Both production and oxidation rates depend on soil composition, nutrient loadings, water content, and plant conditions, but these dependencies are not well characterized. Measurements of CH4 isotope ratios can provide a better understanding of CH4 processes in natural and man- made ecosystems. Here we present progress on the development of a field deployable instrument capable of making precision 13CH4/12CH4 and CH3D/ CH4 isotope ratio measurements of CH4. Moving the instrument out of the lab and into the field will significantly improve the spatial and temporal resolution of data and enhance the study of plant-soil-atmosphere CH4 source and sink processes. Our instrument is a Near-IR (1280-1340 nm) tunable diode laser Cavity Ringdown Spectroscopy (CRDS) system. CRDS is a technique in which the laser injects energy into a high finesse cavity by tuning to one of the cavity resonant modes, resulting in a buildup of energy. At some threshold intra-cavity intensity the injection is stopped, and the intensity decays exponentially due to losses such as absorption by molecules. If the laser is tuned to an absorption line of a sample gas, the concentration of the molecule is proportional to the decay constant (according to the Beer-Lambert law)-—scanning over a frequency range produces an absorption spectrum. Currently our system has a resolution of 150 MHz scanning over a 30 GHz (0.2 nm) region, allowing us to resolve peaks at pressures of 100 torr. Using combinations of CH4 standard (natural isotopic abundance) and a 99% pure 13CH4 standard, we identified several lines in the CH4 HITRAN Database that we attribute to 13CH4. We use these and 12CH4 lines within the same region to measure 13CH4 concentration, 12CH4 concentration, and the isotope ratio (13C/12C and D/H). We present our lab-based prototype system, including our latest isotope ratio performance and measurement precision. In addition, we present the way forward to achieve both our target precision and portability.
B23C-0445
Investigation of Kinetic Fractionation During Evaporation Using the Evaporation Flux Isotope Ratio Measured by a Tunable Diode Laser
Evaporation plays a critical role in isotopic studies of the hydrological cycle and paleoclimate. During the process of evaporation, the kinetic fractionation factors are among the most significant drivers that control the isotopic composition of evaporation (δE). These factors are functions of the ratios of molecular diffusivity coefficients of heavy to light isotopomers in air and the physical processes associated with turbulent diffusion. However, the accuracy of existing estimates of these factors is limited because of the lacks of the δE measurement and the uncertainties in actual water surface temperature by cooling effects during evaporation. This study aims to quantify the kinetic fractionation factors by measuring δE over evaporating water surfaces using continuous measurements of vapor isotope ratios by a tunable diode laser (TDL). We have developed a chamber where relative humidity and temperature of chamber air were monitored. Inside the chamber, water was evaporated from small dishes. The hydrogen and oxygen isotopic compositions of the chamber inlet and outlet air were measured by the TDL and used for calculating δE. The liquid water was sampled at the beginning and the end of experiment and analyzed for the D and 18O compositions. The continuous δE allows us to quantify kinetic fractionation factors and evaluate existing theories on isotopic processes during evaporation.
B23C-0446
Ecosystem Fluxes of Stable Isotopes in Carbon Dioxide and Water Vapor Above a Forest Measured by Laser Spectroscopy
Eddy covariance or flux gradient measurement of stable isotopes in CO2 and H2O would provide a direct measure of ecosystem discrimination and thus indicate the ecosystem fingerprint on the atmosphere's isotopic budget. Eddy covariance measurements of stable isotopes require, however, high-precision and very fast instruments, which are available only since very recently. We use a quantum cascade laser absorption spectrometer (Aerodyne Research Inc.) for the simultaneous measurement of 16O12C16O, 16O13C16O and 18O12C16O isotopologues with a sampling rate of up to 10 Hz. The 1-sec precision for both δ13C and δ18O in CO2 is about 0.20‰. Hourly calibrations are performed with two calibration gases and by dynamically diluting a third calibration gas with CO2-free air. The long-term stability was assessed with repeated measurements of a quality control standard. The 1-σ standard deviation of these measurements over a time period of three weeks was 0.15‰ for both δ13C and δ18O. Using a second laser spectrometer, isotopologues of water vapor (1H16O1H, 1H18O1H and 2H16O1H) are measured at a sampling rate of 0.5 Hz by Off-Axis Integrated Cavity Output Spectroscopy (Los Gatos Research Inc.). The 2-sec precisions of δ2H and δ18O in water vapor are about 1.0‰ and 0.3‰, respectively. A custom made dripping device based on ink jet technology is used to calibrate the isotopic measurements of water vapor. We present the first eddy covariance flux measurements of the stable CO2 isotopologues above a forest ecosystem and assess the feasibility to infer ecosystem discrimination from such measurements. Cospectral analysis of vertical wind speed and the isotopologue mixing ratios show the expected inertial sub- range behavior matching well CO2 measurements with an open-path gas analyzer (LI-7500, Li-Cor Inc.), but also show some dampening in the 0.5-5 Hz range due to tube attenuation. The concurrent measurements of δ2H and δ18O in water vapor show large diurnal and day-to-day variability, but little vertical variation within the canopy indicating strong vertical mixing.
B23C-0447
Development and Validation of an Isotopic Water Vapor Analyzer for Rapid Measurements of 18O/16O and D/H in Ambient Air
Routine measurements of stable oxygen and hydrogen isotopes are crucial to the advancement of hydrological and climate research at the local, regional and global scales. Water isotopes provide critical knowledge about the source, history and age of water supplies, degree of water mixing. Current technology involves collecting and condensing individual water samples, transporting them to a laboratory, and quantifying the isotope ratios via an Isotope Ratio Mass Spectrometer. In practice, this procedure has made long-term continuous monitoring of water supplies prohibitively time-consuming and labor intensive. We will report on the development, deployment and independent validation of a novel isotopic water vapor analyzer, based on cavity-enhanced laser absorption spectroscopy techniques (Off-Axis Integrated Cavity Output Spectroscopy), capable of accurately quantifying 18O/16O and D/H in 1-second time intervals. The analyzer provides continuous monitoring of ambient water vapor over a large range of mixing ratios from 500 ppmv to over 40000 ppmv. Independent validation of the instrument's capabilities at several different locations will be presented and results from recent field tests at the Mauna Loa Observatory will be discussed.
