the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Aug 2025
Novel oxalate-carbonate pathways identified in the tropical dry evergreen forest of Tamil Nadu, India
Abstract. The tropical dry evergreen forest (TDEF) is a vital but endangered ecosystem in India, crucial for supporting cultural services, biodiversity, and organic carbon storage. The oxalate-carbonate pathway (OCP) is an understudied process in which plants and oxalotrophic microorganisms convert atmospheric CO2 into calcium carbonate (CaCO3) within plant tissues or tree-adjacent soils. Yet, despite its significance, the OCP has not been studied in the TDEF of India. This study aimed to assess novel OCP systems associated with three TDEF diagnostic species (Diospyros ebenum, Lepisanthes tetraphylla, Sapindus emarginatus) and one local agroforestry species (Artocarpus heterophyllus) in the restored- and primary-TDEF of Tamil Nadu. Surface soil samples (0–10 cm) were collected from an adjacent and control distance away from trees, along with tree biomass samples, and investigated for oxalate production (microscopy and enzymatic assays), oxalotrophic microbial communities (frc gene sequencing), and tree-induced shifts in soil biogeochemistry. Oxalate was detected in all species (4.4±3.2 % dry weight), accompanied by CaCO3 precipitation on biomass. Oxalotrophic microbial communities were dominated by Actinomycetota (86 %), which were also identified in electron micrographs. Soil biogeochemical shifts indicative of active OCPs were also observed, particularly in the hollowed-out trunks of the TDEF trees. However, differences between adjacent and control soils were less pronounced, suggesting that monsoon conditions leached OCP precipitated CaCO3 from the adjacent soils. This research provides the first evidence of active OCPs in Indian TDEF, highlighting a previously unrecognized mechanism for organic and inorganic carbon cycling in this threatened ecosystem.
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Status: open (until 19 Sep 2025)
- RC1: 'Comment on egusphere-2025-3388', Catherine Clarke, 12 Aug 2025 reply
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RC2: 'Comment on egusphere-2025-3388', Anonymous Referee #2, 09 Sep 2025
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The presented work is excellent, with a complete state-of-the-art on the Oxalate-carbonate pathway presented at full, a clear local Indian/monsoon context, and a transparent experimental design. The results are essential for supporting understanding of our planet's natural carbon removal mechanisms, via plant-microbial-soil interactions, that can be supported in OCP-enabling forest ecosystems, as long as Ca sources are additional, and plant oxalate inputs sufficient for alkalinity.
I find the overall quality of the work to be stellar, with the exception of low replication, though the authors have dealt this with transparent and appropriate statistical methods. In my sense, this work should be considered as a primer, for larger extensive field studies, supported by local Indian scientists and beyond. It showcases the potential of the OCP. Other than that I have no other criticisms. What I find exciting is the potential quantities stored as calcium carbonate in bark, with perhaps another avenue to explore , using bark content as another stable inorganic carbon stock (easily measurable, and verified), in additional soil carbonates, and flushed bicarbonate. Perhaps the authors could dedicate a few sentences in their discussion to showcase a comparison between inorganic carbon stock (in bark), and potential bicarbonate flushing underground. These are essential for project developers, and MRV requirements.
None technical corrections noted.
Citation: https://doi.org/10.5194/egusphere-2025-3388-RC2
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- 1
This study aimed to assess botanical, microbiological, biogeochemical evidence for the oxalate carbonate pathway in the Dry Evergreen Tropical Forest of Tamil Nadu, India. This pathway is interesting as it can potentially convert organic carbon into inorganic calcite which can be a highly stable form of carbon, depending on the environment.
I thoroughly enjoyed reading this paper, it is a very interesting and important piece of research. Quantifying organic-inorganic carbon conversions is necessary to establish if processes like the OCP contribute substantially to global carbon cycles. This is particularly important in an environment of ecosystem disturbance where these cycles could be disturbed or stopped.
My main criticism of the work is already highlighted by the authors in their discussion and that is the shallow sampling depth limits interpretation. Thus, the possibility of deep pedogenic or geogenic carbonates could not be ruled out which makes the interpretation difficult. Despite this I think the paper is worthy of publication and makes an important contribution.
