Assessing the effects of no-till on SOC dynamics throughout the soil profile after grassland renovation and conversion to silage maize

De Los Rios, Josue; Poyda, Arne; Reinsch, Thorsten; Kluß, Christof; Loges, Ralf; Taube, Friedhelm

doi:https://doi.org/10.5194/bg-2022-6

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https://doi.org/10.5194/bg-2022-6

Preprints

07 Feb 2022

| 07 Feb 2022

Status: this discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

Assessing the effects of no-till on SOC dynamics throughout the soil profile after grassland renovation and conversion to silage maize

Josue De Los Rios, Arne Poyda, Thorsten Reinsch, Christof Kluß, Ralf Loges, and Friedhelm Taube

Abstract. Land-use change (LUC) and conventional tillage (CT) result in the reduction of the high soil organic carbon (SOC) stocks stored in grassland ecosystems during their conversion and renovation, contributing to global warming. While plenty studies show the use of no-tillage (NT) as a promising option to increase the topsoil SOC stocks of arable lands, its potential to conserve SOC during grassland conversion and renovation events has been poorly investigated. Further, the effects of LUC and tillage methods on the SOC dynamics have been limited to the topsoil by most studies, thus overlooking their impact on the subsoil where significant amounts of SOC are stored, and changes in vegetation and residue distribution can negatively affect these. In this study, a 10-year-old grassland was converted to continuous silage maize (CM) using NT (NT-CM) and CT (CT-CM), and renovated using NT (NT-GR), while some part remained undisturbed as a control (GC). The systems were either non-fertilized (N0) or fertilized according to a demand of 180 and 380 kg N ha⁻¹ yr⁻¹ (N1) in the silage maize and grassland systems, respectively. SOC stocks were measured annually and annual SOC changes (ΔSOC, in Mg C ha⁻¹ yr⁻¹) were calculated for different soil layers (0–30, 30–60 and 60–90 cm) and across the whole profile (0–90 cm) over a 6-year period (2014–2020). Annual soil carbon inputs (C_i) via plant residues were quantified and related to ΔSOC. Results showed that cropping systems significantly affected SOC dynamics over time. At 0–30 cm, SOC stocks were significantly reduced after conversion using both tillage methods, however, 44 % lower annual losses were obtained in NT-CM (−1.5 Mg C ha⁻¹ yr⁻¹) compared to CT-CM (−2.7 Mg C ha⁻¹ yr⁻¹). Conversely, SOC stocks remained unchanged after NT-GR same as the GC. In the subsoil, SOC stocks increased under GC (1.1 Mg C ha⁻¹ yr⁻¹) and remained unchanged in the other systems. Across the whole profile, SOC stocks increased in GC, remained stable in NT-GR, and decreased in NT-CM and CT-CM with mean annual change rates of 1.3, −0.1, −1.9 and −3.4 Mg C ha⁻¹ yr⁻¹, respectively. The differences in ΔSOC between the unploughed systems (NT-GR, NT-CM and GC) were strongly related to the annual soil C_ifrom plant residues in the topsoil. Our findings highlight the great potential of NT to slow down the annual SOC losses after grassland conversion or renovation, and that C sequestration can occur in the subsoil of permanent grasslands when the topsoil C is already saturated. This strengthens the need to consider the SOC changes occurring in the whole profile after a LUC event.

Received: 09 Jan 2022 – Discussion started: 07 Feb 2022

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Josue De Los Rios, Arne Poyda, Thorsten Reinsch, Christof Kluß, Ralf Loges, and Friedhelm Taube

Status: closed

RC1:
'Comment on bg-2022-6', Anonymous Referee #1, 14 Feb 2022
General comments

This article deals with an issue of great importance globally - i.e., the impact of grassland conversion on SOC and ways to mitigate SOC losses. Despite concerns over its adverse impacts on GHGs, conversion of grasslands to croplands continues to be widespread in many parts of the world, and absent adequate policies to limits such conversions better understanding of how land management can mitigate the SOC losses is important. The study methods and data analysis are generally of good quality, and the manuscript is clearly written. My recommendation is publication following minor revisions.