B23C-0448
Measuring hourly 18O and 2H fluxes in a mixed hardwood forest using an integrated cavity output spectrometer
The 18O and 2H of water vapor can be used to investigate couplings between biological processes (e.g., photosynthesis or transpiration) and hydrologic processes (e.g., evaporation) and therefore serve as powerful tracers in hydrological cycles. A typical method for determining δ18O and δ2H fluxes in landscapes is a 'Keeling Plot' approach, which uses field-collected vapor samples coupled with a traditional isotope ratio mass spectrometer to infer the isotopic composition of evapotranspiration. However, fractionation accompanying inefficient vapor trapping can lead to large measurement uncertainty and the intensive laboring involved in cold-trap make it almost impossible for continuous measurements. Over the last 3-4 years a few groups have developed continuous approaches for measuring δ18O and δ2H that use laser absorption spectroscopy (LAS) to achieve accuracy levels similar to lab-based mass spectrometry methods. Unfortunately, most LAS systems need cryogenic cooling, constant calibration to a reference gas, and substantial power requirements, which make them unsuitable for long-term field deployment at remote field sites. In this research, we tested out a new LAS--based water vapor isotope analyzer (WVIA, Los Gatos Research, Inc, Mountain View, CA) based on Integrated Cavity Output Spectroscopy (ICOS) and coupled this instrument with a flux gradient system. The WVIA was calibrated bi- weekly using a dew point generator and water with known δ18O and δ2H signatures. The field work was performed at Morgan-Monroe State Forest Ameriflux tower site (central Indiana) between August 8 and August 27, 2008. The combination method was able to produce hourly δ18O and δ2H fluxes data with reproducibility similar to lab-based mass spectrometry methods. Such high temporal resolution data were also able to capture signatures of canopy and bare soil evaporation to individual rainfall events. The use of the ICOS water vapor analyzer within a gradient system has the potential to greatly expand the use of continuous δ18O and δ2H fluxes measurements to address a wide range of ecohydrological research topics.
B23C-0449
New estimates on the magnitude of fractionation during photorespiration
We report new estimates of the carbon isotope fractionation during photorespiration for three Senecio species. We determined the contributions of different processes to net 13C discrimination during photosynthesis by comparing observed discrimination to predictions derived from gas exchange measurements. The rate of photorespiration was manipulated by altering the O2 partial pressure in the air surrounding the leaves. The fractionation factors for photorespiration (f) and net carboxylation by Rubisco and PEPc (b), and mesophyll conductance (gi) were treated as unknowns and determined simultaneously for all measurements. We propose this as alternative approach to analyse measurements under field conditions when mesophyll conductance and fractionation factors are not known, or cannot be determined in separate experiments. Good agreement between predicted and observed discrimination was achieved with f of 11.6 permil, b of 26 permil, and gi between 0.22 and 0.27 mol m-2 s-1. Our result for f is close to recent theoretical estimates. Photorespiratory fractionation decreases net 13C discrimination by about 1.2 permil on average under field conditions, which should be taken into account when partitioning net CO2 exchange of ecosystems into gross fluxes of photosynthesis and respiration.
B23C-0450
Diurnal and Seasonal Variation in the Carbon Isotope Composition of Leaf- and Root- respired CO2 in C3 and C4 Species
The carbon isotope signature of leaf (δ13Cl) and root (δ13Cr) dark- respired CO2 records and integrates short-term metabolic changes. Plants with C3 and C4 photosynthetic metabolism are expected to differ in diurnal and seasonal patterns in δ13Cl and δ13Cr because of differences in photorespiration, isotopic fractionation at metabolic branch points and allocation patterns. A thorough understanding of the environmental and metabolic controls on δ13Cl and δ13Cr is necessary to interpret the δ13C of ecosystem respired CO2 and partition the CO2 efflux into autotrophic and heterotrophic respiration sources. We measured δ13Cl in two C3 tree species (Prosopis velutina and Celtis reticulata), a C3 herb (Viguiera dentata) and a C4 grass (Sporobolus wrightii), and δ13Cr in P. velutina and S. wrightii in a semiarid savanna in southeastern Arizona, USA. δ13Cl during the dry pre-monsoon period was relatively enriched in 13C during daytime periods and became depleted in 13C at night relative to daytime values for all species with the exception of S. wrightii, the C4 grass. δ13Cl in S. wrightii was strongly influenced by seasonal differences in water availability with a larger diurnal amplitude in δ13Cl (8.2 +/- 0.6‰) during the wet monsoon period compared to that in the dry pre-monsoon period (4.4 +/- 0.4‰). The δ13C values of starch and lipid fractions remained constant over diurnal periods within the pre-monsoon and monsoon seasons. For C3 species, δ13Cl and δ13C of the cumulative, flux-weighted photosynthate pool estimated from gas exchange were strongly positively correlated, suggesting that progressive 13C-enrichment of leaf-respired CO2 during the daytime period resulted from changes in the δ13C signature of respiratory substrates associated with short-term changes in photosynthetic 13C discrimination. Rapid decreases in δ13Cl following the daytime period was likely caused by decreases in the ratio of PDH:acetyl-CoA oxidation rather than by a shift in respiratory substrate use. Diurnal variation in δ13Cr was observed in P. velutina, but not in S. wrightii. δ13Cr was highly positively correlated with δ13Cl in P. velutina, but only if a 12 hour time lag was applied. Diurnal variation in δ13Cr in P. velutina may potentially result from changes in the ratio of PDH:acetyl-CoA oxidation or from diurnal changes in the δ13C value of carbohydrates transported from leaves. In summary, diurnal and seasonal changes in δ13Cl and δ13Cr were observed among C3 and C4 species in a semiarid savanna. These 13C signals may be useful for probing functional type induced differences in major metabolic pathways within ecosystems.
