B11E-01
Observing 13C Tracer Kinetics in CO2 Respired by a Temperate Grassland Ecosystem: Different Measurement Methods Give Different Answers
To study carbon fluxes in grassland ecosystems, a 13C tracer technique with open-top-chambers was set up for field use and applied to a temperate grassland. The open-top-chambers were optimised with respect to homogeneity of CO2 distribution, exclusion of wind incursion and prevention of biases in soil CO2 efflux due to pressure effects. This included dispersion of air entering the chamber and installing a buffer volume at the top exit, as well as varying the opening diameter at the top and the air flow through the chamber. During tracer experiments in September 2006 and Mai 2007, labelling was accomplished by flushing the chambers during daytime with air containing CO2 at ambient concentration but with 13C of the CO2 differing from ambient by up to 40‰. 13C in the chambers showed little variation during bright daytime, but was depleted by about 1‰ in the early morning and late evening (when assimilation rates and thus tracer uptake were low) compared to bright daytime. Along with day-to-day variation in the same order than during bright daytime, this provided constant labelling conditions in the chambers. The tracer was observed in total ecosystem respired CO2 during nighttime by measuring CO2 concentration and isotopic composition online in the field with two methods: closed static chamber mode and open dynamic chamber mode. The two methods gave significant, reproducible differences in observed 13C of respired CO2. These translated in differences in the fraction of labelled carbon in respired CO2, with a larger fraction of tracer observed in the closed static mode: After two weeks of labelling, in the closed static mode about 70-80% of respired CO2 were labelled, compared to 40-50% in the open dynamic mode. Additional measurements of total ecosystem respiration in a cuvette system in the laboratory (thus avoiding the influence of varying environmental conditions) agreed with the field measurements in the open dynamic mode, but not with those in the closed static mode. Despite the difference between modes, the tracer kinetics of respired CO2 observed in each of the modes supported a two-source model: Respiration was fed from one source which was not affected by the tracer and another source which was turned over by current photosynthesis within several days.
B11E-02
A single-substrate model to interpret high-resolution intra-annual stable isotope signals in tree ring cellulose
High-resolution measurements of the carbon and oxygen stable isotope composition of cellulose in annual tree rings (δ13Ccellulose and δ18Ocellulose, respectively) reveal well-defined seasonal patterns that could contain valuable records of past climate and tree function. Interpreting these signals is nonetheless complex because they not only record the signature of current assimilates, but also depend on carbon allocation dynamics within the trees. Here, we will present a single-substrate model for wood growth in order to interpret qualitatively and quantitatively these seasonal isotopic signals. We will also show how this model can relate to more complex models of phloem transport and cambial activity. The model will then be tested against an isotopic intra-annual chronology collected on a Pinus pinaster tree equipped with point dendrometers and growing on a Carboeurope site where climate, soil and flux variables are also monitored. The empirical δ13Ccellulose and δ18Ocellulose signals exhibit dynamic seasonal patterns with clear differences between years, which makes it suitable for model testing. We will show how our simple model of carbohydrate reserves, forced by sap flow and eddy covariance measurements, enables us to interpret these seasonal and inter-annual patterns. Finally, we will present a sensitivity analysis of the model, showing how gas-exchange parameters, carbon and water pool sizes or wood maturation times affect these isotopic signals. Acknowledgements: this study benefited from the CarboEurope-IP Bray site facilities and was funded by the French INSU programme Eclipse, with an additional support from the INRA department EFPA.
