the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jul 2023
Conservation agriculture increases soil organic carbon stocks but not soil CO2 efflux in two 8-year-old experiments in Zimbabwe
Abstract. Conservation agriculture (CA), combining reduced or no tillage, permanent soil cover and improved rotations, is often promoted as a climate-smart practice. However, our understanding about the impact of CA and its respective three principles on top and sub-soil organic carbon (SOC) stocks and on soil CO2 efflux in low input cropping systems of sub-Saharan Africa is rather limited. The study was conducted at two long-term experimental sites established in 2013 in Zimbabwe. The soil types were abruptic Lixisols at Domboshava Training Centre (DTC) and xanthic Ferralsol at the University of Zimbabwe farm (UZF). Six treatments, replicated four times were investigated: conventional tillage (CT), conventional tillage with rotation (CTR), NT, no-tillage with mulch (NTM), no-tillage with rotation (NTR), no-tillage with mulch and rotation (NTMR). Maize (Zea mays L.) was the main crop and treatments with rotation included cowpea (Vigna unguiculata L. Walp.). SOC concentration and bulk density were determined for samples taken from the 0–5, 5–10, 10–15, 15–20, 20–30, 30–40, 40–50, 50–75 and 75–100 cm depths. Gas samples were regularly collected using the static chamber method during the 2019/20 and 2020/21 cropping seasons and during the 2020/21 dry season. SOC stocks were significantly (p < 0.05) higher under NTM, NTR and NTMR compared to NT and CT in top 5 and 10 cm layers at UZF, while SOC stocks were only significantly higher under NTM and NTMR compared to NT and CT in top 5 cm at DTC. NT alone had a slightly negative impact on top SOC stock. Cumulative SOC stocks were not significantly different between treatments when considering the whole 100 cm soil profile. Regardless of larger organic carbon inputs in mulch treatments, there were no significant differences in CO2 efflux between treatments, but it was higher in maize rows than in inter-rows as a result of autotrophic respiration from maize roots. Our results show the overarching role of crop residue mulching in CA cropping systems in enhancing SOC storage but that this effect is limited to the topsoil.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1233', Anonymous Referee #1, 18 Sep 2023
The paper is very well written, the trial is comprehensive, and methods were explained in detail. The high level of detail in the data presented is a rare find, and valuable insight in the understanding around SOC dynamics in CA systems. I was unable to find any problems in the manuscript, as far as I can tell the trial layout, sampling and analysis are correct, and the manuscript is written in clear language, that convey the message effectively.
Citation: https://doi.org/10.5194/egusphere-2023-1233-RC1 -
AC1: 'Reply on RC1', Armwell Shumba, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1233/egusphere-2023-1233-AC1-supplement.pdf
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AC1: 'Reply on RC1', Armwell Shumba, 07 Nov 2023
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EC1: 'Comment on egusphere-2023-1233', Steven Sleutel, 10 Oct 2023
Publisher’s note: the content of this comment was removed on 10 October 2023 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/egusphere-2023-1233-EC1 -
EC2: 'Comment on egusphere-2023-1233', Steven Sleutel, 10 Oct 2023
The current paper describes how conservation tillage management may or may not impact SOC stocks in two field experiments in Zimbabwe. While this theme is not very novel, the study does stand out in targeting understudied SSA-cropping systems and in the robustness of the methodological approach. It is unfortunate that only little effort was put into quantifying root biomass and overall plant-C inputs as this leaves the analysis rather speculative at points. Another critical point is that the rationale for comparing CO2 emissions between the various combinations of tillage x mulch retention x crop rotation (maize vs. maize & pea) is not really motivated. The net balancing of C-in and outputs results into changes in the SOC stock. So what is then the added value of on top comparing soil CO2 effluxes in the growing season (and once also in the ‘dry season’)? What did the authors hope to learn in addition to what they could not deduce from just looking at the SOC stock data? Most sections are overall in good shape, aside from the discussion. Its structure requires further work and there are a number of important comments on the at times rather limited interpretation of mechanisms behind found changes in SOC between the various treatments. If these points are adequately addressed the current paper could make a good contribution to the state-of-the-art on CA in low-input farming systems.
Detailed/minor comment:
L96 and 67 seem to contradict each other, rephrase perhaps one
L91 ‘have’ instead of ‘has’
L A motorized handheld corer was used to take >30cm soil samples: what diameter was then sampled?
L208 provide more details on the CHN analyzer
Fig2. The quality is rather low, consider something else than the current hatching
Fig.3 I do not see the point in repeating the 0-5cm SOC concentration data here in the bar charts.
