| 24 Jun 2026
Cryogenic Fractionation and Thaw-Gradient Reorganization of Carbon, Nutrient and Trace Element Pools in Permafrost Peatlands
Abstract. Permafrost peatlands are highly vulnerable to climate warming because active layer deepening can mobilize previously frozen solutes into suprapermafrost flow paths and regional hydrological networks. Yet the composition of the frozen porewater reservoir, i.e., pore ice, and its role in regulating C, nutrient and trace-element release during thaw, remain poorly constrained. Here, we characterized dissolved organic matter (DOM), nutrients, and major and trace solutes across the active layer–permafrost interface in a continuous (> 90 % ice) polygonal peatland of the subarctic tundra. Four microtopes representing a thaw gradient (convex polygon → concave polygon→ peat transitional fen → peat-mineral fen) were sampled, with porewater and pore ice (< 0.45 µm) analyzed in active and frozen layers. Pore ice DOM showed a predominantly microbial and aliphatic signature, with elevated dissolved organic carbon (DOC) concentrations, indicating selective preservation of microbially processed, low-aromatic compounds during freezing. Freeze–thaw cycling homogenized active layer porewater chemistry, whereas pore ice retained stronger site-specific geochemical signatures. Overall, DOC, dissolved N and P, and exchangeable cations (Ca, Mn, Ba, Sr) preferentially accumulated in pore ice, while lithogenic elements (Zr, Ga, Hf, Ge, Nb, Cr, Y and REE) were more concentrated in active layer porewaters. Frozen horizons also exhibited lower C:N:P ratios than active layers, highlighting nutrient-enriched stoichiometry in the permafrost compartment. A coupled stoichiometric and decay-rate model suggests that phosphorus is largely consumed in situ during progressive thaw, thereby constraining further carbon and nitrogen processing, whereas substantial fractions of DOC and dissolved N remain available for lateral export. Viewed as a space-for-time thaw sequence, the transition from polygonal bog to fen indicates that permafrost degradation may initially enhance mobilization of labile carbon and nutrients before hydrological redistribution dominates. These results identify pore ice as both a cryogenic archive of past biogeochemical processing and a reactive reservoir capable of amplifying Arctic carbon–nutrient fluxes under continued warming.
Overview
In this manuscript, the authors analyzed porewater chemistry in high-depth resolution along a permafrost thaw gradient in Siberia, targeting both active layer porewaters and pore ice. They found that there are substantial differences in DOC concentration and composition, as well as nutrient pools, in the permafrost layers compared to the active layers. Consumption and potential export rates of nutrients were modeled in order to evaluate the effect of active layer deepening in the future.
In general, I find the results presented in this manuscript very interesting and valuable for the field. The applied methods are described clearly and the figures are in general well understandable. However, the discussion of the results is sometimes not very focused and could use some revisions for clarity. I have some general questions and specific comments for revising the manuscript and I hope that these are helpful to the authors. As most of my questions can be addressed with textual changes but likely require some time, I recommend major revisions.
General comments and questions
Specific comments
L16: The use of “because” in that sentence is not logical. Permafrost peatlands are vulnerable to climate warming which can lead to active layer deepening, etc. I suggest to rearrange that sentence.
L39: Is the amount of C stock that is mentioned relating to the amount of released C from permafrost peatlands or the amount of sequestered C? The first one would be more logical, but in the current way in which the sentence is written, it is not clear. I suggest to rearrange that sentence.
L80: It is not clear to me what the “controls on pore ice formation” would be. It sounds like the authors would aim to study how easily ice forms in the different microhabitats. I suggest to clarify this aim.
L116: Was this whole procedure done directly on-site or after transport to some kind of lab?
L240: The use of the word “contribution” implies that the fraction of organic acids to total DOC are displayed in the figure. However, the data is given in absolute concentrations of mg /L. The authors could consider either changing the wording here or referring to another figure in which the contribution is visible, or simply summarizing the contribution in the text.
L251: What are carboxylic compounds in this case and how were they measured? In case the authors mean organic acids, as shown in Fig S2, then I would stick with that term and not introduce a new one.
L335f: The authors interpret the higher variation of frozen compared to active layer samples in the PCA as a greater variability in pore ice chemistry. However, in the next sentence an example is given that Tz-4 frozen samples show minimal variation along PC1 which shows lower variation within this site. Is that not a contradiction to the previous sentence?
L358f: I do not necessarily agree with the statement that the different chemical signatures between active layer porewater and frozen pore ice are due to freeze-thaw cycles. The successive freezing and thawing should only affect the active layer and not the permanently frozen compartments?
L361: Which compounds do you mean with “certain compound peaks”?
L409: include “potentially chemically labile DOM pool” since this discussion is based only on indicator variables and not on the direct DOM composition or a direct measure of DOM mineralization
L412f: It is not directly clear how is this reference about the amplification of FTC on microbial N turnover is connected to the results of the manuscript. Please consider making the connection to the previous sentence clearer.
L414f: There is a lot of interpretation in this one sentence and I think it would be clearer for the potential readers of this manuscript if you would explain this step-by-step. First, you could start this new paragraph stating that you will talk about the bioavailability of P. Then, you could consider to mention that P can also be organic-bound, mainly by aromatic groups, and that thus the DOM composition affects the bioavailability of P. Finally, you can mention that the more aliphatic-dominated DOM in the pore ice likely leads to more bioavailable P upon thaw.
L525: What does “maximize the coupling” mean? I suggest to avoid such wordy terms.
Figures:
L121, Figure 1: Based on what reference did you identify the different ice forms? Or is this based on the sampled cores?
Figure 1, subplot b: You can barely see the illustrated stream in the figure on the right side (legend item 11). I suggest to elongate this part so you can clearly see the stream if you want to include it.
Figure 1, subplot d: I suggest to specify in the figure caption that the frozen layers are corresponding to the blue overlay in the figure.
L221, Figure 2: The given information on the active layer thickness in the caption is confusing to me. First, it is stated that the lower limit of the active layer thickness is from 35 to 65 cm, which corresponds to the active layer light blue band in the figure. Based on this, I would think that the maximum extent of the active layer is 35 to 65 cm across the cores, but we do not know which core has which extent. Then, different depths are given for the different cores in brackets, which I assumed were the maximum extents of active layer thickness. However, for Tz-1, it says 155 cm, which is well above the 65 cm given before. It would be helpful if the authors clarify that.
L221, Figure 2: put “Tz-1” instead of “Tz-“ (also in L310)
Supplementary information:
Table S4: Please consider adding short explanations on how the N and P limitation factors are calculated.