the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Mixed signals: interpreting mixing patterns of different soil bioturbation processes through luminescence and numerical modelling
Abstract. Soil bioturbation plays a key role in soil functions such as carbon and nutrient cycling. Despite its importance, fundamental knowledge on how different organisms and processes impact the rates and patterns of soil mixing during bioturbation is lacking. However, this information is essential for understanding the effects of bioturbation in present-day soil functions and on long-term soil evolution.
Luminescence, a light-sensitive mineral property, serves as a valuable tracer for soil bioturbation. The luminescence signal resets (bleaches) when a soil particle is exposed to daylight at the soil surface and accumulates when the particle is buried in the soil, acting as a proxy for subsurface residence times. In this study, we compiled three luminescence-based datasets of soil mixing by different biota and compared them to numerical simulations of bioturbation using the soil-landscape evolution model ChronoLorica. The goal was to understand how different mixing processes affect depth profiles of luminescence-based metrics, such as the modal age, width of the age distributions and the fraction of bleached particles.
We focus on two main bioturbation processes: mounding (advective transport of soil material to the surface) and subsurface mixing (diffusive subsurface transport). Each process has a distinct effect on the luminescence metrics, which we summarized in a conceptual diagram to help with qualitative interpretation of luminescence-based depth profiles. A first attempt to derive quantitative information from luminescence datasets through model calibration showed promising results, but also highlighted gaps in data that must be addressed before accurate, quantitative estimates of bioturbation rates and processes are possible.
The new numerical formulations of bioturbation, which are provided in an accompanying modelling tool, provide new possibilities for calibration and more accurate simulation of the processes in soil function and soil evolution models.
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RC1: 'Comment on egusphere-2024-1466', Shannon Mahan, 05 Jul 2024
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GENERAL COMMENTS:
The authors have done a commendable job is taking a system that is very complex and reducing it understandable and concise sections with their ChronoLorica modeling. I could not find much to quibble about with the equations and modeling. Sometimes I was unclear about a parameter or decision but overall in the paper I found the murkiness resolved itself upon further reading. The following comments are meant to help refine small points I found need either editing, additional clarification, or deleting.
Overall the conclusions are justified given the discussion and often confusing scatter in the measurements and model data. I look forward to the next round that includes ants and one site with both quartz and feldspar.
SPECIFIC COMMENTS:
There is an over-use of the word “fundamental in the first paragraph of the introduction. I think that the word “fundamental” is perhaps not quite the correct word to describe what is lacking about our knowledge of soil processes or the way the organisms bioturbate. We actually know a lot about the fundamental details of soil (see the endless journals devoted to it such as Soils Systems, European Journal of Soil Science, Soil Advances, etc.) we just don’t know the depth per dominate organism or the depth is highly regional. We also don’t whether mounding or subsurface dominates at any given depth (some being obvious in the top 10 cm). I think the words you are looking for are a “simple tracer" to measure depth of organism effect on soil processes. You actually say it in lines 65. Rewrite to indicate the “fundamental" question is not about soil processes but the depth and type of processes per a known biological agent.
Table 1. I find it interesting that you used quartz for the termites and feldspar for the worms and ants. Was there no situation which you could also have tested the quartz and feldspar at one condition (I personally would have chosen the worms that had both mounding and subsurface mix)? I understand that you are limited by the source geology but there must have been one site/condition you could have tested both? I’m sure I don’t need to iterate to the authors that knowing the bleaching rates of both minerals in one condition site would be extremely useful.
Lines 129-133. Please define soil particles or luminescence particles by some weight or dimension. At this point in the paper the reader has no idea if we are talking cm and mg or cm and g. Just give some idea of the size and weight if possible. Are they really sand sized particles?
Figure 2. In caption or elsewhere clarify that bioturbation from termites, worms, and ants are definitely not limited to the first meter and indeed in termites’ case may be several meters. Obviously, you had to limit the depth for your model but make it clear you do understand the bioturbation processes vary in depth. Justify why you picked 1m or 1.5 m or 2 m.
Line 175. I thought the model depth was 1 meter not 2 meters? Figure 4 shows 1.5 m depth. I have some confusion about the depth of model and measurements. Were measurements made for an entire 2 m to represent both mounding and subsurface? I missed where the mounding ends and subsurface begins. Line 184 indicates a 1 m depth.
I found the descriptions of the model parameters in the text to be actually quite tedious and wished I had a table to refer to that I could quickly see everything described between lines 175-185. In fact such a table would have been more useful than the current Table 2 which I never looked at again. I did look at the model parameters a couple of times to remind myself and flipping back to text was….tedious. Maybe you could put it in this table the depths of the model, subsurface, and mound depths. Just we are all clear and on the same page. Maybe also set up along the lines of what you describe in lines 350-355.TECHNICAL CORRECTIONS:
Wording is awkward here on lines 45-50 “The luminescence signal accumulates over time due to ionizing radiation emitted from radionuclides of elements within the uranium and thorium decay chains, as well as potassium-40, which are present in the soil, and due to cosmic rays”. It makes it sound like cosmic rays control everything. It could be better stated as “The luminescence signal accumulates over time due to ionizing radiation emitted from radionuclides of elements which are present within the soil. The uranium and thorium decay chains, potassium-40, and cosmic rays all contribute varying amounts to luminescence growth in minerals”.
