Comparison of static chambers to measure N2O, CH4 and CO2
fluxes from soils
Mari Pihlatie1, Jesper Riis Christiansen2, Jukka Pumpanen3, Per Gundersen2, Timo Vesala1
1
Department of Physical Sciences, Division of Atmospheric Sciences, P.O.Box 64, FI-00014
University of Helsinki, Finland
2
Forest & Landscape Denmark, University of Copenhagen, Hørsholm Kongevej 11, DK-2970
Hørsholm, Denmark
3
Department of Forest Ecology, P.O. Box 27, FI-00014 University of Helsinki, Finland
Background
In many environmental studies the soil fluxes of the greenhouse gasses (GHG) nitrous oxide (N2O),
methane (CH4) and carbon dioxide (CO2) are measured by closed static chambers. Due to large
spatial and temporal variability of GHG production and consumption in soils, a large number of
replicate chambers are often required. Static chambers can be constructed using inexpensive
materials, enabling the researcher to install a large number of replicate chambers. Dependent on the
chamber design the magnitude of impact in the surroundings vary, and is difficult to assess in
quantitative terms.
In general, most chamber types tend to underestimate trace gas fluxes from the soil by slowing
down the gas diffusion from the soil to the chamber headspace during the measurement (Norman et
al., 1997; Rayment, 2000; Conen and Smith, 2001; Pumpanen et al., 2004; Livingston et al., 2005).
The chamber type and measurement protocol substantially affect the flux of the target gas, but so
far comparisons between chambers have mainly focused on CO2. These comparisons show
relatively large differences in CO2 fluxes between chamber types or indicate chamber-specific
limitations (Raich et al., 1990; Jensen et al., 1996; Norman et al., 1997; Fang and Moncrieff, 1998;
Gao and Yates, 1998; Janssens et al., 2000; Pumpanen et al., 2003B; Pumpanen et al., 2004). Due to
generally much smaller fluxes of N2O and CH4 from the soil than of CO2 the results obtained from
CO2 chamber calibration are not directly applicable to the other gases. Furthermore, few data are
available of inter-comparisons between different static chamber designs when considering GHG
fluxes.
Other approaches to measure GHG fluxes include eddy covariance (Launiainen et al., 2005; Pihlatie
et al., 2005B; Rinne et al., 2007) and soil gradient techniques (Maljanen et al., 2003; Flechard et al.,
2005; Pihlatie et al., 2007). These measurement techniques operate at different spatial and temporal
scales. The eddy covariance method is a direct and continuous measurement technique and gives
information on the fluxes at ecosystem scale. The soil gradient method relies on the measurement of
gas concentrations inside the soil profile, and the calculation of the gas transport between different
soil layers (Pumpanen et al., 2003A; Pihlatie et al., 2007). The soil gradient method gives
information on the gas production and consumption within a soil profile in small scale. Both the
eddy covariance and the soil gradient methods have been much less used than the static chamber
method to estimate GHG fluxes from soils. Each of the three techniques mentioned has its
limitations, however, the low-tech requirements and inexpensive costs of static chambers are
arguments for their extensive use in gas flux measurements.
Chamber measurements of GHGs in soil-plant ecosystems encompass a range of conditions from no
vegetation to dense plant cover. In particular, GHG flux measurements in ecosystems with high
plant canopies often exclude plants from the chambers. This provides a possibility to study in detail
the formation of GHG directly from the soil. However, this approach neglects the possibility of
plants and living roots participating in GHG formation, consumption or transport and may lead to
over or underestimation of the net ecosystem GHG exchange.
Plants directly affect the fluxes of CH4, CO2 and N2O due to their role in the production or
consumption of GHGs, or as a physical pathway for the GHGs from the soil to the atmosphere
(Bubier 1995; Waddington et al., 1996; Wassmann and Aulakh 2000; Muller 2003; Pihlatie et al.,
2005A). With respect to CO2, the gas exchange between soil and atmosphere is regulated by the
photosynthesis of ground vegetation and the respiration of soil and ground vegetation. Hence the
exchange of CO2 between soil and atmosphere is often studied by using light and dark chambers in
order to estimate the role of the different components. During a chamber measurement, plants inside
the chamber may also indirectly influence the soil GHG fluxes by physically slowing down the
mixing of gases inside the chamber.