B11E-03 INVITED
Measuring carbon and oxygen isotope signals of photosynthesis and respiration: first field results from a chamber system coupled to tunable diode laser spectrometers
Studying the carbon and oxygen stable isotope signals from plants and soils can help us gain insight to mechanistic processes responsible for the net exchange of CO2 and water cycled between terrestrial ecosystems and the atmosphere. Chamber field measurements of component fluxes and their isotopic composition have been reported for a few ecosystems. These observations have revealed that isotopic signals for carbon and oxygen are dynamic over relatively short time scales (hrs and days) for both branches and soils (Seibt et al., 2006a; 2006b; Wingate et al., 2007), and not fully explained by currently available models (Seibt et al., 2006b; Wingate et al., 2007). Ecosystem isotope studies have been limited by flask sampling requirements in the past. To evaluate and refine our models of isotopic fractionation by plants and soil, we need high resolution continuous isotopic measurements over the growing season for different ecosystems. In this study, we coupled chambers with tunable diode laser spectroscopy techniques in the field to continuously capture the isotopic signals from the most important component fluxes contributing to the net ecosystem exchange of CO2 in a Pinus pinaster forest in south-west France. We obtained profiles of the carbon and oxygen isotope content of CO2 within and above the forest canopy. In addition, we measured branch photosynthetic 13C and 18O discrimination alongside the 13C and 18O isotopic composition of the branch, stem and soil respiration during a 6-month period in 2007. In this talk, we will present the first results from this field campaign. References Seibt, U., Wingate, L., Berry, J.A. and Lloyd, J. (2006a) Non steady state effects in diurnal 18O discrimination by Picea sitchensis branches in the field. Plant, Cell and Environment Vol 29, 928-939. Seibt, U., Wingate, L., Lloyd, J. and Berry, J.A. (2006b) Diurnally variable δ18O signatures of soil CO2 fluxes indicate carbonic anhydrase activity in a forest soil. JGR-Biogeosciences, Vol. 111, G04005, doi:10.1029/2006JG000177. Seibt, U., Wingate, L. and Berry, J.A. (2007) Nocturnal stomatal conductance effects on the δ18O of foliage gas exchange observed in two forest ecosystems. Tree Physiology, Vol. 27, 585-595. Wingate, L., Seibt, U., Moncrieff, J.B., Jarvis, P.G. and Lloyd, J. (2007) Variations in 13C discrimination during CO2 exchange by Picea sitchensis branches in the field. Plant, Cell and Environment doi: 10.1111/j.1365-3040.2007.01647.
B11E-04
CO2 and Carbon Isotopes of CO2 Within a Subalpine Forest Snowpack
Stable isotopes are useful to examine a variety of carbon cycle processes, and have been used to link the carbon isotope ratio (δ13C) of CO2 in soil respiration to weather events in summer. Recent studies have shown that many high-elevation and high-latitude ecosystems can lose a significant amount of carbon in the winter by respiration under the snowpack. Very little is known about the carbon isotope content of winter respiration in seasonally snow-covered forests. During winter and spring 2006-2007, CO2 was monitored within a snowpack at the Niwot Ridge AmeriFlux site in the Rocky Mountains of Colorado (3050 m elevation). CO2 and δ13C of CO2 were monitored in vertical profiles within the forest, within the snowpack, at the soil-snow interface, and within the soil, using tunable diode laser spectroscopy. Ancillary measurements included wind and other weather measurements, snow and soil temperature, soil moisture, and snowpack hydrology (density profiles, snow water equivalence or SWE). During the study period, SWE increased from 30 to 50 cm, and then decreased until the snow was fully melted. Snow depth peaked at 140 cm. Soil temperatures in the top few cm of soil ranged from -1 to 0 C, and within the snowpack from -15 to 0 C. Measured CO2 at the soil-snow interface ranged from 1500 to 3500 ppm, and peaked when SWE was maximum in late April. δ13C at the soil-snow interface ranged from – 8 to -21 permil. Within the snowpack, CO2 ranged from 400 to 3000 ppm, and δ13C ranged from -8 to -20 permil. Gradients in both CO2 and δ13C were dependent on wind conditions above the snowpack. Mixing lines between δ13C and CO2 were confined to a relatively narrow range, with an inferred isotope ratio of respiration of -27.1 permil (assuming a 4.4 permil kinetic fractionation). These data will be discussed in the context of expected δ13C of respiration based on measurements in air.
B11E-05 INVITED
Spatial Variability in Instantaneous Photosynthetic Carbon and Oxygen Isotope Discrimination
Stable carbon and oxygen isotope ratios of CO2 are useful tracers in studies of carbon and water cycling between the terrestrial biosphere and the atmosphere. Interpretation of variation in 13CO2 and C16O18O relies on models describing physical and biochemical processes and their associated fractionations. A layer of complexity not currently quantified or accounted for in canopy models is spatial variation in photosynthetic discrimination within a single leaf. A new measurement technique, employing tunable diode laser absorption spectrometry coupled to an open gas exchange system, enables online measurement of photosynthetic discrimination at high temporal resolution. Using this system, photosynthetic 13C and 18O discrimination was measured along leaves of a C3 monocot. For the forage cereal Triticale, 13C discrimination increased by 2‰ and 18O by 20‰ from the base to the tip of mature leaves when measured at saturating irradiance. The increase in 13C discrimination was associated with an increase in the leaf internal conductance of CO2, and 18O discrimination with carry-forward of evaporative enrichment along the leaf. When numerical averages are compared to flux- and area-weighted averages, the portion of the leaf approximately one third of the way from the base can be shown to provide the most representative area for scaling up.
