the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Formation of particulate organic carbon from dissolved substrate input enhances soil carbon sequestration
Qintana Si
Kangli Chen
Bin Wei
Yaowen Zhang
Xun Sun
Junyi Liang
Abstract. Particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), which are two primary components of the soil carbon (C) reservoir, have different physical and chemical properties and biochemical turnover rates. Microbial necromass entombment is a primary mechanism for MAOC formation from fast-decaying plant substrates, whereas POC is typically considered as the product of structural litter via physical fragmentation. However, emerging evidence shows that microbial by-products derived from labile C substrates can enter the POC pool. To date, it is still unclear to what extent labile substrates contribute to the POC formation and the subsequent long-term SOC stock. Our study here, through a 13C-labeling experiment in 10 soils from 5 grassland sites as well as a modeling analysis, showed that up to 12.29 % of isotope-labeled glucose-C (i.e., dissolved C) was detected in POC pool. In addition, the glucose-derived POC was dependent upon 13C-MBC and the fraction of clay and silt, suggesting that the POC formation from newly added labile C is dependent on interactions between soil physical and microbial processes. The modeling analysis showed that ignoring the C flow from MBC to POC significantly underestimated soil C sequestration by 7.79 % – 49.51 % across the 10 soils. The results emphasize that the soil texture-regulated microbial process, besides the plant structural residues, is a significant contributor to POC, acting as a vital component in SOC dynamics.
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Qintana Si et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1483', Anonymous Referee #1, 25 Aug 2023
General comments:
The preprint under review explores the dynamics of microbial carbon contributions to particulate organic carbon (POC) in grassland ecosystems. The study investigates the relationship between microbial-derived carbon from labile sources, like glucose, and its incorporation into POC. While the manuscript offers valuable insights into the intricate connections between microbial processes and POC formation, there are several aspects that warrant attention and further clarification.
In the introduction, the authors could enhance the clarity of their hypotheses by providing more concise predictions based on the research questions. This would provide readers with a clearer roadmap of what to expect in the subsequent sections. Additionally, the intro raises a pertinent point regarding the heavy POC or coarse MAOC fractions, but this fractionation scheme was not utilized in the study. The use of density fractionation to isolate this pool could potentially yield more targeted insights.
The collection of grazed versus ungrazed soil samples is mentioned in the methods section, yet its rationale remains absent from both the introduction and results. Addressing the purpose of this comparison within the context of the research objectives would be beneficial for readers' comprehension. Moving to the results section, the reviewer highlights the intriguing findings regarding glucose-derived POC and its proportion within total POC. However, the text does not mention the relative contribution of glucose-derived carbon to POC and MAOC, which could provide a more holistic perspective.
Furthermore, clarifying whether respiration data solely validated model outputs or served additional purposes would mitigate this ambiguity. Figure 2 requires clearer labeling, including definitions of "in" and "out" to facilitate interpretation. The reviewer could identify significant variations in glucose-derived POC and MAOC among different grazing scenarios, which could enrich the discussion by exploring potential explanations for these disparities.
The intriguing graphical representation in Figure 3, while interesting, prompts the reviewer to suggest an exploration of heavy POC independently, given its importance in understanding silt/clay and dissolved organic carbon (DOC) interactions. Scaling the C sequestration projections to an ecosystem level, both with and without microbial POC mechanisms, could enhance the practical implications of the findings.
In the discussion section, addressing the potential direct interaction of glucose-derived carbon with POC, rather than solely through microbial pathways, adds depth to the interpretation. The reviewer aptly acknowledges the significance of the study's findings in challenging the dichotomy between physical and microbial pathways in POC formation.
In conclusion, this preprint contributes noteworthy insights into the complex interplay between microbial processes and POC dynamics in grassland ecosystems. Addressing the identified gaps and refining the manuscript in line with the reviewer's suggestions would undoubtedly elevate its scientific impact.
Specific comments:
Intro
Would be nice to see concise hypotheses/predictions for your questions.Methods
If you are interested in the heavy POC or coarse MAOC fraction as mentioned in the introduction, why not do a density fractionation to isolate this pool?
Is there a reason you collected grazed vs ungrazed soils? You do not mention anything about it in your introduction or results.
Results
These are very interesting results! It would be helpful to see the data on glucose C remaining as percent C remaining of initial C added. Also, the proportion of total POC that is from glucose C would be good to know. What is the relative contribution of glucose C to POC and MAOC?
Also, there is no mention of the comparison between sites. For example
You measured respiration, but I do not see any results presented on respiration or glucose derived respiration. Was respiration data only used to validate the model ouputs? This needs more explanation.
Figure 2. Need description of in and out.
There are noticeable differences in Glucose derived POC in DLin and DLout, and differences in glucose derived POC and MAOC between HLin and HLout. These seem like interesting results but are not discussed in Results or Discussion. This seems to be a missed opportunity to discuss why the biogeochemistry might be different under grazing vs no grazing.
Figure 3. Interesting way to display the data, I like it. These are interesting data. Again I think this is a missed opportunity to look at heavy POC independently as this is a key fraction in understanding the mechanisms relating silt/clay and DOC.
