the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Quantifying stocks in exchangeable base cations in permafrost: a reserve of nutrients about to thaw
Elisabeth Mauclet
Maëlle Villani
Arthur Monhonval
Catherine Hirst
Edward A. G. Schuur
Sophie Opfergelt
Abstract. Permafrost ecosystems are limited in nutrients for vegetation development and constrain the biological activity to the active layer. Upon Arctic warming, permafrost degradation exposes large amounts of soil organic carbon (SOC) to decomposition and minerals to weathering, but also releases organic and mineral soil material that may directly influence the soil exchange properties (cation exchange capacity and base saturation). The soil exchange properties are key for nutrient base cation supply (Ca2+, K+, Mg2+) for vegetation growth and development. In this study, we investigate the distribution of soil exchange properties within typical Arctic tundra permafrost soils at Eight Mile Lake (Interior Alaska, USA) because they will dictate the potential reservoir of newly thawed nutrients and thereby influence soil biological activity and vegetation nutrient sources. Our results highlight a difference in the SOC distribution within soil profiles according to the permafrost thaw. The poorly thawed permafrost soils (active layer thickness; ALT ≤ 60 cm) present more organic material in surface (i.e., organic layer thickness; OLT ≥ 40 cm) than the highly thawed permafrost soil (i.e., ALT > 60 cm and OLT < 40 cm). In turn, this difference in SOC distribution directly affects the soil exchange complex properties. However, the low bulk density of organic-rich soil layers leads to much lower CEC density in surface (~9 400 cmolc m-3) than in the mineral horizons of the active layer (~16 000 cmolc m-3) and in permafrost soil horizons (~12 000 cmolc m-3). As a result of the overall increase in CEC density with depth and the overall increase in base saturation with depth (from ~20 % in organic surface to 65 % in permafrost soil horizons), the average total stock in exchangeable base cations (Ca2+, K+, Mg2+ and Na+ in g m-3) is more than 2-times higher in the permafrost than in the active layer. More specifically, the stocks in base cations in the upper part of permafrost about to thaw in the following are ~ 860 g m-3 for Caexch, 45 g m-3 for Kexch, 200 g m-3 for Mgexch and 150 g m-3 for Naexch. This first order estimate is a needed step for future ecosystem prediction models to provide constraint on the size of the reservoir in exchangeable nutrients (Ca, K, Mg) about to thaw.
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Elisabeth Mauclet et al.
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RC1: 'Comment on essd-2022-240', Matthias Siewert, 21 Sep 2022
The manuscript by Mauclet et al undoubtedly presents an interesting data set on the vertical distribution of soil exchange properties in permafrost soils at a location in Alaska. The authors provide data on CEC, base saturation and other parameters for 7 soil profiles. This data is relevant and useful for the scientific community and provides a valuable reference point to understand the role of permafrost soils in the global carbon cycle. However, the manuscript needs to be thoroughly revised before being published.
1. I believe that the authors should separate between results and discussion. The jump between both and the comparison with the litertaure is not always clear and leading to confusion. In this case, it made it really hard to read the manuscript, filter out key messages and critically evaluate them. The most interesting and relevant discussion point is only in the last paragraph. A couple of things I would be more interested in are: Is there a statistical difference between the organic layer, active layer and upper permafrost (you have this partly) and what is the relevance in an arctic greening context. What nutrients do plants mine? Are there any trends. Further, how could hydrology be relevant for future changes in these systems.
2. The described correlation (L 206-219) between organic layer and active layer thickness are by no means news. The thermal isolating properties of the OL are well understood. I suggest to reduce this result to 1-2 sentences summarizing this and to remove Fig 2 and 3. This would also help to streamline the manuscript to focus on CEC.
3. The figures need to be reconsidered. Figure 2 and 3 are irrelevant from a scientific point of view, while figure 7,8 and 9 seem to essentially show the same data in 3 different ways.
Also: There are 9 points in Fig 2 but only 7 cores are mentioned in the method section! Are you sure that all results, e.g. L221-225, refer to the mentioned 7 profiles?4. The 7 presented soil profiles are sampled along a gradient that is not explained. There needs to be a better description of the sample location and discussion of the potential soil pedon variability due to pattern ground landforms or environmental gradients. Furthermore, I also would like to see a map of the spatial distribution of the profiles and some sort of indicator reflecting the mentioned gradient, maybe a satellite image. Turbic histic cryosols can show huge variability within a full soil pedon (sensu JL Ping) and the sample location could determine much of the variability of the patterns that you find in your exchange capacity results. This also means a throughout discussion of cryoturbation as a process and explanation for the seen pattern in exchange properties. I am also not sure of the separation between shallow active layer and thick active layer are relevant. It would be much better to express this in terms of soil type or along the mentioned gradient.