Line 177. Two periods. Delete one.Citation: https://doi.org/10.5194/egusphere-2024-1466-RC1 -
CC1: 'Comment on egusphere-2024-1466', Hao Long, 09 Jul 2024
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Publisher’s note: the content of this comment was removed on 11 July 2024 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/egusphere-2024-1466-CC1 -
RC2: 'Comment on egusphere-2024-1466', Hao Long, 11 Jul 2024
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General Comments:
This paper aims to differentiate the impacts of various mixing processes, specifically mounding and subsurface mixing, on soil profiles. The authors have compiled three luminescence-based datasets that illustrate soil mixing by different biota and compared them to numerical simulations of bioturbation using ChronoLorica, a soil-landscape evolution model. The research topic is interesting, and the methods and results demonstrate a considerable level of credibility. The paper is well-organized, with a clear logical progression and concise language. It is undoubtedly deserving of publication; however, some modifications are necessary before it can be published.
For lines:
Line 146: Based on the expression of the formula (Eq. 3-5), it seems to me that BTpot represents something akin to the maximum potential disturbance rate, while BT(z) represents the potential disturbance rate at each depth. If that’s the case, it should be clarified in the text.
Line 175: What does "one-dimensional soil profiles" mean? Based on the discussion about “two to three-dimensional settings” in section 4.2, I presume that "one-dimensional" refers to considering only vertical movement of particles in the soil profile. Is that correct? Anyway, adding a sentence or two to clarify this would improve understanding.
Line 177: There is an extra period at the end of the sentence. Please remove one of them.
Line 178: The simulations were conducted using loess-like soil texture. For the simulation itself, such simplification is, of course, reasonable. However, for the calibration as discussed in section 4.3, it might be worth considering simulating it using the soil composition that closely matches the referenced profiles. This modification might also help explain a portion of the deviation observed between the simulated and expected results.
Line 194: How is the "modal age" calculated? Is it similar to the Central Age Model (CAM)? Providing more details on this would be helpful.
Line 205-206: The statement "with a larger contribution ... decrease." compares the overall characteristics among the three profiles. However, the interquartile ranges and the bioturbated fractions vary significantly within each profile, and there is some overlap in the data range among different profiles. This general statement is therefore inaccurate and confusing. It is recommended to modify or completely remove this sentence.
Line 230: It is unclear where the “increase in scatter” is reflected in the data. As the text (L195) states that the interquartile range reflects the width of the distribution, I suppose it’s also a parameter capable of describing the “scatter in the age distribution”. However, as the depth increases, the interquartile range curves for all functions shift to the left (towards younger ages, Figure 4B), and it’s also mentioned in L232-234 that "interquartile ranges… generally decrease down the profile.". Therefore, I am confused what does the “increase in scatter” exactly mean? More explanation is need here.
Line 253: The description of Figure 6A is inaccurate. The statement "termites... show lower modes of ages compared to worms..." is only evident in the lower part of the graph (e.g., the range from 0.6 to 1 on the y-axis). However, in the upper part of the profile, the mode ages of termites are older or comparable to those of worms. It is mentioned in the text that the results of ants were not used for comparison due to calibration issues. Actually, the comparison between termites and worms is also questionable due to significant differences in the age used for calibration.
Line 435: Delete one of the redundant "leading to".
Regarding sections and equations:
Section 4.1: The discussion on the presence of other types of bioturbations is commendable. To strengthen this point, it is suggested to incorporate a bit more information about the geographical and vegetation environment of the referenced luminescence profiles, followed by further discussions on whether or not the three luminescence profiles used in the study are influenced by uprooting or other factors.
More explanations should be made for Eq.6 and 7.
Concerning the soil mixing tracer, I would like to suggest you refer to a recent work on mollisols from China (Zhang et al., 2024, Reconstructing Mollisol Formation Processes Through Quantified Pedoturbation, GRL; https://doi.org/10.1029/2024GL108189)
Regarding graphs and tables:
I noticed a discrepancy in the labeling of figures. The labels in the figures were marked with lowercase letters (a, b, c), while uppercase letters (A, B, C) were used in the captions and references in the text. Please make sure to check the journal requirements and consider unifying the labeling format accordingly.
Figure 6c, please check the x-axis title.
Table 1, please check the spelling of the luminescence method used for ants “IR50IRIRSLe”.
Citation: https://doi.org/10.5194/egusphere-2024-1466-RC2
Model code and software
Mixed Signals W. Marijn van der Meij https://github.com/MarijnvanderMeij/Mixed-signals_Bioturbation
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