the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Nov 2024
Missing the input: The underrepresentation of plant physiology in global soil carbon research
Abstract. Plant processes regulating the quantity and quality of soil organic carbon inputs such as photosynthesis, above- and belowground plant growth, and root exudation are integral to our understanding of soil carbon dynamics. However, based on a bibliometric analysis including almost 50 000 scientific papers, we found that plant physiology has been severely underrepresented in global soil organic carbon research. Less than 10 % of peer-reviewed soil organic carbon research published in the last century addressed plant physiological processes relevant to soil carbon inputs. Similarly, plant physiology was overlooked by the overwhelming majority (>90 %) of peer-reviewed literature investigating linkages between soil organic carbon, climate change, and land use and management. These findings highlight that our understanding of soil carbon dynamics and hence the carbon sequestration potential of terrestrial ecosystems is largely built on research that neglects the fundamental processes underlying organic carbon inputs. We advocate that the active engagement of plant scientists in soil carbon research is imperative to shed light on this blind spot. Long-term interdisciplinary research will be essential to develop a comprehensive perspective on soil carbon dynamics and to inform effective policies that support soil carbon sequestration.
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RC1: 'Comment on egusphere-2024-3644', Stefano Manzoni, 11 Dec 2024
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Raza and co-authors discuss the lack of plant physiological processes in soil carbon cycling research. This issue is highly relevant as much of soil carbon cycling is driven by plant activity, in particular in the rhizosphere or related to plant symbionts. Their results are clear—the large majority of soil carbon studies do not include plant physiology even though a plant physiological perspective could provide novel or more mechanistic insights on soil carbon processes.
My main concern is that only a few examples of plant physiological processes that matter for soil carbon cycling are mentioned, and similarly not many examples of empirical approaches to characterize plant physiology are reviewed and discussed. For example, what physiological processes are relevant for plant-mycorrhizae interactions, which in turn affect soil carbon processes? What is the fate of rhizodeposits in soil fractions (Cotrufo et al., 2022)? How can isotopic tracer methods be used (only briefly mentioned now)? To make this work more complete, I would suggest going a bit deeper into key processes (or groups of processes) and also check with the bibliometric analysis is some of them are missing more than others. Examples of process groups could be: rhizodeposits, carbon allocation to roots or mycorrhizae, litter quality (chemistry, element ratios), root properties affecting soil structure (aggregation, microporosity).
Most of the examples of plant physiological processes in the manuscript deal with direct plant effects on soil carbon cycling, but there are also indirect effects that might be worth mentioning. For example, plants affect soil carbon cycling indirectly via changes in the soil microenvironment (especially soil moisture), and such effects are mediated by plant physiology (in the example of soil moisture, via stomatal regulation, water redistribution through roots etc.).
Moreover, I wonder if the gap between soil-focused and plant-focused studies extends also to theoretical or modelling papers. For example, there is a rich literature on linkages between roots and soils via rhizodeposition or symbiotic interactions (Cheng, 1999; Darrah, 1991; Franklin et al., 2014; Smith and Wan, 2019; Zelenev et al., 2000), but I would expect the large majority of soil carbon cycling models focus on microbial or soil faunal processes and only consider plants a source of carbon and nutrients (without accounting for plant physiology).
Specific comments
Figure 1: rather generic representation of soil-plant interactions, nearly without plant physiology. I would include more graphical representations of the physiological processes considered relevant (and overlooked)—e.g., there is no mention of nutrient uptake and re-cycling via litter with feedbacks on carbon cycling; mycorrhizae and microbes in the rhizosphere are linked to plants in different ways. Indirect effects of plant physiology on soil carbon cycling are also not shown.
L59: what does “address” mean? Probably hard to say based on bibliometric analysis, but by reading the papers you found, did you notice if plant physiology is perceived as a driver of soil processes, or rather a consequence of soil processes? These different perspectives could add some nuance to the results presented here.
L96: I don’t disagree with the point raised here, but I wonder if soil scientists perhaps focus on the net result of plant physiological processes on soils, rather than those processes per se. For example, in virtually all soil carbon balance calculations litterfall is an input, but it might be less important what specific physiological process led to that amount of litter—e.g., higher litterfall could be due to higher photosynthesis or lower respiration.
References
Cheng, W. X.: Rhizosphere feedbacks in elevated CO2, Tree Physiol., 19, 313–320, 1999.
Cotrufo, M., Haddix, M., Kroeger, M., and Stewart, C.: The role of plant input physical-chemical properties, and microbial and soil chemical diversity on the formation of particulate and mineral-associated organic matter, Soil Biol. Biochem., 168, https://doi.org/10.1016/j.soilbio.2022.108648, 2022.
Darrah, P. R.: Models of the Rhizosphere .1. Microbial-Population Dynamics around a Root Releasing Soluble and Insoluble Carbon, Plant Soil, 133, 187–199, 1991.
Franklin, O., Naesholm, T., Hoegberg, P., and Hoegberg, M. N.: Forests trapped in nitrogen limitation - an ecological market perspective on ectomycorrhizal symbiosis, New Phytol., 203, 657–666, https://doi.org/10.1111/nph.12840, 2014.