B23C-0451
Dynamics of the C and O isotopic signatures of chamber respiration varying with wetting and drying cycles in semi-arid grassland
Our semi-arid grassland research site had coexisting C3 (cool-season) and C4 plant (warm-season) species, with climate characterized by brief periods of water availability with pulsed rainfall events. Understanding respiration dynamics in this ecosystem type is demanded for improving global C cycle models since the majority of stable isotope biogeochemistry research has been focused on C3-ecosystems. We measured CO2 respiration and corresponding δ13C and δ18O values from four different vegetation cover types, C3, C4, mixed C3 and C4 and bare ground patches over growing seasons with wetting and drying cycles, three times during several diurnal campaigns. We evaluated how plant community specific δ13C and δ18O from chamber respiration (δ13CR and δ18OR) varied with moist versus drying conditions or pulse event-driven episodes. Contrasting influences of soil moisture availability on C3 and C4 grasses were obvious for both δ13C and δ18Oƒnmeasurements. During moist conditions, recently fixed carbon assimilates contributed to the apparent different δ13CR values; -22 permil from C3 patches, -16 permil from C4 patches, and -19 permil from mixed C3 and C4 patches. Significant differences in δ18OR between C3 and C4 patches also occurred during moist condition. However, during dry conditions, the apparent vegetation type differences were not distinguishable, suggesting reduced autotrophic activity, soon after the onset of dry conditions for δ13CR. Reduced variations during dry conditions for δ18OR may be explained by three counteracting factors such as root distributions, physiological traits in terms of internal CO2 concentration and CA activity between two species, and a marked gradient in soil water δ18O profile. Small rainfall pulses initiated respiration of labile C and improved leaf gas exchange, greatly influencing δ18O of foliar respiration from patches dominated by C4 plants, which have shallow roots. Incorporating dynamic aspects of labile/stable C sources that vary with wetting and drying cycles should improve regional and global scale C cycle models.
B23C-0452
In search of the mechanisms behind soil carbon metabolism of a Douglas fir forest in complex terrain using naturally abundant 13C
Soil is well known for being highly variable, spatially and temporally, in moisture, texture, nutrients, carbon content and organisms. The magnitude of variation in soil characteristics represented in a study is, in part, determined by the choice in site location. Choosing sites that are topographically flat reduces variability due to environmental gradients, variability that is amplified in sites of complex terrain. We measured soil respiration, an integrative measure of ecosystem biological and physical processes, and its isotopic signature (δ13CR-s) to accomplish two goals: 1. Explore how gradients in temperature and moisture within a steeply sloped watershed affect the flux and isotopic signature of soil CO2 2. Deconvolve the isotopic signature of soil respiration into autotrophic and heterotrophic sources using a multi-source mixing model constrained by samples of soil organic matter and water soluble extracts of leaf foliage. Our site is located in a steep catchment within the central Cascades of Oregon (HJ Andrews LTER) where we made respiration measurements in plots established along side a sensor transect that continuously measures soil moisture and temperature; air relative humidity and temperature; and tree transpiration. There was a distinct difference in soil metabolism between the south and north aspects in the watershed. Temperature-corrected basal respiration of the south facing slope was 1 μmol m-2s-1 greater than the north facing slope. There was also a difference in isotopic signature between the two slopes that could be as great as 2 per mil depending on the period within the growing season. The strength of the correlation between environmental variables and soil carbon flux was non-uniform across the catchment. There was, however, a strong positive correlation between soil flux with recent transpiration rates (0 to 3 days prior) as well as with transpiration rates that occurred up to 9 days previously. This pattern was especially prevalent for locations near the ridge of each slope and dampened with a decrease in plot elevation. The correlation between δ13CR-s and transpiration, as well as vapor pressure deficit, was similar with a high degree of correlation that occurred 0-3 and 8 days before sampling. The correlation analysis suggests that soil flux in this forest is primarily controlled by aboveground inputs throughout the growing season. The source partitioning analysis confirms this observation although the magnitude of the aboveground contribution varies with season and topographic position.
B23C-0453
Using Keeling Plots to Trace the Isotopic Composition of Carbon Dioxide Through Processes of Heterotrophic Respiration, Diffusion and Soil Water Equilibration in Artificial C3- and C4-Grassland Soils
Field studies aimed at better understanding biosphere-atmosphere CO2 fluxes have been advanced by the use of Keeling plots to interpret the isotopic composition of ecosystem respiration, or soil-respired CO2. Keeling plots have also been employed to interpret the isotopic composition of soil CO2, and processes controlling CO2 equilibration with dissolved inorganic carbon species and soil carbonate precipitated in aridland soils and paleosols. This study specifically addresses the need for increased knowledge of the isotopic composition of soil- respired CO2 and its effect on the δ13C and δ18O values of soil CO2 and soil carbonate. We designed artificial soil columns that consist of typical soil horizons (A1, A2, E, a developing Bk, and C), so that we could sample soil CO2 from each horizon under controlled laboratory conditions. We built three replicates of two types of soil profiles (C3 or C4), each containing a homogenized grass litter (δ13C = -29.1 ±0.3‰ and -13.8 ±0.2‰ respectively). Soil CO2 was sampled monthly and analyzed by IRMS and Keeling Plots created with Model II linear regression. Keeling Plot δ13C intercepts for replicates of entire columns (δ13C =-24.1 ±0.3‰ and -10.3 ±1‰ respectively) track the δ13C of source CO2 when corrected with a constant 4.4‰ diffusional fractionation, but change in isotopic value with column depth, indicating curved rather than straight Keeling plot lines. The Keeling plot intercepts for replicates of each horizon from the C3 columns show trends of increasing 13C-depletion from the A1 horizons to the Bk horizons. These trends suggest a departure from the steady-state source-to-background diffusional mixing inherent in interpreting Keeling Plots of soil CO2. These data will be used to assess the utility of steady-state diffusion-production models of soil CO2 equilibration with soil carbonate.
B23C-0454
Partitioning Respiration Fluxes in a Forested Mountainous Ecosystem Using Natural Abundance of Stable Carbon Isotopes
Isotopic mass balance techniques can help overcome challenges to studying ecosystem fluxes in complex terrain. Because scaling up measurements made at the leaf level compounds and magnifies measurement errors, it is desirable that ecosystem fluxes be measured using an integrative technique, such as eddy covariance. However, advective forcing complicates the use of eddy covariance in complex terrain, so we used an isotopic mass balance approach to partition ecosystem respiration flux in a forested site in a mountainous region of northern Idaho, USA. In 2006 and 2007, we analyzed the carbon-13 composition (δ13C) of ecosystem respiration and both the δ13C and magnitude of soil, foliar, and stem component fluxes of ecosystem respiration. Soil- and stem-respired δ13C values were similar, from -27.24 (2.63)‰ (stem) to -26.9 (1.11)‰ (soil). Foliar respiration was consistently more enriched by up to 8‰. We took advantage of nighttime advection to collect samples of air from the forest, then used a Keeling plot approach to determine the isotopic composition of respired CO2. Ecosystem-respired CO2 had a seasonal range from -27.3 (0.67)‰ to -23.5 (1.0)‰. Using a mixing model, we then determined a ratio of soil respiration to ecosystem respiration for the forest. We compared this ratio to a ratio obtained independently by scaling up soil, foliar, and stem respiration rate measurements to the forest level. This work shows that, in the absence of eddy covariance techniques, whole-ecosystem gas exchange can be examined using the natural abundance of stable isotopes. This approach can be further used to test the accuracy of ecosystem carbon models or scaling techniques.
