B11A-0325
Winter time burst of CO2 from the high arctic soils of Svalbard
Though a number of studies have reported CO2 fluxes from the Arctic, few of these include measurements from winter time and it is often assumed that emission rates during winter time are either constant or negligible. These assumptions are often made because no data are available or consist of relatively few measurements which appear to give small and constant emission rates. Further, most studies of the processes behind winter time emission of CO2 conclude that the flux during this time of year can be linked to the respiratory release of CO2 from soil micro organisms, which is temperature dependent and given the low temperatures of the Arctic winter it is reasonable to assume that CO2 production is low. Here we present measurements from a new design of an open top chamber which has show a good potential for continuous measurements of CO2 and other gases even under extreme coldness during the winter at a high arctic location in Svalbard (78°N). Measurements were conducted in the field during the winter season of 2004-2005 and show reliable and continuous measurements of CO2 fluxes down to a level of 0.01 ěmol m-2 s-1 and good correspondence with other types of soil chambers. Our results indicate that a substantial part of the annual CO2 emission from the ecosystem occur during the freeze in period, where more CO2 is emitted from the soil over a few weeks than the accumulated flux for the rest of the winter. During the coldest part of the winter, CO2 emission show a dependency on soil temperature down to -5°C and no dependency below this temperature.
B11A-0326
Regulating factors on continuous winter CO2 flux in black spruce forest soils, interior Alaska
Winter CO2 flux is a significant element to consider when estimating the annual carbon budget on
regional and global scales. Nevertheless, the winter observation frequency is limited due to extreme cold
weather in subarctic and arctic ecosystems. Here, the first continuous monitoring of winter CO2 flux in a
black spruce forest of Alaska was performed using NDIR CO2 sensors at 10, 20, and 30 cm above the
surface during the snow-covered period (DOY 357 to 466) of 2006/7. The atmospheric pressure was divided
into four phases; >1000 hPa (HP: high pressure), 985
B11A-0327
Comparison of the environmental effects on variation in yearly litter decay and soil respiration rates over four years in forested and harvested sites across Canada.
Soil respiration includes CO2 respired by plant roots and by soil biota decomposing plant detritus. Detrital C
stocks and fluxes are being studied at 16 sites at 7 stations of the Fluxnet Canada Research Network,
including paired mature and clearcut forest sites at 5 upland stations (BC, SK, ON, QC, NB). All sites were
instrumented for in situ measurements of soil moisture and temperature and many sites also included
coincident measurements of soil respiration by chambers. Cumulative litter decay was measured using
surface placed litterbags with one of four standard material types (aspen leaves -AL, black spruce needles
BS, Douglas fir needles DF and birch wood sticks BW). Six replicate plots were located at each site, each plot
contained sufficient numbers of surface litterbags of each material type to allow for four annual collections
(2004 – 2007). As well unconfined birch chopsticks were placed at three depths down the soil profile
(surface, 5cm, 15cm) and replaced annually to examine interannual variability of decay. After four years
cumulative decay, litter rank by %mass remaining had AL B11A-0328 What Lies Within: Contributions of the Organic Horizon to Forest Soil Respiration
Soil respiration rates can exhibit tremendous spatial variability, making it difficult to ascertain the proximal
causes of CO2 production. In coniferous forest soils, a large proportion of CO2 is produced in the
organic layer, and better characterizing the organisms and chemical composition of this heterogeneous
horizon may contribute to a predictive understanding of soil respiration rates. In this study we sought to 1)
identify characteristics of organic soil that could serve as predictors of soil respiration rates, including the
presence/absence of fungal mats, and 2) determine the proportion of total surface efflux derived from the
organic horizon seasonally. Working in an old-growth Douglas-fir stand in the Central Oregon Cascades (HJ
Andrews LTER), we found that fungal mats of the Piloderma genus colonized over 56% of the forest
floor, and organic horizons containing these mats had on average 10% higher surface efflux rates than
neighboring non-mat soils. In addition to containing fungal mats, we found the organic horizon contained
significantly more fine root biomass than the top 10cm of mineral soil. By measuring moisture and CO2
concentrations throughout mat and non-mat soil profiles, we evaluated CO2 production within soil
horizons using the principles of Fick's First Law and the diffusion gradient method. We found that the
organic layers produced roughly half of the net CO2 flux and that higher surface efflux rates are
associated with higher percent contributions of production in the organic layer. We also examined
correlations between respiration rate and a suite of soil biological, physical, and chemical characteristics.
