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The oxygen isotope ratio (delta(18)O, SMOW) of atmospheric CO, is a powerful indicator of large-scale CO, exchange on land. Oxygen isotopic exchange between CO, and water in leaves and soils controls the delta(18)O of atmospheric CO, Currently there is little empirical information on the spatial and temporal variation in the delta(18)O of leaf and stem water in tropical ecosystems. We measured the seasonal dynamics of delta(18)O in atmospheric CO, 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 Santarem). Within regions, the source (stem) water delta(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 delta(18)O value of leaf water was positively correlated with leaf height above ground because of associated variation in vapor pressure deficit and the delta(18)O of atmospheric water vapor within forest canopies. Consistent with these observations, the delta(18)O value of leaf cellulose was positively correlated with forest height. Leaf water from pasture grasses was more 180 enriched than leaf water from forest vegetation. There was a tendency for daytime leaf water to be more enriched in 180 during the dry season, reflecting generally lower humidity conditions during the dry season. Nighttime measurements of the oxygen isotope ratio of ecosystem respired CO2 in both forest and pasture vegetation were not consistent with values expected for CO2 in equilibrium with stem (soil) water, despite nighttime vapor pressure deficits close to. zero. Apparently, the delta(18)O of leaf water lagged and did not attain isotopic equilibrium at night. Thus, the deviation of nighttime delta(18)O values of ecosystem respiration from that expected from a CO2 efflux in equilibrium with soil (stem) water increased as delta(18)O values of ecosystem respiration became 180 enriched. Discrimination against CO2 containing 110 (DeltaC(18)OO) during photosynthesis was calculated based on measured leaf water delta(18)O values. Forests had consistently higher modeled DeltaC(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 110 in atmospheric CO2 over the Amazon Basin of Brazil

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