the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jul 2025
Temporary waterlogging alters CO2 flux dynamics but not cumulative emissions in cultivated mineral soils
Abstract. Increasingly variable rainfall patterns expose soils to more frequent waterlogging in humid climates. Yet, the effects of waterlogging on soil organic matter decomposition in mineral soils remain uncertain. We studied the impact of off-season waterlogging on carbon dioxide (CO2) and methane (CH4) production, and dissolved carbon dynamics in controlled greenhouse conditions using 32 soil profiles (h=63 cm, d=15.2 cm) sampled from two agricultural fields (silty clay, sandy loam) in southern Finland. During the 1.5-year study comprising three growth cycles, spring barley (Hordeum vulgare) was grown during the growing seasons. During the off-seasons, half of the monoliths were subjected to waterlogging episodes of seven weeks, while in the control monoliths soil moisture was maintained below field capacity. Additionally, overwintering cover crop (Festuca arundinacea) was grown in half of the monoliths. Soil temperature and moisture were continuously monitored, dissolved organic (DOC) and inorganic carbon (DIC) concentrations in pore water were analyzed at three depths, and CO2 and CH4 fluxes were measured at the surface. Temporary waterlogging did not induce CH4 production in either soil. Contrary to our hypothesis, waterlogging did not increase soil DOC content. Instead, on-going microbial/rhizospheric activity promoted an increase in DIC content while CO2 fluxes declined, indicating an accumulation of respired CO2 in soil pore water. This sustained CO2 production could not be explained solely by mobilization of Fe-associated C, as initially hypothesized. After the initiation of drainage, CO2 fluxes from both soils and plant treatments increased more than predicted based on changes in soil moisture and temperature, likely due to the release of previously accumulated CO2. These post-waterlogging increases in CO2 fluxes roughly equaled the earlier decreases during waterlogging. Thus, although off-season waterlogging strongly influenced the temporal dynamics of CO2 fluxes, it did not alter total cumulative CO2 emissions from the studied agricultural soils.
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RC1: 'Comment on egusphere-2025-2801', Anonymous Referee #1, 18 Aug 2025
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The authors present a thorough and well-described study of saturation treatments in soil monoliths (i.e., 3D soil columns) collected from agricultural field sites. The overall manuscript is very well-written, and the description of data analyses is particularly thorough and of high quality: nicely done! However, there are some areas where it is not clear that the focal concepts and hypotheses align with the study design or expectations, leading to a need for some careful consideration of the relevance of cited literature and the justification for hypotheses/interpretations. Specifically, much of the background information and explanation of relevant processes is tied to the idea of Fe-SOC associations driving mobilization of DOC and higher CO2 mineralization. However, there is a key distinction between prolonged saturation and frequent saturation cycling, because the saturation cycling is likely the mechanism for changes in Fe reducibility and subsequent reduction/release of DOC. Further, the importance of this mechanism is contingent on a measurable amount of the SOC in a system being stabilized via Fe associations to begin, and much of the related work has been done in tropical soils or acidic podzols with very high reactive Fe content. In this study, I would characterize the saturation pattern as one relatively short-term but consistently waterlogged within the treatment periods, so the design does not invoke the mechanism of cyclic saturation underpinning many of the related studies (which is acknowledged in the Discussion), and it is not clear whether initial Fe-SOC interactions are a major contribution to SOC stabilization in this system. While I think the overall conclusions and insights from the study are interesting and important, some reframing of the conceptual underpinning is needed to justify the design and interpretations, and some of the known considerations mentioned in the Discussion need to be introduced more directly early on. These points are explained in more detail below, e.g. on comments on lines 25-26, 56-58, and 428-429.
Specific Comments on Scientific Content:
Line 25-26: A key factor related to waterlogging is the cycling of saturated conditions, in addition to the absolute duration. From the abstract alone, this seems like it was repeated waterlogging events over a seven-week period, but it is not completely clear; from the text, it seems that it is continuous waterlogging for a 7 week duration. Suggest more directly describing the timing and duration of waterlogging (i.e., repeated events vs. one continuous 7-week waterlogging condition) here, as it is important context for the subsequent interpretation of results (see later comments, e.g. on lines 56-58).
