Use of the isotope flux ratio approach to investigate the C<sup>18</sup>O<sup>16</sup>O and <sup>13</sup>CO<sub>2</sub> exchange near the floor of a temperate deciduous forest
Stable isotopologues of CO<sub>2</sub>, such as <sup>13</sup>CO<sub>2</sub> and C<sup>18</sup>OO, have been used to study the CO<sub>2</sub> exchange between land and atmosphere. The advent of new measuring techniques has allowed near-conti...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2012-07-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/9/2385/2012/bg-9-2385-2012.pdf |
| Summary: | Stable isotopologues of CO<sub>2</sub>, such as <sup>13</sup>CO<sub>2</sub> and C<sup>18</sup>OO, have been used to study the CO<sub>2</sub> exchange between land and atmosphere. The advent of new measuring techniques has allowed near-continuous measurements of stable isotopes in the air. These measurements can be used with micrometeorological techniques, providing new tools to investigate the isotope exchange in ecosystems. The objectives of this study were to evaluate the use of the isotope flux ratio method (IFR) near the forest floor of a temperate deciduous forest and to study the temporal dynamics of δ<sup>18</sup>O of CO<sub>2</sub> flux near the forest floor by comparing IFR estimates with estimates of δ<sup>18</sup>O of net soil CO<sub>2</sub> flux provided by an analytical model. Mixing ratios of <sup>12</sup>C<sup>16</sup>O<sub>2</sub>, <sup>13</sup>CO<sub>2</sub> and C<sup>16</sup>O<sup>18</sup>O were measured within and above a temperate deciduous forest, using the tunable diode laser spectroscopy technique. The half-hourly compositions of the CO<sub>2</sub> flux near the forest floor (δ<sup>13</sup>C<sub>F</sub> and δ<sup>18</sup>O<sub>F</sub>) were calculated by IFR and compared with estimates provided by a modified Keeling plot technique (mKP) and by a Lagrangian dispersion analysis (WT analysis). The mKP and IFR δ<sup>18</sup>O<sub>F</sub> estimates showed good agreement (slope = 1.03 and correlation, <i>R</i><sup>2</sup> = 0.80). The δ<sup>13</sup>C<sub>F</sub> estimates from the two methods varied in a narrow range of −32.7 and −23‰; the mean (± SE) mKP and IFR δ<sup>13</sup>C<sub>F</sub> values were −27.5‰ (±0.2) and −27.3‰ (±0.1), respectively, and were statistically identical (<i>p</i>>0.05). WT analysis and IFR δ<sup>18</sup>O<sub>F</sub> estimates showed better correlation (<i>R</i><sup>2</sup> = 0.37) when only turbulent periods (<i>u</i><sup>*</sup>>0.6 m s<sup>−1</sup>) were included in the analysis. The large amount of data captured (~95 % of half-hour periods evaluated) for the IFR in comparison with mKP (27 %) shows that the former provides new opportunities for studying δ<sup>18</sup>O<sub>F</sub> dynamics within forest canopies. Values of δ<sup>18</sup>O<sub>F</sub> showed large temporal variation, with values ranging from −31.4‰ (DOY 208) to −11.2‰ (DOY 221). Precipitation events caused substantial variation (~8‰) in δ<sup>18</sup>O<sub>F</sub> over a period of approximately 24 h. A diel trend of δ<sup>18</sup>O<sub>F</sub> was observed, with more depleted values present during the daytime. Model simulations indicate that the activity of the carbonic anhydrase enzyme was quite variable during the evaluated period. These simulations indicate that more frequent sampling of δ<sup>18</sup>O of soil water could improve the estimates of δ<sup>18</sup>O of net soil CO<sub>2</sub> flux. |
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| ISSN: | 1726-4170 1726-4189 |