While none of these factors correlated directly with surface efflux rates, we found litter depth to be an
important variable that correlated with soil water content and pH, and we found that mat soils tended to have
deeper litter than non-mat soils. This work indicates that in a mature coniferous forest, the organic horizon
serves as a large and important source of habitat and carbon substrate for soil organisms.
B11A-0329 Specific microbial populations thrive under fluctuating redox conditions in tropical soils
The highly weathered soils of upland humid tropical forests are characterized by rapidly fluctuating redox
conditions, dominated by Fe-oxide mineralogy, and have relatively low sulfate availability. To assess how
fluctuating redox conditions and accompanying biogeochemistry impact microbial community structure and
function, we collected soil cores from the Luquillo LTER forest in Puerto Rico and incubated them for 32 days
under one of three redox regimes: static oxic, static anoxic, and 4-day fluctuating redox. Over this time
course we measured CO2, CH4, and N2O production, amorphous iron and Fe(II), and microbial community
structure by high density microarray (PhyloChip) analysis. Static oxic, anoxic, and fluctuating redox soils all
had statistically indistinguishable respiration rates over the course of the experiment. Fluctuating redox
conditions permitted simultaneous methanogenesis, N2O production, and iron reduction, all accompanied by
steady CO2 production. We analyzed the standing and active microbial community using the 16S ribosomal
DNA and RNA biomarkers, identifying 2489 taxa in these soils. Ordination analysis showed significant
separation between the active (RNA-based) and standing (DNA-based) communities, with much more
variation in the active community compared to the standing community. Fluctuating redox conditions
maintained a microbial community structure similar to that of the pre-incubation samples, while static
anaerobic conditions had the most profound effect on the communities. Finally, there was considerable
overlap between the taxa that were the most highly correlated with production of CH4 and Fe(II). Association
of groups of taxa with specific biogeochemical processes begins to identify organisms potentially responsible
for field biogeochemical processing.
B11A-0330 Does warming affect older soil carbon differently than young soil carbon?
Soils are a large reservoir of terrestrial carbon (C), which have the potential to form a positive feedback to
global warming if they release more CO2 to the atmosphere as temperatures increase. It is well known
that warming increases the decomposition rate of new soil organic matter, but the amount of old soil carbon
released by warming is a subject of current debate. Incubation of soils from the Free Air CO2
Enrichment (FACE) sites provides a unique opportunity to study the effects of warming on belowground
carbon cycling. Soils from FACE experimental ecosystem are depleted in both 13C and 14C due to
multi-years' addition of fossil-derived CO2, making young (FACE-labeled C, < 10 y in this study) and
old (pre-FACE fumigation, >10 y) soil carbon easily distinguished by their isotope signatures. Here we use
incubation and isotope measurements of FACE soils to test whether warming affects older (more stable) soil
organic matter (SOM) differently than young (labile) SOM. Similarly, a nitrogen (N) fertilization experiment at
the site allows us to test the effect of nitrogen deposition on the age of carbon respired.
Soils from CO2- and N- elevated and control plots at the Duke FACE site near Durham, North Carolina
were incubated in the laboratory at three temperatures (site mean annual temperature, +10°,
+20°). Respiratory fluxes of CO2 were determined periodically, and measured for
δ13C and Δ14C content. In addition to increasing respiratory carbon losses, warming
released older CO2 (more enriched in Δ14C) relative to the control temperature in mineral
soils. In contrast, there was no effect of warming on the age of carbon respired from the litter layer despite
similar warming-induced respiratory increases. N fertilization resulted in proportionally more old carbon
respired from mineral soils, and negatively interacted with the temperature response. This indicates that
nitrogen fertilization can influence old carbon loss with increasing temperature.
B11A-0331 Changing Precipitation Pattern can Offset Warming Effects on Soil Respiration
Since soil respiration is a major flux in the global carbon cycle, potential global warming effects have received
great attention. Field soil warming studies focused on the difference in soil respiration under actual and
elevated (future) soil temperatures. Accordingly, we warmed the topsoil of a mature spruce forest by
4°C compared to the actual soil temperature during the growing seasons since 2005. We observed a
constant 40 - 45 percent increase in total soil respiration on warmed plots. Root and heterotrophic respiration
reacted similarly to elevated soil temperature. However, besides rising soil temperature, decreasing
precipitation during summer is predicted for our location in the northern limestone Alps (Austria). During July
2008 we simulated a one-month summer drought by building roofs over warmed and control plots. A
reduction of soil respiration was observed on plots where drought was simulated. Until abstract submission,
the simulated summer drought offset the positive warming effects.
