The information about carbon cycle over Amazonian ecosystems are particularly important to identify terrestrial sinks of atmospheric CO 2, overall due to the large extension of still undisturbed primary tropical forest in this area. Discussions of the effectiveness and the extend of a suggested forest carbon uptake accumulation has placed a interrogation dot on this issue, is this system really growing at the proposed rates? An important input to this question is the replacement of forested areas by pastureland, first by transferring the carbon stocks from the forests directly to the atmosphere and second by changing an important mechanism of atmospheric CO 2 recycling. Several techniques are being used to predict and model the carbon cycle in terrestrial ecosystems, but complementary tools are necessary to better define sources and sinks. The ecosystems conversions in the tropics from forest (mainly C3 plants) to pasture (C4 gramineae) can produce effects on the physical characteristic of the atmospheric CO 2 due to different metabolism of C3 and C4 reflected on the respired CO 2 by these ecosystems. We have been using isotope techniques to characterize the respiration signal of the carbon dioxide at Brazilian National Primary Forest (FLONA), and in a pasture at Santarém, PA, Brazil, officials LBA sites. Oxygen Isotope ratio of respired CO2 in Amazon Forest and Pasture Ecosystems Jean Ometto 1,a, Marcelo Moreira 1, Tomas Domingues 2, Luiz Martinelli 1, James Ehleringer 2, Larry Flanagan 3 Modeling Isotope Composition The discrimination associated to the photosynthetic gas exchange (ª A ) in forest and pasture ecosystem can be calculated using Farquhar and Loyd (1993) model. During the photosynthetic gas exchange two major processes influence the isotope fractionation against the C 18 O 16 O. The first process is diffusion, when the heavier CO 2 molecules containing 18 O diffuse in a slower ratio than lighter molecules containing only 16 O. On the second processes, a portion of the CO 2 that enter into the leaf and equilibrate with chloroplast water is not fixed into carbohydrate by the photosynthetic pathway and diffuses back to the atmosphere with an altered oxygen isotope ratio. The dissolution of the CO 2 in the chloroplast water is catalyzed by the enzyme carbon anhydrase (CA) which enhance the hydration of CO 2 and dehydration of the HCO 3 -. Thus the leaf temperature, the oxygen isotope ratio of the chloroplast water and the fractionation associate to the CO 2 diffusion will determine the isotope ratio of the CO 2 leaving the leaf (Flanagan, 1998). The ecosystem respired CO 2 has a 18 O signal related to the leaf tissue water oxygen isotope ratio from when the carbohydrate was fixed, mixed with the CO 2 released by the soil surface. ril (1) Cena / USP, Brazil (2) University of Utah, USA (3) University of Lethbridge, Canada (a) C3C3  18 O wl leaf water 18 O enrichment CO 2  18 O wa  18 O wl = R wl /R SMOW -1  18 O wl organic matter CO 2 diffusing out of leaf C4C4 C 3 and C 4 plants contribute different  C 18 O 16 O signals Discrimination against CO 2 containing 18 O In the tropics the conversion from forest to pasture is predicted to increase C 18 O 16 O discrimination, which in theory, reduces the likelihood that the C3 forest to C4 pasture conversion are contributing to the decreasing global trend in  18 O of atmospheric CO 2. Predicted  18 O LW and ∆C 18 O 16 O values for forests and pastures in Amazonia  18 O LW ∆C 18 O 16 O CA eq. C 3 forest-5.6 ‰2.8 ‰100 % C 4 grassland+2.3 ‰6.7 ‰ 38 % Smaller difference between  18 O of stem and leaves for February 2000, showing a lower isotopic enrichment of leaf water, wich is in accordance with a more enriched  13 C and  18 O from ecosystem respiration, suggesting a water excess stress (La Niña year). ftp://ecophys.biology.utah.edu