Specific comments

Line (L) 9: Should replace "global warming" with more general "climate change" (which also includes e.g. changes in preciptitation, sea level, and even cooling at some times/locations).

L9: "of" missing at end of line

Repeated us of the term "whole profile" implies that samples were taken all the way to bedrock, which does not seem to be the case. This term should be clarified as referring specifically to "a depth of 0.9 m", and replced with this more precise phrase wherever convenient.

L23 SOC increases under GC indicate that the plot is still responding to historic distrubance or LUC. Implications of this need to be considered in the discussion.

L76 "most likely as a result of historical erosion and sedimentation processes" - and/or downslope transport of DOC

L77 "To exclude these site-specific effects, this study only reports results from the upper half of the experimental site". This was an unfortunate choice (that can't be remedied now). Better would have been to include slope location as an explanatory variable in the analysis. The exclusion of lower sites introduces a new confounding effect (similar to measuring only topsoil, but excluding subsoils in the earlier studies cited). I.e. an increase in SOC on the upper slopes could be accompanied by a corresponding decrease in downslope soc, if the greater accumulation on higher ground is partly accounted for by reduced tranport processes. This possibility and how it affects potential interpretation of the results (i.e. quanitifcation of this effect was not possible within the current study and would require further research to account for differential transport processes between treatments) needs to figure in the Discussion. The paragraphs on L260-75 would be a relevent place to raise this issue.

Section 2.2.2: I appreciate how thoroughly the biomass inputs were assessed, and commend the authors on this. Too few studies show this level of thoroughness, which is important.

L303 "maintain SOC stocks" should be replaced with "to mitigate losses of SOC stocks"

Fig 1: needs error bars on the data points and a confidence interval or s.e. ribbon on the regression lines
Citation: https://doi.org/10.5194/bg-2022-6-RC1
- AC1: 'Reply on RC1', Josue De Los Rios, 23 Mar 2022
  
  Dear Referee,
  Thank you very much for your suggestions for improvement.
  Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
  Point 1
  Line (L) 9: Should replace "global warming" with more general "climate change" (which also includes e.g. changes in preciptitation, sea level, and even cooling at some times/locations).
  L9: "of" missing at end of line
  Response:
  The words “global warming” has been replaced by “climate change”
  Point 2:
  Repeated use of the term "whole profile" implies that samples were taken all the way to bedrock, which does not seem to be the case. This term should be clarified as referring specifically to "a depth of 0.9 m", and replaced with this more precise phrase wherever convenient.
  Response
  We have eliminated the word “whole”, and referred to the specific soil depth throughout the text to avoid this confusion.
  Point 3
  L23 SOC increases under GC indicate that the plot is still responding to historic disturbance or LUC. Implications of this need to be considered in the discussion.
  Response
  Added this in the manuscript (lines 297-303).
  “Before establishing the grassland in 2004, the study site was dominated by an arable cropping system. Since then, the field ceased to be ploughed and a multispecies perennial forage mixture with different rooting depth was established, which favours both the increase and distribution of SOC to deeper layers over time observed in GC”
  Point 4
  L76 "most likely as a result of historical erosion and sedimentation processes" - and/or downslope transport of DOC
  Response:
  We have added “and/or downslope transport after “as a result of historical erosion”
  