B23C-0455
Physical Controls on the Isotopic Composition of Soil Respired CO2
Isotopic measurements of soil and soil-respired δ13CO2 are valuable tool in ecosystem carbon- cycling research. While steady-state work has been indispensable in understanding the implications of diffusive transport on soil CO2 isotopic composition, natural systems may rarely achieve isotopic steady state. In non-steady state diffusive environments such as soil, dynamic fractionations result from differential equilibration of isotopologues following a change in system physical parameters (diffusivity, production rates etc). These fractionations are transient in nature, but failure to recognize them may potentially lead researchers to misinterpret isotopic data. We have been using both 1-D and 3-D isotopologue-based transport models and continuous isotopic measurements in experimental systems to assess the character and magnitude of dynamic fractionation effects associated with changes in environmental parameters such as CO2 production rate and soil moisture, and various lab and field-based measurement methodologies. Here we present data from a subset of these modeling and laboratory studies that illustrate the nature and importance of dynamic fractionation processes. Time varying soil characteristics were indeed found to induce non-steady state gas transport conditions, and in all cases, modeled and measured data showed strong correspondence. In systems with realistic variability in CO2 production rate and soil moisture, we observed transient disturbances in the isotopic signature of soil and soil-respired δ13CO2, by up to several permil in the case of diurnal changes in CO2 production, and potentially more following rain events. Most measurement methodologies (open and closed chambers, for example) biased isotopic data to some extent, however these biases were not constant; changeable soil properties such as porosity, moisture, diffusivity, production rate, and concentration gradient, all exerted an influence upon the measured isotopic signature. Hardware-related measurement biases were typically smaller than 2 permil, but could be magnified when analytical techniques such as Keeling plots were applied to the data, or when these methodologies are used in conjunction with open-bottom laboratory columns. These dynamic fractionations are theoretically not limited to soil environments, but apply to all diffusive systems large or small. Although these isotopic transport dynamics are complex, modeling efforts appear to properly reproduce measured patterns, and can be used to unravel data complexities, or to develop appropriate sampling strategies and interpretation techniques.
B23C-0456
Isotope end-member mixing models are non-linear in non-steady state diffusive environments
Linear end-member mixing models (ie. Keeling) are commonly used to interpret isotopic data from environments where diffusion is the dominant mode of mass transport. While their applicability in situations where isotopologues of the same element have the same transport rates (ie. convective mixing) is known, researchers have not considered the effects of differential rates of isotopologue transport on the linearity of these mixing models. Transport related non-linearities may lead to significant biases in end member estimates, especially when diffusive fractionation is large. Using measurement of the isotopic composition of soil respired carbon dioxide as an example, we show through numerical modeling and laboratory experiments that differential isotopologue transport rates can significantly affect linearity. Both diffusivity and production rate of carbon dioxide were found to be important controls on the non-linearity of the mixing line for this particular system. Additionally, bottom boundary effects imposed by lateral 3-D diffusion skewed mixing lines more significantly than was observed under 1-D diffusion conditions. Without consideration, these non-linearities could impose significant skew on results gained using linear mixing models and affect the conclusions made about biological and/or geochemical processes.
B23C-0457
Differential Usage of Summer Versus Winter Precipitation by Vegetation Growing in Central Colorado
Montane ecosystems rely on two distinct water sources throughout the year: winter and summer precipitation. While winter snow is typically the dominant contributor to ecosystem water budgets, summer rain can account for a substantial fraction of total water inputs. Superimposed on this broad hydrological context, there is evidence of changes in the timing and amount of both water inputs in many montane ecosystems, with unknown impacts on vegetation. In this work, I present water isotope data for snow, rain, and streamflow at the the Rocky Mountain Biological Laboratory in central Colorado. Summer rain is shown to be isotopically enriched compared to winter snow, thus allowing an estimate of the fractional contributions of each precipitation source to xylem water. This isotopic approach provides evidence for differential use of winter versus summer precipitation for several tree species growing in this area. Subalpine fir is shown to rely much more on summer water inputs compared to Englemann spruce and aspen. These isotopic data are supported by growth data for each species. These results have potentially important implications for the ecological success of these species in a warming climate.
B23C-0458
Impacts of physiological response and species composition on ecosystem respiration
Stable isotope analyses have provided insights into physiological controls of ecosystem carbon balance.
Here we present measurement updates of a nation-wide network of carbon-13 ratios of ecosystem respiration
(δ13CR) observed in 10 ecosystems, many of which are part of AmeriFlux. Systematic
variations in δ13CR have been reported with an average intra-seasonal variation of 2.3
± 0.5‰ in the Wind River Experimental Forest in southern Washington, USA over the period
of 2001 - 2007. This seasonal pattern of δ13CR was consistently observed in all three
coniferous forests of relatively uniform lifeform in the Pacific Northwest USA (Wind River Canopy Crane,
Mary-Fir, and Metolius). In contrast, we observed no apparent seasonal differences in the
δ13CR value at Harvard Forest, a mixed deciduous forest, during the same period. Two
possible mechanisms have been suggested to explain inter- and intra-annual variations in
δ13CR: 1) canopy physiological response (stomatal closure) to drought and 2) shifts in
species composition within an ecosystem. The former is most tractable for the observed
δ13CR variation in stands with a uniform lifeform, such as seen at the Wind River Canopy
Crane location. The Harvard Forest is a mixed deciduous forest where shifts are occurring in the proportion
of major species, Red Oak (ring-porous) and Red Maple (diffuse-porous). Shifts in species dominance within
an ecosystem may result in δ13CR values where species effects and environmental
response effects offset each other. Alternatively, ring-porous species may exhibit much less dynamic
δ13CR patterns than diffuse-porous or tracheid-dominated forest species. On a seasonal
basis, the relationships between δ13CR variations and environmental conditions are in
general consistent with leaf-level physiological response, regardless of ecosystem types. Relative impacts
resulting from physiological response and changes in species dominance on δ13CR
variation may be used as constraints to characterize ecosystem respiration, which can cascade to result in
detectable patterns in the atmosphere at continental scales. A canopy-process model will be developed and
tested against these ideas using multi-year δ13CR measurements from a variety of
ecosystems.