B11A-0332 Mechanism and Environmental Control of Soil Respiration During and After Rainfall Events in Agricultural Ecosystem
The response of soil respiration to rainfall is not well understood due to the difficulty in making reliable
CO2 measurements during and after rainfall in a short crop. This study was designed to (1)
characterize dynamic patterns of soil respiration in response to rainfall; and (2) identify soil temperature and
soil water content that drive variation of soil respiration using the soil gradient method and a soil automated
chamber in two type of vegetation in Georgia, USA. The dynamic patterns of daily soil respiration showing
decrease with increase in soil water content during rainfall was likely due to decreased diffusivity. A few days
later, the soil respiration increased and then declined to the pre-rain value, but flux enhancement was larger
in heterotrophic respiration plot as compared with total soil respiration plot. The magnitude of soil respiration
loss after rainfall was related to soil water content. The soil respiration may have been limited by large
amount of rainfall events during rainfall, but the magnitude of their contribution after rainfall was greater due
to the soil remaining wet for a longer period of time. This was probably caused by dissolving soil organic
matter by soil water content, consequently stimulating microbial activity. Moreover, variations in soil
respiration could be explained by the fluctuation in soil temperature and soil water content. Soil water content
had a strong influence on daily total soil respiration in both fields, but soil temperature had little correlation
with daily total respiration.
B11A-0333 Biotic and Abiotic Factors Controlling Soil Respiration Partitioning in a Tall Grass Prairie
Soil respiration is a major component of ecosystem respiration, and small changes in soil CO2 efflux
can have a major impact on the amount of carbon released back to the atmosphere. Contributions to soil
respiration come both from free-living soil microorganisms decomposing soil organic matter (heterotrophic
respiration) and from roots and rhizosphere microorganisms (autotrophic respiration). Little is known about
the factors underlying the partitioning of soil respiration into auto- and heterotrophic components, although
increasing evidence shows that these components respond differently to vegetation, climate and
environmental factors. We investigated biotic and abiotic factors that affect auto- and heterotrophic
respiration in a 19-year-old restored tall grass prairie at Fermi National Accelerator Laboratory in Batavia, IL.
Four automated soil respiration chambers continuously measured soil CO2 efflux during the growing
season. Total soil respiration was separated into its hetero- and autotrophic components by using a stable-
isotope mass balance technique based on the differential 13C/12C ratio of the respired CO2
from roots and soil at the site. Keeling plot measurements and root and soil incubations were conducted
every three weeks at midday and at night to detect temporal changes in the end members that affect mass
balance calculations during the growing season. Continuous measurements were also made of soil moisture,
soil temperature, and net ecosystem exchange derived from eddy correlation techniques. Preliminary results
show that regardless of diurnal variation, the proportion of autotrophic respiration to total respiration was
larger during the earlier part of the growing season while the proportion of nighttime heterotrophic respiration
to total respiration increased later in the growing season. Our results suggest that differential controlling
factors for soil respiration components might be operating differently in time.
B11A-0334 Soil Respiration in a Coastal California Pine Forest: Partitioning Foggy Results
Radiocarbon (14C) is a useful tool for partitioning sources of respiration in terrestrial ecosystems. This is
because there are often large differences between the mean 14C signature of CO2 respired by plants versus
CO2 respired from microbes. When combined with automated chamber measurements of soil respiration,
14C source partitioning techniques can be a potentially powerful approach for identifying both the
environmental and biological controls on soil respiration over hourly to monthly time intervals. This
presentation will include preliminary results from a study in a unique conifer ecosystem on the Channel
Islands, California, in which summertime soil moisture inputs are derived exclusively from numerous small fog
drip events. Fog drip occurs when cloud water droplets accumulate on vegetation and fall to the ground.
