A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Oxygen isotope ratio of CO 2 in forest and pastures ecosystems in the Amazon BasinOmetto, J.P.H.B. (1)(2) , Ehleringer, J.R. (2) , Martinelli, L.A. (1) , Domingues, T.F. (2) ,Flanagan, L. (3)(1) CENA/USP, Brasil(2) University of Utah, USA.(3) University of Lethbridge, CanadaThe Amazon Basin in South America represents the largest extent of tropical forest in theworld, with high species diversity and an estimate forest stock of carbon over 169Mg Cha -1 . Forest-to-pasture conversions and logging activities are expected to have an impacton the carbon balance within the Amazonian Basin, resulting in landscapes consisting ofprimary forest, logged forest, and pasture ecosystems. Stable isotope ratio analyses ofatmospheric CO 2 provide useful information regarding the balance betweenphotosynthetic carbon gain and respiratory carbon loss in each of these ecosystem types.The oxygen isotope ratio of the CO 2 emitted by the biosphere is entirely dependent on the18 O/ 16 O of the water associated to soil and plants within that ecosystem and on relativehumidity. Our studies over the past 2 years have shown that there was an enrichment on18 O of leaf water above source water in leaves from all ecosystems, with upper canopyleaves being more 18 O enriched than lower canopy leaves. A seasonal shift of 5 to 10 ‰has been observed between wet and dry seasons, which was not due to a change in thesource water but instead to changes in humidity. The leaf water 18 O enrichment can beaccurately modeled and a permanent record of this labile signal was reflected in the 18 Osignal of cellulose through the canopy profile. We have not identified a strong differencebetween the nighttime 18 O of respired CO 2 between adjacent forests and pastureecosystems, although daytime values are different between these ecosystem types.CENA/USPAv. Centenário, 303Piracicaba, SP, Brasil13416-970jpometto@cena.usp.br