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the Creative Commons Attribution 4.0 License.
| 30 Sep 2024
Cultivation reduces quantities of mineral-organic associations in the form of amorphous coprecipitates
Abstract. Mineral-organic associations are crucial carbon and nutrient reservoirs in soils. However, soil cultivation disrupts these associations, leading to carbon loss and reduced soil fertility. Although, identifying the specific type(s) of mineral-organic associations susceptible to destruction or transformation upon cropping remains challenging, it is essential for devising strategies to preserve organic matter in croplands. Here we aimed to determine the predominant mineral-organic associations and to identify which types of associations are transformed upon cultivation. To achieve this, we sampled an andosol from both a forested and a cultivated area. We then analyzed cultivation-induced changes in soil physicochemical parameters and characterized mineral-organic associations using an array of spectro-microscopic techniques (TEM-EDX, TEM-EELS, and STXM), for comprehensive structural and compositional analysis. At the micro and nanoscale, we observed mineral-organic associations in the form of coprecipitates composed of amorphous oligomers containing Al, Si, and Fe (referred to as nanoCLICs for nanosized coprecipitates of inorganic oligomers with organics). Down to a few hundred nanometers, the nanoCLICs displayed elemental enrichments with C+Al+Si, C+Fe+Al+Si, or Al+Si dominance with less C. In contrast, organic matter exhibited various C speciation without compound-specific enrichments. These findings suggest that mineral-organic associations in andosols are nanoCLICs-type coprecipitates rather than organic matter associated solely with secondary minerals. NanoCLICs were present in both forest and crop andosols, and while cropping led to a 50 % decrease in nanoCLICs, it did not alter their nature. This novel conceptualization of mineral-organic associations as nanoCLICs shifts our understanding of their persistence in andosols and demonstrates their vulnerability to crop-induced changes.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-2933', Anonymous Referee #1, 08 Nov 2024
General Comments
Manuscript quality
The manuscript is well-written overall, and the study seems sound. The authors used microscopy and spectroscopy techniques to evaluate the co-precipitates of amorphous oligomers containing Fe, Al, and Si and organic matter in Andosols. This study focuses on understanding organomineral associations and how they are affected by land use change, especially in volcanic soils with amorphous minerals. I believe the work could be interesting for the readers of Soil, and the manuscript could be accepted after some revision.
Limitations on land-use comparisons
The study builds up mainly on the detailed characterization of co-precipitates at the nanoscale, which helps us better understand how organic matter is stabilized in these soils. Perhaps the main limitation of this work is that the authors only sampled one soil profile in each land use, which makes comparisons between these lands more limited. However, once this manuscript focused more on understanding the nature of these organo-mineral associations rather than directly comparing land uses, I believe this limitation is acceptable. Yet, a few comments on this issue could be added to topic 4.4. Also some conclusions about abundance of nanoclics should be modified acknowledging these limitations (see specific comments Line 379).
NanoCLICS conceptualization:
The authors describe co-precipitates of amorphous oligomers containing Al, Si, Fe, and organic matter as nanoCLICS, as Tamrat et al. (2019) first suggested. I am confused when the authors discuss nanoclics and their comparison with short-range order minerals (SROs), as it is unclear whether nanoclics are a separate category of mineral structure or still part of SROs. The authors suggest they are a separate category for having a more amorphous and disorganized structure than assumed for SROs, as they also did in Jamoteau et al. (2023). Nonetheless, I wonder whether the observed nanoclics are simply not at the far end of the disorder spectrum of SROs. I believe this aspect is a bit ambiguous in the manuscript and the authors should state more clearly in what step we are regarding the understanding of these concepts. I believe we currently cannot say either they are or not a different type of structure, as detailed, side by side comparisons of structural attributes are missing.
Specific comments
Materials and methods: include the type of tillage. Also, specify in which seasons the plants were cultivated and when the fallow period took place. Also specify if this system has changed or maintained consistent throughout all the 30 years from the conversion.
Figure 1: I would change the type of graph from lines to bars/columns/boxplot. Because the lines suggest a change along time and the graph contain only two points with a straight line connecting them. This illustration suggests these changes are linear, which is not possible to know based on this comparison.
