Sequestering carbon in the subsoil benefits crop transpiration at the onset of drought

Turek, Maria Eliza; Nemes, Attila; Holzkämper, Annelie

doi:https://doi.org/10.5194/egusphere-2023-1077

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https://doi.org/10.5194/egusphere-2023-1077

Preprints

19 Jun 2023

| 19 Jun 2023

Sequestering carbon in the subsoil benefits crop transpiration at the onset of drought

Maria Eliza Turek, Attila Nemes, and Annelie Holzkämper

Abstract. Increasing soil organic carbon is promoted as a negative emission technology for the agricultural sector with a potential co-benefit for climate adaptation due to increased soil water retention. Field-scale hydrological models are powerful tools to evaluate how the agricultural systems would respond to the changing climate in upcoming years and decades, to predict impacts, and look for measures that help decrease drought-driven crop stress under current and future climatic conditions. We quantified how different levels of soil organic carbon (SOC) additions at varied soil depths are expected to influence drought-induced transpiration reduction (Tred_dry) in maize cultivated in Switzerland. Parameterization of the model based on a pedotransfer function (PTF) was validated against soil moisture data from a long-term lysimeter experiment with a typical Swiss soil and the model was subsequently applied under climate forcing between 1981 until 2099 representative of three distinct climatic sites of Switzerland. We used the same PTF to indirectly assess the effects of SOC additions in different depths on soil hydraulic properties. We found a threshold in both added amount of SOC (2 % added) and in the depth of sequestering that SOC (top 65 cm) beyond which any additional impact appears to be substantially reduced. However, adding at least 2 % SOC down to at least 65 cm depth can reduce Tred_dry in maize, i.e. increase transpiration annually, but mostly at the onset of summer drought by almost 40 mm. We argue that SOC increases in subsoils can play a supporting role in mitigating drought impacts in rain-fed cropping in Switzerland.

Received: 23 May 2023 – Discussion started: 19 Jun 2023

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Maria Eliza Turek et al.

Status: closed

RC1:
'Comment on egusphere-2023-1077', Anonymous Referee #1, 30 Jun 2023
Overall comments:
This study evaluated how variation in SOC enhancements across the soil profile could contribute to alleviating drought stress on maize at a Swiss site using an agro-hydrologic model where key soil hydraulic parameters were modified based on highly validated pedo-transfer functions. The authors also tested the effects of these SOC changes under future climate conditions using downscaled climate projections. Overall, the authors found that moderate increases in SOC down to 65cm depth could provide important drought adaptation benefits in the early summer, but with diminishing returns after these thresholds.
This was a very interesting and thoughtfully designed study that adds value to the literature on SOC-water-plant-climate interactions in a few key ways. First, the authors examine the sensitivity of feedbacks of enhanced SOC (taken as given) on plant transpiration and interactions with drought conditions – an interaction that is still highly uncertain and not always a chief topic of study in hugely expanding literature on SOC and climate change. Second, the authors tackled an important question dealing explicitly with varying over depth of SOC additions – also important as emphasis increases on building SOC (particularly for climate change mitigation). Where SOC is enhanced in the soil column matters both biogeochemically and biogeophysically. Third, the study looks at these interactions and the efficacy of building SOC under current and future climate conditions – a critical consideration that should be systematically investigated with additional model runs both at the site-level and regionally (notwithstanding limitations on ascertaining SHPs at larger spatial scales). These responses obtained were explained decently, and seem to be consistent with emerging findings across the literature the authors’ cited that while SOC can provide benefits in times of drought, these benefits can be limited and are regionally heterogenous.
There are a few clarifications and smaller points that I raise below for the authors to consider. I consider these to be minor revisions, however, and I’m happy to support publication of the manuscript after these are addressed.
Minor comments:
Introduction:
Lines 58-63: there is discussion of soil structure here and I understand that one way SOC in part impacts the soil water retention is through changes in porosity, and structural characteristics. However, the PTFs seem to only consider soil texture, and this does also play a leading role in soil water storage and flows – an overview of the role of texture and SOC influences does not really appear in your intro despite being the basis for your PTFs, and so I wonder if a few quick sentences should be added here. I’ll note that structure is rarely explicitly represented in regional, gridded models.

