1. The Amazon Basin in South America represents the largest extent of tropical forest in the world, with high species diversity and an estimate forest stock of carbon over 169Mg C ha -1. Forest-to-pasture conversions and logging activities are expected to have an impact on the carbon balance within the Amazonian Basin, resulting in landscapes consisting of primary forest, logged forest, and pasture ecosystems. Stable isotope ratio analyses of atmospheric CO 2 provide useful information regarding the balance between photosynthetic carbon gain and respiratory carbon loss in each of these ecosystem types Oxygen isotope ratio of CO 2 in forest and pastures ecosystems in the Amazon Basin Jean Pierre Ometto 1,2, James Ehleringer 2, Luiz Martinelli 1,Tomas Domingues 2, Larry Flanagan 3 C3C3 C4C4 The enrichment of leaf water in the canopy profile was identified at the forests sites. Seasonal changes in leaf water enrichment was not due to a change in the source water but instead to changes in humidity. ftp://cptec.inpe.gov.br Leaf water profile in Santarém and Manaus forest canopies Cellulose 18 O enrichment through the canopy height Measurements of the 18 O/ 16 O ratio of CO 2 can provide a constraint on the gross (one-way) exchange of CO 2 between the atmosphere and the terrestrial biosphere. Carbon dioxide exchanges oxygen with water when it is dissolved in liquid water. This exchange is very rapid in the presence of carbonic anhydrase, such as in the leaves of plants when the stomates are open during photosynthesis. Isotopic exchange also occurs in soils while the respired CO 2 diffuses toward the atmosphere. The water in leaves tends to be isotopically enriched with respect to the source water of the plants because isotopically light water (H 16 O 16 O) evaporates slightly faster than heavy water (H 18 O 16 O). Therefore, CO 2 that has been in contact with leaves will be isotopically heavier than respired CO 2. This distinction offers the potential to unravel one-way exchanges of CO 2 with plant canopies over large spatial scales.  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 Leaf water precipitationsoil waterxylem water Becomes enriched in 18 O during transpiration No fractionation Humidity (VPD) has a significant impact on leaf water 18 O values The leaf water 18 O enrichment can be accurately modeled and a permanent record of this labile signal was reflected in the 18 O signal of cellulose through the canopy profile Identifying different respiratory signals is important to the regional isotopic balance of the CO 2. We did not find a strong difference between the nighttime 18 O of respired CO 2 between adjacent forests and pasture ecosystems, although daytime values are different among these ecosystem types and the open river water. (1) Cena / USP, Brazil (2) University of Utah, USA (3) University of Lethbridge, Canada (a) jpometto@cena.usp.br The “Keeling Plot” technique was used to identify the oxygen isotope signature of the respired CO 2