Factors affecting nitric oxide and nitrous oxide emissions from ...

Plate 3.2 Closed gas circuit used for NO flux measurement, showing soil core inside gas
sampling Mason jar.
In the field, NOx flux measurements were taken by first switching the LMA-3D on and
allowing it to warm-up and for the ambient reading to stabilise. It was then connected to a
Teflon ® coated stainless steel chamber which had vacutainer stoppers (red with no additives,
Becton-Dickinson, NJ) installed in the top of the chambers as septa. To take the flux reading,
the chamber was placed onto a PVC ring (19.5cm diameter) previously inserted into the
middle of each urine patch (section 7.2.1, Plate, 7.1). The Teflon ® chamber was 20 cm in
diameter and 11 cm high (3454 cm 3 ). On each sampling day, NOx flux determinations were
made between 12.00 and 15.00 hours. The concentration readings were taken every five
seconds until the rate of increase in the NOx concentration declined. Figure 3.5 shows the plot
of NO-N concentration versus time from a field experiment. The chamber was left in position
measuring the increasing concentration of NOX in the headspace until the readings stabilised.
Then the chamber was removed to allow the dispersion of any gases that had accumulated in
the headspace prior to any subsequent N2O flux determinations (see below). The value of
dC/dt was calculated by using only the linear phase of the ‘concentration vs time’ plots and
calculating CA, using the calibration plot, at two times to generate ‘dC’ with ‘dt’ the
corresponding time interval.
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