the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Feb 2025
Warming accelerates the decomposition of root biomass in a temperate forest only in topsoil but not in subsoil
Abstract. Global warming could potentially increase the decomposition rate of soil organic matter (SOM), not only in the topsoil (< 20 cm) but also in the subsoil (> 20 cm). Despite its low carbon content, subsoil holds on average nearly as much SOM as topsoil across various ecosystems. However, significant uncertainties remain regarding the impact of warming on SOM decomposition in subsoil, particularly root-derived carbon, which serves as the primary organic input at these horizons. In the Blodgett Forest warming experiment (California, USA), we investigated whether warming accelerates the decomposition of root-litter at three depths (10–14, 45–49, and 85–89 cm) by using molecular markers and in-situ incubation of 13C-labelled root-litter at each depth. Our results reveal that the decomposition of added root-litter was only accelerated in the topsoil (10–14 cm) but not in the subsoil (45–49 and 85–89 cm) with warming. In subsoil, although the decomposition rate of root-litter derived carbon did not differ significantly between ambient and warmed plots, the underlying reasons for this similarity are distinct. With molecular marker analysis, we found higher microbial activity, indicated by higher concentration of certain fatty acid monomers that could be originally microbial-derived such as octadecanoic acid (C18:0 fatty acids), octadecenoic acid (C18:1 fatty acids), and hexadecanoic acid (C16:0 fatty acids) than those originally derived from roots in ambient subsoil. With warming, the higher concentration of long-chain (C number > 20) π-hydroxy acids and diacids left after 3 years of root incubation suggested a lower turnover rate and this could be due to lower microbial abundance and lower soil moisture induced by warming. Our study demonstrates that the impact of warming on the decomposition of root-litter in a temperate forest is depth-dependent. The slower turnover rate of long-chain π-hydroxy acids and diacids shows that they are more persistent compared to bulk root mass and could be preserved in subsoil for longer time as long as the environmental conditions are unfavorable for decomposition with warming.
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Status: open (until 20 Mar 2025)
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RC1: 'Comment on egusphere-2025-299', Anonymous Referee #1, 07 Mar 2025
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General Comments:
1. This study investigates the effects of whole soil warming on the decomposition of root litter by depth in the soil profile in Blodgett Experimental Forest. The authors found, after 3 years of +4 degree C warming, distinct depth-dependent decomposition of labeled root litter where warming accelerated the topsoil root litter, but not the subsoil root litter. This is an interesting and novel contribution to the field.Β
2. The introduction and results are clear and well-structured, but the discussion is very long.Β
3. Please acknowledge the limitations in study design regarding the large spatial heterogeneity of soil properties (plus microbial community differences and conditions at depth) at this site and the low sample size of n=3. C inputs (amount and type) and the effects of warming are expected to differ by depth in the soil profile, yet the study design does not account for this. Since there was increasing variability in microbial communities with depth, this should be more directly addressed since the main conclusion of this work is the depth-dependent responses.Β
4. The use of a single root litter type from an annual grass is also a limitation, as this coniferous forest can be expected to have root contributions from fine roots of conifers which should have different chemical composition like lignin and lipids. Please justify the use of wild oat roots instead of conifer roots.
5. If the natural temperature gradient was maintained, there may still be artifacts of the heating coil that are not accounted for, like differentials in soil drying by depth and consequent influence on decomposition dynamics at different depths. There could also be differences in microbial community distribution in close proximity to the heating coils vs. further away.
6. Measuring after 3 years does not capture short-term priming effects, which would be expected more immediately than 3 years later. Please take this into account when addressing priming in the results and discussion sections. For example, the statement on L439, βThus, positive priming occurred,β from my perspective, cannot be so definitive.Β
7. Regarding statistical analysis and model selection, LME and AIC were used to assess the best fit models but it is not directly stated which models were compared. This could be added in a supplementary section. It is also unclear whether the depth, temperature, and their interaction were modeled as fixed or random effects in all cases.
8. The study suggests that microbial activity was higher in ambient subsoil compared to warmed subsoil, based on the accumulation of mid-length fatty acids, but if microbial activity was higher in ambient conditions, one would expect greater decomposition of SOM and added root material. Instead, results suggest greater root-litter preservation in ambient plots. Perhaps there are alternative explanations for the accumulation of fatty acids, such as selective preservation, microbial necromass accumulation, or sorption to mineral surfaces.
9. While bulk root-litter decomposition was not significantly different between warmed and ambient plots, lipid composition changed, and fatty acid accumulation occurred under ambient conditions. The authors state that subsoil decomposition was unaffected by warming, but this contradicts their molecular marker results showing that microbial metabolism and decomposition pathways did shift.
Specific Comments:Β
10. L76-77: Unclear what is meant by βharnessing rootsβ
11. L113-114: It is unclear if soil depths were heated the same amount or not from this sentence. I recommend explaining what the natural temperature gradient is rather than refer to another paper.Β
12. L121-146: The coring system, excavation and root additions are a little confusing. For each depth, was a soil core extracted, then the labelled roots added to the hole, then the soil replaced for that depth? Or was the soil inside the core, plus the core itself, left in the hole for the duration of the experiment?
13. L148-150: Why were those specific depths chosen? Is it because of the known rooting depths of conifers in that forest? If so, this should be described in the site description section at the top of methods.
14. L222: Specify what is meant by region-specific in this context.
15. L322: The figure text is very small and hard to read. Instead of separating the ambient and warmed graphs, it would be better to have the sets of bars next to each other for direct comparison (e.g., ambient and warmed 10-14 cm, ambient and warmed 45-49, etc.).Β
16. L339: Line about the error bars is not needed in the text since itβs in the figure captions.
17. L345: Was this difference statistically significant? Specify either way.Β
18. L467-468: For clarity, instead of βThis argues for co-metabolic decomposition of the added root litter,β βthis indicatesβ¦β
19. L642-644: What is meant by βthe warming was heterogeneousβ in this sentence?Β
Technical Corrections:Β
20. L46: missing word: ββ¦biotic factors THAT could changeβ¦β
21. L50: grammatical errors: βMoreover, roots impact on SOM dynamics in subsoil in two way:βΒ
22. L50-52: What is meant by βThey are more likely to form stable SOM to aboveground plant biomassβ?Β
23. L54-55: Grammar revisions needed.
24. L68-69: βBesidesβ is an awkward way to start a sentence.
25. L 70: grammatical errors
26. L71: Missing the word βtheβ
27. L96-97: Revise second hypothesis for clarity and maintain consistency in tense used. Relative accumulation to what?
28. L176: Write out the word dichloromethane for clarity and consistency with other acronyms.
29. L188: Remove extra space after min
Citation: https://doi.org/10.5194/egusphere-2025-299-RC1
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