Lines 77-92: McDermid et al 2022 also leveraged a set of PTFs for SHP calculations that included SOC, which they varied over for global simulations and found SOC declines reduced soil moisture. However, to your interesting point, they found decreases in conductivity with SOC declines, which as you highlight is textbook but opposite of what you found. I see you offer an explanation in your Discussion – and I’ve added a comment on this below.

Methods:
I don’t see suction (h) as predicted with your PTFs in Table 3? Is this changing in response to SOC too?

Likewise, there can be important interactions between bulk density and SOC. However, I realize these are difficult to disentangle. Are you actually accounting for these feedbacks here? And if not, how would you expect these feedbacks to modify your results, if at all?

I would provide a few sentences on the formulation and bit more motivation for using Tred(dry). I think this is a fine metric for analysis but not everyone may be familiar with it and there are many ways of evaluating the impacts of water limitation on crops/plants. Another way to look at this too would be to pull out of the 990 simulations those drought (low precipitation and/or high VPD) years and composite those responses compared to “average” years (excluding the most impacted years).

Results:
So my most major comment is on Figure 5 – I think the presence of multiple lines of the same color, one showing the absolute interannual Tred and the other the time-varying offset, is a bit confusing. I understand that the same color links the SOC scenario, but it takes a couple of looks to get it all straight. The authors could leave this figure as is, but I think the figure would benefit from some slight re-working. E.g. maybe using a slightly different color for the offset lines (e.g. bright yellow vs duller yellow or dashed yellow) or maybe plotting separately just one number for the long-term mean Tred across the SOC scenarios (since there does not appear to be huge variability in this) and then leaving the interannual offset trend lines as they are. I also think the “offset” terminology doesn’t quite capture the value of the measure here – when I think offset I’m thinking displacement of some sort. Maybe just the word “change” or “delta” would suffice or maybe even the Tred “gain” or “benefit”

Discussion:
Lines 408-421: Per my comments on the Intro, I appreciate your explanation for the reduced conductivity with SOC gains – very interesting. You provide a possible mechanism, but it’s not completely clear how this maps onto your results by way of what processes are included in your model. Would these interactions – the tortuosity of the conductive pathways – be what mediates this response in your model specifically?

Maybe a quick word on how your experiments be impacted with dynamic vegetation (prognostic LAI, since it appears your LAI was prescribed?)?

Lines 466-469: Given your explanation here on the early season match between available water and available water capacity, I was wondering if and how increasing winter (or early spring) precipitation factor into the increasing offset of Tred shown in Figure 5 (and from Figure 6, it would seem this is driven by early season)?

Lines 486-490: May also summarized in Powlson (see refs below)

References:
Powlson, D. et al (2014) Limited potential of no-till agriculture for climate change mitigation, Nature Climate Change volume 4, pages 678–683 https://www.nature.com/articles/nclimate2292
McDermid, S.S., E. Weng, M. Puma, B. Cook, T. Hengl, J. Sanderman, G.J.M. De Lannoy, and I. Aleinov, 2022: Soil carbon losses reduce soil moisture in global climate model simulations. Earth Interact., 26, no. 1, 195-208, doi:10.1175/EI-D-22-0003.1.
Citation: https://doi.org/10.5194/egusphere-2023-1077-RC1
- AC1: 'Reply on RC1', Maria Eliza Turek, 31 Aug 2023
  
  Dear Referee #1,
  We appreciate very much your detailed and constructive comments, which greatly helped us to improve our manuscript. The detailed responses to your suggestions and comments is placed on the supplement file of this response.
  
