First on-line isotopic characterization of N<sub>2</sub>O above intensively managed grassland
The analysis of the four main isotopic N<sub>2</sub>O species (<sup>14</sup>N<sup>14</sup>N<sup>16</sup>O, <sup>14</sup>N<sup>15</sup>N<sup>16</sup>O, <sup>15</sup>N<sup>14</sup>N<sup>16...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-04-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/12/2517/2015/bg-12-2517-2015.pdf |
| Summary: | The analysis of the four main isotopic N<sub>2</sub>O species
(<sup>14</sup>N<sup>14</sup>N<sup>16</sup>O, <sup>14</sup>N<sup>15</sup>N<sup>16</sup>O,
<sup>15</sup>N<sup>14</sup>N<sup>16</sup>O, <sup>14</sup>N<sup>14</sup>N<sup>18</sup>O) and especially the
intramolecular distribution of <sup>15</sup>N ("site preference", SP) has been
suggested as a tool to distinguish source processes and to help constrain the
global N<sub>2</sub>O 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 N<sub>2</sub>O isotopic species at trace levels and is
suitable for in situ measurements.
<br><br>
Here, we present results from the first field campaign, conducted on an
intensively managed grassland site in central Switzerland. N<sub>2</sub>O 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 N<sub>2</sub>O 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 δ<sup>15</sup>N<sup>α</sup>, δ<sup>15</sup>N<sup>β</sup>
and δ<sup>18</sup>O, respectively. Simultaneous eddy-covariance N<sub>2</sub>O flux
measurements were used to determine the flux-averaged isotopic signature of
soil-emitted N<sub>2</sub>O.
<br><br>
Our measurements indicate that, in general, nitrifier-denitrification and
denitrification were the prevalent sources of N<sub>2</sub>O during the campaign
and that variations in isotopic composition were due to alterations in
the extent to which N<sub>2</sub>O was reduced to N<sub>2</sub> rather than to other pathways, such
as hydroxylamine oxidation. Management and rewetting events were
characterized by low values of the intramolecular <sup>15</sup>N site preference
(SP), δ<sup>15</sup>N<sup>bulk</sup> and δ<sup>18</sup>O, suggesting that nitrifier-denitrification and incomplete heterotrophic bacterial denitrification
responded most strongly to the induced disturbances. The flux-averaged isotopic
composition of N<sub>2</sub>O from intensively managed grassland was
6.9 ± 4.3, −17.4 ± 6.2 and 27.4 ± 3.6‰ for SP,
δ<sup>15</sup>N<sup>bulk</sup> and δ<sup>18</sup>O, respectively. The approach
presented here is capable of providing long-term data sets also for other
N<sub>2</sub>O-emitting ecosystems, which can be used to further constrain global
N<sub>2</sub>O inventories. |
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| ISSN: | 1726-4170 1726-4189 |