Responses of dissolved organic carbon to freeze-thaw cycles associated with the changes in microbial activity and soil structure
Abstract. Arctic warming accelerates snowmelt, exposing soil surfaces with shallow or no snow cover to freeze-thaw cycles (FTCs) more frequently in early spring and the late autumn. FTCs influence Arctic soil C dynamics by increasing or decreasing the amount of dissolved organic carbon (DOC); however, mechanisms-based explanation of DOC changes considering other soil biogeochemical properties is limited in previous research. To understand the effects of FTCs on Arctic soil responses, we designed microcosms with surface organic soils from Alaska and investigated several soil biogeochemical changes under seven-successive temperature fluctuations of freezing at -9.0±0.3 °C and thawing at 6.2±0.3 °C for 12 h each. Our study found that FTCs significantly changed the following soil variables: soil respiration, DOC and total dissolved nitrogen (TDN) contents, two DOC quality indices, micro-aggregate distribution, and small-sized mesopore volume. Multivariate statistical analyses supported that the FTCs improved soil structure and functions which led to facilitated DOC decomposition by soil microbes, and changes in DOC quantity and quality by FTCs. This study showed that FTCs affected DOC characteristics without negatively impacting soil microbial respiration activity, as soil microbes had previously adapted to temperature fluctuations in the Arctic. Soil micro-aggregation enhanced by FTCs and the subsequent increase in soil respiration and small-sized pore volume could promote DOC decomposition, eventually decreasing the DOC content in the soil solution. This study provides a mechanism-based interpretation of how FTCs alter DOC characteristics in Arctic soil by incorporating its structural changes and microbial responses, ultimately improving our understanding of Arctic soil C dynamics.
You Jin Kim et al.
Status: final response (author comments only)
RC1: 'Comment on tc-2023-3', Anonymous Referee #1, 06 Mar 2023
- AC1: 'Reply on RC1', You Jin Kim, 11 May 2023
RC2: 'Comment on tc-2023-3', Liam Heffernan, 10 Mar 2023
- AC2: 'Reply on RC2', You Jin Kim, 12 May 2023
You Jin Kim et al.
You Jin Kim et al.
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The paper couples soil physical properties with dissolved organic carbon parameters to establish a mechanism-based understanding of soil C dynamics and changes to DOC during freeze-thaw cycles in early spring and late autumn arctic tundra soils. Significant findings include increased soil respiration and soil microaggregation following 7 freeze-thaw cycles. The paper provides a much needed and valuable contribution to the freeze-thaw literature. The authors expand the pre-established concept of changeability of the physical arrangement of the soil to dissolved organic carbon response (also generally studied in separation in freeze-thaw experiments). The paper is well written and high quality, the experimental design is elegant and well-supported, the results are impactful, and the discussion is insightful, further emphasizing the value of the authors' contribution. The paper is well suited for publication in the Cryosphere. There are some points of clarification that should be addressed prior to publication, outlined below.
I agree that 12 hours is enough to freeze and thaw 120 g of soil, but please cite another incubation for the method or provide test data where you found that soil was able to completely freeze and thaw in that time. It will make your method more citable/reproducible.
I am unclear on the significance of 7 freeze-thaw cycles in the context of your paper. Ma et al., 2021 found unpredictable freeze-thaw response up to 7 freeze-thaw cycles, after which freeze-thaw resulted in increased pore connectivity. Placing your experiment on this cusp, at 7 freeze-thaw cycles, is a great contribution, but I think it needs to be a little bit more clear why you chose 7 freeze-thaw cycles. Was it because you were targeting that unpredictable pore network response that ends at around 7 freeze-thaw cycles? (line 94)