the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Feb 2025
High biodegradability of water-soluble organic carbon in soils at the southern margin of the boreal forest
Abstract. Water-soluble organic carbon (WSOC) is an important component of the organic carbon pool in boreal ecosystems. However, the biodegradability of WSOC across various soil depths in boreal ecosystems remains unclear. Here, based on spectroscopic techniques, we conducted a 28-day laboratory incubation to analyze the molecular composition, biodegradability, and compositional changes of WSOC at different soil depths in a southern region of the boreal forest. The results showed that in the upper 2 m soils, the average content of biodegradable WSOC was 0.228 g/kg with an average proportion of 86.41 % in the total WSOC. In the deep soils below 2 m, the average content of biodegradable WSOC content was 0.144 g/kg, comprising 80.79 % of the total WSOC. Spectroscopic analysis indicates that the WSOC in the upper soils is primarily composed of highly aromatic humic acid-like matter with larger molecular weights than those in deep soils. Both the aromaticity and molecular weight decrease with depth, and the WSOC is mainly composed of fulvic acid-like matter in the deep soils, suggesting high biodegradability of WSOC in the deep soils. Overall, our results suggest that the water-soluble organic carbon in the boreal forests exhibits high biodegradability both in the shallow layer and deep soils.
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RC1: 'Comment on egusphere-2025-126', Anonymous Referee #1, 31 May 2025
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General Comments
This manuscript investigates the content, molecular composition, and biodegradability of water-soluble organic carbon (WSOC) across a 7.4 m soil profile at the southern boundary of the boreal forest. The authors employ a combination of spectroscopic analyses and laboratory incubations to evaluate depth-resolved changes in WSOC properties and their relation to biodegradability. The study is timely and relevant given the sensitivity of boreal carbon stocks to climate change and permafrost degradation. It contributes valuable data on deep soil carbon dynamics, particularly from a region (southern margin of the boreal forest in Northeast China) that is underrepresented in current literature.
The manuscript is generally well-structured and clearly written, with a sound experimental design and appropriate data analysis. However, a few aspects require clarification or improvement to enhance the manuscript's scientific rigor and presentation. These include a better contextualization of the study’s novelty, clearer methodological justifications, and refinement of certain interpretations to avoid potential overgeneralization.
Specific Comments
Novelty and Contribution:
The manuscript would benefit from a clearer articulation of how this study advances current understanding beyond existing work (e.g., studies from Alaska, Siberia). The novelty lies in deep-profile in situ WSOC characterization at the southern boreal margin—this should be emphasized more explicitly in both the abstract and introduction.
Methodological Justification:
The use of nutrient amendments (NH₄NO₃ and K₂HPO₄) in biodegradability assays should be more critically discussed in the Methods and Discussion sections. While this standardizes microbial activity, it may inflate BWSOC estimates compared to natural field conditions.
The use of extracted microbial inoculum per soil layer could introduce variability due to differential microbial biomass and viability. Were microbial abundance or composition controlled or measured?
Depth Resolution:
The authors group certain depths for discussion (e.g., “upper 2 m” vs. “below 2 m”), but patterns are often non-linear across depths. Consider incorporating a more nuanced, depth-wise interpretation where appropriate (especially for Fig. 6 and Table 2).
The high BWSOC% at 60–180 cm is striking—please clarify if this is a consistent finding or potentially due to sampling or incubation artifacts.
Spectroscopic Interpretation:
The authors use SUVA254 and E250/E365 as proxies for aromaticity and molecular weight, respectively. This is acceptable, but readers would benefit from a brief discussion on their limitations and the role of fluorescence indices (e.g., FI, HIX) that were not used.
Data Interpretation:
The discussion of negative correlations between BWSOC and environmental factors like NH₄⁺, NO₃⁻, and EC is somewhat speculative. The proposed mechanism (e.g., nutrient suppression of microbial degradation) should be more cautiously framed unless further supported.
Clarify whether BWSOC and degradation constants (k) are significantly correlated with SUVA254/E250:E365 using regression statistics.
Conclusions:
The final statements about climate-driven SOC loss extrapolate from WSOC data. It would be more appropriate to caution that WSOC is a proxy for labile SOC, but not equivalent to total SOC vulnerability under field conditions.
Technical Corrections
Abstract: Define "BWSOC" at first use (L16).
L60–61: "in situ conditions" – Specify if this refers to field conditions vs. extracted WSOC.
L194: "formulas ... in Supporting Information" – Ensure these are provided.
Table 2: Units: "g/kg" should be "g kg⁻¹" for consistency; "%" should be "%.
Fig. 2: Axis labels are garbled (e.g., "8-10cm" vs. "0-10cm"). Correct depth labels and ensure variables are clearly defined in the caption.
Fig. 6: Y-axis label "BWSOC (g/kg)" → "BWSOC (g kg⁻¹)".
L263–264: Incomplete sentence ("resulted in very...").
Citation: https://doi.org/10.5194/egusphere-2025-126-RC1
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