CIRAS-3 Operation Manual V. 1.09
141
Simulated Soil Respiration Measurements
With the CIRAS-3 software a quadratic relationship (y = a + bx + cx
2
) is fitted between the cuvette
concentration (C=y) and time (T=x) from the start of measurement. Note that there is a delay after the
chamber is first placed on the soil to allow for the establishment of stable gradients before the
measurements begin.
Then
2
cT
bT
a
C
+
+
=
The true assimilation rate will be calculated from dC/dT at T=0.
Now dC/dT = b + 2cT
so at T=0 dC/dT = b
(2)
A comparison of b and cT gives an indication of the leakage. Within the software, cT must be less than
20% of b or the “Non-linear“ error message is given. This is believed to be a better approach than
lowering the CO
2
value at the start of the measurement.
FCO
2
Units for measurement of Soil CO
2
Efflux
From 1 and 2:-
R = b x V/A
To give the flux in mass/unit area/unit time then b must be measured in mass/unit volume.
The CO
2
analyzer measures the volume/volume ratio (= ppm by volume = micro-liter/liter = micro-bar/bar
= micro-mol/mol).
Now one kg mol of gas (44.01kg of CO
2
) at STP (0 ºC, 1013.25 mb) occupies 22.41 m
3
.
C
u
vet
te C
o
n
cen
tr
at
io
n
Time
Cn
No Leakage
With Leakage
Co
To
Tn