Thus, in contrast to most Mediterranean ecosystems, surface litter/soil moisture is supplemented over the
warm summer. The study aims to separate how these fog inputs versus wintertime rain inputs, along with
other environmental variables, influence plant and microbial activity and their respective CO2 fluxes. In our
first summer of measurements, data show noticeable differences in the rate and 14C signature of soil
respiration: (1) between the day and night, (2) immediately before and after fog events, and (3) over the
course of the summer. This indicates changing plant and microbial source contributions with small moisture
pulses, and over both diel and seasonal timescales.
B11A-0335 Sources of Variation in the Carbon Isotopic Composition of Root Respiration
Soil CO2 efflux is comprised of CO2 from root respiration, rhizosphere microbes and heterotrophic
respiration from soil organic matter. Isotopic approaches at partitioning autotrophic and heterotrophic
respiration require determining source signatures. The signature of root tissue has been commonly used as
the root end-member. However, this signature corresponds to all cellular constituents, which are not expected
to be part of root respiration. Also, there is some evidence of fractionation during dark respiration so that the
use of dried bulk roots may not accurately represent respired CO2. Incubation of excised roots and
subsequent isotope analysis offers a simple alternative that can be of use for experimental purposes. To
assess the applicability of this approach we measured respiration rates and 13C signatures of the respiration
of excised roots from an ongoing field experiment studying the effects of warming and elevated CO2
concentrations and their interactions on soil organic matter dynamics. The experiment uses Free-Air
CO2 enrichment and generates continuous labeling with 13C depleted CO2 in the elevated
CO2 treatments. Roots were collected from two different depths in the spring and summer seasons and
isotopic analyses of respiration and whole tissue were performed. Analyses to date have found no
differences in the respiration rates of roots from any treatment at any date. d13C (delta Carbon 13) values of
respiration were lower under elevated CO2 and there was no effect of warming or root depth. There
was a greater difference between the respiration signatures of roots from elevated CO2 and ambient
CO2 in the samples collected in the summer than in the samples collected in the spring because values
in the ambient CO2 treatment were higher in the summer. d13C values became significantly more
positive with time since collection in all treatments. We hypothesize that root metabolism immediately after
excision utilizes labile C (sugars and starch) that produces more depleted CO2, whereas after 2-3
weeks it relies on more structural compounds, producing more enriched CO2. Ongoing experiments are
investigating the source of these variations. This research will improve isotope-based estimates of
autotrophic respiration rates and should therefore provide insights into autotrophic controls over soil
respiration.
B11A-0336 Vegetation Type Regulates Soil CO2 Production at Multiple Temporal Scales
Soil CO2 efflux (F0) constitutes a significant component of CO2 fluxes from terrestrial ecosystems, but the
mechanistic understanding of F0 remains unclear because of the complexity of processes involved. F0, in
fact, is the combination of the production of CO2 (Pi) at different depths from autotrophic and heterotrophic
components.. While F0 provides information about the total carbon loss to the soil into the atmosphere
through a diffusion process, Pi provides a direct measurement of biological activity. We used automated
measurements of soil CO2 concentrations to calculate Pi in three vegetation types (mature woody vegetation,
young woody vegetation and herbaceous vegetation) influenced by the same atmospheric forcings (light,
precipitation, temperature). The study was done within an area of 800 m2 in a mixed temperate forest in
California, USA. We used a wavelet approach and coherence analysis to investigate how temperature and
soil moisture influence Pi at multiple temporal scales over two years of measurements. Our results suggest
that vegetation type plays a critical role in how Pi responds to changes in soil temperature and soil moisture
at multiple temporal scales. In particular an increasing lag between Pi and temperature has been observed at
daily period passing from herbaceous vegetation to mature woody vegetation sites.
B11A-0337 Responses of Plant Respiration to Pleistocene Changes in Atmospheric CO2 Concentrations
Vegetation plays a crucial role on the terrestrial C cycling through the processes of photosynthesis and
respiration. At a global scale, these two processes are essential components of the C cycle, because 30% to
70% of the CO2 fixed by photosynthesis is released back to the atmosphere each year by plant
respiration. Therefore, small changes in these two fluxes can have a significant impact on atmospheric
CO2 concentration. Changes in CO2 concentrations in the atmosphere have prompted plant
evolutionary responses that have resulted in novel physiological photosynthetic adaptations such as the
photosynthetic C oxidation pathway or the rise of C4-photosynthesis. However, little is known about the role
of respiration on the nature of plant acclimation and adaptation to different CO2 scenarios when the
photosynthesis-to-respiration ratio is low. Plant respiration is further complicated by the presence of the
alternative pathway that burns photosynthate without producing chemical energy (ATP). Here, we explore the
effects of Pleistocene levels of CO2 on plant respiration and on the activity of the alternative pathway.
