the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Feb 2025
Soils signal key mechanisms driving greater protection of organic carbon under aspen compared to spruce forests in a North American montane ecosystem
Abstract. Soil organic carbon (SOC) is often retained more effectively in aspen-dominated forests compared to coniferous forests in North America, yet the reasons why are unclear. A potential driver could be differences in SOC protection mechanisms. Over decades to centuries, chemical (e.g., mineral association) and physical (e.g., aggregation) processes can work to preserve SOC stocks, which can vary across cover types. To investigate this hypothesis, we evaluate controls on SOC concentrations in the Coal Creek watershed (CO, USA), a montane ecosystem dominated by quaking aspen and Engelmann spruce and underlain by granite and sandstone. We examined a combination of biological, chemical, physical, and environmental conditions to evaluate potential abiotic and biotic mechanisms of SOC preservation at multiple depths. As expected, we observed greater SOC under aspen compared to spruce. Growing season soil moisture, temperature, and CO2 and O2 varied with slope position and aspect, and thus forest cover type. Dissolved organic carbon (DOC) was lower under aspen compared to spruce. Exo-enzyme data indicate that aspen soil microbes exhibited greater effort to seek organically-bound resources; consistent with this, soil organic N exhibited higher δ15N values, hinting at a greater degree of organic matter processing. Finally, aspen roots exhibited greater root abundance, and aspen mineral soils revealed smaller mean aggregate diameters compared to conifer sites. Our data suggest enhanced biotic activities in aspen-dominated forest soils that promote both chemical and physical protection of SOC in aspen- relative to spruce-dominated forests, and associated limitations on DOC export.
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RC1: 'Comment on egusphere-2025-70', Anonymous Referee #1, 07 Mar 2025
General comments
The presented manuscript deals with the question why there is generally more organic C (SOC) in soils under aspen than under spruce trees in North America. The strength of this work is that its methods are quite comprehensive (and described with enough details), which makes it possible to assess complex interactions. A relative weakness of the study is the limited number of sites and especially the fact that aspen and spruce sites have (naturally) different positions in the landscape, which makes it impossible to extract the "pure" effect of the tree species. The authors are perfectly honest in acknowledging this fact and the discussion is written accordingly. The main conclusions are therefore not so much based on direct proofs but rather on clusters of indicators.
The article is generally well written (for a reviewer using English as a third language). There are, however, several long, sometimes complicated sentences that would deserve a simplification (see details below).
The bibliography list is extremely long. Could it be shortened without affecting the understandability of the text?
Details
L. 44: more SOC under aspen than under spruce: is this per soil mass or per area? Or both?
L. 49: repeating "root" not necessary.
L. 66: "SOC regulates" may be misunderstood as SOC being the sole factor regulating these properties.
L. 102: the effect of coarse roots on aggregates is certainly small simply because there are by far less coarse than fine roots.
L. 142: the word "tandem" is mostly used for a two-fold combination, i.e. its use here is not wrong but a bit surprising.
L. 180: there are several shapes, therefore rather "shapes represent" (plural). The whole legend of this figure is quite long.
L. 193–194: it's not clear what "average yearly minimum and maximum" are. From the wording itself, it should be taking each year the minimal/maximal recorded temperature and then averaging over years. The given range is, however, very narrow for that, even more for a continental climate. Are these perhaps minimum and maximum monthly averages, then averaged over years?
L. 226: how many coring locations are there within these 100 m?
L. 235: I would always encourage to refer to concentrations by using the word concentrations, and not indirectly by using the measurement unit like %N or %C. One could actually also express the same parameter in e.g. g/kg. Note also that the C:N ratio is not "measured" but calculated.
L. 242: it would be better to specify from the beginning on that this is extractable DOC (by opposition to a DOC concentration that would be measured by lysimetry in the field).
L. 256: prefer the SI unit Hz (s-1) to rpm.
L. 258 ff.: long sentence.
L. 271 ff.: as nitrate in soils is subject to strong variations with time, please specify the vegetation and hydrology conditions at sampling time.
L. 316: consider writing "oven-dry" with an hyphen.
L. 318: "2-4.76": an en dash should be used here instead of an hyphen (even if one could argue that this is the job of the typesetter and not of the authors...)
