Effects of increased residue biomass under elevated CO<sub>2</sub> on carbon and nitrogen in soil aggregate size classes (rice-wheat rotation system, China)
Bibliographic record
Abstract
In order to study how soil carbon and nitrogen contents of different aggregate size fractions are affected by an increase in crop biomass and additional carbon inputs to soil due to elevated [CO2], a field experiment under a free-air CO2 enrichment (FACE) system was conducted. The experiment was set up with two CO2 levels [ambient CO2 and elevated CO2 (ambient + 200 ?mol mol-1)] and three N levels [low N (LN), 150 kg N ha-1 and 90 kg N ha-1, normal N (NN), 250 kg N ha-1 and 180 kg N ha-1, and high N (HN), 350 kg N ha-1 and 250 kg N ha-1, during the rice season and the wheat season, respectively] and straw was added at the same soil:biomass ratio as in the field during the rice and wheat seasons. Compared with ambient CO2, little change was observed in the percentage distribution of soil fractions, carbon and nitrogen content, and C:N ratio in each soil fraction under elevated CO2 from 2001 to 2003. However, after the soil was cultivated with straw at two CO2 levels, as a ratio of biomass to field area for 1 yr, elevated CO2 decreased the percentage distribution of the macroaggregate (> 250 µm) and microaggregate (53-250 µm) fractions by 28.9% (P < 0.01) and 27.8% (P < 0.01), respectively, and increased that of the clay- and silt-sized (< 53 µm) fraction by 38.2% (P < 0.01). Elevated CO2 increased the carbon concentration by 4-41% and increased the nitrogen concentration by 0-30% compared with ambient CO2, with the largest increases of 41.2% (P < 0.01) and 30.2% (P < 0.05), respectively, in the macroaggregate fraction with NN. Elevated CO2 decreased the contributions of soil carbon and nitrogen contents, respectively, in each fraction, to the whole soil by 13.1 and 17.2% in macroaggregates, by 12.9% (P < 0.05) and 16.9% (P < 0.01) in microaggregate, but increased them, on average, by 47.6% (P < 0.01) and 44.4% (P < 0.01), respectively, in the clay- and silt-sized fractions. The changes in soil C:N due to elevated CO2 were largest in the <53 µm fraction with LN and in the >250 µm fraction with NN and HN. These results suggest that adding straw is an important factor for soil structure and function with regard to soil carbon and nitrogen storage and cycling under FACE.Key words: Elevated CO2, nitrogen levels, aggregate turnover, carbon and nitrogen fractions, rice-wheat cropping, addition of straw
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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 | 0.000 |
| 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".