58 Ecological Applications, 15(1), 2005, pp. 58–70  2005 by the Ecological Society of America OXYGEN ISOTOPE RATIOS OF WATERS AND RESPIRED CO 2 IN AMAZONIAN FOREST AND PASTURE ECOSYSTEMS J. P. H. OMETTO, 1,2 L. B. FLANAGAN, 3 L. A. MARTINELLI, 1 AND J. R. EHLERINGER 2,4 1 Centro de Energia Nuclear na Agricultura, Avenida Centena ´rio 303, Cep 13416-000, Piracicaba SP, Brazil 2 Stable Isotope Ratio Facility for Environmental Research, Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, Utah 84112-0840 USA 3 Department of Biological Sciences, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta T1K 3M4, Canada Abstract. The oxygen isotope ratio ( 18 O, SMOW) of atmospheric CO 2 is a powerful indicator of large-scale CO 2 exchange on land. Oxygen isotopic exchange between CO 2 and water in leaves and soils controls the  18 O of atmospheric CO 2 . Currently there is little empirical information on the spatial and temporal variation in the  18 O of leaf and stem water in tropical ecosystems. We measured the seasonal dynamics of  18 O in atmospheric CO 2 and water in different ecosystem compartments in both primary forest and pasture ecosystems in three different regions of the Amazonian Basin of Brazil (Ji-Parana ´, Manaus, and Santare ´m). Within regions, the source (stem) water  18 O values for primary forests and pastures were similar; neither vegetation type exhibited distinct wet–dry season patterns. Daytime leaf water isotope ratios were strongly correlated with predictions of the Craig- Gordon model. The  18 O value of leaf water was positively correlated with leaf height above ground because of associated variation in vapor pressure deficit and the  18 O of atmospheric water vapor within forest canopies. Consistent with these observations, the  18 O value of leaf cellulose was positively correlated with forest height. Leaf water from pasture grasses was more 18 O enriched than leaf water from forest vegetation. There was a tendency for daytime leaf water to be more enriched in 18 O during the dry season, reflecting generally lower humidity conditions during the dry season. Nighttime measurements of the oxygen isotope ratio of ecosystem respired CO 2 in both forest and pasture vegetation were not consistent with values expected for CO 2 in equilibrium with stem (soil) water, despite nighttime vapor pressure deficits close to zero. Apparently, the  18 O of leaf water lagged and did not attain isotopic equilibrium at night. Thus, the deviation of nighttime  18 O values of ecosystem respiration from that expected from a CO 2 efflux in equilibrium with soil (stem) water increased as  18 O values of ecosystem respiration became 18 O enriched. Dis- crimination against CO 2 containing 18 O(C 18 OO) during photosynthesis was calculated based on measured leaf water  18 O values. Forests had consistently higher modeled C 18 OO values than pastures. The daytime isotope effects we calculate for photosynthesis and respiration were consistent with previous model predictions of a strong depletion of 18 O in atmospheric CO 2 over the Amazon Basin of Brazil. Key words: carbon cycle; carbon dioxide (CO 2 ); oxygen isotope ratio; photosynthesis; respi- ration; stable isotope ratio; tropical forests. INTRODUCTION The annual increase in atmospheric carbon dioxide concentration varies substantially from year to year, despite relatively constant anthropogenic CO 2 emis- sions from fossil fuel burning (Marland et al. 1999). This has led to the hypothesis that environmentally induced variation in carbon dioxide exchange in ter- restrial ecosystems is largely responsible for the ob- served interannual variation in the rise of atmospheric CO 2 (Fung 2000). Measurement and analysis of the stable carbon isotope composition of atmospheric CO 2 Manuscript received 12 February 2003; revised 14 January 2004; accepted 15 January 2004; final version received 7 June 2004. Corresponding Editor: A. H. Goldstein. 4 Corresponding author. E-mail: ehleringer@biology.utah.edu also support large yearly variation in terrestrial net eco- system carbon uptake (Tans and White 1998). The mechanisms responsible for annual changes in terres- trial ecosystem CO 2 exchange are not well understood, but El Nin ˜o/La Nin ˜a events contribute substantially to yearly variation in temperature and precipitation with likely significant effects on ecosystem function in trop- ical regions (Tian et al. 1998, Behrenfeld et al. 2001). Strong interest has been expressed in using additional analyses of the concentration and stable isotope ratio of atmospheric carbon dioxide to infer changes in large- scale biosphere activity and to increase understanding of the controlling processes (Canadell et al. 2000). The oxygen isotope ratio ( 18 O) of atmospheric CO 2 is a powerful indicator of large-scale CO 2 exchange on land (Farquhar et al. 1993, Ciais et al. 1997a, b, Gillon