The information about carbon cycle over Amazonian ecosystems are particularly

important to identify terrestrial sinks of atmospheric CO2, 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 CO2 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 CO2 due to different metabolism of C3 and C4 reflected on

the respired CO2 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 Ometto1,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) 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 C18O16O. The first process is diffusion, when the heavier CO2

molecules containing 18O diffuse in a slower ratio than lighter molecules containing only

16O . On the second processes, a portion of the CO2 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 CO2 in the chloroplast water is catalyzed by the enzyme carbon anhydrase (CA) which

enhance the hydration of CO2 and dehydration of the HCO3-. Thus the leaf temperature, the

oxygen isotope ratio of the chloroplast water and the fractionation associate to the CO2

diffusion will determine the isotope ratio of the CO2 leaving the leaf (Flanagan, 1998).

The ecosystem respired CO2 has a 18O signal related to the leaf tissue water oxygen isotope

ratio from when the carbohydrate was fixed, mixed with the CO2 released by the soil

surface.

r il

(1) Cena / USP, Brazil(2) University of Utah, USA

(3) University of Lethbridge, Canada

(a)ometto@biology.utah.edu

C3

18Owl

leaf water18O enrichment

CO2 18Owa

Rwl * kRwx

ei ebei

kbRwx

eb eaei

Ra

eaei

18Owl = Rwl/RSMOW -1

18Owl

organicmatter

CO2 diffusingout of leaf

C4

.

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12108642 121086421999 2000

Time, Month of Year

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Ecosystem Respiration

Stem Water

C3 and C4 plants contribute different C18O16O signals

Discrimination against CO2 containing 18O

In the tropics the conversion from forest to pasture is predicted to increase C18O16O discrimination, which in theory, reduces the likelihood that the C3 forest to C4 pasture conversion are contributing to the decreasing global trend in 18O of atmospheric CO2 .

Predicted 18OLW and ∆C18O16O valuesfor forests and pastures in Amazonia

18OLW ∆C18O16O CA eq.

C3 forest -5.6 ‰ 2.8 ‰ 100 %

C4 grassland +2.3 ‰ 6.7 ‰ 38 %

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18O

PD

B

CO2 SW

CO2 LW

Smaller difference between 18O of

stem and leaves for February 2000,

showing a lower isotopic enrichment

of leaf water, wich is in accordance

with a more enriched 13C and 18O

from ecosystem respiration, suggesting

a water excess stress (La Niña year).

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1999 2000 Normal

ftp://ecophys.biology.utah.edu

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