OXYGEN ISOTOPE RATIOS OF WATERS AND RESPIRED CO <sub>2</sub> IN AMAZONIAN FOREST AND PASTURE ECOSYSTEMS
Bibliographic record
Abstract
The oxygen isotope ratio (δ18O, SMOW) of atmospheric CO2 is a powerful indicator of large-scale CO2 exchange on land. Oxygen isotopic exchange between CO2 and water in leaves and soils controls the δ18O of atmospheric CO2. Currently there is little empirical information on the spatial and temporal variation in the δ18O of leaf and stem water in tropical ecosystems. We measured the seasonal dynamics of δ18O in atmospheric CO2 and water in different ecosystem compartments in both primary forest and pasture ecosystems in three different regions of the Amazonian Basin of Brazil (Ji-Paraná, Manaus, and Santarém). Within regions, the source (stem) water δ18O values for primary forests and pastures were similar; neither vegetation type exhibited distinct wet–dry season patterns. Daytime leaf water isotope ratios were strongly correlated with predictions of the Craig-Gordon model. The δ18O value of leaf water was positively correlated with leaf height above ground because of associated variation in vapor pressure deficit and the δ18O of atmospheric water vapor within forest canopies. Consistent with these observations, the δ18O value of leaf cellulose was positively correlated with forest height. Leaf water from pasture grasses was more 18O enriched than leaf water from forest vegetation. There was a tendency for daytime leaf water to be more enriched in 18O during the dry season, reflecting generally lower humidity conditions during the dry season. Nighttime measurements of the oxygen isotope ratio of ecosystem respired CO2 in both forest and pasture vegetation were not consistent with values expected for CO2 in equilibrium with stem (soil) water, despite nighttime vapor pressure deficits close to zero. Apparently, the δ18O of leaf water lagged and did not attain isotopic equilibrium at night. Thus, the deviation of nighttime δ18O values of ecosystem respiration from that expected from a CO2 efflux in equilibrium with soil (stem) water increased as δ18O values of ecosystem respiration became 18O enriched. Discrimination against CO2 containing 18O (ΔC18OO) during photosynthesis was calculated based on measured leaf water δ18O values. Forests had consistently higher modeled ΔC18OO values than pastures. The daytime isotope effects we calculate for photosynthesis and respiration were consistent with previous model predictions of a strong depletion of 18O in atmospheric CO2 over the Amazon Basin of Brazil.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".