Smith, G. R. and Wan, J.: Resource-ratio theory predicts mycorrhizal control of litter decomposition, New Phytol., 223, 1595–1606, https://doi.org/10.1111/nph.15884, 2019.
Zelenev, V. V., van Bruggen, A. H. C., and Semenov, A. M.: “BACWAVE,” a spatial-temporal model for traveling waves of bacterial populations in response to a moving carbon source in soil, Microb. Ecol., 40, 260–272, 2000.
Citation: https://doi.org/10.5194/egusphere-2024-3644-RC1 -
RC2: 'Comment on egusphere-2024-3644', Grant A. Campbell, 13 Dec 2024
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Reviewed by Grant Campbell, University of Aberdeen
General Comments
Really interesting piece of research highlighting the lack of awareness into plant physiology in global soil carbon research. The authors have shown strong understanding in where plant physiology could be utilised more in the literature and have presented in clear fashion the process they undertook in evaluating and assessing this as part of a review topic. I also thought the graphics were really well presented on the whole and were easy to interpret (minus one small adjustment).
At this moment, only minor revisions are required and some of these are seen as technical/discussion ideas for the authors to consider. I attach my comments of the manuscript under supplementary PDF.
I place my comments under the particular headings also in order to help the authors shape the revised version of which I am happy to look over again if required. I am also happy to take any comments from the authors should they be not clear of my thoughts.
Specific comments and technical corrections
Line 1 - Suggest title change to "Missing the input: The lack of awareness in plant physiology in global soil carbon research
Line 13 - land use and management. Remove the first "and".
Line 13/14 - I would reword this a little bit. Something like "understanding of both soil carbon dynamics and subsequently carbon sequestration potential"
Line 15 - "continued? dynamic? ever developing? Suggest using one of these words to improve this sentence
Line 17 - and design? (when talking about implementing policies)
Line 22 - Bolden Figure 1 text
Line 31 - such as? Flooding? Drought? Other?
Line 32 - I would reword this sentence a bit more clearly. Something like"Environmental conditions not only influence the stabilization of soil organic carbon but also significantly affect plant physiology, which in turn impacts the quality and quantity of carbon inputs to the soil."
Line 32 - 33 - This sentence could do with a rejig as at present it looks clunky. Suggest something like ""For instance, a global meta-analysis encompassing both natural and managed ecosystems across various biomes demonstrated that rising temperatures result in a shift of carbon allocation from shoots to roots, particularly in drier climates"
Line 40 - you mention temperature but what about precipitation or snowmelt? Suggest adding a source or two to discuss other conditions
Line 43 - Bolden Figure 1
Line 45 - 51 - I would just keep Figure 1 in this text as bold only. I am also worried that we have too much text here to discuss the figure., Suggest, if possible to simplify the text.
Line 54 - Underrepresented as not being discussed in science overall too or just with respect to purely soil organic carbon? Perhaps make this distinction clearer.
Line 57-58 - Bolden Supplementary Table S1
Line 59 - I would place the 3,907 out of 49.,971 here with (8%) in brackets to make this a bit clearer and to make sure the reader is drawn purely to just Figure 2A. "revealed that out of 49,971 publications, just 3,907 (8%)..." Also bolden Figure 2A
Line 64- Bolden Figure 2A
Line 65 - 69 Perhaps suggest a different colour here, in Graphic A, to accommodate for colour blind people/photocopy/printing? Avoid greens if possible. I would keep only Figure 2 boldened and keep rest of text as normal text
Line 73 - Bolden Supplementary Table S1
Line 75 - Bolden Figure 2B
Line 80 - Bolden Supplementary Figure S1
Line 84 - A couple of policy items should be noted here to potentially add onto this:
The European Green Deal
EU Soil Thematic Strategy
UN Sustainable Development Goals
Paris Agreement
EU CAP reforms perhaps as well?Line 87 - Bolden Figure 2B
Line 90 - Bolden Supplementary Figure S1 - change from Supplemental
Line 93 - this is definitely true in Europe. Perhaps add more sources to back up this point?
Line 95 - Bolden Figure 2 and Supplementary Figure S1
Line 102 - Add (3D) next to three-dimensional
Line 103- Suggest the use of FAPAR as well as a possible option for a toolset - https://en.wikipedia.org/wiki/Fraction_of_absorbed_photosynthetically_active_radiation
Line 104 - Break up a bit what we mean by mathematical models? I take it we mean classification and regression approaches? Machine learning overall?
Line 109-110 - Agree. Suggest noting the use of MRV standardisation as a potential concept for this (Monitoring, Reporting and Verification)
Line 124 - Investment in scientific funding but also in terms of industry interest too I would argue. Especially for working farming environment
Additional comments (do not to be addressed but should be considered)
Line 41 - Has there been any discussion about the effect of seasonality in this work? Might be good to address or look at contrasts between even summer and winter months.
Line 62 - Does this suggest an increased awareness in plant physiological interactions with soil organic carbon OR also greater accessibility to access more information/data? Something to think about perhaps rather than to address directly.
Line 118 - 120 Should this become a requisite in future soil surveying? European activities such as LUCAS should in my view look at this but also the development and movement of the EU Soil Monitoring Law could play a factor here in whether this could be effectively achieved. I would also argue that the need for continued development and expert knowledge should be being passed over across interested people.
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