CIRAS-3 Operation Manual V. 1.09
140
Section 11. Closed System Measurements
Soil CO
2
Efflux and Net Canopy CO
2
Flux
The
SRC-1 and SRC-2 Soil Respiration Chambers
can be used with the CIRAS-3 Portable CO
2
/H
2
O
Gas Analysis System for closed system measurement of soil CO
2
efflux. The
CPY-4 and CPY-5 Canopy
Assimilation Chamber
can also be used with the CIRAS-3 for closed system measurement of net
canopy CO
2
flux
. Important Note. When using the SRC-1, SRC-2, CPY-4 and CPY-5 with the
CIRAS-3, the reference sampling pump must be sealed to ensure leak-tight seal for closed system
operation. Please contact PP Systems if you have any questions.
Theory
The respiration/assimilation is measured by placing a closed chamber on the soil and measuring the rate
of increase of the CO
2.
concentration inside the chamber. Then, assuming a well-mixed and sealed
system :
(
)
A
V
Tn
Co
Cn
R
*
−
=
(1)
Where R is the respiration/assimilation rate (CO
2
flux, or moles of CO
2
unit area
-1
unit time
-1
), Co is the
CO
2
concentration at T=0 and Cn is the concentration at a time Tn later. A is the area of soil exposed and
V the total system volume.
It has been suggested that to make accurate measurements of the assimilation it is essential to start with
a CO
2
concentration in the cuvette below (respiration)/above(photosynthesis) ambient and measure until
the concentration is above/below ambient, presumably with the intention of getting some compensation
for leakage. However, this leakage can only take place at ground level, where the CO
2
concentration is
unknown and most certainly will not be what we would consider as ambient.
Over the short period of measurement and with the relatively small CO
2
concentrations in the chamber
compared with the soil concentrations, we would expect the assimilation to be a constant flux, giving a
constant rate of change in the cuvette CO
2
concentration.
Any leakage should be a function of the concentration difference between the cuvette and the exchange
air. Due to leakage, the apparent assimilation rate decreases with time.
