TARGAS-1 Operation Manual V. 1.02
157
Soil Respiration Measurements
The TARGAS-1 software assumes a quadratic relationship (y = a + bx + cx
2
) between the chamber
concentration (C=y) and time (T=x) from the start of measurement to account for the non-linearities
caused by leakage. 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.
(2.2)
The quadratic equation is
𝑪𝑪
=
𝒂𝒂
+
𝒃𝒃𝑻𝑻
+
𝒄𝒄𝑻𝑻
𝟐𝟐
Where
𝑪𝑪
and
𝑻𝑻
are a series of chamber CO
2
concentration measurements made over time, and
𝒂𝒂
,
𝒃𝒃
,
𝒄𝒄
are coefficients calculated from a least square fit of the data.
The respiration rate will be calculated from the rate of change of CO
2
at time zero or
𝑪𝑪𝑪𝑪
/
𝑪𝑪𝑻𝑻
at
𝑻𝑻
=0.
(2.3)
Differentiating equation
(1.2)
yields
𝑪𝑪𝑪𝑪
𝑪𝑪𝑻𝑻
=
𝒃𝒃
+
𝟐𝟐𝒄𝒄𝑻𝑻
(2.4)
And evaluated at T = 0,
𝑪𝑪𝑪𝑪
𝑪𝑪𝑻𝑻
=
𝒃𝒃
A comparison of
𝒃𝒃
and
𝒄𝒄𝑻𝑻
gives an indication of the magnitude of the non-linearity of the C vs T data. The
TARGAS-1 software indicates a "non-linear" error message whenever the value
𝒄𝒄𝑻𝑻
is greater than 20% of
𝒃𝒃
. This is believed to be a better approach than lowering the CO
2
value at the start of the measurement.
In addition to the quadratic assumption and calculation, the TARGAS-1 software also calculated the
respiration (or assimilation) using a linear assumption.
(2.5)
The linear equation is
𝑪𝑪
=
𝒂𝒂
+
𝒃𝒃𝑻𝑻
(2.6)
which evaluates again to
C
u
vet
te C
o
n
cen
tr
at
io
n
Time
Cn
No Leakage
With Leakage
Co
To
Tn