Experimental nitrogen addition alters structure and function of a boreal bog: critical load and thresholds revealed
Why this work is in the frame
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Bibliographic record
Abstract
Abstract Bogs and fens cover 6% and 21%, respectively, of the 140,329 km 2 Oil Sands Administrative Area in northern Alberta. Development of the oil sands has led to increasing atmospheric N deposition, with values as high as 17 kg N·ha −1 ·yr −1 ; regional background deposition is <2 kg N·ha −1 ·yr −1 . Bogs, being ombrotrophic, may be especially susceptible to increasing N deposition. To examine responses to N deposition, over five years, we experimentally applied N (as NH 4 NO 3 ) to a bog near Mariana Lake, Alberta, unaffected by oil sands activities, at rates of 0, 5, 10, 15, 20, and 25 kg N·ha −1 ·yr −1 , plus controls (no water or N addition). Increasing N addition: (1) stimulated N 2 fixation at deposition <3.1 kg N·ha −1 ·yr −1 , and progressively inhibited N 2 fixation as N deposition increased above this level; (2) had no effect on Sphagnum fuscum net primary production ( NPP ) in years 1, 2, and 4, but inhibited S. fuscum NPP in years 3 and 5; (3) stimulated dominant shrub and Picea mariana NPP ; (4) led to increased root biomass and production; (5) changed Sphagnum species relative abundance (decrease in S. fuscum , increase in S. magellanicum , no effect on S. angustifolium ); (6) led to increasing abundance of Rhododendron groenlandicum and Andromeda polifolia , and to vascular plants in general; (7) led to increasing shrub leaf N concentrations in Andromeda polifolia , Chamaedaphne calyculata , Vaccinium oxycoccos , V. vitis‐idaea , and Picea mariana ; (8) stimulated cellulose decomposition, with no effect on S. fuscum peat or mixed vascular plant litter decomposition; (9) had no effect on net N mineralization rates or on porewater NH 4 + ‐N, NO 3 − ‐N, or DON concentrations; and (10) had minimal effects on peat microbial community composition. Increasing experimental N addition led to a switch from new N being taken up primarily by Sphagnum to being taken up primarily by shrubs. As shrub growth and cover increase, Sphagnum abundance and NPP decrease. Because inhibition of N 2 fixation by increasing N deposition plays a key role in bog structural and functional responses, we recommend a N deposition critical load of 3 kg N·ha −1 ·yr −1 for northern Alberta bogs.
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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) | 0.003 | 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 it