Line 347: I would modify these first two sentences because I believe it is well known that organo-mineral associations are prone to destabilization due to a variety of factors, even in buried soils as Shimada et al. 2022 evaluated.
Line 281 – 283: I think this phrase kind of suggest that nanoclics completely replace SROs in some cases, which I think is stepping longer than the leg.
Line 379: I am not sure whether the analyses made by the authors allow them to conclude that more than 50% of the lost C was in the form of nanoclics, because they evaluated the mineral associated organic C in the fraction below 20 microns, which can comprise different type of interactions than just nanoclics. Also I am not sure whether they can infer much about the abundance of these nanoclics because: 1) C contents in the microscopy analyses was not much different between forest and cultivated soils; 2) microscopy analyses are not the ideal tool to infer about amounts as they lack representativeness, 3) they only sampled one replicate in each land use.
Citation: https://doi.org/10.5194/egusphere-2024-2933-RC1 -
RC2: 'Comment on egusphere-2024-2933', Anonymous Referee #2, 18 Nov 2024
Overview Comments:
Jamoteau et al. present a variety of high spatial-resolution analyses probing organic and mineral phases in a Fe-poor Andosol, focusing specifically on distribution of C within either SRO or amorphous phases. The imaging and spectroscopy approaches are strong and presented well (though with some methodological detail expansion needed), and in particular, the finding regarding secondary interactions between an SRO phase and associated (potentially adsorbed) nanoCLIC phases is quite an interesting result! The contrast between a forested and cultivated setting is based on single-point measurements (based on my understanding of the methods and data presented), which means that some of the overall interpretations regarding land-use impacts need to be more constrained in their presentation. In addition, some broader context for prior work focusing on amorphous phases in Andosols would help strengthen the knowledge gap addressed. Examples of these considerations are outlined in the detailed comments below.
Detailed Comments:
Line 20: It's unclear what is being referred to in terms of "predominant mineral-organic associations." In a specific system, or croplands in general?
Line 22: It is important to establish upfront how the contrast between the forested and cultivated area is being made: are these two adjacent areas on the same site? In other words, is the forested area a fair reference point?
Line 22: As with any study contrasting management or ecosystems, it is important to justify that differences are due to cultivation per se (rather than other potential correlated factors). Is the contrast specific to cultivation (i.e., tillage) or overall conversion from forest to agricultural systems, which alters input type, root systems, organic surface layers, etc.
Line 26: “Down to a few… “ What does this refer to specifically? A few hundred nanometers in spatial resolution? NanoCLIC "features" a few hundred nanometers in size? Some clarification needed here.
Line 27: The wording describing these distinct elemental compositions is unclear and I think could be presented more simply. My interpretation from is that there are three primary regions with compositions of (1) C+Fe+Al+Si, (2) C+Al+Si, or (3) Al+Si. Is that correct?
Line 28: “Exhibited various C…” Not sure I follow what is meant by this statement. Does this imply that organic matter speciation (rather than quantity) is not tied to the distribution of other elements, but occurs randomly throughout the soil samples/NanoCLICs?
Lines 28-30: Since the phrasing of the previous two sentences describing results is unclear, I'm not sure I follow exactly what they indicate amorphous coprecipitate-like structure rather than surface associations with crystalline minerals. There seems to be an indication that these elemental compositions/distributions and C speciation are in line with coprecipitates specifically, but the direct connection between the two is unclear.
Lines 30-31: How were NanoCLICs quantified? A bit more information is needed in the abstract to be able to assess this conclusion (and based on the single measurements, as I interpret it, this value needs to be considered with caution).
Line 31: I am not sure that the conceptualization as a "nanoCLIC" is fundamentally different than the description of amorphous organo-mineral materials in Andosols, which has significant historical focus. The observations and description of nanoCLIC composition and structure is novel information, but there is historical precedence for considering Andosol organo-mineral associations as amorphous phases (perhaps within a range of different related processes, such as organo-metal complex formation and precipitation, e.g. as discussed in Wada 1985 (https://link.springer.com/chapter/10.1007/978-1-4612-5088-3_4). I suggest highlighting the compositional and structural insights from the study more clearly in the abstract (such as interactions between amorphous phases and SRO phases, which I thought was really interesting!) to highlight more clearly how this improves our understanding of their persistence and vulnerability.
