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 O-2. The relative importance of the two processes, particularly under varied partial pressures Of O-2, is not always known. This paper focuses on the influence of O-2 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 N-15 and were incubated during 14 days at different O-2 partial pressures: 0, 0.35, 0.76, 1.5, 4.3 and 20.4 kPa. A N-15 tracing technique was used to quantify nitrification and denitrification rates. (N2O)-N-15 and N-15(2) 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 O-2 decreased from 20.4 to 0.35 kPa. They were highly correlated with O-2 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 N-15 tracing using isotope composition of NH4, NO3 and N2O, the other based on the measurement of the (N2O)-N-15: N-15(2) 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 O-2 pressure, and they were due to nitrification. Nitrification was the main source of N2O at O-2 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 O-2 concentration: it varied from 0.16 to 1.48% when O-2 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 O-2 concentration. (C) 2004 Elsevier Ltd. All rights reserved.