http://www.sci.sdsu.edu/biomet/pmwiki.php/PmWiki/Home
B23C-0459
BASIN Synthesis and Spatial Mapping of Keeling Plot Data Using an Artificial Neural Network
The "Keeling plot" method has proven to be a robust and highly informative measure of ecosystem-
atmosphere interactions, particularly with respect to photosynthesis, respiration and water use efficiency of
terrestrial ecosystems. Applied over many years and locations, the archive of Keeling plot data is steadily
increasing, especially in light of recent coordinated collection efforts and advances in laser-based
technologies. However, meta-analyses of this valuable and potentially informative record remains challenging
because of the discontinuous nature of the largely campaign-based and site-specific collections over the
years. One of the main objectives of the Biogeosphere-Atmosphere Stable Isotope Network (BASIN) is to
facilitate the synthesis and exchange of stable isotope information related to ecosystem processes in carbon
and water cycles at various scales. Towards this goal, we have initiated a BASIN-wide effort for routine
synthesis of past and future Keeling plot data in the context of an objective and statistically based approach
using an artificial neural network (ANN) to help elucidate coherent patterns in the inherently disparate data.
Predictive relationships between Keeling plot intercepts and climate and vegetation developed with this
method can help to not only reveal patterns in the data that may lead to future process-based research, but
can also provide the means to efficiently translate site-specific, campaign-based data into spatial and
temporally continuous maps of Keeling plot intercepts. Using this data-intensive approach, the ANN can be
continually updated to increase its accuracy and resolution as new data from more sites becomes available.
We will describe the various sites and datasets currently available (BASIN, SIBAE, DOE-TCP, etc.), results
related to the training and site-specific validation of the ANN, functional responses of Keeling plot intercepts
to environmental conditions and vegetation status as revealed through the ANN, and finally, spatial maps
produced with the ANN when applied with global meteorological data and satellite observations of vegetation
status.
http://basinisotopes.org/
B23C-0460
Concentrations and 14C age of nonstructural carbon in California oaks
Plants store photosynthetic assimilates as nonstructural carbon (NSC), mainly glucose, fructose, sucrose, and starch. NSC fuels processes such as respiration and growth. Research suggests that NSC represents a significant fraction of a plant's annual C budget, but temporal dynamics of NSC are poorly understood. We used concentration and radiocarbon (14C) measurements of NSC to investigate how temporal dynamics of NSC vary with life strategy and throughout a species' range. In Mediterranean environments, oaks have developed two strategies (evergreen and deciduous) to cope with drought. Within California, the uncertainty of annual winter rain increases from north to south. We compared two evergreen and deciduous species: Coastal and Interior live oak (Quercus agrifolia and wislizenii) and Valley and Blue oak (Q. lobata and douglasii). Samples (4 mm cores to 20 cm depth at dbh) were taken in 2008 before leaf-out and fall at five sites which represent an inland to coast temperature gradient from high to low summer temperatures as well as a north- south precipitation gradient. Sugars were isolated by shaking in methanol-water and quantified using a spectrometric micro-plate technique. Starch was isolated by boiling in ethanol followed by HCl digestion and quantified manometrically. 14C contents were measured by AMS. Preliminary findings indicate that in live oaks, winter sugar concentrations are constant throughout the tree and across sites, while 14C concentrations increase towards a tree's center. This suggests that the NSC pool oaks is not well mixed. Future work will elucidate whether plants can access these older NSC stores.
B23C-0461
Coupled Oxygen and Hydrogen Isotope Analysis of Water Along the Soil-Plant- Atmosphere Continuum
The oxygen and hydrogen isotope compositions of water within a plant vary with transpiration rates and the isotopic composition of soil water. Both of these parameters are affected by temperature and relative humidity. A controlled-temperature, growth-chamber experiment was conducted to determine the relationships among temperature, relative humidity, soil water evaporation and plant-water isotope composition in cattails and horsetails. Typha, a cattail species that grows in wetland conditions, and Equisetum, a horsetail species that prefers dry soils, were each grown in four chambers at 15, 20, 25 and 30 degrees Celsius. The oxygen and hydrogen isotope compositions of watering water, soil water, vapour in the growth chambers and plant water from the leaves and stems were analyzed throughout the eight-month long artificial growing season. Although the oxygen isotope composition of the watering water remained constant, the soil water, atmospheric vapour and plant water were progressively enriched in oxygen-18 and deuterium in each of the four chambers from low to high temperatures as a result of increasing evaporation. The oxygen isotope composition of plant water along the length of a single stem or leaf was increasingly enriched in the heavier isotopes towards the apex. There was no significant difference in the magnitude of this trend between species. These results indicate that the isotopic composition of plant water is primarily controlled by environmental conditions. The oxygen isotope composition of the water vapour in the growing chamber increased with temperature, consistent with equilibration between the vapour and the oxygen-18 enriched soil and plant water reservoirs. The magnitude and interaction of these variables, as measured for these modern samples of cattails and horsetails, should be useful in calibrating paleoclimate proxies based on fossilized plant materials (e.g., cellulose, phytoliths).
B23C-0462
Forest Harvesting Impacts on the Water and Isotope Balance of a Mountainous Watershed in Northern Idaho USA
The impacts of timber harvesting practices on the flow regime, water and isotope mass balance (2H, 2O) in a mountainous catchment were studied over the period from 2004 to 2007. The Mica Creek Experimental Watershed (MCEW) is located in northwestern Idaho, USA (97 km2, 975 - 1,750 m a.s.l.). It includes three sub watersheds of comparable physical conditions with clear-cut (100 % removal in 50 % of the area, CC), partial-cut (50 % removal in 50 % of the area, PC), and unimpacted (control forest, CF) sites. Precipitation, spring water, stream flow, soil water, and sap flow were collected for stable isotope analyses on a monthly basis during the growing season. Snow, which is the dominant water input in this region, was intensively studied during winter 2005/06. A base flow sampling campaign was conducted during low flow in September 2006. Weekly analyses of precipitation indicate isotopic variations ranging between -3.9 ‰ and -22.0 ‰ and -42 ‰ and -170 ‰ for δ2O and δ2H, respectively. Isotopic composition of snow samples obtained in 2006 from snow profiles at snow courses varied between -13.8 ‰ and -17.5 ‰ for δ2O and δ2H values varied between -102 ‰ and -129 ‰. The isotopic composition of snow reflects enrichment due to intercepted snow sublimation and evaporation in the dense forest sites. Stream flow samples are in the range of -15.7 ‰ and -113 ‰ for δ2O and δ2H, respectively. Mean soil water concentrations of the upper 20 cm for the CC, PC and CF sites show values of -15.4 ‰, -14.1 ‰, -13.3 ‰ and -123 ‰, -114 ‰, -109 ‰ for δ2O and δ2H, respectively. Sap flow appeared to reflect differential canopy interception losses, with greater enrichment under the densest canopies (-14.4 ‰, -14.2 ‰, -13.8 ‰ for δ2O of CC, PC, CF and -130 ‰, -124 ‰, -119 ‰ for δ2H of CC, PC, CF, respectively). Presumably, variations reflect fluctuating rates of precipitation, deposition, ablation and sublimation of the snow cover. Stream water, soil water and plant water follow these patterns indicating that water isotope concentrations vary throughout physiographically similar areas as a result of land cover differences. Water isotope fluxes were calculated and allowed improved estimates of evapotranspiration of the forest treatments.