  Point 5
  L77 "To exclude these site-specific effects, this study only reports results from the upper half of the experimental site". This was an unfortunate choice (that can't be remedied now). Better would have been to include slope location as an explanatory variable in the analysis. The exclusion of lower sites introduces a new confounding effect (similar to measuring only topsoil, but excluding subsoils in the earlier studies cited). I.e. an increase in SOC on the upper slopes could be accompanied by a corresponding decrease in downslope soc, if the greater accumulation on higher ground is partly accounted for by reduced tranport processes. This possibility and how it affects potential interpretation of the results (i.e. quanitifcation of this effect was not possible within the current study and would require further research to account for differential transport processes between treatments) needs to figure in the Discussion.
  The paragraphs on L260-75 would be a relevent place to raise this issue.
  Response: Added to the manuscript (303-312[J1] ).
  “These results, however, ignore the potential soil erosion effects caused by the slope gradient present on the site, excluded from our analysis by using exclusively the upper half of the site. Due to significant colluvial processes occurring at the site over time, large amounts of SOC were observed in the 30-60cm and 60-90 cm layer in several plots, with similar values to those observed in the 0-30 cm, confounding some treatment effects (not shown). These resulted in the lower half differing in SOC stocks and SOC distribution between the soil layers and treatments compared to the upper half (not shown). Slope gradients have previously shown to have an important effect on the distribution of SOC, favouring the downward movement and accumulation of SOC in the depositional area including the subsoil (Holz and Augustin, 2021; Li et al., 2019; Vos et al., 2019). The presence of a considerable slope gradient could, thus, override the treatment effects induced by the study factors tillage and N rates. Further research is required to account for the slope gradient effects and the different transport processes between the treatments.”
  Point 6
  Section 2.2.2: I appreciate how thoroughly the biomass inputs were assessed, and commend the authors on this. Too few studies show this level of thoroughness, which is important.
  Response:
  We are thankful for the positive feedback.
  
  Point 7
  L303 "maintain SOC stocks" should be replaced with "to mitigate losses of SOC stocks"
  Response:
  We have replaced "maintain SOC stocks" with "to mitigate losses of SOC stocks"
  
  Point 8
  Fig 1: needs error bars on the data points and a confidence interval or s.e. ribbon on the regression lines
  Response:
  We have changed Figure 1, by adding the mean ± the S.D.s as error bars for the measured SOC values. We have done the same to Figure 3 to be consistent in the whole manuscript.
  The confidence bands could not be added into the regression models of Figure 1, because of the different variances given by mixed models. However, Figure 2 and 4 reflect the uncertainties surrounding the slopes (± 95 % confidence intervals).
  
  Additional changes made by the authors:
  We have modified the X-axe name of Figure 1 and 3, to make them consistent with the statistical analysis.
  
  We are happy to answer further questions and include your recommendations to our manuscript.
  Sincerely,
  Josue De Los Rios
  
  Citation: https://doi.org/10.5194/bg-2022-6-AC1
- AC2: 'Reply on RC1', Josue De Los Rios, 23 Mar 2022
  
  Dear Referee,
  Thank you very much for your suggestions for improvement.
  Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
  
  Point 1
  Line (L) 9: Should replace "global warming" with more general "climate change" (which also includes e.g. changes in preciptitation, sea level, and even cooling at some times/locations).
  L9: "of" missing at end of line
  Response:
  The words “global warming” has been replaced by “climate change”
  
  Point 2:
  Repeated use of the term "whole profile" implies that samples were taken all the way to bedrock, which does not seem to be the case. This term should be clarified as referring specifically to "a depth of 0.9 m", and replaced with this more precise phrase wherever convenient.
  Response
  We have eliminated the word “whole”, and referred to the specific soil depth throughout the text to avoid this confusion.
  