Lines 26-27: As described, it is unclear whether the same half of monoliths received both overwinter cover crop and waterlogging treatments. As described in the text, these are a factorial design; please clarify here.
Line 33: “both soils and plant treatments…” Wording is unclear: please revise for clarity. In addition, how the plant treatments overlap with waterlogging treatments is not yet clear, as mentioned above.
Lines 56-58: An important overarching consideration for this work in the context of "temporary" waterlogging is the frequency of saturation cycles, in addition to the total duration of saturation. The process of Fe-associated OM destabilization is described nicely below, but one of the additional mechansims underpinning some of the observations in the cited studies is the progressive increase in overall Fe reducibility from repeated dissolution/precipitation processes; e.g., as discussed in Ginn et al. (2017) (https://pubs.acs.org/doi/full/10.1021/acs.est.6b05709) and mentioned in the Discussion section. Prolonged vs. fluctuating saturation is one important factor conceptually distinguishing short-term but consistent waterlogging (in this study) from in-situ or manipulated variable moisture conditions, and relevant literature.
Lines 90-92: While I agree that many of the more targeted experiments have been conducted in laboratory scale incubation, there have been field studies evaluating fluctuating water conditions related to OM mobilization, e.g. Possinger et al., 2020 (cited in the Discussion). However, field-only observations also have limited ability to have controlled comparisons across treatments, for example. Thus, the value of a middle ground between controlled lab incubations and in-situ studies could be more clearly identified here.
Lines 97-98: An important assumption underpinning the Fe-C hypothesis is that a significant proportion of mineral-stabilized C in this system is actually associated with Fe to begin. Are these soils abundant in redox-active Fe-rich mineralogy? Note that many (not all) of the cited studies were conducted in tropical soils with high Fe oxides (e.g. much of the work by Aaron Thompson et al.), acid forest podzols dominated by Fe- and Al-C complexes, etc. While the more detailed soil description and characterization is more relevant for the methods, the justification for the overall hypotheses and study design/concept is important here, and sets up the subsequent interpretations. See also comment on lines 608-609 in the Discussion on the importance of this assumption and related interpretations.
Lines 112-113: More detail and justification for soil hypotheses related to Fe-C stabilization is needed. (1) In my opinion, “tentative” classification is insufficient: how was this assigned (by soil maps)? Do these soils align with characteristics of the assigned soil types? (2) Building on the comment regarding soil Fe abundance and Fe-OM associations, neither of these orders are archetypically associated with especially high Fe abundance or Fe-dominated OM stabilization to my knowledge. This isn't to say that they do not have abundant reactive Fe, but providing the reader some context for the oxalate-extractable Fe and Al presented in Table 1 would help to justify that there is significant contribution of these phases to OM stabilization in this system. (3) Is there a fundamental difference in the saturation regime between the Stagnosol and Cambisol/Umbrisol? In addition to texture, this seems a key factor that could influence interpretation of subsequent response to waterlogging and should be clarified here.
Lines 121-122: This is a minor point and mostly a difference in usage/terminology, but a monolith in my understanding is a 2D representation of a profile surface, rather than a 3D cylindrical core. I would interpret the soil systems as described as soil columns or mesocosms. I would suggest describing once early on in the text that you're referring to columns; e.g. "soil monoliths (i.e., 3D soil columns)..." or similar.
Lines 162-164: How was waterlogging maintained after full saturation was reached throughout the 54 and 50 day periods, especially between cover crop and no cover crop treatments? If the columns were watered subsequently, did partial drying occur during watering events?
Line 214: With high TDC solutions, freezing can sometimes induce coagulation/flocculation of organic solids which do not always resuspend following thaw. In addition, I am not familiar with the efficacy of freezing alone for DIC, which is sensitive to exposure to atmosphere. Could you comment a bit further on the preservation approach and potential limitations, if any, for TDC/DOC/DIC analyses?