3.3 first part could be condensed further
Table 1 lowercase letter to designate significantly different means should not be placed in superscript
L382 ‘at UZF’ comes in a bit late in this sentence, move forward. Same remark also for the next sentence and ‘at DTC’
L384 so also provide likewise figures for UFZ
L391 ‘had’ instead of ‘has’; overall the sentences in this section 387-398 read strangely ‘treatment x had net loss or accumulation of SOC’ rephrase please in a more active form.
Use the same manner to denote significance level in Tables 1 & 2
L418 rewrite this sentence
L430 ‘0.05’ I assume?
L465 instead of > 50%, write maybe ‘over half’
L469 ‘treatment effects’ what sort of treatments?
L472 long and difficult to understand sentence
L491 add ‘and’ before ‘hence’
L497 ‘do not’ instead of ‘does not’, ‘add SOC’ is also not very well phrased further on in the sentence
L514 Be clear, supposedly you are referring to an N-fertilization effect onto maize, but that is not spelled out here. Do then also report on the found (in)differences in maize crop residue returns.
L527-535 shorten and rewrite so that this reason for finding no difference in SOC stock when considering deeper soil profiles could potentially also be in part attributed to lack of replicates.
L545 best to refer once more to DTC here
Citation: https://doi.org/10.5194/egusphere-2023-1233-EC2 -
AC3: 'Reply on EC2', Armwell Shumba, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1233/egusphere-2023-1233-AC3-supplement.pdf
- AC4: 'Reply on EC2', Armwell Shumba, 07 Nov 2023
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AC3: 'Reply on EC2', Armwell Shumba, 07 Nov 2023
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RC2: 'Comment on egusphere-2023-1233', Anonymous Referee #2, 10 Oct 2023
An overall relevant contribution on use of conservation agriculture practices to improve maize crop productivity and raise carbon stocks (and in turn fertility) in Zimbabwe. The study relies on two well-established field trials and standard yet well executed assessments were used to reveal how tillage, mulching and crop rotation in isolation or jointly influence SOC in these understudied systems. While the approach is overall robust – aside from limited attention to crop C inputs – at times the interpretation remains rather superficial. I have listed a number of comments below:
L103 & 108 One aspect of the hypotheses is best elaborated a bit more: while it is clear that crop residue retention and adoption of no-tillage are likely to improve SOC stocks, this is less obvious so for crop rotation. Crop rotation as such rather seems like a means to combat pests, but why should there necessarily be any beneficial effect onto SOC? Indirectly perhaps, via enhanced crop growth?
L300 Why carry out statistical comparisons for each individual date? This does not provide an overall view on the treatment effects and complicates drawing conclusions.
In the UFZ site, Fig 3 reveals a rather conspicuous treatment effect of including a rotation 90cm depth (and to a lesser extent at 60cm as well). Perhaps relevant to discuss as well, even though these trends were apparently not (yet) significant. Maybe the introduction of peas particularly led to deeper root proliferation and derived C-inputs at depth… perhaps the authors could try using contrasts in the ANOVA to pool the three ‘rotation’ treatments vs. the other treatments and demonstrate such effect?
The discussion section requires improvement:
The second half of 4.1 does not make much of a connection with own observations (L476-483) – it is not clear what message the authors wanted to bring here.
Section 4.2 is lengthy - Perhaps this section could benefit from further subdivision into subparts that deal with the various treatment aspects (tillage, mulching, crop rotation).
4.2 lacks a clear calculation of conversion efficiency of C-inputs into SOC. There have been ample studies on removal or incorporation of maize residues (in light of research into the value of root vs. shoot C inputs) – such studies have generally revealed that aboveground biomass C inputs are far less effective in sustaining SOC – and this body of literature should be used to complement the discussion here. Even though much of the C-inputs are just derived from yield data, it would still be relevant to see some estimates of the efficiency by which these C-inputs formed or sustained SOC.
The discussion lacks considerations on the potential mechanisms through which the inclusion of peas in the crop rotation might influence maize C inputs. With a legume introduced there may well be more N-available for the maize crop – not trivial given the very low doses of mineral N-supplied. If so, we may expect this to hold an effect on the soil C balance not only through an overall stimulation of maize productivity (and that is currently not clearly explained in the current discussion) but possibly also by effects onto the maize rooting pattern. These aspects should at least be commented upon, especially in light of the rather conspicuous difference in SOC at 75 or 90cm depth when a rotation is included (at least such would seem so from Fig 3, but unfortunately no comparison is given of the 75-100cm SOC stocks).
Towards the end of the discussion and in the conclusion clearly the specificity of the here established crop rotation needs to be stipulated. By no means could we simply extrapolate found results to other ‘crop rotations’ – with different crops and plant C-inputs.
