the amazon basin in south america represents the largest extent of tropical forest in the world,...
TRANSCRIPT
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 CO2 provide useful
information regarding the balance between photosynthetic carbon gain
and respiratory carbon loss in each of these ecosystem types
Oxygen isotope ratio of CO2 in forest and pastures ecosystems in the Amazon Basin
Jean Pierre Ometto1,2 , James Ehleringer 2, Luiz Martinelli 1,Tomas Domingues 2, Larry Flanagan 3
C3
C4
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 18O enrichment through the canopy height
Measurements of the 18O/16O ratio of CO2 can provide a constraint on the
gross (one-way) exchange of CO2 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 CO2
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 (H16O16O) evaporates slightly faster than heavy water (H18O16O).
Therefore, CO2 that has been in contact with leaves will be isotopically heavier
than respired CO2. This distinction offers the potential to unravel one-way
exchanges of CO2 with plant canopies over large spatial scales.
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
Leaf water
precipitation soil water xylem water
Becomesenriched in18O during
transpiration
No fractionation
Humidity (VPD) has a significant impact on leaf water 18O values
The leaf water 18O enrichment can be accurately
modeled and a permanent record of this labile signal
was reflected in the 18O signal of cellulose through the
canopy profile
Identifying different respiratory signals is important to the regional isotopic balance of the CO2. We did not find a
strong difference between the nighttime 18O of respired CO2
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)[email protected]
The “Keeling Plot” technique was used to identify the oxygen isotope signature of the respired CO2