Use of the isotope flux ratio approach to investigate the C18O16O and 13CO2 exchange near the floor of a temperate deciduous forest

[摘要] Stable isotopologues of CO₂, such as ¹³CO₂ and C¹⁸OO,have been used to study the CO₂ exchange between land and atmosphere.The advent of new measuring techniques has allowed near-continuousmeasurements of stable isotopes in the air. These measurements can be usedwith micrometeorological techniques, providing new tools to investigate theisotope exchange in ecosystems. The objectives of this study were toevaluate the use of the isotope flux ratio method (IFR) near the forestfloor of a temperate deciduous forest and to study the temporal dynamics ofδ¹⁸O of CO₂ flux near the forest floor by comparing IFRestimates with estimates of δ¹⁸O of net soil CO₂ fluxprovided by an analytical model. Mixing ratios of ¹²C¹⁶O₂,¹³CO₂ and C¹⁶O¹⁸O were measured within and above atemperate deciduous forest, using the tunable diode laser spectroscopytechnique. The half-hourly compositions of the CO₂ flux near the forestfloor (δ¹³C_F and δ¹⁸O_F) were calculatedby IFR and compared with estimates provided by a modified Keeling plottechnique (mKP) and by a Lagrangian dispersion analysis (WT analysis). ThemKP and IFR δ¹⁸O_F estimates showed good agreement(slope = 1.03 and correlation, R² = 0.80). The δ¹³C_F estimatesfrom the two methods varied in a narrow range of −32.7 and −23‰; the mean(± SE) mKP and IFR δ¹³C_F values were −27.5‰ (±0.2)and −27.3‰ (±0.1), respectively, and were statistically identical(p>0.05). WT analysis and IFR δ¹⁸O_F estimates showedbetter correlation (R² = 0.37) when only turbulent periods (u^*>0.6 m s⁻¹) 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 studyingδ¹⁸O_F dynamics within forest canopies. Values of δ¹⁸O_F showed large temporal variation, with values ranging from−31.4‰ (DOY 208) to −11.2‰ (DOY 221). Precipitation events causedsubstantial variation (~8‰) in δ¹⁸O_F over a periodof approximately 24 h. A diel trend of δ¹⁸O_F was observed,with more depleted values present during the daytime. Model simulationsindicate that the activity of the carbonic anhydrase enzyme was quitevariable during the evaluated period. These simulations indicate that morefrequent sampling of δ¹⁸O of soil water could improve theestimates of δ¹⁸O of net soil CO₂ flux.

[学科分类] 地球化学与岩石