General comments

This preprint presents an application of complex electrical conductivity to estimate soil organic carbon (SOC) at greater depths, with the objective of supporting more realistic soil carbon stock assessments. The topic is timely and relevant, and the work has the potential to contribute to a better understanding of deep soil carbon pools, which are still poorly quantified. I agree with the authors that geophysical methods offer a valuable opportunity to collect quasi-continuous information about subsurface properties in a non-invasive manner, which represents a major advantage over conventional sampling approaches.

However, I would like to preface my review by noting that my expertise lies in the direct analysis of SOC in surface soils, and not in geophysical methods. Therefore, I am not able to fully evaluate the technical aspects and methodological robustness of the CC approach. My comments mainly concern the clarity and effectiveness of the scientific communication.

In its current form, the manuscript follows the structural requirements of BG Letters, but in my opinion the overall message would benefit from improved contextualization and clearer explanation of the methodological basis supporting the conclusions. In particular, the introduction does not clearly convey why quantifying SOC at depth is critically important for carbon budget assessments. Since this aspect appears to be central to the significance of the study, a more explicit rationale would strengthen the manuscript.

Moreover, while the authors state that their results demonstrate the ability of the CC method to map SOC pools at varying depths, the discussion does not convincingly communicate the robustness of the evidence supporting this claim, especially for readers unfamiliar with the technique.



Specific comments

1.    Importance of deep SOC

The introduction presents the commonly known limitations of estimating SOC in topsoil, but it remains unclear why mapping SOC in deep soils (>100 cm) is particularly relevant. The authors should explicitly state the implications of deep SOC for long-term carbon storage, carbon budget accounting, or soil management strategies. Clarifying this would help readers appreciate the relevance of the study.

2.    Description of the method and presentation of results

Although the methods are described in the appendix, as required by the BG Letters format, the manuscript would greatly benefit from a brief explanation in the main text summarizing how the experimental design allows testing whether the CC method is suitable for estimating SOC at depth. Providing a brief explanatory sentence or two in the main text about the rationale of the approach and how the measurements link to SOC estimates would significantly aid comprehension. This is especially important for a Letter format, where the main message must be quickly accessible.

The manuscript would also benefit from explicit references to the appendix where methodological details are provided. This would guide readers who wish to understand the technical basis of the results.

Considering the limited methodological explanation in the main text, the robustness of the results is not clearly communicated. The authors should better articulate how the presented data support the main conclusion that the CC method allows mapping SOC pools at depth. In addition, the manuscript would benefit from a more explicit discussion of potential limitations or constraints of the presented evidence, such as sources of uncertainty, methodological assumptions, or conditions under which the approach may be less reliable.

3.    Figure 3

Figure 3 is difficult to interpret for readers not familiar with the technique. Since it seems essential for supporting the reliability of the conclusions, improving its readability (e.g., clearer legends, explanations, or visual simplification) would significantly enhance the manuscript.



Technical corrections

•    Please write out all acronyms when first mentioned.

•    Line 14: “we show that an increase the polarization effect” → grammatical error.

•    Line 82: “with lower values ~20 mS/m)” → misplaced parenthesis.

•    Line 137: “our sample sis beyond” → typo (“sample is”?).

This manuscript presents an interesting application of complex electrical conductivity imaging to identify deep soil organic carbon stocks. The authors showed that the increase of polarization effect at low frequencies (< 5 Hz) is linearly related to high concentrations of SOC. They also measured a catchment with silty loam soils, where the geometry of a deep carbon stock (between 4 and 6 m depth) was identified by CC images, and validated through laboratory analysis of soil samples. The overall structure is clear, and the integration of geophysical and geochemical data is compelling. However, the manuscript would be improved by substantial clarification of the methodological details and data interpretation.

Major Comments

1. Insufficient explanation of the mechanisms linking σ′ and σ″, and their relationships with SOC and σ″. The introduction section did not adequately explain the theoretical background of σ′ and σ″. Each parameter represents what and how the two are related in the context of SOC interpretation. 

2. The conclusion that the polarizable anomaly “can only be explained by variations in SOC (Lines 119–120)” is too strong, given the range of possible contributors to σ″ in mineral soils. The frequency dependence of σ″ is central to the interpretation, yet the manuscript did not sufficiently justify why SOC-driven polarization should peak at ≤1 Hz, particularly in light of previous studies reporting variable frequency responses. Please provide quantitative or experimental evidence that rules out alternative explanations. I would also recommend providing more comparisons between your observed frequency patterns and those reported in previous studies, along with a clearer justification for why the low-frequency peak is diagnostic in this case.

3. Lack of quantitative uncertainty analysis for the inversion results and SOC estimates. Although the manuscript acknowledges the mismatch in sampling volumes, no quantitative uncertainty assessment is provided. Additional analysis of uncertainty or sensitivity is necessary, given that the interpretation relies on the subtle differences in σ″ between B1 and B2.

4. A number of typos were found in the text, so the language and grammar should be further checked throughout the manuscript. 

Minor Comments

Line 56: is “66 ha catchment” correct?

Lines 94–96: The authors note that CEC values are similar in B1 and B2, but they do not discuss the potential implications of this observation, particularly given that mineralogy plays a major role in SOC stabilization.

Line 134: Please confirm whether the correct spelling is “McAnalen” or “McAnallen”.

Line 137: “sample sis” seems to be a misspelling.

Line 147: “soil organic caron” should be “soil organic carbon.”

Line 229: Figure A2: The unit μS/m should be made consistent with the main text, where mS/m is used in the preceding sections.