CIRAS-3 Operation Manual V. 1.09 141 support@ppsystems.com
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
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
.
Cuvette Concentration
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