The relationship between 18O/16O and 13C/12C ratios of ambient CO2 in two Amazonian tropical forests

Abstract

Oxygen and carbon isotopic compositions of ambient carbon dioxide in two tropical forests in the Amazonian basin were determined at the beginning and the end of the dry season. One site, Fazenda Vitoria, is typical of a seasonal forest with a pronounced dry season from May to November whereas the other site, Reserva Ducke, has a milder dry season. Samples collected at night revealed that in the beginning of the dry season, when the forest is still relatively wet, isotopic exchange between carbon dioxide and wet surfaces or fog is possible. Oxygen isotope ratios of ambient carbon dioxide collected during the day were correlated with the carbon isotope ratios. These correlations are representative of the mixing line between tropospheric and respired CO2. The mixing lines were used to extrapolate the δ18O values of soil-respired CO2. These values were compared with those derived from the isotopic composition of bulk soil water (as reflected by plant stem water) and a diffusion isotopic effect of 8.8°/∞. It was observed that δ18O values of respired CO2 derived by extrapolation were greater than those calculated by using δ18O values of bulk soil water and the associated diffusion isotope effect. The results show that in tropical forests, assumptions used in previous studies of temperate regions to derive the oxygen isotopic composition of respired CO2 do not hold.