We concentrated in plants that have a low photosynthesis-to-respiration ratio such as plants grown in shade
and CAM plants, and on Arabidopsis thaliana plants that were selected at Pleistocene CO2 levels
(200ppm), current (360ppm) and projected (680 ppm) atmospheric levels of CO2. Our results, indicate
that regardless of the overall respiration response to CO2 levels the activity of the alternative pathway
was inversely correlated with atmospheric CO2 concentration in all plants. Because alternative pathway
activity is not coupled to ATP production and does not support maintenance or growth processes as
effectively as normal respiration, plants exposed to Pleistocene CO2 levels will run respiration more
efficiently than plants exposed to current or higher CO2 levels. The effectiveness of respiration can
either play a survival role at low CO2 levels, or return excess photosynthate to the atmosphere as
CO2 at high CO2 concentrations in the atmosphere.
B11A-0338 Dependence of ecosystem respiration on soil temperature, moisture and plant biomass in a semi-arid grassland
The Mongolian steppe zone is one of the main components of central-Asian grasslands. Ecosystem
respiration rates between the atmosphere and the steppe ecosystem were measured in a Mongolian semi-
arid grassland in July 2004, May 2005, July 2005, September 2005, and June 2006 by using a closed-
chamber technique. The study area is dominated by Poaceous grasses and was enclosed by a fence (300 m
x 300 m) in June 2004 to prevent livestock from grazing. Along with the respiration measurements, soil
temperature, precipitation, and soil volumetric water content were continuously measured. Aboveground
plant biomass (AGB) was also determined by clipping vegetation for each observation period. From the
measured data of ecosystem respiration and environmental variables, their quantitative relationships were
examined. Individual rates of respiration showed the highest rank correlation coefficient with AGB. The
correlation between respiration rate and soil moisture was higher at 3 cm depth than at 10 cm depth, which
suggested that soil moisture near the soil surface was better proxy than that at deeper layer for accounting
for variations in respiration rate. The respiration rate was exponentially related to the soil temperature and
the relationship modified by soil moisture. The amount of respiration rate and its temperature sensitivity
(Q10) declined with decreasing soil moisture. Standardized rates of respiration at 20°C were
expressed well as a bivariate function of AGB and soil moisture at 3 cm depth. Approximately 88% of the
fluctuation in standardized respiration rate was explained by the interaction of these two factors.
B11A-0339 Assessment of CO2 flux measurements in different soil types
Accurate measurements of soil CO2 efflux are extraordinarily challenging due to the very properties of CO2
transport in a porous medium of soil. The most commonly used method today is the chamber method, which
provides direct measurements of CO2 efflux at the soil surface, but it can not measure the soil CO2 flux
continuously. In order to develop new measurement methods in soil CO2 efflux, small solid-state CO2
sensors have been used to continuously to monitor soil CO2 profiles by burying these sensors at different
soil depths. Using this method we compared soil CO2 efflux of four different soil types: forests soil, grassland
soil (collected in Maryland) commercial potting soil and pure sand as control. CO2 concentration varied
between 500 ppm in sand and 8000 ppm in forest soil at depth 12 cm. CO2 flux had the following order:
Forest (0.3~0.4 mg CO2 m-2 s-1), potting soil (0.1~0.14 mg CO2 m-2 s-1 ), grassland (0.03~0.05 mg CO2
m-2 s-1), sand ( 0 mg CO2 m-2 s-1 ). Exponential relationship between temperature and CO2 flux was
established for forest soil and potting soil only. Leaf litter, often thick layer in many terrestrial ecosystems and
a significant source of CO2 production, is not part of the of the diffusivity models. We are currently
conducting experiments which include the effect of leaf litter and soil invertebrates into soil respiration.
B11A-0340 Assessment of the soil CO2 gradient method to measure soil CO2 efflux in an agricultural ecosystem
This paper evaluates the feasibility of using the soil gradient method to estimate soil CO2 efflux in an
agricultural ecosystem. The soil CO2 efflux estimated using six different models in the relative gas
diffusion coefficient calculation were compared with the soil CO2 efflux measured using a soil chamber
respiration system. The estimated soil CO2 efflux using the gradient method differs by 3 and 176%
from fluxes obtained using the soil chamber method depending on the choice of the model used. Harmonic
averaging is used to estimate the soil CO2 diffusion coefficient, yielding the best estimate of soil surface
CO2 efflux. Our findings suggest that the soil CO2 efflux was best described by an exponential
function of soil temperature and a linear function of soil moisture at a depth of 0.12 m, with the Q10 of
2.43.
