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

In the laboratory, N2O samples were taken from the Mason jars, as described above, a further
20 minutes after the NOx flux had been measured. Checks were made to ensure that the N2O
gas concentration increased at a linear rate over the twenty minute period. The N2O flux was
calculated as follows:
F
N 2O
( C
t
Ci
) V
Mwt
R T
A
N 2O
where, FN2O is the N2O flux (g N2O m -2 h -1 ), Ct and Ci are the N2O concentrations at time t
and time zero (L N2O L -1 ), V is the headspace volume (L), MwtN2O is the mass of a mole of
N2O (g N2O mol -1 ), R is the universal gas constant (8.314472 J K -1 mol -1 ), T is the
temperature (K) and A the soil surface area (m 2 ).
3.6 Soil measurements
3.6.1 Soil pH
Soil surface pH was measured using a Hanna HI 9025C portable pH meter fitted with a soil
surface probe (Broadley-James Corporation, Irvine, CA, USA). This was calibrated with
(3-2)
buffer solutions in the laboratory prior to use in the field. The soil surface was moistened with
one drop of deionised (DI) water prior to placing the probe on the surface.
Bulk soil pH was measured by diluting soil samples 1: 2.5 with deionised water, shaken by
hand for approximately 30 seconds. The pH was measured with a pre-calibrated pH meter
equipped with a Broadley James pH electrode. The pH meter was calibrated using buffer
solutions of pH 4 and 7.
3.6.2 Soil inorganic-N determinations
Soil inorganic-N concentrations were determined by extracting the soil with 2 M KCl
(Mulvaney, 1996) and analysing the extracts with colorimetric flow injection methods for
NH4-N, NO2-N and NO3-N (Alpkem, FS 3000).
3.6.3 Water-filled pore space
The water-filled pore space was determined using gravimetric soil water content (θg) and soil
bulk density. The determination of gravimetric soil water contents (θg) was performed on subsamples
of soil. This was done by drying soil at 105 o C for 24 h and recording the changes in
mass. In the field, soil bulk density was determined, by taking soil cores (5 cm diameter) to a
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