Seasonal and Interannual Variation in Oxygen Isotope Discrimination in a Temperate Grassland Ecosystem

Our objective was to analyze the effects of seasonal and interannual climate variation on the oxygen isotope composition of carbon dioxide exchanging with the atmosphere in a temperate grassland ecosystem. On-line photosynthetic discrimination measurements were combined with analyses of the oxygen isotope composition of leaf water and respired carbon dioxide. In addition, we made continuous measurements of net ecosystem carbon dioxide exchange using the eddy covariance technique. Our study period included a year with near average precipitation (1999), and a year with below average precipitation (2000). The dry conditions in the summer of 2000 resulted 4\permil higher leaf water isotopic enrichment than was observed in 1999. Reduced stomatal conductance caused chloroplast carbon dioxide concentrations to be 30 ppm lower on average, so oxygen isotope discrimination during photosynthesis was only moderately higher in 2000 than in 1999. On-line discrimination during photosynthesis was approximately 35\permil in 2000. The oxygen isotope composition of ecosystem respired carbon dioxide did not differ between the study years. The effect of higher oxygen isotope discrimination during photosynthesis in 2000 was also partly reduced by the lower rates of gross photosynthesis caused by the drought.