B11A-0341 Contribution of herbaceous understory vegetation to ecosystem respiration in a semi-arid ecosystem undergoing woody encroachment
Understanding the response of arid and semi-arid systems to changes in woody plant cover is an area of
active research. Shifts in vegetation structure or function in these water-limited systems can have important
and non-linear affects on ecosystem function and biogeochemical cycling. Most studies, however, focus on
the grass and woody plant end-members of this transition. Conversion of grassland to woodland can also
result in a more diverse and complex community with a mosaic of plant functional types. We took a
manipulative approach to determine the contribution of annual and ephemeral herbaceous vegetation to
above and below-ground ecosystem carbon flux in a riparian system undergoing woody encroachment.
Annual and herbaceous ephemeral plants in this system are present primarily in areas with significant woody
plant encroachment, are active during late summer in response to the summer monsoon precipitation, and
die back after the first frost. Although they represent a temporally-limited contribution to the aboveground
biomass, they can fill in 65 percent of the understory with plants 1 m in height and an average leaf area index
of 2.5 (meters leaf/meters ground). Given this abundance of aboveground photosynthetic biomass, at their
peak seasonal activity annual and ephemeral plants should have an important effect on ecosystem carbon
flux, primary productivity, and water-use efficiency. We compared measurements at two sites with different
densities of woody plant cover, and created plots with herbaceous plant removal to compare to nearby paired
plots (<3m distant) with an intact herbaceous cover. We used chamber measurements of plot and leaf-
level gas exchange to measure above- and below-ground respiration and photosynthetic rates. During
periods of peak photosynthesis, we found that plots with an intact herbaceous understory could show rates of
CO 2 uptake of up to 5 μmol m-2 s-1 compared to the release of -4 μmol
CO2m-2 s-1 in the paired removal plots. In unmanipulated plots, aboveground respiration from
the herbaceous plants on average contributed half of the total plot respiration. When compared to daily rates
of uptake and respiration at the ecosystem level, these plants may contribute up to 30 percent of ecosystem
CO2 flux during periods of peak biomass and activity.
B11A-0342 Constraining a Carbon – Water Flux Model for a Sagebrush Ecosystem With Multiple Data Sources
Modeling water and carbon fluxes in a non-mesic system like sagebrush-steppe ecosystem has always been
a challenging proposition. The study site located at south-central Wyoming (WY) northeast of the Sierra
Madre Mountains is dotted with sagebrush and graminoids with stark differences in the kinetics of their
physiological response. Initial attempt to model both in the months of June and July in 2005 using only gsref
while being successful for water (r2 = 0.85), left much room for improvement in case of net ecosystem
exchange (NEE). NEE (- (photosynthesis – respiration)) was over predicted under low soil moisture conditions
towards the end of July. This could be either due to over-prediction of photosynthesis or under-prediction of
soil respiration. The former could be due to lack of sensitivity of photosynthesis parameters to low soil
moisture condition which was addressed by fitting a soil water potential constraint to Vcmax. Alternatively
under-prediction of soil respiration could be due to uncertainties in parameter estimates. A Bayesian
framework using Markov Chain Monte Carlo simulation was applied to soil respiration data obtained from
three sources at half-hour interval: eddy covariance data which measures ecosystem level respiration, soil
collar data which measures below ground respiration and trench data which measures heterotrophic
respiration. The Bayesian analysis was developed to sample the posterior distribution of the soil respiration
parameters and partition soil respiration into autotrophic and heterotrophic respiration with the null
hypothesis that there is no difference in parameter estimates for soil respiration obtained from different
sources.
