Influence of vegetation and soil CO2 exchange on the concentration and stable oxygen isotope ratio of atmospheric CO2 within a Pinus resinosa canopy.

Measurements were made of the concentration and stable oxygen isotopic ratio of carbon dioxide in air samples collected on a diurnal basis at two heights within a Pinus resinosa canopy. Large changes in CO2 concentration and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong vertical stratification in the forest canopy, with higher CO2 concentrations and more negative δ18O values observed closer to the soil surface. The observed daily increases in δ18O values of forest CO2 were dependent on relative humidity consistent with the modelled predictions of isotopic fractionation during photosynthetic gas exchange. During photosynthetic gas exchange, a portion of the CO2 that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered oxygen isotopic ratio. The oxygen isotope ratio of CO2 diffusing out of a leaf depends primarily on the 18O content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the δ18O values of forest CO2 at night, because CO2 released from the soil has equilibrated with soil water which has a lower 18O content than leaf water. The observed relationship between diurnal changes in CO2 concentration and oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO2 fluxes associated with photosynthesis, respiration and turbulent exchange between the forest and the bulk atmosphere.