  Sincerely,
  Maria Eliza Turek, Attila Nemes, and Annelie Holzkäemper
  
  Citation: https://doi.org/10.5194/egusphere-2023-1077-AC1
RC2:
'Comment on egusphere-2023-1077', Anonymous Referee #2, 02 Aug 2023
This very interesting modelling study presented by Maria Eliza Turek and co-workers examines the potential benefits of SOC addition to Swiss maize cultivation on crop transpiration.
The manuscript itself it nicely written and well-structured – the ideas behind the study are reasonable and well explained and the proceeding during the modelling approach is described in a clear way. The outcomes of the study show that there is a (small) benefit of SOC addition on water retention, and that incorporation into depths below 65 cm do not lead to additional gains for the presented Swiss sites.
However, I am a bit doubtful about the scenarios considered in this study. I understand that the addition of SOC in the topsoil is the main focus and certainly what will happen under adapted management practices. The subsoil so far is still kind of a debate and a lot of research is happening at the moment to better evaluate the importance and future of subsoil SOC under future agricultural practices and climate change. Some studies, however, already point out that an SOC enrichment in the topsoil is likely to go along with a (slight) depletion of SOC in the subsoil (and directly below the enrichment layer, respectively). However, this certainly is dependent on the agricultural measure and most studies (and meta-analyses) of recent years have focussed on tillage (e.g. Krauss et al. 2022: https://doi.org/10.1016/j.still.2021.105262; Meurer et al. 2018: https://doi.org/ 10.1016/j.earscirev.2017.12.015). Nevertheless, a recent study from Germany (Skadell et al. 2023: https://doi.org/10.1016/j.agee.2023.108619) showed that, averaged over a variety of management practices, SOC stocks still increase in the upper and lower subsoil. Still, the variation across the soil profile was very different between different management practices. If I understand the modelling correctly, the management was kept the same during the simulations, while the SOC content in the soil increases. I actually think that the presented scenarios are kept too simple and too optimistic. The authors should consider to extend them towards a potential “subsoil depletion” scenario.
Some minor comments:
Figure 1: I assume that it is minimum and maximum temperature that is shown in the top panel?
249 – 261: do I understand correctly that no calibration and “only” validation was performed?

272 – 277: I am not sure if “reduction factors” is a good expression here. My first thought was that the authors assume a reduction in SOC below the enrichment layer (which has also been shown in some studies, see studies above), but from what I understand from Table 4, so was simply the increase reduced. What about the increase of topsoil SOC at the expense of subsoil SOC (see my comment above)?

337 ff: these results are interesting. However, does this mean that the authors assumed unlimited access to (ground-)water? What was the reduction if yield given the changes in precipitation (and certainly temperature) patterns?

368 – 370: this is not clear to me. From what I understood did the authors rather assume a constant SOC level in the soil, but not a constant SOC addition – the latter would lead to a build-up over time. Or was the SOC level adapted annually within the model?

475 – 477: yes, that´s an important point. How was this handled in the model? Did the roots reach a maximum depth/length?
Citation: https://doi.org/10.5194/egusphere-2023-1077-RC2
- AC2: 'Reply on RC2', Maria Eliza Turek, 31 Aug 2023
  
  Dear Referee #2,
  All of your comments and suggestions were very much appreciated and addressed in this review. In the attached document, we placed the detailed responses for all the issues raised. We thank you very much for your contributions.
  Sincerely,
  Maria Eliza Turek, Attila Nemes, and Annelie Holzkäemper
  
  Citation: https://doi.org/10.5194/egusphere-2023-1077-AC2

Status: closed

RC1:
'Comment on egusphere-2023-1077', Anonymous Referee #1, 30 Jun 2023
Overall comments:
This study evaluated how variation in SOC enhancements across the soil profile could contribute to alleviating drought stress on maize at a Swiss site using an agro-hydrologic model where key soil hydraulic parameters were modified based on highly validated pedo-transfer functions. The authors also tested the effects of these SOC changes under future climate conditions using downscaled climate projections. Overall, the authors found that moderate increases in SOC down to 65cm depth could provide important drought adaptation benefits in the early summer, but with diminishing returns after these thresholds.
This was a very interesting and thoughtfully designed study that adds value to the literature on SOC-water-plant-climate interactions in a few key ways. First, the authors examine the sensitivity of feedbacks of enhanced SOC (taken as given) on plant transpiration and interactions with drought conditions – an interaction that is still highly uncertain and not always a chief topic of study in hugely expanding literature on SOC and climate change. Second, the authors tackled an important question dealing explicitly with varying over depth of SOC additions – also important as emphasis increases on building SOC (particularly for climate change mitigation). Where SOC is enhanced in the soil column matters both biogeochemically and biogeophysically. Third, the study looks at these interactions and the efficacy of building SOC under current and future climate conditions – a critical consideration that should be systematically investigated with additional model runs both at the site-level and regionally (notwithstanding limitations on ascertaining SHPs at larger spatial scales). These responses obtained were explained decently, and seem to be consistent with emerging findings across the literature the authors’ cited that while SOC can provide benefits in times of drought, these benefits can be limited and are regionally heterogenous.
There are a few clarifications and smaller points that I raise below for the authors to consider. I consider these to be minor revisions, however, and I’m happy to support publication of the manuscript after these are addressed.
Minor comments:
Introduction:
Lines 58-63: there is discussion of soil structure here and I understand that one way SOC in part impacts the soil water retention is through changes in porosity, and structural characteristics. However, the PTFs seem to only consider soil texture, and this does also play a leading role in soil water storage and flows – an overview of the role of texture and SOC influences does not really appear in your intro despite being the basis for your PTFs, and so I wonder if a few quick sentences should be added here. I’ll note that structure is rarely explicitly represented in regional, gridded models.