L497: what about the whole idea that reduction of tillage would promote physical protection of OM inside microaggregates – should be commented on here as well.
L510 SOC storage does not seem very site specific but rather rotation-specific + ‘the differences in the response to SOC changes’ makes little sense -> rephrase
L518 ‘the net SOC loss in CTR was due to seasonal exposure to oxidative losses (SOC mineralization) through disruption of soil macroaggregates by tillage’ this makes little sense since these numbers are actually obtained by comparison with the CT treatment in which there is an equally intensive disruption of soil structure.
L547 what about other SOC stabilization mechanisms?
4.4 as it is stands alone and really does not bring a clear message. It may be better to integrate this section into 4.2 (which then gets even lengthier and is therefore best split).
The conclusion section is fine.
Citation: https://doi.org/10.5194/egusphere-2023-1233-RC2 -
AC2: 'Reply on RC2', Armwell Shumba, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1233/egusphere-2023-1233-AC2-supplement.pdf
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AC2: 'Reply on RC2', Armwell Shumba, 07 Nov 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1233', Anonymous Referee #1, 18 Sep 2023
The paper is very well written, the trial is comprehensive, and methods were explained in detail. The high level of detail in the data presented is a rare find, and valuable insight in the understanding around SOC dynamics in CA systems. I was unable to find any problems in the manuscript, as far as I can tell the trial layout, sampling and analysis are correct, and the manuscript is written in clear language, that convey the message effectively.
Citation: https://doi.org/10.5194/egusphere-2023-1233-RC1 -
AC1: 'Reply on RC1', Armwell Shumba, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1233/egusphere-2023-1233-AC1-supplement.pdf
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AC1: 'Reply on RC1', Armwell Shumba, 07 Nov 2023
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EC1: 'Comment on egusphere-2023-1233', Steven Sleutel, 10 Oct 2023
Publisher’s note: the content of this comment was removed on 10 October 2023 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/egusphere-2023-1233-EC1 -
EC2: 'Comment on egusphere-2023-1233', Steven Sleutel, 10 Oct 2023
The current paper describes how conservation tillage management may or may not impact SOC stocks in two field experiments in Zimbabwe. While this theme is not very novel, the study does stand out in targeting understudied SSA-cropping systems and in the robustness of the methodological approach. It is unfortunate that only little effort was put into quantifying root biomass and overall plant-C inputs as this leaves the analysis rather speculative at points. Another critical point is that the rationale for comparing CO2 emissions between the various combinations of tillage x mulch retention x crop rotation (maize vs. maize & pea) is not really motivated. The net balancing of C-in and outputs results into changes in the SOC stock. So what is then the added value of on top comparing soil CO2 effluxes in the growing season (and once also in the ‘dry season’)? What did the authors hope to learn in addition to what they could not deduce from just looking at the SOC stock data? Most sections are overall in good shape, aside from the discussion. Its structure requires further work and there are a number of important comments on the at times rather limited interpretation of mechanisms behind found changes in SOC between the various treatments. If these points are adequately addressed the current paper could make a good contribution to the state-of-the-art on CA in low-input farming systems.
Detailed/minor comment:
L96 and 67 seem to contradict each other, rephrase perhaps one
L91 ‘have’ instead of ‘has’
L A motorized handheld corer was used to take >30cm soil samples: what diameter was then sampled?
L208 provide more details on the CHN analyzer
Fig2. The quality is rather low, consider something else than the current hatching
Fig.3 I do not see the point in repeating the 0-5cm SOC concentration data here in the bar charts.
3.3 first part could be condensed further
Table 1 lowercase letter to designate significantly different means should not be placed in superscript
L382 ‘at UZF’ comes in a bit late in this sentence, move forward. Same remark also for the next sentence and ‘at DTC’
L384 so also provide likewise figures for UFZ
L391 ‘had’ instead of ‘has’; overall the sentences in this section 387-398 read strangely ‘treatment x had net loss or accumulation of SOC’ rephrase please in a more active form.
Use the same manner to denote significance level in Tables 1 & 2
L418 rewrite this sentence
L430 ‘0.05’ I assume?
L465 instead of > 50%, write maybe ‘over half’
L469 ‘treatment effects’ what sort of treatments?
L472 long and difficult to understand sentence
L491 add ‘and’ before ‘hence’
L497 ‘do not’ instead of ‘does not’, ‘add SOC’ is also not very well phrased further on in the sentence
L514 Be clear, supposedly you are referring to an N-fertilization effect onto maize, but that is not spelled out here. Do then also report on the found (in)differences in maize crop residue returns.
L527-535 shorten and rewrite so that this reason for finding no difference in SOC stock when considering deeper soil profiles could potentially also be in part attributed to lack of replicates.
