Isotopic composition and concentration of total nitrogen and nitrate in xylem sap under near steady‐state hydroponics
Bibliographic record
Abstract
Abstract After root uptake, nitrate is effluxed back to the medium, assimilated locally, or translocated to shoots. Rooted black cottonwood (Populus trichocarpa) scions were supplied with a NO3−‐based (0.5 mM) nutrient medium of known isotopic composition (δ15N), and xylem sap was collected by pressure bombing. To establish a sampling protocol, sap was collected from lower and upper stem sections at 0.1–0.2 MPa above the balancing pressure, and after increasing the pressure by a further 0.5 MPa. Xylem sap from upper stem sections was partially diluted at higher pressure. Further analysis was restricted to sap obtained from intact shoots at low pressure. Total‐, NO3−‐N and, by difference, organic‐N concentrations ranged from 6.1–11.0, 1.2–2.4, and 4.6–9.4 mM, while discrimination relative to the nutrient medium was −6.3 to 0.5‰, −23.3 to −11.5‰ and − 1.3 to 4.9‰, respectively. There was diurnal variation in δ15N of total‐ and organic‐N, but not NO3−. The difference in δ15N between xylem NO3− and organic‐N suggests that discrimination by nitrate reductase is near 25.1 ± 1.6‰. When this value was used in an isotope mass balance model, the predicted xylem sap NO3−‐N to total‐N ratio closely matched direct measurement.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| 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 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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 source (direct Gemma or distilled Codex), 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".