5. It is unclear if bulk density was measured or only interpolated from previous BD measurements? Please clarify this in L 128ff. If direct BD measurement is still possible on the samples, then this should be the preferred method and added to the manuscript. Then stocks of different parameters were measured, but it is unclear to which depth. Calculation of stocks should be done to a specific depth for all profiles. I suggest 1m. Otherwise you compare apples with oranges.
6. What was the average weight of the elemental analysis sample? Were loss on ignition measurements (LOI) performed on a larger sample to confirm the representativness of the elemental analysis sample? If not, then you need to discuss this.
7. I find the interpretation outlined in L213-219 relating the SOC distribution to permafrost degradation very speculative and not supported. How are these soils more degraded? I assume you don’t mean eroded or slope processes, but rather a thermal driven thickening of the active layer. Even differences in the thickness of the active layer may not imply a stronger degradation of the permafrost for these profiles relative to others. Most likely their thermal forcing is rather similar at the scale of the study area. Permafrost soils are highly variable and the seen changes may be observed within a less then a meter for any given soil profile for turbic soils. The difference may also reflect different soil development along the mentioned gradient (that was reduced to two soil types, motivation?). Both would by no means be related to permafrost degradation and even loss of SOC from the system. Most likely they rather reflect inter pedon variability or less SOC accumulation due to environmental gradients. If you have a different opinion on this, then I would expect a detailed chemical or structural analysis of this and a throughout explanation of the mechanisms. Again in L306, I dont think this premise of a significantly reduced organic layer due to thawing is supported by the data shown in the manuscript, the study area description or the cited literature. For instance, Schuur et al 2021 report a loss of 781.6 g C m−2 since the switch to a C source in 1990 and a cumulative projection of 4.18–10.00 kg C m−2 by 2100. Your differences in SOC for individual pedons surpasses potential loss as GHG within a century timescale.8. Language is overall very good, but would benefit from rephrasing a couple of sentences. e.g. L 25, 31,58-63, 291...
I hope the authors find these comments helpful and constructive. Iam looking forward to see the results eventually published as I believe that it is a relevant contribution to the field
//Matthias Siewert
Minor comments:L 14 Which arctic tundra soils are typical?
L16 - ‘poorly thawed’ – never seen this, please use a different expression. How about: permafrost affected soils with a shallow active layer and soils with a thick active layer?
L 19 - CEC is not defined in the abstract.
L24 To what depth did you count the permafrost for the stocks?
L 37 You should mention the word cryoturbation here.
L 62 Do you mean lateral in the soil as ground water or in streams?
L 71 I would argue that active layer thickness changes are fairly well understood and quantified.
L79 What do you mean by contrasted? I think you should be more specific here? Do you mean a range of ALT values, or two groups? What are the mean and the SD for these in cm?
L87 Which months represent the growing season?
L 88 Your profiles Mod2, Mod3 and EXT3 have less than 35 cm thick organic horizon. The results for SOC also indicate rather different soil types. What soil type are these then?
I assume you used the US soil taxonomy system?L101 What determined the sample location? How did you chose the gradient? How representative are these locations?
L 102 what are the ranges?
L 143 and 146 What do Min1, Mod3, EXT,… stand for?
Table S1, what does n stand for?
L145 Please motivate the profile selection.
L165 Please motivate the profile selection.
L180 Contrasted ALT – The two selected profiles have almost the same ALT 60 and 65 and are the deepest and shallowest in there respective ALT group. Maybe you could give another reason why you selected exactly these two profiles.
L200 This increase in SOC due to cryoturbation is typical for the bottom of the active layer and top of the permafrost section.
Fig 1. It is impossible to distinguish individual profiles. I suggest to differentiate each line by color, or make an average and min/max lines. The lines are also rather thin.
Fig 4 Any idea why Ca may behave different?
L236 By surface you mean organic? A suggest to statistically compare the organic layer, the mineral layer and the mineral permafrost layer. It would actually be interesting to see if there is a trend across all 7 profiles.
Fig 5 Again, it is hard to see any trends in this figure.
Are Fig 7, Fig 8 and Fig 9 essentially showing the same data?
