DISCRIMINATION AGAINST (COO)-O-18-O-16 DURING PHOTOSYNTHESIS AND THE OXYGEN-ISOTOPE RATIO OF RESPIRED CO2 IN BOREAL FOREST ECOSYSTEMS

Our objective was to analyze factors that influence changes in the oxy gen isotope ratio (delta(18)O) of atmospheric CO2 within boreal forest ecosystems. We made measurements in the three major forest types (bla ck spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a large r study, the Boreal Ecosystem-Atmosphere Study (BOREAS). In terrestria l ecosystems the delta(18)O value of atmospheric CO2 is strongly influ enced by isotope effects that occur during photosynthesis and respirat ion, Of primary importance is an equilibrium isotope effect that occur s between oxygen in CO2 and oxygen in soil water and plant chloroplast water. During the equilibrium reaction the oxygen isotope ratio of CO 2 becomes enriched in O-18 relative to that of water. We measured seas onal changes in the oxygen isotope ratio of(1) water input to the ecos ystems (precipitation), (2) water taken up by the major plant species from the soil (plant stem water), and (3) water in plant leaves. We us ed this information in calculations of isotope discrimination during p hotosynthesis and soil respiration. Discrimination against (COO)-O-18- O-16 during photosynthetic gas exchange (Delta(A)) (influenced by equi libration with chloroplast water) averaged approximately 21 parts per thousand at midday and was similar for all forest types. In contrast, CO2 released during plant and soil respiration had an average delta(18 )O value of -14.4 parts per thousand but was less depleted in O-18 tha n would be expected for respired CO2 in isotopic equilibrium with soil water. This effect was most pronounced in black spruce sites because of the extensive coverage of moss on the ground surface and the observ ation that water in the upper moss layers can have an oxygen isotope r atio substantially different from water in deeper soil layers.