Influence of vegetation and soil CO exchange on the concentration and stable oxygen isotope ratio of atmospheric CO 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 CO 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 CO concentrations and more negative δO values observed closer to the soil surface. The observed daily increases in δO values of forest CO 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 CO 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 CO diffusing out of a leaf depends primarily on the O content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the δO values of forest CO at night, because CO released from the soil has equilibrated with soil water which has a lower O content than leaf water. The observed relationship between diurnal changes in CO concentration and oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO fluxes associated with photosynthesis, respiration and turbulent exchange between the forest and the bulk atmosphere.