Line 32: Please clarify what is meant by crop-induced. Is the effect one from cultivation (tillage), a particular crop, being an agricultural system in general...? It is not clear what system is being contrasted.
Line 45: Coprecipitation of "amorphous metal-organic complexes" is also a well-known variant of copreciptation (which doesn't necessarily fall within the "short-range ordered" mineral phases), e.g. as mentioned in Chen et al. 2014 (https://doi.org/10.1021/es503669u) and references within, and mentioned in Kleber 2015 chapter cited. While most of the mentioned studies focus on laboratory coprecipitation experiments, I think these concepts align well with observations (some spatially resolved, some not) of "amorphous" organo-mineral associations in a variety of "coprecipitation" environments - e.g., spodic horizons: there is extensive literature related to podzolizaiton which is inherently a copreciptiation process, e.g. see Buurman and Jongmans 2005, https://www.sciencedirect.com/science/article/abs/pii/S0016706104001855). In all, I think that the premise that coprecipitation is a traditionally a dominantly SRO mineral-related process is not well-supported, and hence the distinction of a "nanoCLIC" from "amorphous organo-metal complex" is less clear. I don't think the two are synonymous - a "complex" implies a particular binding mechanism and molecular arrangement that I think is more ambiguous with the term nanoCLIC - but I think it is important to put this concept in the context of prior conceptualizations of amorphous organo-mineral coprecipitates.
Lines 50-51: These studies cited prior (Kinyangi, Wan, Solomon, etc.) do not to my knowledge speak to stability under agricultural practices - it should be made more clear that this is a potential application that is being leveraged in this study.
Lines 52-53: The link between C stabilization by SRO minerals to the data presented by the cited study is not clear.
Lines 55-57: This statement seems like a good summary of the key question. However, there are a number of potential considerations here in terms of variation among Andosols (age and development, for example, e.g. in Torn et al. 1997 and many other variations) - I think some qualifying language here is needed (e.g., "in some situations Andosols may have a more amorphous constitution than earlier proposed models" or similar).
Lines 60-61: It is stated above that this is already known to be the case ("Instead, organic carbon is primary associated in the form of nanosized coprecipitates...") - further clarification of this section is needed to highlight the specific knowledge gaps.
Line 64: “the one…” It's not likely that there is only one type of MOA susceptible to transformations under disturbance. Suggest editing to "types of MOAs" or similar.
Line 66: “crop soils…” In general, the terms cultivation, cropping, crop, etc. are used interchangeably, and the set-up would benefit from some consistency and clarification of what exactly is meant by this in the context of MOA disturbance. To me, "cultivation" speaks more towards physical disturbance (tillage) while "crop/croplands" is a more general distinction. A definition of what the authors have in mind as the primary contrast/focus as it relates to MOA disturbance would help frame this more clearly.
Lines 67-68: Intensified root and microbial activities after cultivation/tillage? Intensified root and microbial activities within cropping systems more generally? The direction of this effect seems specific to the nature of the cropping system. This ties into the comment earlier about setting some boundaries on the primary contrast of interest.
Lines 70-71: Do these citations (Li, Newcomb, etc.) speak to MOA destabilization in general, or with respect to cultivation/agricultural management? Please specify.
Line 79: “This study aims…” From the abstract, it seems that the focus of this study is a contrast between a forested area and an adjacent cultivated agricultural field within one site (and from below, a site specifically with Fe-poor parent material). While this is probably just needing rewording here (this study isn't determining MOAs within Andosols generally, but Andosols within the specific context of the study site), the application of the findings in Andosols in general seems to pop up in other locations in the abstract (such as the last sentence of the abstract) as well as in the justifying material above. Within Andosols, there is a huge variation in mineralogy and by extension possible MOA types based on age, parent material, climate, etc., and it's important to frame the findings with more specificity (particularly to the "Fe-poor" characteristic).