  Point 3
  L23 SOC increases under GC indicate that the plot is still responding to historic disturbance or LUC. Implications of this need to be considered in the discussion.
  Response
  Added this in the manuscript (lines 297-303).
  “Before establishing the grassland in 2004, the study site was dominated by an arable cropping system. Since then, the field ceased to be ploughed and a multispecies perennial forage mixture with different rooting depth was established, which favours both the increase and distribution of SOC to deeper layers over time observed in GC”
  Point 4
  L76 "most likely as a result of historical erosion and sedimentation processes" - and/or downslope transport of DOC
  Response:
  We have added “and/or downslope transport after “as a result of historical erosion”
  Point 5
  L77 "To exclude these site-specific effects, this study only reports results from the upper half of the experimental site". This was an unfortunate choice (that can't be remedied now). Better would have been to include slope location as an explanatory variable in the analysis. The exclusion of lower sites introduces a new confounding effect (similar to measuring only topsoil, but excluding subsoils in the earlier studies cited). I.e. an increase in SOC on the upper slopes could be accompanied by a corresponding decrease in downslope soc, if the greater accumulation on higher ground is partly accounted for by reduced tranport processes. This possibility and how it affects potential interpretation of the results (i.e. quanitifcation of this effect was not possible within the current study and would require further research to account for differential transport processes between treatments) needs to figure in the Discussion.
  The paragraphs on L260-75 would be a relevent place to raise this issue.
  Response: Added to the manuscript (303-312[1] ).
  “These results, however, ignore the potential soil erosion effects caused by the slope gradient present on the site, excluded from our analysis by using exclusively the upper half of the site. Due to significant colluvial processes occurring at the site over time, large amounts of SOC were observed in the 30-60cm and 60-90 cm layer in several plots, with similar values to those observed in the 0-30 cm, confounding some treatment effects (not shown). These resulted in the lower half differing in SOC stocks and SOC distribution between the soil layers and treatments compared to the upper half (not shown). Slope gradients have previously shown to have an important effect on the distribution of SOC, favouring the downward movement and accumulation of SOC in the depositional area including the subsoil (Holz and Augustin, 2021; Li et al., 2019; Vos et al., 2019). The presence of a considerable slope gradient could, thus, override the treatment effects induced by the study factors tillage and N rates. Further research is required to account for the slope gradient effects and the different transport processes between the treatments.”
  
  Point 6
  Section 2.2.2: I appreciate how thoroughly the biomass inputs were assessed, and commend the authors on this. Too few studies show this level of thoroughness, which is important.
  Response:
  We are thankful for the positive feedback.
  
  Point 7
  L303 "maintain SOC stocks" should be replaced with "to mitigate losses of SOC stocks"
  Response:
  We have replaced "maintain SOC stocks" with "to mitigate losses of SOC stocks"
  
  Point 8
  Fig 1: needs error bars on the data points and a confidence interval or s.e. ribbon on the regression lines
  Response:
  We have changed Figure 1, by adding the mean ± the S.D.s as error bars for the measured SOC values. We have done the same to Figure 3 to be consistent in the whole manuscript.
  The confidence bands could not be added into the regression models of Figure 1, because of the different variances given by mixed models. However, Figure 2 and 4 reflect the uncertainties surrounding the slopes (± 95 % confidence intervals).
  
  Additional changes made by the authors:
  We have modified the X-axe name of Figure 1 and 3, to make them consistent with the statistical analysis.
  
  We are happy to answer further questions and include your recommendations to our manuscript.
  Sincerely,
  Josue De Los Rios
  
  Citation: https://doi.org/10.5194/bg-2022-6-AC2
RC2:
'Comment on bg-2022-6', Anonymous Referee #2, 13 Mar 2022

The manuscript reports annual SOC stocks over a period of 6 years comparing different management systems. However, bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative. Further, SOC stocks must be calculated on equivalent soil mass basis based on measured bulk densities. It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting. Further, total C is reported as SOC without indicating test for carbonates. More comments and suggestions in an annotated manuscript attached.

Citation: https://doi.org/10.5194/bg-2022-6-RC2
- AC4: 'Reply on RC2', Josue De Los Rios, 23 Mar 2022
  
  Dear Referee,
  
  Thank you very much for your suggestions for improvement.
  
  Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
  
  We are happy to answer further questions and include your recommendations to our manuscript.
  
  Sincerely,
  
  Josue De Los Rios
  
  General comment
  
  The manuscript reports annual SOC stocks over a period of 6 years comparing different management systems. However, bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative. It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting. Further, total C is reported as SOC without indicating test for carbonates. More comments and suggestions in an annotated manuscript attached.
  
  (1) Regarding: “bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative.”
  
  Response:
  
  The dynamics in SOC stocks we are reporting are only linked to changes in SOC concentrations, not to changes in bulk density.
  
  (2) Regarding: “It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting”
  
  Response:
  
  The SOC stocks in the control (GC) are not in equilibrium at 30-60 cm depth. Before grassland establishment in 2004, the study site was dominated by an arable cropping system with a 3-year rotation. It is probable that since then, the SOC stocks increase is ongoing. We are addressing that point in the new version.
  