Would be nice to see the C sequestration projections scaled to ecosystem scale (i.e. MgC/ha/yr) with and without microbial POC mechanisms.
Discussion
Again, you discuss heavy POC, but did not separate this fraction. You show that MBC is correlated to Glucose C, but this explains only a small portion of the glucose C in POC. Is it possible that glucose C is somehow directly sticking to POC and not passing through microbes?
These are significant findings that provide evidence that physical transfer pathway and microbial DOC pathway are not distinct. Microbial contributions to POC in these grasslands are significant, I’m interested to know if it is the result of microbial biofilms promoting heavy POC stabilization or possibly fungal bodies larger than 53um? This research opens the door to more exciting research, good work!
Technical corrections:
L31: Consider changing ‘As opposed to this’ to ‘In contrast’ -just a suggestion, take it or leave it.
L33: Instead of ‘roughly’ use physically. -just a suggestion, take it or leave it.
L34: Remove ‘to operate’ -just a suggestion, take it or leave it.
L42: low molecular weight is more accurate than small-molecular.
L45: sentence is awkward consider changing. i.e. “However, the potential for microbial products derived from labile C to stick to semi-decomposed plant residues and connect with minerals to become POC has received much less attention.”
L48: Remove ‘the’ in ‘the POC’ both instances.
L87: rather than ‘the other’ – and the >53 um fraction was considered POC
L140: remove duplicate ‘effects of’
L172: efficiency to efficient.
Citation: https://doi.org/10.5194/egusphere-2023-1483-RC1 -
RC2: 'Comment on egusphere-2023-1483', Anonymous Referee #2, 27 Sep 2023
Si et al. present data from a laboratory study in which they incubated different soils with 13C-labeled glucose and measured the label after the end of the incubation in microbial biomass, particulate organic matter (POM), and mineral-associated organic matter (MAOM). The authors find fluxes of C from the glucose to microbial biomass and subsequently to POM and MAOM. The results indicate that flows from dissolved organic C to POM may be relevant to soil C sequestration. The study is timely and provides interesting insights into flows of C from DOM to POM and MAOM. However, I believe that communication of the results could be improved. The authors write at various locations that dissolved compounds form particulate organic C (POC), which connotes that this POC or POM is formed de novo. This is, however, not supported by the data, and it is much more likely that microbes that metabolize the labeled glucose and
colonize POM are responsible for the 13C recovered in that fraction. Moreover, data (e.g., on total POM/POC before and after the incubation) that could substantiate the authors' claims are not provided. I thus suggest that the authors revise their title, abstract, discussion, and conclusions and refrain from using the term "formation" and rather refer to flows of added C to POM (or MAOM). Moreover, the grammar throughout the manuscript should be checked and errors corrected.
Specific comments below.L28 I suggest not referring to stabilization when writing about POC.
L33-25 grammar broken
L37ff It remains unclear why heavy POC can be a precursor for MAOM formation. I suggest stating the potential formation pathways of MAOC in the text above, for clarity.
L40ff I believe the studies cited refer to rhizodeposits and not to root exudates (not sure if Cotrufo et al., 2013 thematize above- or belowground inputs at all). The statement here should be revised accordingly. It could also be mentioned here that root exudates can destabilize C as well (e.g., Keiluweit et al., 2015, Nat. Geosci.).
L59ff What was the rational behind sampling these sites? Did site/management have an influence on C flows?
L69ff How was glucose added and how did the authors assure that it was uniformly distributed in the soil?
L85ff Reference for the methods?
L132ff How were the models validated?
L149/150 What proportion of initially added glucose is this?
L151 glucose-derived POC and MAOC were correlated, not dependent.
L155 Compared to what did the model under-/overestimate turnover?L170ff I believe that this statement is erroneous, i.e., the correlation of glucose-derived POC with glucose-derived MBC indicates that microbial processing/metabolization of DOM and subsequent colonization of POM by these microorganisms explains the observed pattern.
That is, DOM or microorganisms do not form POM de novo but "attach" to existing POM. So what the authors observe in their study is basically the decomposition process and no de-novo formation of POC/POM. The authors could use data on total POC/POM values in their soils before and after the incubation to substantiate their claims, but such data are absent. As such, the title of the manuscript is misleading as well. I thus suggest that the authors frame their discussion differently and refrain from using the term POC formation, which connotes that DOC forms POC de novo, but only refer
to flows of C to POC. The abstract and conclusions should be adapted accordingly.
L172 The study by Sokol does not support the claims by the authors since inputs by living roots encompass structural compounds, e.g., sloughed-off cells, which can be a substantial contributor to POM, and not just exudates.
L172 The authors use the term POC formation, which I believe is misleading since they only show POC derived from glucose but no overall POC or POM values that would substantiate additional formation of POC.
L174 "was positively correlated"; this correlation could indicate that the higher the clay and silt content, the more aggregates and the more POM is protected from decomposition. In the following lines, the authors indirectly refer to the aggregation process, which could be explicitly referred to as such.Citation: https://doi.org/10.5194/egusphere-2023-1483-RC2
Qintana Si et al.
Qintana Si et al.
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