EGM-5 Operation Manual V. 1.04 125 support@ppsystems.com
Soil Respiration Measurements
The EGM-5 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.
(1.2) The quadratic equation is
= + +
W
here and are a series of chamber CO
2
concentration measurements made over time, and , ,
are coefficients calculated from a least square fit of the data.
T
he respiration rate will be calculated from the rate of change of CO
2
at time zero or / at =0.
(1.3) Differentiating equation (1.2) yields
= +
(1.4) And evaluated at T = 0,
=
A comparison of and gives an indication of the magnitude of the non-linearity of the C vs T data. The
EGM-5 software indicates a "non-linear" error message whenever the value
i
s greater than 20% of .
This is believed to be a better approach than lowering the CO
2
value at the start of the measurement.
I
n addition to the quadratic assumption and calculation, the EGM-5software also calculated the
respiration (or assimilation) using a linear assumption.
(
1.5) The linear equation is
= +
(1.6) which evaluates again to
=
In this case the value of / is calculated from a linear regression of the C and T data.
Correction for water vapor increase on CO
2
efflux
In addition to the influx of CO
2
from soil, a closed chamber system can also experience an increase in
H
2
O due to evapotranspiration from soil or plants. This added H
2
O dilutes the remaining CO
2
and
requires a compensation in a similar way to equation (A.13).
For total moles of air in the chamber of ,
(1.7) the moles of H
2
O added to the chamber is =
(
)
(
)
,
where
and
are H
2
O concentrations in mmol mol
-1
.
To Next Page
Loading...
Need help?
General
