Consequences and recovery after nutrient enrichment in the boreal forest understory
Bibliographic record
Abstract
Atmospheric nitrogen deposition poses a serious threat to plant communities globally. Furthermore, nitrogen-induced shifts in plant community composition may create positive feedbacks via litter decomposition by the soil microbial community. These feedbacks could prevent the recovery of plant communities, even in the absence of further nutrient addition, because nutrient availability and cycling remain high. We investigated the role of nutrients and herbivores in regulating plant and soil microbial communities in the boreal forest understory in northwestern Canada. We used an experiment that began in 1990 where plots were fertilized, exclosed (herbivore reduction), or both, in a 2 x 2 factorial design. In early 2000, plots were divided in half; treatments were continued on one half, and discontinued on the other half. In 2009, we sampled plant community composition, along with carbon/nitrogen ratios and total phenolics in the plant tissue. Using phospholipid fatty-acid analyses and extracellular enzyme activity assays, we estimated the soil microbial community composition and activity. Lastly, we measured soil pH and chemistry. Overall, fertilization had significant impacts on the variables we measured; herbivore exclosures mostly had no detectable impacts. In fertilized plots, species richness declined and the plant community became dominated by Epilobium angustifolium and Mertensia paniculata. Total phenolics and the carbon/nitrogen in the plant tissue declined. The total microbial biomass declined, as did the ratio of fungi to bacteria, indicating a more bacteria-dominated food web in the soil. Extracellular enzymes involved in the breakdown of cellulose increased in activity, but those involved in the acquisition of nitrogen and phosphorus were mostly unaffected, except urease, which declined in activity. Soil pH declined significantly, and fertilizer increased the availability of many nutrients. In recovery plots, the results do not fit the predictions of a plant-soil feedback hypothesis. Instead, the system follows a “cascade of responses”, where soil chemistry recovers first, then plant tissue chemistry, followed more slowly by plant community composition. After 10 years, the soil microbial community has yet to show significant signs of recovery. These results highlight the sensitivity of the boreal forest to nutrient enrichment and demonstrate that recovery of these ecosystems may take decades.
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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 | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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".