  (3) Regarding: “total C is reported as SOC without indicating test for carbonates”
  
  Response:
  
  We did not find carbonate content within the samples after using HCl. Therefore total C is referred as total SOC.
  
  Further observations added to the manuscript are documented in the attached document.
  
  Citation: https://doi.org/10.5194/bg-2022-6-AC4
AC3: 'Comment on bg-2022-6', Josue De Los Rios, 23 Mar 2022

Dear Referee,
Thank you very much for your suggestions for improvement.
Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
We are happy to answer further questions and include your recommendations to our manuscript.
Sincerely,
Josue De Los Rios

General comment
The manuscript reports annual SOC stocks over a period of 6 years comparing different management systems. However, bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative. It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting. Further, total C is reported as SOC without indicating test for carbonates. More comments and suggestions in an annotated manuscript attached.

(1) Regarding: “bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative.”
Response:
The dynamics in SOC stocks we are reporting are only linked to changes in SOC concentrations, not to changes in bulk density.

(2) Regarding: “It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting”
Response:
The SOC stocks in the control (GC) are not in equilibrium at 30-60 cm depth. Before grassland establishment in 2004, the study site was dominated by an arable cropping system with a 3-year rotation. It is probable that since then, the SOC stocks increase is ongoing. We are addressing that point in the new version.

(3) Regarding: “total C is reported as SOC without indicating test for carbonates”
Response:
We did not find carbonate content within the samples after using HCl. Therefore total C is referred as total SOC.

Further observations added to the manuscript are documented in the attached document.

Citation: https://doi.org/10.5194/bg-2022-6-AC3

Status: closed

RC1:
'Comment on bg-2022-6', Anonymous Referee #1, 14 Feb 2022
General comments

This article deals with an issue of great importance globally - i.e., the impact of grassland conversion on SOC and ways to mitigate SOC losses. Despite concerns over its adverse impacts on GHGs, conversion of grasslands to croplands continues to be widespread in many parts of the world, and absent adequate policies to limits such conversions better understanding of how land management can mitigate the SOC losses is important. The study methods and data analysis are generally of good quality, and the manuscript is clearly written. My recommendation is publication following minor revisions.

Specific comments

Line (L) 9: Should replace "global warming" with more general "climate change" (which also includes e.g. changes in preciptitation, sea level, and even cooling at some times/locations).

L9: "of" missing at end of line

Repeated us of the term "whole profile" implies that samples were taken all the way to bedrock, which does not seem to be the case. This term should be clarified as referring specifically to "a depth of 0.9 m", and replced with this more precise phrase wherever convenient.

L23 SOC increases under GC indicate that the plot is still responding to historic distrubance or LUC. Implications of this need to be considered in the discussion.

L76 "most likely as a result of historical erosion and sedimentation processes" - and/or downslope transport of DOC

L77 "To exclude these site-specific effects, this study only reports results from the upper half of the experimental site". This was an unfortunate choice (that can't be remedied now). Better would have been to include slope location as an explanatory variable in the analysis. The exclusion of lower sites introduces a new confounding effect (similar to measuring only topsoil, but excluding subsoils in the earlier studies cited). I.e. an increase in SOC on the upper slopes could be accompanied by a corresponding decrease in downslope soc, if the greater accumulation on higher ground is partly accounted for by reduced tranport processes. This possibility and how it affects potential interpretation of the results (i.e. quanitifcation of this effect was not possible within the current study and would require further research to account for differential transport processes between treatments) needs to figure in the Discussion. The paragraphs on L260-75 would be a relevent place to raise this issue.

Section 2.2.2: I appreciate how thoroughly the biomass inputs were assessed, and commend the authors on this. Too few studies show this level of thoroughness, which is important.