B11E-06
Simulating Stable Carbon Isotopic Signatures and Exchange of a C4 Canopy: Comparison with a 'Big-Leaf' Model
Considerable uncertainties remain in using stable isotope techniques to partition net ecosystem CO2 exchange into its component fluxes. Many of these uncertainties arise from the determination of isotopic signatures (i.e., ecosystem isotope discrimination and isotope ratio of ecosystem respiration) and the simplifying assumptions associated with the 'big-leaf' analogy in the isotopic approach. It is necessary to rigorously test these assumptions in order to improve the isotopic partitioning. In this study, a multilayer canopy model was adapted to simulate the dynamics of the isotopic signatures of a C4 corn canopy to test several assumptions in a 'big-leaf' isotopic partitioning model for the growing season of 2003. Results showed that: 1) The bulk canopy conductance inverted from the Penman-Monteith equation departed significantly from the modeled canopy stomatal conductance when the canopy was highly decoupled from the atmosphere. For such conditions, the use of bulk canopy conductance as a proxy for the canopy stomatal conductance presents a potential limitation for the isotopic approach. 2) Sunlit and shaded leaves showed different isotope discriminations in response to different CO2 leakiness of the bundle sheath cells. Since the estimation of isotope discrimination is highly sensitive to the leakiness factor, care needs to be taken to determine an appropriate leakiness factor for the entire canopy. 3) The assumption on the equivalence between the daytime and nighttime isotope ratio of ecosystem respiration (δR) is problematic, because daytime δR may differ by up to 2 per mil over the diurnal period. Variations in δR were caused by changes in the contributions of component respiration. Daytime ecosystem respiration was more enriched as a result of the increased contribution from the relatively enriched foliar and rhizosphere respiration. Our modeling of δR provided opportunities for investigating the quantitative relationship between δR and environmental factors, which can subsequently be used to estimate daytime δR to improve the isotopic partitioning. Future research involving automated chambers and isotope laser spectroscopy will be used to help validate our modeled values of daytime δR.
B11E-07
Seasonal isotopic fractionation of atmospheric CO2 reveals a mismatch between observations and model simulations
Here we present a novel approach in using the carbon isotopic composition of the atmosphere and the biosphere to identify inconsistencies in model predictions of water and carbon exchange between the biosphere and atmosphere. We evaluated different methods for estimating the isotopic signature of source CO2 (δs) on seasonal timescales. We determined that the most effective method for retrieving information about isotopic discrimination by the terrestrial biosphere was by analyzing the residual differences between CO2 and δ13CO2 observations from the planetary boundary layer and the free troposphere. Seasonal patterns of δs derived from atmospheric sampling sites in the Northern Hemisphere showed a coherent increase in δs during the Boreal summer. Intra-annual measurements of δ13C in the cellulose of Pinus taeda growing in a pine forest of the Southeastern, US also indicated consistent increases in δs during the growth season. However, an identical analysis of isotopic tracers in a global biosphere-atmosphere transport model indicated a decline in δs during the growth season. The mismatch between observations and model predictions was greatly improved when stomatal conductance was allowed to vary as a function of vapor pressure deficit instead of relative humidity. These results suggest that model predictions of conductance may be inconsistent, especially in terrestrial regions where vapor pressure deficit and relative humidity are temporally covariant.
B11E-08 INVITED
Process Model for Studying Regional 13C Stable Isotope Exchange between Vegetation and Atmosphere
The variation of the stable isotope 13CO2 in the air in exchange with land ecosystems results from fractionation processes in both plants and soil during photosynthesis and respiration. Its diurnal and seasonal variations therefore contain information on the carbon cycle. We developed a model (BEPS-iso) to simulate its exchange between vegetation and the atmosphere. To be useful for regional carbon cycle studies, the model has the following characteristics: (i) it considers the turbulent mixing in the vertical profile from the soil surface to the top of the planetary boundary layer (PBL); (ii) it scales individual leaf photosynthetic discrimination to the whole canopy through the separation of sunlit and shaded leaf groups; (iii) through simulating leaf-level photosynthetic processes, it has the capacity to mechanistically examine isotope discrimination resulting from meteorological forcings, such as radiation, precipitation and humidity; and (iv) through complete modeling of radiation, energy and water fluxes, it also simulates soil moisture and temperature needed for estimating ecosystem respiration and the 13C signal from the soil. After validation using flask data acquired at 20 m level on a tower near Fraserdale, Ontario, Canada, during intensive campaigns (1998–2000), the model has been used for several purposes: (i) to investigate the diurnal and seasonal variations in the disequilibrium in 13C fractionation between ecosystem respiration and photosynthesis, which is an important step in using 13C measurements to separate these carbon cycle components; (ii) to quantify the 13C rectification in the PBL, which differs significantly from CO2 rectification because of the diurnal and seasonal disequilibriums; and (iii) to model the 13C spatial and temporal variations over the global land surface for the purpose of CO2 inversion using 13C as an additional constraint.