First on-line isotopic characterization of N<sub>2</sub>O above intensively managed grassland

Abstract

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 δ15Nα, δ15Nβ 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), δ15Nbulk 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, δ15Nbulk 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.

Real-time analysis of <i>δ</i><sup>13</sup>C- and <i>δ</i>D-CH<sub>4</sub> in ambient air with laser spectroscopy: method development and first intercomparison results

“…However, highprecision measurements of low abundance isotopic species of trace gases (such as δD-CH 4 ) at ambient mole fractions require preconcentration when using direct absorption techniques (Bergamaschi et al, 1998a). The strategy of trace gas preconcentration prior to isotopic analysis by quantum cascade laser spectroscopy (QCLAS) has been demonstrated for nitrous oxide (N 2 O) isotopologues (Mohn et al, 2010(Mohn et al, , 2012 and was applied in an extended field campaign (Wolf et al, 2015).…”

“…1) is based on the "Medusa" system (Miller et al, 2008), which was later adopted for the preconcentration of N 2 O and its subsequent isotope analysis by QCLAS (Mohn et al, 2010(Mohn et al, , 2012Waechter et al, 2008;Wolf et al, 2015). The main advantages over previously developed systems (Brand, 1995) are the low trapping temperatures in combination with its independence from liquid nitrogen.…”

“…Finally, the preconcentration unit can be applied for the analysis of mole fraction and isotopic composition of other trace gases, e.g., N 2 O and VOCs. The potential of this technique for N 2 O isotopes was recently demonstrated in an extended field campaign (Wolf et al, 2015).…”

Constraining N cycling in the ecosystem model LandscapeDNDC with the stable isotope model SIMONE

“…) were measured and are described in detail by Wolf et al. (). The ic of N 2 O emissions was determined using keeling plots of N 2 O ic of ambient air (height of measurement inlet 2.2 m).…”

“…) and considers SP, (2) to link it to LandscapeDNDC and analyze the agreement between measured (Wolf et al. ) and simulated N 2 O ic emitted from an intensively managed grassland site in Switzerland, and (3) to assess if this approach can identify weaknesses in the model process parameterization.…”