the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2025
Subsoil particulate organic matter is more responsive to 10 years of whole–soil warming than mineral–associated organic matter in a temperate forest
Abstract. Global average temperatures are forecast to increase by 4 °C by 2100 under the Intergovernmental Panel on Climate Change SSP5–8.5 scenario. This warming could accelerate soil organic carbon (SOC) mineralization, net loss of soil carbon to the atmosphere, and consequently exacerbate global warming through a positive feedback loop. It is generally assumed that mineral–associated organic matter (MAOM) is less sensitive to warming compared to particulate organic matter (POM), especially in subsoil; yet more empirical data investigating the whole–soil response to warming is still required to test this assumption.
Our study was conducted in a whole–soil field warming experiment in a temperate mixed–conifer forest at Blodgett Forest Research Station, University of California, Berkeley, which had been subjected to 10 years of warming. Soils taken at three depths (10–20, 40–50, and 80–90 cm) were separated into three density fractions, and we then investigated the SOC concentration (elemental analysis) and composition of bulk soil and fractions with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. We found that a decade of experimental warming shifted subsoil bulk SOC composition towards lignin and C–H aromatic bonds. warmed plots had significantly lower mass of the POM fractions relative to control plots in the subsoil (80–90 cm), but there was no difference in the topsoil, which could occur if higher decomposition losses of POM were obscured by fresh plant inputs. In contrast, the mass of MAOM and its chemical composition was not different between warmed and control treatments, but did shift along the depth gradient. This study thus supports the assumption that POM is more responsive to warming than MAOM, which was particularly evident in the subsoil at Blodgett Forest.
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Status: open (until 02 Jan 2026)
- RC1: 'Comment on egusphere-2025-5483', Shang Wang, 16 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-5483', Anonymous Referee #2, 19 Dec 2025
reply
General Comments
This article reports the effects of ten years of experimental warming on soil carbon fractions at three depths from Blodgett Forest. Subsoil (80-90 cm depth) particulate organic matter (POM) mass was reduced and bulk soil carbon shifted composition toward more recalcitrant compounds (lignin and aromatic bonds), while mineral-associated organic matter (MAOM) remained unchanged, indicating that POM is more responsive to warming than MAOM.
The study has important implications for impacts of global change on soil carbon cycling, and represents a novel contribution to the field by reporting specific changes in carbon compound and organic matter composition with sustained warming.
However, there is a concerning over-interpretation of statistically non-significant results. The confidence intervals reveal massive uncertainty, which should preclude strong conclusions about subsoil carbon loss at specific depths. In addition, the reported marginally significant results are hard to justify within the confidence intervals that span a huge range of negative to positive values. At the very least, these parts of the results section should be reworded to reflect the high uncertainty instead of the supposedly “marginal significance.” The statistical power of the comparisons is weak with n=3 samples throughout, and there were no reported corrections for the multiple comparisons made.
There is weak evidence for the bulk SOC composition shifts (Section 3.3, Fig. 3), which are based largely on visual PCA inspection. There are no formal statistical tests for whether warmed vs. control groups differ significantly in composition.
There are a few sentences in the results section that start with the word “basically,” which detracts from the meaning of the sentence. Recommend removing or rephrasing.
Throughout the discussion section, my suggestion is to separate the pattern description from statistical inference.
The authors do a good job of putting the results from their study in the context of previous studies.
Throughout the manuscript, the authors explain topsoil patterns by inferring warming-induced increases in plant inputs (e.g., lines 377-384, 432-436, 485-487). However, plant inputs were not measured in this study and the Ofiti et al. (2021) study they reference reported decreased root biomass at this site under warming, contradicting the proposed mechanism. Alternative explanations (moisture effects, substrate limitation, redistribution) receive insufficient consideration and the Abstract and key sections present this inference as established rather than hypothetical. I recommend revising to: (a) clearly identify increased inputs as a hypothesis rather than observation, (b) address the contradiction with Ofiti et al.'s root data, (c) discuss alternative mechanisms with appropriate weight, and (d) acknowledge this limitation explicitly. Consider tempering mechanistic conclusions in the Abstract and Conclusions to reflect this uncertainty.
