Geoscience Reference
In-Depth Information
rubber air strips. Two fans per
flux chamber were continuously running, ensuring
well-mixed headspace air. Automatic chamber closure (once per hour) was made
possible by use of a pneumatic system regulated by the valve manifold box. Air
fl
fl
flow between the chamber and the FTIR was initiated by a pump placed behind the
measurement cell and set to 1 L min
1
per minute. The air from the
fl
ux chamber
was measured every 2 min (2 min-spectra) for 20 min. The
fl
flux chamber closed and
opened after respectively 4 and 18 min. Gas
fl
fluxes were calculated by:
RST
d
C
VP
F
¼
ð
2
Þ
d
z
wherein V is the volume of the chamber (m
3
), P the chamber air pressure (Pa), R
the gas constant (8
314 m
3
Pa K
1
mol
1
), S the soil area (m
2
), T the chamber air
:
temperature
(K)
and
d
C
=d
t
is
the gas concentration change over
time
mol mol
1
s
1
).
(
l
3 Flux Gradient- and Flux Chamber Measurements
The FTIR measurement precisions for 10-minute average values are given in
Table
2
(Grif
th and Galle
2000
). Precisions for spectra with shorter averaging
times are estimated (Table
2
). Measurement of greenhouse gas
fl
uxes by
fl
ux
chambers, as well as by the
flux gradient, are based on concentration differences,
where an eventual instrumental drift plays a minor role. For reliable
fl
fl
flux gradient
measurements, a minimum concentration difference of 2
r
is needed. Typical dif-
fusion coef
cient values under unstable conditions (Obhukov-length < 0) range
between 0.1 and 0
4m
2
s
1
m
1
(Foken and Nappo
2008
). For reliable
:
fl
ux chamber
measurements, also a minimum difference of 2
r
between measurements is required.
Minimum detectable
fl
fluxes for the
fl
flux gradient technique (3 min-spectra) and for
the
flux chamber technique (2 min-spectra) are given in Table
2
. Increasing the time
per spectrum decreases the required minimum concentration difference and could
be considered in small magnitude
fl
fl
ux-ecosystems.
fluxes during chamber closure, due to altering the
concentration gradient and the air pressure (Hutchinson and Livingston
2002
;
Pumpanen et al.
2004
). To minimize these effects, tall, well mixed chambers were
chosen. During data evaluation, concentration increase was checked for logarithmic
behavior and only linear regression
Flux chambers can disturb
fl
fits with R
2
0
:
9 were used.
[
ux
chambers) was tested in the restored peatland Himmelmoor (Quickborn, Germany).
The
From August to November 2012, the setup (a
fl
flux gradient system and two
fl
flux chambers were placed on a dry position in the wetland, but later in the
experiment were moved to a wet location. The
fl
flux gradient system overlooked an
area with mostly wet locations. In Fig.
2
, hourly CO
2
-
fl
fl
uxes (from
fl
ux chamber- and
Search WWH ::
Custom Search