B23C-0463
Canopy photosynthesis estimated from sapflux and stable carbon isotope ratios in northern Idaho
Canopy-scale estimates of photosynthesis have traditionally required either scaling up from a sample of leaf measurements or scaling down from eddy flux measurements contaminated by opposing carbon dioxide fluxes. We propose an alternative based on transpiration estimates using the well established Granier sapflux sensor and scaled by modifications of standard measurements of forest structure. The resulting sapflux estimates are converted to carbon uptake measurements by first estimating canopy conductance and then using stable carbon isotope ratios to estimate the ratio of carbon to water exchange. Carbon isotope ratios were measured on leaf bulk material, phloem contents, and the highly concentrated stem CO2 pool. As found elsewhere, leaves were highly depleted and did not provide adequate estimates. We used transfer conductances estimated in other work to adjust the carbon isotope ratios prior to estimating carbon-water exchange ratios. The resulting estimates were 11 Mg C ha-1 yr-1, well within the range to be expected based on net primar production (3.6 Mg C ha-1 yr-1) in these stands. We observed seasonal variation caused by both canopy conductance and changes in \d13C. This method of estimating canopy photosynthesis provides an important test of one of the key, and hitherto poorly constrained, components of carbon budget analyses.
B23C-0464
Using stable isotopes to monitor water and carbon relations at the small-watershed scale
The influence of topography on microclimate, vegetation patterns, disturbance history, and hydrology has
been noted and characterized for many decades. More recently, research reveals that vegetation
processes, e.g. water and carbon relations, are extremely variable in steeply-sloped landscapes, even when
there is little obvious variation in vegetation cover or soil properties. We have found, for example, that the
responses of vegetation to inter-annual variability in climate can vary dramatically over distances of a few
tens of meters. How is this fine-scale spatial heterogeneity in vegetation processes manifest at the scale of
entire basins? Can the environmental response of vegetation processes at the whole-basin scale be
characterized by a sum of responses at the plot level, or are there emergent properties of system responses
at larger scales? A long-term goal of our research team is to develop tools to measure and monitor
vegetation processes at the landscape (small basin) scale in mountainous terrain in order to address such
questions. Intensive measurements in a small steeply-sloped, headwater catchment in the western Cascades
of Oregon, U.S.A. (Watershed 1 in the H.J. Andrews Experimental Forest) reveal that nocturnal cold air
drainage in this 96 ha basin is common in spring-fall; the air drainage is deep, well mixed, and frequently
carries more than 90% of respired CO2 from the entire ecosystem advectively out of the mouth of the
watershed. We reported previously that the isotopic composition of respired CO2 appears to be highly
correlated with stomatal conductance of over-story vegetation one to four days previous. Here we consider in
more detail the relationships between the temporal and spatial variability of carbon and water processes at
the plot scale with measurements at the whole-basin scale, and we explore the possibility of monitoring GPP
at the basin scale from a combination of plot-level measurements of transpiration and basin-scale
measurements of isotopes in respired CO2.
http://oregonstate.edu/feel
B23C-0465
Examining the Role of Topography and Climate on Forest Water-use Efficiency
The role topography plays in forest water relations is poorly understood. Quantifying the spatial variation in forest water use with regard to topographic position is central to understanding the influence of complex terrain on ecosystem function. We investigated both the intra- and inter-annual response of water use by young, mature Douglas-fir stands to annual and seasonal changes in climate and soil moisture properties. We measured transpiration, soil moisture, foliar carbon and nitrogen, and microclimate throughout the summers of 2005 and 2006 in plots along a ridge-to-ridge transect in a steep, headwater catchment in the western Cascades of Oregon, U.S.A. (Watershed 1 in the H.J. Andrews Experimental Forest). In addition, the carbon isotopic composition of tree ring cellulose and water soluble foliar extracts was examined in multiple trees per plot. From May through October 2006, daily average transpiration in upslope plots was approximately 40% greater than that of valley bottom plots (1.0 mm per day vs. 0.6 mm per day, respectively); however, greater water loss did not lead to greater carbon gain when measured in basal area increment. Carbon isotope composition of tree ring cellulose, another measure of water-use efficiency, ranged from - 23.8 to -27 per mil with differences of up to 2 per mil among topographic locations and showed similar trends in water-use efficiency as indicated by measures of transpiration and basal area increment. The south-facing, down-slope plot location had the greatest water-use efficiency and was the least responsive to inter-annual climate variation. The tree ring isotopic composition varied more inter-annually for north facing plots than south facing plots. North facing plots were also characterized as having lower soil moisture and higher hydraulic conductivities than the south facing counterparts. Our findings suggest that tree physiological processes are highly variable across short distances, as well as, through time, and that topographic position alone is a poor predictor forest water use behavior. Geomorphic gradients that influence soil texture, soil drainable porosity, and microclimate are likely responsible for the large variation in forest water use over very small distances.
