The dynamics of mineral N species involved in the processes of denitri
fication were studied in laboratory systems using a soil from permanen
t pasture. Experiments were carried out to test a mechanistic model li
nking C mineralization and denitrification. This involved the use of c
ompetitive Michaelis-Menten type enzyme kinetics to simulate the reduc
tion of NO3- and the formation of NO2-, N2O and N2. The rate of reduct
ion of each N oxide was assumed to be dependent upon its concentration
and on a weighting factor for competition between electron acceptors.
Concentrations of 20, 200 and 2000 mg NO3--N kg-1 were applied to a s
oil slurry and anaerobically incubated for 5 days at 25-degrees-C. The
production of CO2, N2O and N2, and concentrations of NO3- and NO2- we
re monitored daily. The ratio between CO2 produced and nitrate reduced
was 0.7; much lower than the value expected if glucose was the only C
substrate and all nitrate was reduced to N2. The nitrite concentratio
n remained below 0.5 mg NO2--N kg-1 in each treatment indicating that
all NO2- formed, was very rapidly reduced to N2O. The reduction of N2O
was much slower: no N2 was formed within the first day when 200 mg NO
3--N kg-1 was applied, and the rate of N2O production remained higher
than the rate of N2O reduction for the first 4 days of the incubation.
Only 75% of the N2O formed was reduced within 1 day when 20 mg NO3--N
kg-1 was added. The dynamics of the different forms of nitrogen gener
ated in the denitrification process at different concentrations of ava
ilable nitrate could be simulated using a weighting factor or an affin
ity of 1 for NO3-, around 1000 for NO2- and 0.75 for N2O. It was not n
ecessary to include inhibitory effects of nitrate or nitrite in the mo
del to simulate the observed patterns of mineral N dynamics. It was co
ncluded from this study that low affinity for N2O as compared to the h
igh affinity for NO2-could lead to high losses of nitrous oxide in the
field even if only low quantities of NO3- were available.