Lines 77-92: McDermid et al 2022 also leveraged a set of PTFs for SHP calculations that included SOC, which they varied over for global simulations and found SOC declines reduced soil moisture. However, to your interesting point, they found decreases in conductivity with SOC declines, which as you highlight is textbook but opposite of what you found. I see you offer an explanation in your Discussion – and I’ve added a comment on this below.

Methods:
I don’t see suction (h) as predicted with your PTFs in Table 3? Is this changing in response to SOC too?

Likewise, there can be important interactions between bulk density and SOC. However, I realize these are difficult to disentangle. Are you actually accounting for these feedbacks here? And if not, how would you expect these feedbacks to modify your results, if at all?

I would provide a few sentences on the formulation and bit more motivation for using Tred(dry). I think this is a fine metric for analysis but not everyone may be familiar with it and there are many ways of evaluating the impacts of water limitation on crops/plants. Another way to look at this too would be to pull out of the 990 simulations those drought (low precipitation and/or high VPD) years and composite those responses compared to “average” years (excluding the most impacted years).

Results:
So my most major comment is on Figure 5 – I think the presence of multiple lines of the same color, one showing the absolute interannual Tred and the other the time-varying offset, is a bit confusing. I understand that the same color links the SOC scenario, but it takes a couple of looks to get it all straight. The authors could leave this figure as is, but I think the figure would benefit from some slight re-working. E.g. maybe using a slightly different color for the offset lines (e.g. bright yellow vs duller yellow or dashed yellow) or maybe plotting separately just one number for the long-term mean Tred across the SOC scenarios (since there does not appear to be huge variability in this) and then leaving the interannual offset trend lines as they are. I also think the “offset” terminology doesn’t quite capture the value of the measure here – when I think offset I’m thinking displacement of some sort. Maybe just the word “change” or “delta” would suffice or maybe even the Tred “gain” or “benefit”

Discussion:
Lines 408-421: Per my comments on the Intro, I appreciate your explanation for the reduced conductivity with SOC gains – very interesting. You provide a possible mechanism, but it’s not completely clear how this maps onto your results by way of what processes are included in your model. Would these interactions – the tortuosity of the conductive pathways – be what mediates this response in your model specifically?

Maybe a quick word on how your experiments be impacted with dynamic vegetation (prognostic LAI, since it appears your LAI was prescribed?)?

Lines 466-469: Given your explanation here on the early season match between available water and available water capacity, I was wondering if and how increasing winter (or early spring) precipitation factor into the increasing offset of Tred shown in Figure 5 (and from Figure 6, it would seem this is driven by early season)?

Lines 486-490: May also summarized in Powlson (see refs below)

References:
Powlson, D. et al (2014) Limited potential of no-till agriculture for climate change mitigation, Nature Climate Change volume 4, pages 678–683 https://www.nature.com/articles/nclimate2292
McDermid, S.S., E. Weng, M. Puma, B. Cook, T. Hengl, J. Sanderman, G.J.M. De Lannoy, and I. Aleinov, 2022: Soil carbon losses reduce soil moisture in global climate model simulations. Earth Interact., 26, no. 1, 195-208, doi:10.1175/EI-D-22-0003.1.
Citation: https://doi.org/10.5194/egusphere-2023-1077-RC1
- AC1: 'Reply on RC1', Maria Eliza Turek, 31 Aug 2023
  
  Dear Referee #1,
  We appreciate very much your detailed and constructive comments, which greatly helped us to improve our manuscript. The detailed responses to your suggestions and comments is placed on the supplement file of this response.
  