B23C-0466
Variation in Foliar δ13C of Desert Plant Reaumuria soongorica (Pall.) Maxim. among Different Environments in Northwestern China
Reaumuria soongorica is a dominant desert shrub species in arid regions of northwest China, it playing an important role in the maintenance of the stability and continuity of desert ecosystem. The objectives of this study were to investigate the distribution characteristics of foliar δ13C value in R. soongorica, establish the correlations between foliar characteristics and environmental factors, and identify the major factor controlling the variations of foliar δ13C among different environments. Leaves of R. soongorica were collected from 21 natural populations in its major distribution area in northwestern China, across a range of mean annual precipitation from 27 to 328 mm, at altitudes from 394 to 1987 m above sea level, at latitudes from 36°N to 45°N and at longitudes from 81°E to 107°E. We measured the leaf nitrogen (LN), phosphorus (LP), potassium content (LK), leaf water content (LWC) and foliar δ13C in leaves of 407 individuals, and the soil physicochemical properties including nitrogen (SN), phosphorus (SP), soil organic matter (SOM), soil water contents (SWC) and total dissolved solids (TDS). Mean annual precipitation (MAP), mean annual temperature (MAT), evaporation, mean relative humidity (MRH) and duration of sunshine (DS), were collected from the Cold and Arid Environmental and Engineering Research Institute, Chinese Academy of Sciences. We observed that the foliar δ13C values increased significantly with the decreasing of MAP (r = -0.623, P = 0.003) and MRH(r = -0.702, P = 0.002), and decreased with decreasing DS and evaporation. No significant correlation with MAT was detected in δ13C values of R. soongorica. The correlations between foliar δ13C value and the soil factors demonstrated that the foliar δ13C values in R. soongorica significantly increased with the decreasing SWC (r = - 0.470, P = 0.037) and increasing TDS (r = 0.507, P = 0.022) in soil. There were no significant correlations between the foliar δ13C values and soil pH, total SN, SOM, total SP and available SP in soil. The main soil factors affecting δ13C values in the desert halophyte R. soongorica were SWC and TDS. Leaf δ13C values was significantly correlated with the contents of LK, LWC, and proline (P<0.001). Correlation with the contents of LK content was most profound (r=0.793), followed by that with LWC (r=0.786), indicating that the variation of leaf δ13C value could reflect the degree of drought stress and the variation in leaf δ13C values of R. soongorica were likely caused by stomatal conductance, rather than by nutrient-related changes in photosynthetic efficiency under extremely low available water conditions. The observed significant correlations between foliar δ13C values in R. soongorica and SWC and soil TDS reinforced that R. soongorica is a super-halophyte in terms of adaptive strategies to arid environments.
B23C-0467
Enrichment in δ13C of soil CO2 with increased tree height, atmospheric VPD, and seasonal water stress in mixed conifer stands located in the complex mountainous setting of northern Idaho, U.S.A.
The analysis of δ13C from respired CO2 provides an integrative measure of tree water use efficiency (WUE) at the stand and basin scale. According to the hydraulic limitations theory, tall trees should experience reduced stomatal conductance as xylem pathlength increases. We used soil respired δ13C to examine the influence of stand height and environmental factors on the WUE of trees growing in nine mixed conifer stands located in the complex mountainous setting of northern Idaho, U.S.A. We hypothesized that soil-respired CO2 in forests with tall trees should have less negative δ13C, indicative of higher WUE compared to similar forests composed of short trees. During the summer of 2003, 2004, and 2005 we sampled soil-respired δ13C from the headspace of soil chambers embedded in the forest floor. We observed a significant enrichment of soil respired δ13C with increasing tree height in stands that varied from 6 to 30m in canopy height (p = 0.0364, linear mixed effects model). Across the three-year sampling period, mean soil respired δ13C averaged -27.1 (0.2), -26.5 (0.2), and -25.9 (0.3) ‰ respectively for the short, intermediate, and tall stands in our study. We observed the highest correlation between soil respired δ13C and canopy conductance (GS) at time lag of two days. Highest correlations between soil respired δ13C and VPD and SWC% were also noted for a time lag of two days. This suggests that soil-respired δ13C can be used to assess both short-term and long-term changes in canopy conductance and photosynthesis. Furthermore, for the montane mixed conifer stands we examined, our results indicate that increasing tree height is correlated with an increase in canopy level WUE.
B23C-0468
SF6 Tracer and CO2 Advection into a local drainage creek at the Niwot Ridge Ameriflux Site
To further understand advective fluxes of CO2 in complex terrain, sulfur hexafluoride (SF6) tracer studies were conducted at the Niwot Ridge Ameriflux site in the Colorado Rocky Mountains. The site is located on a broad sloping ridge where nocturnal drainage flows are prevalent under stable nighttime conditions. Initial tracer tests in 2003 and 2004 and other studies at Niwot Ridge indicated that a small drainage, Como Creek, located immediately N of the Niwot tower array might produce local advection of nighttime CO2 into the creek drainage. In 2004 and again in 2007, additional tracer tests were conducted with tracer releases immediately next to the creek and to the side of the creek in order to investigate the local nighttime drainage patterns. This paper presents results from these tracer studies where the tracer dispersion data are used to help determine the source footprint of CO2 within the drainage, and to investigate the extent to which CO2 is advected into the drainage from the area of the Niwot tower array.
B23C-0469
The Influence of Soil Properties and Local Characteristics on the Distribution, Migration and Potential Bioavailability of Radio-Cesium in Bavarian Forest Ecosystems More Than 20 Years After the Chernobyl Accident
Soil properties and local characteristics of landscapes and ecosystems influence the behaviour of Radio- Cesium. Humic horizons are a main factor in understanding the migration and potential bioavailability of radio-nuclides in soils. Until 1962 and in the year 1986, nuclear arms tests in the Pacific and the Chernobyl reactor accident emitted persistent radionuclides in the atmosphere that are stored in several European ecosystems. Short-term high as well as long-term low immissions lead to enrichments and increasing contamination of the environment up to superposition effects in certain ecosystems. South German forest ecosystems like the Bavarian Forest or the Northern pre-Alps are subareas of the cesium fallout affected sites after the Chernobyl accident. Cesium-137 is constantly contained in the vegetation and food chain in spite of decreasing local doses. Investigations have shown that the enrichment of cesium is mainly restricted to the organic top layers of the forest soils. Examples of several Bavarian forest ecosystems are given. Horizontal and vertical forest soil distributions of the cesium contamination and its bioavailability were determined to provide a default-document how to act in case of a repetition of a nuclear accident. Such a guideline has been created by order of the Bavarian State Government and its scope is presented here.
