Nitrous oxide production by nitrification and denitrification in soil aggregates as affected by O2 concentration

Nitrous oxide emitted by soils can be produced either by denitrification in anoxic conditions or by nitrification in presence of O2. The relative importance of the two processes, particularly under varied partial pressures of O2, is not always known. This paper focuses on the influence of O2 concentration on N2O production by nitrification and denitrification in an arable Orthic Luvisol. Soil aggregates (2–3 mm size), water unsaturated, received 116 mg N kg−1 as ammonium sulphate labelled with 15N and were incubated during 14 days at different O2 partial pressures: 0, 0.35, 0.76, 1.5, 4.3 and 20.4 kPa. A 15N tracing technique was used to quantify nitrification and denitrification rates. 15N2O and 15N2 were measured. Oxygen pressure appeared to strongly influence both nitrification and denitrification rates and also N2O emissions. Nitrification rates were reduced by a factor of 6–9 when O2 decreased from 20.4 to 0.35 kPa. They were highly correlated with O2 consumption rates. Denitrification mainly occurred in complete anoxic conditions. The proportion of N2O emitted by denitrification was estimated by two independent methods: one based on 15N tracing using isotope composition of NH4, NO3 and N2O, the other based on the measurement of the 15N2O:15N2 ratio. The two methods gave close results. The highest N2O emissions were obtained under complete anoxic conditions and were due to denitrification. However, N2O emissions almost as important were obtained at day 14 with 1.5 kPa O2 pressure, and they were due to nitrification. Nitrification was the main source of N2O at O2 concentrations greater than 0.35 kPa. The amounts of N2O-N emitted by nitrification were linearly related to the amounts of N nitrified, but the slope of the regression was highly dependent on O2 concentration: it varied from 0.16 to 1.48% when O2 concentration was reduced from 20.4 to 0.76 kPa. Emissions of N2O by nitrification may then be quite significant if nitrification occurs at a reduced O2 concentration.