First on-line isotopic characterization of N₂O above intensively managed grassland

Abstract. The analysis of the four main isotopic N₂O species (¹⁴N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁴N¹⁸O) and especially the intramolecular distribution of ¹⁵N ("site preference", SP) has been suggested as a tool to distinguish source processes and to help constrain the global N₂O budget. However, current studies suffer from limited spatial and temporal resolution capabilities due to the combination of discrete flask sampling with subsequent laboratory-based mass-spectrometric analysis. Quantum cascade laser absorption spectroscopy (QCLAS) allows the selective high-precision analysis of N₂O isotopic species at trace levels and is suitable for in situ measurements.

Here, we present results from the first field campaign, conducted on an intensively managed grassland site in central Switzerland. N₂O mole fractions and isotopic composition were determined in the atmospheric surface layer (at 2.2 m height) at a high temporal resolution with a modified state-of-the-art laser spectrometer connected to an automated N₂O preconcentration unit. The analytical performance was determined from repeated measurements of a compressed air tank and resulted in measurement repeatability of 0.20, 0.12 and 0.11‰ for δ¹⁵N^α, δ¹⁵N^β and δ¹⁸O, respectively. Simultaneous eddy-covariance N₂O flux measurements were used to determine the flux-averaged isotopic signature of soil-emitted N₂O.

Our measurements indicate that, in general, nitrifier-denitrification and denitrification were the prevalent sources of N₂O during the campaign and that variations in isotopic composition were due to alterations in the extent to which N₂O was reduced to N₂ rather than to other pathways, such as hydroxylamine oxidation. Management and rewetting events were characterized by low values of the intramolecular ¹⁵N site preference (SP), δ¹⁵N^bulk and δ¹⁸O, suggesting that nitrifier-denitrification and incomplete heterotrophic bacterial denitrification responded most strongly to the induced disturbances. The flux-averaged isotopic composition of N₂O from intensively managed grassland was 6.9 ± 4.3, −17.4 ± 6.2 and 27.4 ± 3.6‰ for SP, δ¹⁵N^bulk and δ¹⁸O, respectively. The approach presented here is capable of providing long-term data sets also for other N₂O-emitting ecosystems, which can be used to further constrain global N₂O inventories.