L545 best to refer once more to DTC here
Citation: https://doi.org/10.5194/egusphere-2023-1233-EC2 -
AC3: 'Reply on EC2', Armwell Shumba, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1233/egusphere-2023-1233-AC3-supplement.pdf
- AC4: 'Reply on EC2', Armwell Shumba, 07 Nov 2023
-
AC3: 'Reply on EC2', Armwell Shumba, 07 Nov 2023
-
RC2: 'Comment on egusphere-2023-1233', Anonymous Referee #2, 10 Oct 2023
An overall relevant contribution on use of conservation agriculture practices to improve maize crop productivity and raise carbon stocks (and in turn fertility) in Zimbabwe. The study relies on two well-established field trials and standard yet well executed assessments were used to reveal how tillage, mulching and crop rotation in isolation or jointly influence SOC in these understudied systems. While the approach is overall robust – aside from limited attention to crop C inputs – at times the interpretation remains rather superficial. I have listed a number of comments below:
L103 & 108 One aspect of the hypotheses is best elaborated a bit more: while it is clear that crop residue retention and adoption of no-tillage are likely to improve SOC stocks, this is less obvious so for crop rotation. Crop rotation as such rather seems like a means to combat pests, but why should there necessarily be any beneficial effect onto SOC? Indirectly perhaps, via enhanced crop growth?
L300 Why carry out statistical comparisons for each individual date? This does not provide an overall view on the treatment effects and complicates drawing conclusions.
In the UFZ site, Fig 3 reveals a rather conspicuous treatment effect of including a rotation 90cm depth (and to a lesser extent at 60cm as well). Perhaps relevant to discuss as well, even though these trends were apparently not (yet) significant. Maybe the introduction of peas particularly led to deeper root proliferation and derived C-inputs at depth… perhaps the authors could try using contrasts in the ANOVA to pool the three ‘rotation’ treatments vs. the other treatments and demonstrate such effect?
The discussion section requires improvement:
The second half of 4.1 does not make much of a connection with own observations (L476-483) – it is not clear what message the authors wanted to bring here.
Section 4.2 is lengthy - Perhaps this section could benefit from further subdivision into subparts that deal with the various treatment aspects (tillage, mulching, crop rotation).
4.2 lacks a clear calculation of conversion efficiency of C-inputs into SOC. There have been ample studies on removal or incorporation of maize residues (in light of research into the value of root vs. shoot C inputs) – such studies have generally revealed that aboveground biomass C inputs are far less effective in sustaining SOC – and this body of literature should be used to complement the discussion here. Even though much of the C-inputs are just derived from yield data, it would still be relevant to see some estimates of the efficiency by which these C-inputs formed or sustained SOC.
The discussion lacks considerations on the potential mechanisms through which the inclusion of peas in the crop rotation might influence maize C inputs. With a legume introduced there may well be more N-available for the maize crop – not trivial given the very low doses of mineral N-supplied. If so, we may expect this to hold an effect on the soil C balance not only through an overall stimulation of maize productivity (and that is currently not clearly explained in the current discussion) but possibly also by effects onto the maize rooting pattern. These aspects should at least be commented upon, especially in light of the rather conspicuous difference in SOC at 75 or 90cm depth when a rotation is included (at least such would seem so from Fig 3, but unfortunately no comparison is given of the 75-100cm SOC stocks).
Towards the end of the discussion and in the conclusion clearly the specificity of the here established crop rotation needs to be stipulated. By no means could we simply extrapolate found results to other ‘crop rotations’ – with different crops and plant C-inputs.
L497: what about the whole idea that reduction of tillage would promote physical protection of OM inside microaggregates – should be commented on here as well.
L510 SOC storage does not seem very site specific but rather rotation-specific + ‘the differences in the response to SOC changes’ makes little sense -> rephrase
L518 ‘the net SOC loss in CTR was due to seasonal exposure to oxidative losses (SOC mineralization) through disruption of soil macroaggregates by tillage’ this makes little sense since these numbers are actually obtained by comparison with the CT treatment in which there is an equally intensive disruption of soil structure.
L547 what about other SOC stabilization mechanisms?
4.4 as it is stands alone and really does not bring a clear message. It may be better to integrate this section into 4.2 (which then gets even lengthier and is therefore best split).
The conclusion section is fine.
Citation: https://doi.org/10.5194/egusphere-2023-1233-RC2 -
AC2: 'Reply on RC2', Armwell Shumba, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1233/egusphere-2023-1233-AC2-supplement.pdf
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AC2: 'Reply on RC2', Armwell Shumba, 07 Nov 2023
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Armwell Shumba
Regis Chikowo
Christian Thierfelder
Marc Corbeels
Johan Six
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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