A few questions:
1) Did the parent materials initially contain carbonates? I could not find this information in the supporting Information, but given their origin I guess it is possible? I see Calcisols are mentioned in the underlying Manaveli clay (Lines 155/6). I think this is quite important to mention as it affects the Ca source and calcite cycling.
2) Related to this, I was also wondering about the influence of termites. I assume they exist in this environment? How did the soil enter the trunk? In other environments it is often through termite activity and if so, is there the possibility that calcite-rich subsoils were brought up into the trunks as castings? S. Fig1-C looks a bit like it could be a termite casting? More information on the trunk soil and its potential source is very important for the study, as this is one of the main pieces of evidence that supports the hypothesis.
There are a few more specific comments below
Lines 90-95: it feels like locations could be listed more eloquently.
Lines 100 – 105: I like hypothesis testing, but I would leave aspects like underexplored and undetected out of the central hypothesis, those are hard to test. The hypothesis should just read …… the OCP is a substantial C sink in India’s TDEF ecosystems. This can be tested. The underexplored and undetected can be added elsewhere to highlight the research gap.
Section 2.2.1: I would appreciate the explanation of “Dry” in the name of the TDEF, the rainfall is very high, so is dry used to highlight the ustic climate with a dry winter? When I read the title, I was thinking semi-arid until I saw the rainfall data.
I am a little confused by Figure 1. If I look at site D in Fig 1B then it looks like it is on the sandstone, but in 1C it is shown as alluvium?
Lines 150 – 155: Could you mention if the parent materials contained calcite in them? Also maybe expand on the clay soils that can contain calcisols?
Section 2.2.2: I think there needs to be more information on the control soils? Was it bare soil/grassland or were there other tree species. Was the surrounding vegetation checked for oxalates? Also, some pictures in SI of the trees and controls would be great.
Lines 160-165: Some more information on the trunk soil is required, is it the same colour and texture as the adjacent topsoil? How did it get there? Termites? They often bring deep subsoil material up to build protective castings in trees. If so, this might be an alternative explanation of how the carbonates got there.
Lines 170-175: The soils were milled, but it is unclear what these soils were used for?
Lines 120-180: It is mentioned that plant material was milled prior to SEM analysis. Were the samples rinsed prior to milling to remove dust particles. I am not sure if windblown marine dust aerosols could add CaCO3 to plant parts?
Lines 195 – 200: I see cobalt hexamine was used for exchangeable cation extraction, with the reason that it is less aggressive towards CaCO3. It is also worth mentioning that Ca oxalate is also susceptible to aggressive extraction agents.
Line 214: was should be were
S. Table 7: I think the sum of basic cations have been calculated incorrectly for this table?
Lines 380-385: When you talk about the CEC are you referring to sum of total cations? This would be best described as effective cation exchange capacity (ECEC). CEC is measured in a buffered salt usually at pH 7? It is remarked that the CEC is highest in the trunk and this was driven by higher Ca and Mg concentrations. I think the choice of words here is incorrect. CEC is not driven by cations, rather a soil can accommodate more cations because it has more exchange sites. Was there a textural difference between the soils in the trunk and the adjacent soils, more clay maybe? Looking at the substantially higher SOC in the trunk, I think the higher ECEC is probably related to the exchange sites on the SOC? Is the SOC higher in the trunk because it contains decaying plant material?
My final point on the cation data is: Given how large the difference is between the pH in KCl and DI, I am surprised that the exchangeable Al is so low. I am not familiar with Co hexamine, but usually exchangeable Al+ H+ is measured in a 1M KCl extract. I mention this as Al does feature in your discussion later and the reserve acidity associated with exchangeable Al may be the reason for the similar pH of the control and adjacent soils.
Lines 544 – 545: It is noted that the current study area has lower rainfall than the site used by Herve et al (2018), but the rainfall described in the methods would suggest its higher (1225 vs 1100 mm/y)?
Finally just a comment: It is really interesting that the pH of the adjacent soils and the control soils were no different, you would think over time, there would be some changes even to the topsoils. Maybe another metric to include in these high rainfall regions would be exchangeable acidity or some estimation of reserve acidity. This would then show if any sort of pH buffering was occurring in the adjacent soils.