Line 215: The data analysis and statistical approaches are very thorough. Nicely done! In some cases, they could be slightly trimmed down for readability when discussing conversions (if standard to the field), e.g. in the section on DIC change calculations.
Lines 229-230: Can you expand a little more on this interpolation? What is the linear part of the interpolation based on (between two sampling dates)? I can guess at how na.approx might work for this, but it's not immediately clear how this would be done with a sequence of measurements over time. It's not a major point, but a little clarification would be helpful for replicability of the method.
Lines 239-241: A sentence before explaining the overall approach for using a modeled CO2 efflux would help provide the reader some context (e.g., to compare measured and modeled efflux to test XYZ...).
Line 432-434: Figure 2 caption: While the modeling procedure was explained in the text, it would be helpful to remind the reader what "monitored monoliths" means here - it's not immediately intuitive and adds a degree of confusion that may not be needed. In addition, the importance of the difference between Cycle 2 and Cycle 3 is highlighted by having them in two separate panels, but how these experimentally-induced cycles relate to important experimental conditions/phases (e.g., the removal of biomass in Cycle 3) isn't really clear in this figure or how the results are presented in the text. Some additional labeling/timeline explanation in the figure would be helpful to follow the results as presented in the text.
Lines 380-382: It is interesting to see the change in DIC accounting for the main change in TDC. Given the question about storage and sample preservation mentioned above, just as a check: do these values align with typical ranges of DIC in these soil types (especially the values close to or slightly higher initially than DOC in the silty clay soil)?
Lines 428-429: Following my comments on the introduction, it's great to introduce the distinction between prolonged vs. fluctuating anaerobosis here, but I would suggest accounting for this distinction earlier in the text with respect to relevant literature and expectations for the experiment, as it did not apply fluctuating saturation and the related implications for increasing Fe reducibility are less applicable (e.g., see Ginn et al., 2017: https://pubs.acs.org/doi/full/10.1021/acs.est.6b05709).
Lines 433-434: Given the dependence of anaerobic response on soil type as mentioned above, initial characteristics of the soils are critical for interpretation of differences (or lack thereof). For example, though these soils do differ in Fe oxide content, it is not clear from the presented soil information whether Fe oxides in general are a major contributor to overall soil mineralogy in either soil, and further if they contribute meaningfully to SOC stabilization in this system. While the absolute values for the extractable Fe oxides are presented and the reader could convert the mmol per kg values and look up ranges for other soil types, it would be helpful for the reader to have it stated upfront (before or as part of the hypotheses) that these soils are appropriate for testing the hypothesized response due to the contribution of Fe oxides to SOC in this system.
Lines 463-465: Here and throughout, it would be helpful to have a little more guidance in the text about what the reader should take away from statements about observations in a certain study cycle. The meaning of the reference to the second and third study cycle is not immediately understandable in terms of what that means for the conditions of the experiment; in other words, what are the critical differences between these phases relevant to the target processes? It seems to me that in the third cycle the monoliths have already experienced waterlogging in the first and second cycles, so that is one important difference, as is the removal of biomass, but it's a little bit challenging as a reader to follow these processes.
Lines 608-610: In line with comments above, this take-away is stronger if the experiment were targeting Fe-associated SOC stabilization and dissolution: i.e., with relatively high contribution of Fe-associated SOC (which is unclear from the data presented) and fluctuating conditions that result in subsequently higher Fe reducibility. Consequently, some nuance regarding this take-away might be needed.
Minor Editorial Suggestions:
Line 198-199: The detail of CO2 flux conversion can be omitted, unless there was a non-standard conversion used.
Line 275: "similarly than with..." Awkward wording here, suggest rewording for clarity.
Lines 288-289: Suggest clarifying that this refers to "higher than the non-saturated treatment."
Citation: https://doi.org/10.5194/egusphere-2025-2801-RC1
Data sets
Supporting data: Temporary waterlogging alters CO2 flux dynamics but not cumulative emissions in cultivated mineral soils Reija Kronberg, Markku Koskinen, Tatu Polvinen https://doi.org/10.5281/zenodo.14438980
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