B11A-0343 Assessing the determinism of the seasonal variations of trunk CO2 efflux by combining field-isotopic composition monitoring and process-based modeling
Trunk CO2 efflux is a major component of total CO2 forest ecosystem efflux but its determinism is
still poorly understood. This CO2 flux could originate from different carbon sources (respiration of newly
assimilates or reserves; xylem sap flow dissolved CO2). These potential CO2 sources of the
ecosystem vary at a diurnal and seasonal time scale. They follow distinct metabolic pathways within the tree
and could potentially differ in terms of stable C isotopes composition (δ13C). During this last
decade, new techniques such as tunable diode laser absorption spectroscopy (TDLAS) has enabled to track
both the δ13C and rate of CO2 fluxes at a high temporal frequency compared to conventional
isotope ratio mass spectroscopy and chamber-based techniques. In this context, our objective is to examine
the diurnal and day-to-day variations of δ13C trunk CO2 efflux and to test if they are driven by
climate, xylem sap flow and photosynthetic activity.
A TDLAS (TGA100A, Campbell Sci., UT) was installed in early July 2008 in a mature oak (Quercus
petraea, L.) stand of the Barbeau forest (France, Carboeurope site). It has been connected to three opened
trunk chambers placed at breast height. Before each chamber measurement, which occurred every six
minutes, the analyzer was calibrated with four calibration gas bottles with known CO2 concentration (in
air) and δ13C values. Concurrently to trunk CO2 efflux rate and δ13C, xylem sap flow
rate, air and trunk temperatures, and vapor pressure deficit above canopy were recorded. Data for the
summer and fall seasons will be presented and discussed. Preliminary results showed that in summer both
trunk CO2 efflux rate and CO2 followed the time evolution but at a different level among trees. The
mean hourly averages of CO2 of trunk CO2 efflux values ranged from -29.6‰ to -
23.2‰, and hourly means of CO2 efflux were positively and linearly linked to trunk temperature.
The diurnal variations of δ13C of CO2 efflux are less obvious that for CO2 efflux rate,
since they occurred during several days and averaged around 1‰.
The current CO2 values of trunk CO2 efflux were compared to the CO2 of phloem sugars
simulated by ISOCASTANEA, a forest process-based flux model (Dufrene et al., 2005), which integrates the
model of photosynthetic discrimination described in Farquhar et al. (1989) and the carbon translocation from
canopy to trunk. Over the studied period, the modeled CO2 of sugars appeared to be mainly
determined by VPD values whereas the observed variations in δ13C of CO2 efflux are more
related to temperatures. Further field data are required, particularly during the end of the growing season in
order to highlight the processes driving the variation of trunk CO2 efflux.
B11A-0344 Identification of Robust Models for co2 xcEhange at Canopy Scale
Nonstationary and nonlinear dynamic time series analysis tools are applied to numerous sites and years of
eddy covariance and meteorological measurements provided by the worldwide network FLUXNET with the
intention to identify robust models for the CO2 exchange at canopy scale. The analysis tools based on
Kalman filtering and smoothing techniques help to extract the dominant behaviour of daily gross
photosynthesis and respiration in relation to their principally modulating drivers. Based on these findings
semi-parametric models are formulated and optimized against over one hundred FLUXNET site-years. The
only input variables are soil temperature, insolation and precipitation or evaporative fraction as moisture
availability measure. The resulting well-defined parameters are analyzed for patterns that relate the
parameters to site characteristics as vegetation and climate class. Despite their simplicity the retrieved model
structures fit the measured data well and provide reliable annual CO2 budgets for forests and grasslands.
B11A-0345 Simulating Carbon Dioxide Exchange Among Deciduous Forest Species: A General Soil Moisture Response?
Species-specific differences in the carbon exchange response to soil moisture remains difficult to simulate in
heterogeneous deciduous forests. A combined three-dimensional mechanistic model was parameterized at
the organ scale using data obtained from in situ organ-scale measurements of both diurnal and seasonal
changes in carbon dioxide exchange to predict carbon accumulation and respiration under both irrigated and
water-stressed conditions. Among species, we observed variability in carbon dioxide exchange rates under
watered and water stressed conditions. However, carbon estimate validation indicated that differences
between measured values and model estimates were within 6% to 12% under watered conditions and the
use of a general soil moisture model resulted in estimates within 2% to 25% under water stress conditions.
Results confirmed that a general soil moisture response can be used to estimate carbon dioxide exchange
among temperate deciduous tree species when organ-level physiology is quantified. Therefore, to improve
our predictive understanding of deciduous forest ecosystem carbon exchange while at the same time
minimizing model complexity this work presents a general model capable of quantifying the organ-specific soil
moisture response among deciduous species.