the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Aug 2026
CO2 influx and efflux circadian cycles in bare dry lake sediments
Abstract. Permanent and temporary drying of inland waters expands the surface area of exposed sediments, potentially altering total ecosystem CO₂ fluxes. However, the response of CO₂ fluxes from dry sediments to episodic rewetting, and the relative roles of biotic and abiotic exchange mechanisms in shaping net fluxes, remain poorly understood. Here, we conducted a field rewetting experiment on long-term exposed sediments of Lake Gallocanta (Spain) to assess rewetting-induced variations in CO₂ fluxes. CO₂ exchanges between sediments and the atmosphere were measured before and after rewetting using closed gas chambers, and the isotopic composition of emitted CO₂ (δ¹³C-CO₂) was analyzed. Sediment samples were also collected to characterize key physicochemical properties (e.g., water activity, temperature, pH) and to assess changes in microbial community composition. In addition, a numerical model integrating gas flux and isotope data was developed to disentangle biotic and abiotic contributions to total CO₂ emissions. Contrary to expectations, rewetting had minimal influence on CO₂ flux magnitudes, which instead exhibited a pronounced circadian pattern of efflux and influx. Although rewetting substantially altered microbial community structure, these changes did not indicate that biological carbon fixation explained the observed CO₂ influx. Model simulations incorporating both biotic and abiotic processes indicated that abiotic mechanisms, most likely carbonate weathering, dominated CO₂ influx, whereas aerobic respiration accounted for midday efflux. Overall, our results highlight the critical role of abiotic processes in regulating CO₂ dynamics from long-term desiccated lake sediments and suggest that net CO₂ fluxes may not reliably indicate organic carbon remobilization under dry lake conditions.
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Status: open (until 20 Oct 2026)
- RC1: 'Comment on egusphere-2026-5010', Anonymous Referee #1, 31 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-5010', Anonymous Referee #2, 20 Sep 2026
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General comments
This manuscript investigates the diel dynamics of CO2 exchange in long-term exposed lake sediments and further explores the relative contributions of biotic and abiotic processes using CO2 flux, stable isotope, physicochemical, microbial, and modeling approaches. The experimental design is well conceived and comprehensive, with multiple complementary approaches that allow the authors to examine both the observed CO2 flux patterns and their potential underlying mechanisms. The study provides an interesting perspective on CO2 dynamics in dry lake sediments and highlights the potentially important role of abiotic processes in regulating CO2 exchange. The finding that relatively stable abiotic CO2 uptake, together with temporally variable biological respiration, contributes to the periodic transition between net CO2 uptake and emission is potentially valuable for understanding carbon cycling in desiccated inland waters.
Overall, the manuscript is well organized and the results are generally convincing. I therefore recommend minor revision. However, several points should be clarified or better presented to strengthen the manuscript. My specific comments are listed as follows:
P1 L1: The title seems somewhat broad because the observed circadian CO2 influx–efflux pattern may be particularly relevant to carbonate-rich/calcareous sediments and may not necessarily be generalizable to bare dry lake sediments as a whole. Moreover, the current title does not highlight one of the key findings of the study, namely that abiotic processes, rather than microbial processes, appear to play a dominant role in regulating the observed CO2 flux pattern. The authors may consider revising the title to more accurately reflect both the environmental context and the main mechanistic finding.
P1 L34–35: The Abstract does not sufficiently convey the central mechanistic conclusion of the study. In particular, the periodic transition between net CO2 source and sink appears to result from the interaction between temporally variable biological respiration and relatively persistent abiotic CO2 uptake. This interaction is an important conceptual contribution of the study, but it is currently not clearly stated in the Abstract. I suggest that the authors explicitly incorporate this mechanism into the Abstract.
P5 L148: The procedure used to seal the chambers should be briefly described. Because chamber sealing is critical for accurately measuring CO2 exchange between sediments and the atmosphere.
P8 L247: The isotope model assumes a fixed enrichment factor for CO2 uptake. This assumption may be reasonable when a single carbonate precipitation process dominates under relatively stable temperature and reaction conditions. However, isotope fractionation associated with carbonate precipitation or other CO2 uptake processes may vary with temperature and reaction conditions, particularly when temperature or precipitation rates change substantially. Please clarify how the observed temperature variation during the experiment may affect the validity of this fixed-enrichment-factor assumption and discuss the associated uncertainty in the model results, if appropriate.
Citation: https://doi.org/10.5194/egusphere-2026-5010-RC2
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General comments
Arı et al. present an interesting study about CO2 fluxes in dry lakebed sediments, making a novel contribution to our understanding of dry inland water GHG cycling. In particular, the partitioning of abiotic vs biotic contributions to dry CO2 emissions is valuable and understudied, as well as the link to microbial communities. Below, I outline some specific comments regarding expanding the discussion of the drivers of sediment CO2 emissions, elaborating on the 2nd hypothesis, and providing justification for some choices in the experimental design. There are also some small technical corrections to be made. Overall, with some minor revisions I believe this article presents a valuable contribution to the literature on dry inland water biogeochemical cycling.
Specific comments
51-52 sediment moisture and sediment texture are other important variables influencing dry CO2 emissions. See a summary of important variables in Silverthorn et al., 2023 Freshwater Biology https://doi.org/10.1111/fwb.14172
L112-113 I think your last hypothesis might be too vague, can you hypothesize which process might be more important and why?
L121-122 “compromising the water sources” sounds a bit vague. Can you describe how the water is being compromised? E.g. enhanced nutrient inputs or water abstraction?
L134 Why was 28mm of rainfall chosen? Why was distilled water chosen? The chemical composition of distilled water will be quite different from rainwater and may result in different biogeochemical processes.
L176-177 I presume water activity is similar to sediment moisture content? It could be worthwhile to briefly describe as this is perhaps is a less-familiar variable.
L310 For an additional visual aid, you could add a small symbol of sun and moon to the first white/grey bars.
L443-452 Could the use of distilled water potentially also help explain the lack of an observed Birch effect?
L504 Was organic matter content of the sediments measured? Organic matter availability could perhaps help explain some of your observed trends with temperature and moisture.
Technical corrections
L26 CO2 exchange (delete the ‘s’)
L29 I would briefly mention what method(s) you used to measure microbial communities at the end of this sentence, i.e. 16S rRNA amplicon sequencing.
L41 Here, and in some cases throughout, the accent on the ‘e’ in Marcé et al. is missing.
L305 For clarity, you could specify “the fixed effect of sampling time and its interaction with treatment exerted a significant effect on CO2 fluxes…”.
L340 In Figure 2A the points can be difficult to distinguish when they are overlapping, I would suggest adding a think back outline to the points. Perhaps it also needs to be described in the caption that the “Main” effect is the effect of the precipitation treatment?
L377 “treatment” and “time” are lowercase here but capitalized elsewhere (e.g. L305, L311), I would suggest remaining consistent and not capitalizing the model terms.
L404 diel?
L436 Perhaps a personal choice, but I would prefer the term ‘diel cycle’ over ‘circadian cycle’, as circadian has a biological association while CO2 fluxes here have both biotic and abiotic drivers.
L490 and L569 subscript 2
L585 I suggest making the R code available for peer review and including a link or DOI for the dataset.