LI-COR LI-8100A - Page 67

4-12
Theory of Operation
respiration, including above ground respiration and soil respiration. It is a key
variable for understanding the carbon balance of an ecosystem. NCE can be
readily measured with the eddy covariance method over a large uniform field or
with the chamber-based method for short and small canopies. If no plants are
present inside the clear chamber, then the measured NCE is the soil respiration.
LAI
V
S
C
s
C
i
C
c,
w
c
g
l
g
s
Figure 4-3. Diagram showing a chamber volume of V (m
3
) and surface area S (m
2
) sitting
over the soil with plants inside. The leaf area inside chamber is LAI.
g
s
and g
l
are soil
conductance and leaf conductance (m s
-1
). C
i
is the intercellular CO
2
mole fraction (mol mol
-
1
), C
s
is the CO
2
mole fraction of soil air (mol mol
-1
). C
c
is the CO
2
mole fraction inside
chamber (mol mol
-1
).
For a clear chamber, the mass balance for CO
2
inside the chamber would be (see
Equation 4-7 for more information);
NCE
V
S
w
c
t
c
c
c
= −
ρ
∂
∂
()
'
1
4-18
where 
c
is the air number density inside the chamber (mol m
-3
), S is the surface
area inside the chamber (m
2
), and w
c
is the chamber air water vapor mole fraction
(mol mol
-1
). C
c
’ is chamber CO
2
mole fraction corrected for water vapor dilution.
For soil CO
2
flux f
c
f
c
= 
c
g
s
(C
s
– C
c
) = 
c
g
s
(1-w
c
)(C’
s
– C’
c
) 4-19
Where f
c
is soil CO
2
flux (mol m
-2
s
-1
), and all variables have the same definitions as
described in this section for opaque chambers.
When you have green plants with leaf area index, (LAI, defined as the ratio of total
green leaf area inside the clear chamber to ground area), the CO
2
uptake rate via
photosynthesis (f
p
) would be