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CD-02 Abstract

Carbon and Oxygen Isotope Ratio CO2 Flux Analyses at the Soil, Canopy, and Landscape Scales

James Ehleringer — University of Utah (US-PI)
Luiz Antonio Martinelli — CENA - Centro de Energia Nuclear na Agricultura (USP) (SA-PI)


We propose studies in carbon storage and exchange. We propose to work in disturbed and

undisturbed sites along a C3/C4 gradient (savanna to rainforest).

Project Overview

We propose to measure stable isotope ratio measurements of atmospheric CO2

to infer changes in terrestrial ecosystem photosynthesis and respiration (sink or source

strength) on local and regional scales. These measurements will be made in conjunction

with soil respiration rates and collaborative studies with eddy flux tower studies. Within

terrestrial ecosystems, photosynthesis and respiration have different and contrasting

effects on the carbon and oxygen isotope ratio of atmospheric CO2. Monitoring

shifts in both the concentration and stable isotope ratio of atmospheric CO2

can be used as a tool to study large scale terrestrial ecosystem gas exchange processes,

but the linkages of ecosystem and regional isotope ratio signals have not been attempted

in the past. Regional and global interpretation of the stable isotope signal recorded by

the atmosphere requires an understanding of fractionation processes that occur during

ecosystem gas exchange processes. While we have gained a mechanistic understanding of

leaf-level isotope effects during photosynthesis and respiration, we need to extend and

apply our knowledge of isotope effects to the ecosystem- and regional-level in order to

constrain models so that calculations of large scale ecosystem gas exchange processes can

be made with confidence.

We have developed a nested sampling scheme for measuring isotope effects associated

with photosynthesis and respiration at three spatial scales: (i) individual chamber at the

soil surface; (ii) canopy tower for canopy and/or ecosystem level; (iii) aircraft sampling

of changes in the convective boundary layer for the regional level. Our research plan will

allow us to determine the impacts of land-use changes (conversion of C3 forest to C4

pasture grasses) to regional productivity because the C3 and C4 plants have very different

isotope effects during photosynthesis. Interannual variability associated with El Niño/

La Niña events should also have substantial effects on the timing and magnitude of

ecosystem gas exchange processes and associated isotope effects. Direct measurements of

the effects of El Niño/ La Niña events on isotope effects during photosynthesis and

respiration are needed for future large scale isotope modeling studies.

Proposed Measurements

The proposed study will make several key LBA-ECO measurements at each of the three

primary regions along the east Amazônia transect between Brasília and Manaus, including:

  • soil respiration rates

  • carbon isotope ratio analyses of soil organic matter

  • carbon and oxygen isotope ratio analyses of leaf material

  • carbon and oxygen isotope ratios of CO2 within the soil

  • carbon and oxygen isotope ratios of CO2 released from the soil surface

  • carbon and oxygen isotope ratios of total ecosystem respiration

  • carbon and oxygen isotope ratios of ecosystem photosynthetic discrimination

  • carbon and oxygen isotope ratios of landscape-level respiration

  • carbon and oxygen isotope ratios of landscape-level photosynthetic discrimination

  • carbon and oxygen isotope ratios and profiles of CO2 within the convective

    boundary layer

The measurements are proposed to be collected 2-4 times per year, depending on

parameter complexity and other team-project considerations. Our studies will be conducted

in close association with simultaneous eddy covariance measurements, made by other

researchers, in order to obtain information about ecosystem photosynthesis, respiration

and turbulent exchange. The leaf- level, canopy- level, and regional-level models of

isotope fractionation can be used to determine the effect of different terrestrial

ecosystems on the carbon and oxygen isotope ratio of atmospheric CO2, because

of differences in photosynthetic pathway, ecosystem composition, and environmental

influences. The information gathered using our isotope studies would help in determining

the role of the Amazon Basin as a net sink or source of CO2 to the atmosphere.

Preferred Sites

Our interest is in spanning from seasonally dry cerrado ecosystems (Brasília) to

seasonal rain forests (Pará) and through aseasonal rainforest ecosystems (Manaus).

Primary research sites in each of three major areas along this geographic transect will be

chosen in (a) primary rainforest or cerrado, (b) pastures derived from primary forest

conversion, and (c) secondary growth forest. This provides us with 9 different sites with

which to examine ecosystem and land-use changes (canopy and soil levels) and with 3

regions across Amazônia with which to examine landscape processes. Strong isotope ratio

gradients exist at each of the major sites that should allow us to differentiate carbon

turnover and respiration sources. Clear C3/C4 differences in species composition exist

between forest and pasture species along this entire gradient. Virtually all of the

pasture species (native and introduced) have C4 photosynthesis and all of the forest

species have C3 photosynthesis. A potentially strong C3 - C4 gradient exists as well in

the transition between tropical rainforest and cerrado ecosystems.


This proposal provides a strong education component, which includes training Brazilian

graduate and postgraduate scientists. We plan to provide lecture/laboratory opportunities

for Brazilians to participate in the Utah Stable Isotope Ecology course, as well as

exploring development of a similar course in Brazil. There is also the opportunity for

other LBA investigators to receive stable isotope ratio training at SIRFER.

Last Updated: May 18, 1998

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