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

Abstract. 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-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 δ¹⁸O of CO₂ flux near the forest floor by comparing IFR estimates with estimates of δ¹⁸O of net soil CO₂ flux provided by an analytical model. Mixing ratios of ¹²C¹⁶O₂, ¹³CO₂ and C¹⁶O¹⁸O were measured within and above a temperate deciduous forest, using the tunable diode laser spectroscopy technique. The half-hourly compositions of the CO₂ flux near the forest floor (δ¹³C_F and δ¹⁸O_F) 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 δ¹⁸O_F estimates showed good agreement (slope = 1.03 and correlation, R² = 0.80). The δ¹³C_F estimates from 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 showed better 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 studying δ¹⁸O_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 caused substantial variation (~8‰) in δ¹⁸O_F over a period of approximately 24 h. A diel trend of δ¹⁸O_F 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 δ¹⁸O of soil water could improve the estimates of δ¹⁸O of net soil CO₂ flux.