the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Sep 2025
Imaging the deep carbon stocks with complex electrical conductivity
Abstract. Due to the limited penetration depth of standard sampling methods, mapping of soil organic carbon (SOC) is usually restricted to the top 100 cm of soils. Hence, current models underestimate SOC due to the unexplored deep carbon stocks. Moreover, standard methods only offer punctual data relying on interpolation to investigate extensive areas. We demonstrate here that subsurface images of the complex electrical conductivity (CC) can delineate the presence and geometry of SOC reaching a depth of a few tens of meters below the surface. We demonstrate a positive correlation between imaginary CC at low frequencies (< 5 Hz) and SOC content permitting a quantitative interpretation of geophysical images. We present measurements from 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.
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Status: open (extended)
- RC1: 'Comment on egusphere-2025-4015', Anonymous Referee #1, 26 Nov 2025 reply
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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”?).