Lines 80-82: This study doesn't seem to be comparing parent materials: is this objective primarily in contrast to prior observations (and is prior work representative of the variation within Andosols as mentioned above...?)
Line 85: “in Andosols…” Andosols of one specific age, parent material, climate, etc. How can this contrast be made if you're not comparing Andosols with varying degrees of amorphous mineralogy vs. SRO phases (vs. more crystalline phases)? The framing of these hypotheses would benefit from higher specificity and ties to the actual contrasts being made in the study.
Lines 89-91: While it's reasonable to assume some degree of similarity between these locations, and justified to contrast them with respect to land-use history, there is a caveat about interpretations of "change" if the initial characteristics of the cultivated side weren't determined before cultivation began three decades ago. This is a general note as a suggestion for considerations/limitations of interpretation of study findings.
Lines 96-98: Were these equivalent pedogenic horizons at 10-20 cm depth? Did the forest site have an organic horizon, for example, that would alter the absolute depth to a mineral horizon? Since the primary conclusions are between these single samples, the validity of the contrast needs further development. How many samples were collected for each site? It reads as one bulk sample; while sample limitation is a common situation for these high intensity imaging and characterization analyses, the sampling procedure and any replication/sample compositing relevant to interpretations needs to be clarified here.
Lines 100-101: Following the comment above, changes relative to the baseline need to be carefully justified as a "change" without initial samples for contrast. Also, missing period between sentences.
Lines 102-103: It is not immediately clear that this step is not what was used for imaging analysis. Please clarify that this process was used for bulk characterization.
Lines 119-122: While all sample preparation procedures have benefits and limitations, I think it's important to consider potential artifacts from the separation and preparation process that may cause the observed materials to be altered relative to the intact soil structure. For example, could there be a difference in the stability against sonication of nanoCLIC-type associations in comparison to surface adsorption of OM on SRO minerals? Could air-drying induce aggregation artifacts and lead to more clustered spatial structures? (This is especially important considering the secondary interactions between SRO phases and co-precipitates, which is super interesting!) Consideration of potential limitations or biases is needed for final interpretations of the findings by the reader.
Lines 130-131: Why the imbalance in number of observations? This could affect overall interpretations of the degree of variability within the sample, based on more observations across a wider sample area, for example.
Lines 132: What is meant here by micrometric representativeness? Please clarify.
Lines 141-145: To fully assess potential for beam damage, the relevant metric is that of electron dose (not duration, though that is also helpful information). Can electron dose (e.g. e per area per time) be calculated from available imaging parameters?
Lines 169-173: With the high resolution imaging emphasis, a large number of replicate samples is not feasible, but for these bulk analyses, it's important to keep in mind that these values are only for one sample - the range around these values is unknown and a direct quantitative contrast (e.g., 50% decrease) should be made with caution given the lack of assessment of variation in properties within the site-by-management combination. I think this information is useful for reference, but should be very clearly stated that it is a very limited sample set.
Lines 176-179: Where is the range (50-70%) coming from? Differences in each element? There is no clear "quantitative" difference with no replication or statistical tests. Since the magnitude of the change is large, that is worth mentioning, but needs to be carefully explained in the text.
Lines 183-186: These are interesting and relatively high magnitude changes, but presenting data in figure format and stating these values as such in the text with no assessment of variability across the site (these seem to just be one profile/point) could allow for a broader take-away than is justified by the scope of the study. Again, I think it's helpful to have these values for context for the high-resolution imaging results, but need to be more constrained in how they are stated/interpreted.
Figure 1: Following comments in the text, the presentation of single-measurement data as such needs to be more constrained.