Fig 9 Not sure if the figure is effective at communicating the message. What is the typical profile shallow or thick ALT?Citation: https://doi.org/10.5194/essd-2022-240-RC1 -
RC2: 'Comment on essd-2022-240', Anonymous Referee #2, 14 Dec 2022
The authors quantified the variability with depth of soil exchange complex properties (constituents, cation exchange capacity and base saturation), and the stocks in exchangeable cations in permafrost. The authors found that the CEC density, base saturation, and the average total stock in exchangeable base cations increased with depth. Overall, this dataset is valuable, and the paper is well written. I enjoyed reading this paper.
However, my main concern is that although the authors highlighted the importance of the potential reservoir of newly thawed nutrients to promote plant productivity, the permafrost thaw upon warming also increase the potential of the losses of these base cations through leaching. Thus, I think the authors should also highlight this point. Besides, I have some comments though I believe the authors should address in more detail in the results and discussion to make it clear. Please see the following detailed comments.
L21 Please have a brief explanation for the base saturation here.
L34–35 This sentence seems redundant in this paragraph. The authors may think about moving it to the place when talking about the transient layer in the section of results and discussion (L327).
L37–39 I think this sentence can be removed to the section of “Results and discussion” to support the results of increase in SOC content at depth (up to 30% OC; L199–200).
L37 Soil acidity sometimes is also one of the mechanisms that leads to high SOM accumulation although low temperature and O2 limitation are the main factors.
L48–50 This sentence influences the overall flow of this paragraph. Suggest removing it.
L115 Do you have any data or evidence to show there is no difference in pH by using 1:5 proportion vs. 1:15 proportion?
L148 Could you please provide the details on alkaline fusion method?
L155 Please explain the differences in measuring Ca and K in soil by using the non-destructive portable X-ray fluorescence device vs. ICP-OES. Which data did you report in the results?
Fig. 2 I was confused about the total number of the points in this figure. Total seven soil cores were sampled. But why were there more than seven points in this figure?
L214–215 It is unclear here. How can the difference in patterns of SOC distribution between the “organic-thick” and “organic-thin” permafrost soil profiles suggest a potential loss in C with permafrost degradation? Do you mean the organic-thin permafrost soil profiles have a more potential loss in C with permafrost degradation?
L283: Please explain why accumulation of Fe-oxides can increase potential CEC.
L315 Please consider change the subtitle as the following paragraphs did not mainly talk about the influence of total reserve in bases on the base saturation.
L316 Please explain base saturation here to help remind the readers.
L320–330 I was confused about the discussion on the overall increase in BS with depth. The authors first referred to another study showing larger concentration of exchangeable K and Ca within permafrost than active layer soils. However, I don’t think these results are consistent as the author used the BS in this study to compare with the exchangeable K and Ca in another study. Later, the authors stated that the rare thaw events have likely favored the leaching of the soil base cations of this layer. However, the thaw is less frequent (as mentioned), and then what the mechanism for the leaching was in this transient layers.
L340–341 Based on the above statement, it seems that the data only supported Al3+ and H+ in more acidic soil surface, but not the Ca2+, Mg2+, and K+ in more acidic soil surface unless the authors can show these data.
L342–344 Please explain this mechanism. Does the presence of exchangeable acid cations refer to Al3+ as KCl could extract more Al3+ from soil particles than water that caused the lower values of pHKCl than pHH2O?
L347 Again, it is worthy to mention here what the total reserve in base to help remind the readers, especially when reading a long paper.
L376–378 It seems that the difference in distribution of base cation stocks between the organic-think vs. organic-thin permafrost soils was due to the thickness of the organic layer. Is this right? Or what’s the explanation or implication for this difference?
L382–384 Although the permafrost thaw upon warming provides newly thawed pool of nutrient base cations, it also increases their potential of the losses via leaching.
L410–411 I don’t think there was enough evidence to support the argument that K is a plant limiting nutrient due to its higher stock in the organic surface horizons than in mineral soil horizons.
L417 How about the potential of loss for the exchangeable base cations upon permafrost thaw?
Citation: https://doi.org/10.5194/essd-2022-240-RC2 -
AC1: 'Author Comment on essd-2022-240', Maëlle Villani, 23 Jan 2023
We thank the reviewers for their careful reading and the constructive comments that will improve the quality and the readability of the manuscript. We are happy to apply revisions to improve our manuscript. Please, find the answers to the referee comments in the attached .pdf.
Maëlle Villani and co-authors
Elisabeth Mauclet et al.
Data sets
Characterization of permafrost soil exchange properties and quantification of stocks in soil exchangeable base cations Mauclet, Elisabeth; Villani, Maëlle; Monhonval, Arthur; Hirst, Catherine; Schuur, Edward A. G.; Opfergelt, Sophie https://doi.org/10.14428/DVN/FQVMEP
Elisabeth Mauclet et al.
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