  Sincerely,
  Maria Eliza Turek, Attila Nemes, and Annelie Holzkäemper
  
  Citation: https://doi.org/10.5194/egusphere-2023-1077-AC1
RC2:
'Comment on egusphere-2023-1077', Anonymous Referee #2, 02 Aug 2023
This very interesting modelling study presented by Maria Eliza Turek and co-workers examines the potential benefits of SOC addition to Swiss maize cultivation on crop transpiration.
The manuscript itself it nicely written and well-structured – the ideas behind the study are reasonable and well explained and the proceeding during the modelling approach is described in a clear way. The outcomes of the study show that there is a (small) benefit of SOC addition on water retention, and that incorporation into depths below 65 cm do not lead to additional gains for the presented Swiss sites.
However, I am a bit doubtful about the scenarios considered in this study. I understand that the addition of SOC in the topsoil is the main focus and certainly what will happen under adapted management practices. The subsoil so far is still kind of a debate and a lot of research is happening at the moment to better evaluate the importance and future of subsoil SOC under future agricultural practices and climate change. Some studies, however, already point out that an SOC enrichment in the topsoil is likely to go along with a (slight) depletion of SOC in the subsoil (and directly below the enrichment layer, respectively). However, this certainly is dependent on the agricultural measure and most studies (and meta-analyses) of recent years have focussed on tillage (e.g. Krauss et al. 2022: https://doi.org/10.1016/j.still.2021.105262; Meurer et al. 2018: https://doi.org/ 10.1016/j.earscirev.2017.12.015). Nevertheless, a recent study from Germany (Skadell et al. 2023: https://doi.org/10.1016/j.agee.2023.108619) showed that, averaged over a variety of management practices, SOC stocks still increase in the upper and lower subsoil. Still, the variation across the soil profile was very different between different management practices. If I understand the modelling correctly, the management was kept the same during the simulations, while the SOC content in the soil increases. I actually think that the presented scenarios are kept too simple and too optimistic. The authors should consider to extend them towards a potential “subsoil depletion” scenario.
Some minor comments:
Figure 1: I assume that it is minimum and maximum temperature that is shown in the top panel?
249 – 261: do I understand correctly that no calibration and “only” validation was performed?

272 – 277: I am not sure if “reduction factors” is a good expression here. My first thought was that the authors assume a reduction in SOC below the enrichment layer (which has also been shown in some studies, see studies above), but from what I understand from Table 4, so was simply the increase reduced. What about the increase of topsoil SOC at the expense of subsoil SOC (see my comment above)?

337 ff: these results are interesting. However, does this mean that the authors assumed unlimited access to (ground-)water? What was the reduction if yield given the changes in precipitation (and certainly temperature) patterns?

368 – 370: this is not clear to me. From what I understood did the authors rather assume a constant SOC level in the soil, but not a constant SOC addition – the latter would lead to a build-up over time. Or was the SOC level adapted annually within the model?

475 – 477: yes, that´s an important point. How was this handled in the model? Did the roots reach a maximum depth/length?
Citation: https://doi.org/10.5194/egusphere-2023-1077-RC2
- AC2: 'Reply on RC2', Maria Eliza Turek, 31 Aug 2023
  
  Dear Referee #2,
  All of your comments and suggestions were very much appreciated and addressed in this review. In the attached document, we placed the detailed responses for all the issues raised. We thank you very much for your contributions.
  Sincerely,
  Maria Eliza Turek, Attila Nemes, and Annelie Holzkäemper
  
  Citation: https://doi.org/10.5194/egusphere-2023-1077-AC2

Maria Eliza Turek et al.

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Short summary

In this study, we systematically evaluated prospective crop transpiration benefits of sequestering soil organic carbon (SOC) under current and future climatic conditions based on the model SWAP. We found that adding at least 2 % SOC down to at least 65 cm depth could increase transpiration annually, by almost 40 mm, which can play a role in mitigating drought impacts in rain-fed cropping. Beyond this threshold, additional crop transpiration benefits of sequestering SOC are only marginal.


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