the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Nov 2024
Calcium is associated with specific soil organic carbon decomposition products
Abstract. Calcium (Ca) may contribute to the preservation of soil organic carbon (SOC) in more ecosystems than previously thought. Here we provide evidence that Ca is co-located with SOC compounds that are enriched in aromatic and phenolic groups, across different acidic soil-types and locations with different ecosystem properties. In turn, this co-localised fraction of Ca-SOC is removed through cation-exchange, and the association is then only re-established during decomposition in the presence of Ca (Ca addition incubation). Thereby highlighting a causative link between decomposition and the co-location of Ca with a characteristic fraction of SOC. Incorporating this mechanism into conceptual and numerical models can improve our understanding, predictions, and management of carbon dynamics in natural and managed soils, and account for their response to Ca-rich amendments.
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RC1: 'Comment on egusphere-2024-3343', Anonymous Referee #1, 17 Dec 2024
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The authors have put together a study that elucidates Ca-C interactions in acidic soils, the composition of C in these interactions, and the mechanisms involved. Overall, this manuscript constitutes an advancement in our understanding of SOC dynamics and our evolving understanding of the roles of calcium in SOC dynamics. It is within the scope of SOI, and quite relevant to its readership. The manuscript is short and well written, the introduction cites relevant articles and sets the reader up nicely to understand the context of the results. The figure are well made and nicely visualize the authors' findings. The main findings are nicely supported by the supplementary materials. I would have personally like for some of the table to be moved from SI to the main text to allow a more streamline reading experience. I recommend publishing this article after addressing the minor comments below.
Intro:
L39 - the associations between Ca and C in acidic soils is an interesting finding and indeed contrary to conventional thinking. However, if this is more prevalent than we assume, why do the concentrations of C and Ca not found to be important in acidic soils in data syntheses, e.g. Rasmussen et al 2018?
L55 - how does the co-localization imply a microbial process? While microbial processes could very well be involved in many Ca-C interactions, no microbial measurements are presented here regarding the Rowley et al 2023. Could it not be solely the result of preferential associations between Ca and aromatic and phenolic C that form as OM decomposes in the presence of Ca? To me it makes sense to frame it such that is the hypothesis that requires testing, and to test it you performed the removal/addition/decomposition on several soils to evaluate whether it is a general feature.
Also, could it also be the Ca that originated from the OM vs pedogenic Ca? Plant litter can contain significant amounts of Ca. Fig S5 shows agreement between Ca-C and total C in the litter and biomass, perhaps more carboxylic in Ca regions. Does this imply that Ca-C co localization at least partially results from decomposition of OM rich in Ca?
L56 - instead of characteristic fraction of SOC, perhaps state which fractions? (i.e. aromatic and phenolic rich)
L65 - please elaborate on what you expect to find from a comparison of a soil with low Ca and a soil with high Ca.
Results:
L70 - are the litter and biomass C spectra shown in Fig S5 different than those of the Ca-C areas in the soil samples? If they are similar, one can pose a counterargument to whether this association is inherited from the composition of plant/litter.
L74 - this is total Ca not exchangeable Ca, correct? It would make more sense to compare the exchangeable fraction, if that is the fraction that was removed and then re-added.
L76 - does this mean C was roughly 77-83% Fe-associated and the rest Ca-associated? Suggesting Fe-C is quantitatively more important in these acidic soils? See my comment about the importance of Ca in acidic soils in the intro section.
L86 - it is unknown if DOC export has been prevented but perhaps that can be rephrased e.g. "consistent with reports of decreased DOC leaching..."
L95 - why do you suppose addition of Ca to K-exchanged soils did not re-establish the original C-Ca interactions, while incubating fresh soils with Ca did. Could the K exchange process removed some of the POM that was a precursor to the aromatic/phenolic-C interacting with Ca? Other reasons?
L113 - the authors synthesize their results into a conceptual model that builds upon the continuum model (Lehmann and Kleber 2015) which is very useful. Their assumption is that Ca binds to C compounds mostly through carboxyl groups, which, as the authors state, are partially protonated at these pH values. Indeed compounds often contain both phenolic and aromatic groups (lignin derivatives), however the hypothesis that microbial processing increases the carboxylic content thus supporting interactions with Ca is somewhat contrary to enrichment of Ca-C regions with aromatic and phenolic compounds. Could it be that Ca is interacting directly with phenolic and aromatic functional groups?
Fig 2A and Fig S9A - it appears that the exchange indeed removed phenolic C preferentially (~286 eV), but it also removed aliphatic C (~287 eV). Please address this point.
Methods:
The authors provide STXM images showing the effects of K exchange on removal of Ca, but I wonder if they measured Ca (and C!) in the rinse to attempt to quantify Ca-K exchange and how much C was removed through it.
Have the authors measured exchangeable Ca? This would seem to be more relevant to this study than total Ca.
Citation: https://doi.org/10.5194/egusphere-2024-3343-RC1 -
RC2: 'Comment on egusphere-2024-3343', Anonymous Referee #2, 02 Jan 2025
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The manuscript provides novel mechanistic insights into a relevant soil process. After thorough inspection of the draft, my comments and recommendations mainly concern the presentation of the results, which should be well improved after significant revision related to the following points:
The manuscript is build up on a previous study and relates an earlier observation with some further experimental evidence. The presentation is kind of chronological which hinders the explanation unfolding topic-wise (e.g., in the first paragraph of the introduction the third sentence (l. 39) already jumps into fine-scale NEXAFS observation instead of elucidating the state of the art and the mechanisms further (and how fine-scale techniques provide particular insights based on the heterogeneous soil matrix), which I would recommend to strengthen the logical structure). Also the additional experimental steps (the washing) are inherently important to point out why these were done. This would improve reflecting on the following question: To which extent does the presented evidence suggest that the Ca is present at specific spots triggering an association with dissolved organic compounds or could the Ca be derived from organic compounds itself (and be inherited from the plant or microbial origin?). The microbial transformation is noted here and there but it does not become clear how it is specifically involved since specific aspects of the microbial role are less discussed (e.g., in l. 110 “arises from coupled biogeochemical processes involving microbial decomposition” or in l. 127&128 a lot of different microbial processes are noted but not interrelated with the actual observations). Maybe the authors should consider adding a graphical schema which briefly summarizes the specific details of the suggested mechanism, which I would leave up to them. However, I would suggest Fig. S10 is too speculative and simple since the molecular sizes were not measured. The explanation in l. 117-121 provides the key understanding and should be definitely elucidated further. Also, there is a lot of observations on the different mineralization rates found in the appendix methods and the SI, which is not well connected to the main body of the manuscript
Further comments:
l. 1: The title could be misread as if Ca decomposes specific compounds. Also it might be advantageous to add that the results are related to acid soils.
l. 19: What kind of different ecosystem properties?
l. 22: What kind of “conceptual and numerical models” are referred here? It might be better to directly describe the actual suggested mechanism and delineate it from other processes related with Ca. This comment is also related to l. 131 where it is not clear how “Earth System Models” are specifically influenced?
l. 98 It should be noted that a monovalent treatment does not seem to have been conducted in the reference cited here.
Fig. 1 and Fig. 2: It would be great of the panels had some subheadings, especially in Fig. 2. Also in Fig. 2 the x axis regions may need to be more clearly delineated.
Citation: https://doi.org/10.5194/egusphere-2024-3343-RC2
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