Diurnal fluxes and the isotopomer ratios of N2O in a temperate grassland following urine amendment

There is an urgent need to provide an accurate, up-to-date estimate of N2O fluxes in order that national policies can be developed to reduce emissions of N2O from soils. There are only limited data on temporal and diurnal patterns of N2O fluxes to the atmosphere, mainly due to constraints in the measurement techniques. In this paper we present the first terrestrial source values of N2O isotopomers and have measured and quantified the temporal and diurnal variability in N2O fluxes following urine addition to a grassland system in the UK. The experiment was carried out over a 2-week period on an artificially drained grassland system at the Institute of Grassland and Environmental Research (IGER), North Wyke, UK. Duplicate samples of urine, each of 2 L, were collected from dairy cows and applied to chambers (of area 0.16 m(2)). The N2O diurnal fluxes from urine and control (no urine) plots were measured by an automatic closed chamber technique. The isotopomers of N2O were obtained by analysing the gas samples collected during a peak emission phase. Soil and meteorological data were also collected. The results showed strong diurnal variations in N2O fluxes with minimum fluxes generally occurring between 7:00 and 14:00 hrs. The total cumulative flux of N2O for the whole experimental period was higher by a factor of >2 compared with estimates based on the daytime (between 10.00-16.00 hrs) measurements only. Therefore, measurements of N2O fluxes based on daily single exposure and expressed on a 24-h basis could impose a considerable bias and inaccuracy to the emission estimates, depending on when it was taken. The measured site preference values (difference between the centre (delta N-15 alpha) and the end (delta N-15 beta) N atom of the N2O molecule) for soil-emitted N2O measured during our study were always lower than the tropospheric value. This work confirms that the enhanced tropospheric N2O site preference value could be the result of the back injection from the stratosphere. The intramolecular isotope ratios of nitrogen (delta N-15) and oxygen (delta O-18) and the site preference of the emitted N2O indicated that there was a shift of processes during the measurement period. Copyright (C) 2001 John Wiley & Sons, Ltd.