Lines 203-205: For consideration: what fundamentally defines a "co-precipitate"? To me, this implies a process, rather than a characteristic of a material - i.e. the nanoscale amorphous association that was observed. What is the lower size cutoff for a mineral phase with surface adsorption/"non-coprecipitate" properties vs. coprecipitates? For example, ferrihydrite has been described as granular structures just as small as 2 nm in size (in synthesized standards, albeit not in natural systems, see https://www.science.org/doi/full/10.1126/science.1142525?casa_token=QzpbDPaVRzsAAAAA%3A919dCIOhgBNdJLb_o2nG2zh0hH__NQdrI7z4njEFnHcVtdTWe3Kvi4_cXwGVXCHWiHBKgq8fxFangL3s), and nano-goethite forms etc. (e.g., as described in Aaron Thompson's work, e.g. https://www.sciencedirect.com/science/article/pii/S0016703710005740?casa_token=LwKJiKoK_FUAAAAA:sopD36o1OQCTo3EEXacXJ7FwRwpHtDUua4DKvNekNBreNX0NhGhxpR7aXIeVDQHHez2b2WjFkyal). (I recognize these are both Fe phases that are less expected in this system, but just examples that come to mind). I don't disagree that the observations resemble a more homogeneous distribution that is co-precipitation processes, but I'm not sure that it is as clearly defined as stated. More rationale here as to definition of these specific process-based forms is needed for clarity.
Figure 2: The line profiles I think nicely show the increase in C signal in the amorphous phase, but I wonder about characteristics directly at the interface of the rod-like structure. The line plot implies that there is a non-linear increase in C right at the transition between phases (as indicated by the Si line profile). Were any interesting patterns found with respect to the boundary between phases? It may be partially an artifact of deposition and drying of samples, but it seems that the amorphous phases are also distributed around the edges of the rod-like mineral, which suggests there might be a secondary interaction that is important here in overall stabilization of OC in the crystalline-non-crystalline association. Note: I see this is introduced in the discussion, but more direct identification of this result is needed earlier.
Line 231: As mentioned above, I think of ferrihydrite phases being smaller in size, but admittedly much of this is based on synthetic studies rather than direct observations in natural soils.
Lines 233: “Consisting of a few atoms…” There is a jump from the lower end of resolution (15 nm) to sizes of a few atoms as described for nanoCLICs. I think that you can definitively say that they are less than 15 nm in size, but can you distinguish between size classes below that minimum resolution?
Figure 3: Given the focus of the paper on comparisons between forest and cultivated soil, it seems like it would contribute to the overall conclusions/interpretations to include contrasting images when they are available between the two systems in the main text.
Lines 270-272: An alternate way of looking at this is that the non-C only regions had a relatively high enrichment of 286.6 (phenolic/ketones), especially the C+Al+Fe rich area. The spectra seem to be of high enough quality that deconvolution procedures to more semi-quantitatively compare relate intensity in these regions would be a useful addition to more comprehensively compare relative abundance of C functional groups with regions of different element proportions, which I think is a really interesting element of this work.
Figure 5: What does "C=H" represent? Please clarify what functional groups are being associated with the 285 region.
Lines 279-383: As mentioned above, take into consideration work done to characterize amorphous organo-metallic complexes in Andosols, as well as the size variation possible within these SRO minerals (nano-sized FH, proto-imogolite, etc.) and how these might fall (or not fall!) within the concept of a "nanoCLIC". As an example: https://www.sciencedirect.com/science/article/abs/pii/S0016706122001276
Lines 289-290: I agree that C is much more spatially coupled to the amorphous phases. But, there also seem to be secondary associations between the amorphous phases and the imogolite phase (as shown in Fig. 2), i.e. an association between the coprecipitated phases and the imogolite surface, as you illustrate in Fig. 6. It's not clear through the text up to Fig. 6, though, and in the set-up/abstract the potential role of interactions between SRO phases and truly amorphous phases is not thoroughly highlighted/clear. I think that is a really exciting finding and is somewhat diluted by the takeaway in the abstract and elsewhere (e.g., line 28-30).
Lines 330-331: This is a large change, but as noted above, without any replication this value should be presented with extreme caution.
Minor Editorial Comments:
Line 2: Minor edit: strike "," after "Although"
Line 21: Throughout, I would suggest Andosol should be capitalized.
Line 31: Minor edit: Suggest editing to "cultivated"
Line 40: Suggest editing to "loss of C"
Line 38-39: Suggest editing to "which is essential for..."
Line 69: Suggest editing to "which can"
Line 88: Suggest editing to "organic matter adsorption onto..."
Line 248: Suggest editing to "further investigated"
Citation: https://doi.org/10.5194/egusphere-2024-2933-RC2
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