L. 331: how does ImageJ recognise roots? This is not trivial at all.
L. 346: use the full word "minutes" or the abbreviation "min", but not "mins".
L. 352 ff: this calibration procedure is not clear. Transforming ppm to % should be just dividing by 10000. This works in all cases: if these ratios are for moles, for volume or for partial pressure.
L. 393: be a bit more specific here than just writing "more variable". The text should essentially be understandable also without accessing the supplementary material.
L. 445: "similar range across depth" is not really clear to me as the ranges vary with depth according to the previous sentence, and the value given here is per area, i.e. apparently cumulated over all soil depths.
L. 451 ff: the text goes from the topic "DOC vs. SOC" to a second topic "additive vs. interaction" then back to the first topic and finally again to the second one. Consider rearranging this.
L. 469–471: complicated sentence.
L. 476: this range of C:N values goes from extremely low to quite high. Any comment about such a low value?
Fig. 9: the increases in CO2 should be expected to relate to the decreases in O2 by a relatively constant respiratory quotient. This is not the case here. Is the respiratory quotient really so different or are there other processes, or some measurement errors?
L. 539: it does not make much sense for me to compare aspen with granite. Or do you mean spruce?
Fig. 10: the aggregate size classes seem not to add to 1, i.e. there is also a non-aggregated fraction. It would be at least as interesting to know the proportion of non-aggregated soil than to give the proportions of the size classes.
L. 564: the word "further" is not really wrong as the verb "suggests" follows the verb "indicate" on L. 559. However, the sentence is so long that it comes quite in a surprising manner here.
L. 608 ff.: long sentence. This is a bit counter-intuitive as larger aggregates would be expected to split into small aggregates and thus be at least as well able to protect DOC than such small aggregates. Does this deserve any comment?
L. 648: the word "confounded" would be good if this became a problem for a regression analysis. Here it is just about describing the situation and the word "related" would probably be better.
L. 670 ff.: long sentence.
L. 723: I don't understand the use of the word "import" in this context.
Citation: https://doi.org/10.5194/egusphere-2025-70-RC1 -
RC2: 'Comment on egusphere-2025-70', Anonymous Referee #2, 13 Mar 2025
The authors present a comprehensive experimental investigation of the mechanisms driving higher SOC stocks under aspen compared to spruce land cover in the Coal Creek Watershed, Colorado, USA. They examine a range of biological (e.g., microbial biomass, enzymes, roots), chemical (e.g., pH, CEC), and physical (e.g., texture, aggregate size distribution) factors influencing SOC persistence across the two land cover types. Their findings suggest that increased microbial activity, coupled with enhanced mechanisms protecting SOC transformation products, contributes to greater SOC stocks under aspen. This study provides valuable insights into local-scale controls on SOC persistence and offers a rich dataset integrating biological, chemical, and physical factors, which can support future research, including modeling efforts. My comments are mostly minor, focusing on clarifications and suggestions for future investigations.
I believe the sites are only discussed in the caption of Figure 1. I would include a brief mention of the sites and reference again to Figure 1 at the end of section 2.
The introduction frequently alternates between MAOC and SOC, sometimes implying they are interchangeable. The authors note that a large fraction of SOC is composed of MAOC in aspen (lines 74–77), but is there data on the extent to which MAOC dominates the SOC pool in aspen forests? What about in spruce?
It is unclear how DOC loss was estimated or inferred at the two sites. Were volumetric water content (VWC) and DOC profiles used to calculate DOC leaching, or was some form of water and DOC balance applied? If leveraging VWC data, measurements of the hydraulic conductivity curve could be highly informative.
The analysis of aggregate sizes and SOC is particularly interesting but requires further elaboration. The authors attribute greater SOC stability in aspen soils to their smaller aggregate sizes. However, soil aggregation generally enhances SOC persistence by forming physical barriers against decomposition. Wouldn’t larger aggregates offer greater protection? Additionally, in field conditions, smaller aggregates may either exist independently or be nested within larger aggregates in a hierarchical structure. How might this organization affect the role of aggregation in SOC protection?
Finally, the interpretation of Figure 11 requires caution, as the presented ratio may be misleading without information on SOC distribution across different size fractions. A useful approach for future studies would be to measure SOC within each size fraction. I believe this would provide the information and understanding of the patterns Figure 11 aims to illustrate.
Citation: https://doi.org/10.5194/egusphere-2025-70-RC2
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