To obtain information on the differences between static chambers we will organize a chamber
calibration campaign. The purpose of the campaign is to test different types of static chambers for
N2O, CH4 and CO2 fluxes under controlled conditions using a constant flux calibration tank. The
overall aims of the campaign are to quantitatively assess the uncertainties and errors related to static
chamber measurements, and to bring together and discuss these problems with researchers working
in the field of GHG fluxes from different ecosystems.
Aims and objectives
The overall aim of the measurement campaign is to improve measurement quality in gas fluxes of
CH4, N2O and CO2 from soils, to provide guidelines for chamber designs and sampling procedures,
and to share knowledge on chamber measurement techniques between flux measurement
communities. For the comparison, 10-15 chambers with different dimensions, material and
sampling protocol will be selected and tested.
The objectives of the measurement campaign are:
1) To compare different static chamber designs, under controlled conditions, used for N2O and
CH4, particularly, but also CO2 flux measurements with and without plants and at different
flux levels.
2) To improve understanding on the potential systematic errors associated with chamber
measurements
3) To quantify the possible errors in the chamber measurement for a number of chamber
designs.
Measurement system
The chamber calibrations will be carried out at Hyytiälä Forestry Field Station in southern Finland
(61◦51’N, 24◦17’E) during August-October 2008. The calibration of static chambers is conducted
using a chamber calibration tank designed by Jukka Pumpanen (see Pumpanen et al., 2004). The
calibration tank is a cylindrical stainless steel tank with a diameter of 1130 mm and a height of 1000
mm. It was originally built for CO2 chamber calibration and is presented in detail in Pumpanen et
al. (2004). The tank has a 20 mm thick lid made of high-density polyethylene perforated with holes
of 7mm in diameter. Before a calibration, a layer of quartz sand with known pore size and total
porosity will be place on the top of the lid.
At the start of the calibration a known concentration of each of the target gases, N2O, CH4 and CO2,
are injected inside the calibration tank. The air inside the tank is continuously mixed, and the
concentrations of the gases inside the tank are monitored at time intervals for CH4 and N2O, or
continuously for CO2. For CH4 and N2O the gas concentrations inside the calibration tank
monitored with a gas chromatograph (GC, Agilent 7890A) equipped with an electron capture
detector (ECD) and a flame ionization detector (FID) with a methanizer. Gas samples from inside
the calibration tank are taken with an automatic gas sampler every 5 minute into 12 ml glass vials
(Labco Exetainer). The gas samples are analyzed for N2O and CH4 by a GC.
The flux from the tank through the sand bed to the atmosphere is calculated from the decrease of
gas concentration inside the tank. During the monitoring period, chamber measurements are
conducted on the sand bed of the calibration tank. Gas samples are taken from the headspace of
each chamber in the same way as during normal chamber measurements for each chamber design.
The gas samples are injected into glass vials (Labco Exetainer) and analyzed for N2O, CH4 and CO2
by a GC. The fluxes measured by the chambers are calculated from the change in gas concentration
inside the chamber during the enclosure. Then the actual flux from the calibration tank is compared
to the flux measured by the chamber. This provides estimates on the under- or overestimation of
fluxes for each chamber tested.
Applicability of the results
A quantitative description and understanding of how a chamber works under controlled conditions
is valuable because it provides a possibility to assess the limitations and uncertainties in the
experimental setup and sampling procedure. The uncertainty estimates for flux measurements could
further be used for interpretation of GHG flux time series and as a tool to evaluate and develop
process based mathematical models, such as the DNDC (Li et al. 1992) and CoupModel (Jansson &
Karlberg, 2004).
The results from the calibration campaign can be used in the ongoing research projects
NitroEurope, NinE, Carboeurope, Fluxnet, IMECC and Scandinavian network of flux towers (e.g.
NECC). As the chamber calibration aims at providing error estimates for static chambers, this
campaign gives tools to compare flux estimates across Europe from a wide range of locations and
experimental setups.
We expect that studies of GHG fluxes from soils in a wide variety of ecosystems will increase in
number in the near future due to increasing focus on global warming, specification of national GHG
inventories and climate change mitigation strategies. Thus, the objectives of the workshop are also
aimed at future research and results can be used by a wide range of researchers. Future projects
could use the results as guidelines for chamber design and measurement protocol in the field. We
also believe that the calibration campaign and the following workshop can promote cooperation
across research community boundaries as well as creating a common platform of knowledge for
static chamber measurements.
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