Overall, the claim that MAOM remained stable under warming, across depths, is well-supported. The claim that POM is more responsive than MAOM to warming is somewhat supported, with a consistent pattern but mixed statistical significance. The claim that subsoil carbon loss is driven by POM depletion is overstated based on the evidence presented. While the bulk SOC interaction is significant, the individual subsoil depth reductions are not significant and the confidence intervals include substantial gains, not just losses. The claim that bulk SOC shifted toward lignin/aromatic bonds in warmed subsoils is also not well-supported, since it is based on visual interpretation of PCA only and Fig 3 shows trends but overlapping confidence ellipses.
Specific Comments
Were there changes in soil moisture with warming and by depth? This is important for the discussion of decomposition dynamics.
L257-258: This could be more clearly worded to indicate that it is the interaction effect that is significant. Perhaps “Warming reduced SOC concentration in the subsoil but not the topsoil (warming × depth interaction: p = 0.002).”
L259-262: Reporting these statistics as marginal effects may be inaccurate, since the confidence interval is very large and the p-value close to non-significant. Recommend rephrasing to reflect the high uncertainty instead. Within that confidence interval, the effect could be anything from a huge loss to a moderate gain.
L271: Awkward/unclear wording “affected distinguishably”
L271-273: Again, the large confidence intervals detract from the significance that is claimed. A non-significant increase could actually be a decrease, increase, or major increase, but the directionality of it is highly uncertain.
L292-294: Unclear what is meant by “main effects” in this sentence.
L314-318: "warmed subsoils displayed a trend towards increased AUC values" – this is descriptive, not statistically validated
L334: Specify which PCA plot is being referenced.
L348-349: Reword for clarity.
DSI analysis (Section 3.5) shows no significant warming effect for bulk soil despite claims.
L355-361: Any mention of significance or non-significance should be accompanied by a p-value or p-value range.
L382: If the relationship wasn’t significant, it should not be interpretated as though it was. It is also unclear whether the non-significance is related to just one or more of those relationships listed.
L434: There is nothing in the code/data availability section. However, the authors are commended on their inclusion of significant amounts of data in the supplemental.
Technical Revisions
L26: period instead of comma or else missing capitalization
L375: Significant not significantly
L396: Subsoil not subsoils
Citation: https://doi.org/10.5194/egusphere-2025-5483-RC2
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Dear editor and authors, thank you for your invitation and sorry for the delay comments.
This manuscript presents an interesting investigation of deep soil carbon stability under warming conditions, based on the well-established field experiment at Blodgett Forest. The study is valuable, and the experimental design and methodological approach are generally sound. However, several aspects require substantial improvement before the manuscript can be considered further.
In particular, data quality should be carefully re-evaluated, as large standard errors are observed for several parameters. In addition, the figures and tables need to be reorganized, and statistical analysis results should be clearly indicated or labeled in figures and tables. The English language requires further polishing, and the citation format in the main text should be carefully checked and revised, as the current format occasionally impedes readability and comprehension.
Please see my detailed comments below.
General Comments
Introduction
The Introduction would benefit from additional background on SOC functional groups, as these constitute one of the main parameters measured in this study. Moreover, free light POM and occluded POM are not sufficiently introduced and should be described more clearly.
Results
This section is generally difficult to follow. First, many results are described primarily based on tables and figures presented in the Supplementary Material rather than in the main text. The authors should consider integrating key results into the main text or consolidating them into clearer, more informative figures.
Second, numerous non-significant results are reported, which is not necessary and detracts from the main findings.
In addition, I strongly recommend conducting ANOVA followed by appropriate post hoc tests for parameters across different soil depths, and indicating statistically significant differences using uppercase or lowercase letters in figures and tables.
Discussion
The Discussion requires further improvement. The authors should focus more strongly on their own results and novel findings, rather than extensively mixing their interpretations with results from other studies, particularly those conducted at the same site. This approach currently makes the narrative confusing and diminishes the perceived novelty of the study.
Specific Comments