B23C-0470
Variation in the Carbon Isotope Compositions of Phytoliths Across a Climate Gradient
The carbon isotope composition of plant organic matter within a single species may vary in response to changes in temperature, relative humidity, precipitation amount, altitude, nutrient availability, light levels and amount of canopy. All of these factors affect the rate of carbon assimilation during photosynthesis. Silica phytoliths, which form in the cells and intercellular spaces of terrestrial plants, occlude some of the plant's organic matrix. Carbon sequestered in phytoliths is protected from decay and may therefore be preserved in soils after most other plant material has decomposed. The carbon isotope composition of phytoliths may therefore have potential as an archive of climatic conditions during soil accumulation. In this study, the carbon isotope compositions of modern plant tissues and their phytoliths are compared for the C4 grass species Calamovilfa longifolia across the climate gradient of the North American prairies. The carbon isotope compositions of C. longifolia tissues ranged from -15 to -10 permil, with lower values being most typical of leaf tissues and with greater variability occurring in samples from lower latitudes. Carbonaceous compounds occluded in the phytoliths, by comparison, were depleted of carbon-13 by 5 to 15 permil relative to the tissues from the same plant. Understanding the causes of this offset, which is significantly larger and more variable than reported in previous studies, is necessary before the full potential of the carbon-isotope phytolith proxy can be realized.
B23C-0471
Stable Oxygen isotopes in otoliths to reconstruct salmon and striped bass habitat use within the San Francisco Bay estuary
Understanding the habitat use of anadromous fish species within major riverine and estuarine settings can provide useful information for protecting these fish populations. The inner ear bone, or otolith, of these fish is an accretionary carbonate structure that contains a high-resolution record of the life history of the fish, including certain chemical properties of the ambient waters occupied by the fish. Stable isotopes measured in the daily-accreting otolith layers can provide highly-resolved histories of habitat occupation. For the salmon, the juvenile phase is a critical life history period and researchers, as well as agencies charged with protecting these fish, seek detailed information about habitat use during this phase. We have measured isotopic ratios of 18O/16O (δ18O permil) in the otoliths of Chinook Salmon and striped bass, sampling along the growth axis, to produce a history of habitat use by these fish. The 18O/16O ratios of the carbonate otolith samples are primarily influenced by the 18O/16O ratio of the surrounding waters (which range from ~0 permil near the Golden Gate to -11 permil for river water), modified by temperature (- 0.326 permil/°C, so a range of approximately 1.6 permil over the course of a year. We have modeled the expected values for carbonate samples for locations throughout the estuary based upon seasonally averaged salinity and temperature values for these locations; for example, we expect delta 18O values of about -7.5 permil in otoliths for fish at the entrance to the estuary and about 2.7 permil in the ocean. We find good agreement between the δ18O data and 87/86Sr data collected earlier on the same fish samples (which also varies as a function of salinity). The value of the oxygen isotope data is that they provide great dynamic range in the brackish to saline portion of the estuary. The combined data provide a record of where the fish spent significant portions of their lives.
B23C-0472
Density Banding in Coral Skeletons: A Biotic Response to Sea Surface Temperature?
Density bands in the CaCO3 (aragonite) skeleton of scleractinian corals are commonly used as chronometers, where crystalline couplets of high and low density bands represent the span of one year. This provides a sensitive reconstructive tool for paleothermometry, paleoclimatology and paleoecology. However, the detailed mechanisms controlling aragonite nucleation and crystallization events and the rate of skeletal growth remain uncertain. The organic matrix, composed of macromolecules secreted by the calicoblastic ectoderm, is closely associated with skeletal precipitation and is itself incorporated into the skeleton. We postulate that density banding is primarily controlled by changes in the rate of aragonite crystal precipitation mediated by the coral holobiont response to changes in sea surface temperature (SST). To test this hypothesis, data were collected from coral skeleton-tissue biopsies (2.5 cm in diameter) extracted from four species of Montastraea growing on the fringing reef tract of Curacao, Netherlands Antilles (annual mean variation in SST is 29° C in mid-September to 26° C in late February). Samples were collected in the following three contextual modes: 1) at two sites (Water Plant and Playa Kalki) along a lateral 25 km spatial transect; 2) across a vertical bathymetric gradient from 5 to 15 m water depth at each site; and 3) at strategic time periods spanning the 3° C annual variations in SST. Preliminary results indicate that skeletal density banding is also expressed in the organic matrix, permitting biochemical characterization and correlation of the organic matrix banding to the skeletal banding. In addition, both surficial and ectodermal mucins were characterized in terms of total protein content, abundance and location of their anionic, cationic, and neutral macromolecular constituents. Furthermore, the ratio of mucocytes in the oral ectoderm to gastrodermal symbiotic zooxanthellae has permitted estimates of seasonal carbon allocation by the coral holobiont. Our nanometer-scale optical analyses of crystal morphology, arrangement, and densities have revealed consistent changes between high and low skeletal density bands. Mass spectrometry, newly developed immunohistochemical staining, fluorescence and polarized light microscopy are in progress to further quantify and model these observations.
B23C-0473
Insights on Coral Adaptation from Polyp and Colony Morphology, Skeletal Density Banding and Carbonate Depositional Facies
As one of the core reservoirs of primary production in the world's oceans, tropical coral reefs support a complex ecosystem that directly impacts over ninety percent of marine organisms at some point in their life cycle. Corals themselves are highly complex organisms and exhibit a range of growth forms that range from branching to massive, foliaceous, columnar, encrusting, free living and laminar coralla. Fierce competition over scarce resources available to each individual coral species creates niche specialization. Throughout the Phanerozic geological record, this has driven speciation events and created distinct skeletal growth morphologies that have differential abilities in feeding strategy. In turn, this has presumably led to the development of niche specialization that can be quantitatively measured through hierarchical morphological differences from the micrometer to the meter scale. Porter (1976) observed significant differences in skeletal morphology between Caribbean coral species that reflects an adaptive geometry based on feeding strategy. Within the Montastraea species complex there are four major morphologies; columnar, bouldering, irregular mounding, and skirted. Each morphotype can be found forming high abundance along the bathymetric gradient of coral reefs that grow along the leeward coast of Curacao, Netherlands Antilles. We have undertaken a study to determine the relative relationships amongst coral morphology, skeletal density and feeding strategy by comparing the morphometric measurements of individual polyps as well as the entire colony along spatial and bathymetric gradients. Polyp diameter, mouth size, interpolyp area, and interpolyp distance were measured from high-resolution images taken on a stereoscope, and evaluated with AxioVision image analysis software. These high-resolution optical analyses have also revealed new observations regarding folded tissue structures of the outer margin of polyps in the Montastrea complex. Skeletal densities were measured in vertical cross-sections of each whole corallum using standard X-ray techniques utilizing a calibrated step wedge to portray banding and overall density. The combination of the stereoscope and X-ray analyses across spatial and temporal gradients provide insight into how coral reef carbonate depositional facies are affected by changes in key environmental parameters, such as increased pollution, or changing photosynthetic activity with depth or sea surface temperature fluctuations.