L303 "maintain SOC stocks" should be replaced with "to mitigate losses of SOC stocks"

Fig 1: needs error bars on the data points and a confidence interval or s.e. ribbon on the regression lines
Citation: https://doi.org/10.5194/bg-2022-6-RC1
- AC1: 'Reply on RC1', Josue De Los Rios, 23 Mar 2022
  
  Dear Referee,
  Thank you very much for your suggestions for improvement.
  Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
  Point 1
  Line (L) 9: Should replace "global warming" with more general "climate change" (which also includes e.g. changes in preciptitation, sea level, and even cooling at some times/locations).
  L9: "of" missing at end of line
  Response:
  The words “global warming” has been replaced by “climate change”
  Point 2:
  Repeated use of the term "whole profile" implies that samples were taken all the way to bedrock, which does not seem to be the case. This term should be clarified as referring specifically to "a depth of 0.9 m", and replaced with this more precise phrase wherever convenient.
  Response
  We have eliminated the word “whole”, and referred to the specific soil depth throughout the text to avoid this confusion.
  Point 3
  L23 SOC increases under GC indicate that the plot is still responding to historic disturbance or LUC. Implications of this need to be considered in the discussion.
  Response
  Added this in the manuscript (lines 297-303).
  “Before establishing the grassland in 2004, the study site was dominated by an arable cropping system. Since then, the field ceased to be ploughed and a multispecies perennial forage mixture with different rooting depth was established, which favours both the increase and distribution of SOC to deeper layers over time observed in GC”
  Point 4
  L76 "most likely as a result of historical erosion and sedimentation processes" - and/or downslope transport of DOC
  Response:
  We have added “and/or downslope transport after “as a result of historical erosion”
  
  Point 5
  L77 "To exclude these site-specific effects, this study only reports results from the upper half of the experimental site". This was an unfortunate choice (that can't be remedied now). Better would have been to include slope location as an explanatory variable in the analysis. The exclusion of lower sites introduces a new confounding effect (similar to measuring only topsoil, but excluding subsoils in the earlier studies cited). I.e. an increase in SOC on the upper slopes could be accompanied by a corresponding decrease in downslope soc, if the greater accumulation on higher ground is partly accounted for by reduced tranport processes. This possibility and how it affects potential interpretation of the results (i.e. quanitifcation of this effect was not possible within the current study and would require further research to account for differential transport processes between treatments) needs to figure in the Discussion.
  The paragraphs on L260-75 would be a relevent place to raise this issue.
  Response: Added to the manuscript (303-312[J1] ).
  “These results, however, ignore the potential soil erosion effects caused by the slope gradient present on the site, excluded from our analysis by using exclusively the upper half of the site. Due to significant colluvial processes occurring at the site over time, large amounts of SOC were observed in the 30-60cm and 60-90 cm layer in several plots, with similar values to those observed in the 0-30 cm, confounding some treatment effects (not shown). These resulted in the lower half differing in SOC stocks and SOC distribution between the soil layers and treatments compared to the upper half (not shown). Slope gradients have previously shown to have an important effect on the distribution of SOC, favouring the downward movement and accumulation of SOC in the depositional area including the subsoil (Holz and Augustin, 2021; Li et al., 2019; Vos et al., 2019). The presence of a considerable slope gradient could, thus, override the treatment effects induced by the study factors tillage and N rates. Further research is required to account for the slope gradient effects and the different transport processes between the treatments.”
  Point 6
  Section 2.2.2: I appreciate how thoroughly the biomass inputs were assessed, and commend the authors on this. Too few studies show this level of thoroughness, which is important.
  Response:
  We are thankful for the positive feedback.
  
  Point 7
  L303 "maintain SOC stocks" should be replaced with "to mitigate losses of SOC stocks"
  Response:
  We have replaced "maintain SOC stocks" with "to mitigate losses of SOC stocks"
  
  Point 8
  Fig 1: needs error bars on the data points and a confidence interval or s.e. ribbon on the regression lines
  Response:
  We have changed Figure 1, by adding the mean ± the S.D.s as error bars for the measured SOC values. We have done the same to Figure 3 to be consistent in the whole manuscript.
  The confidence bands could not be added into the regression models of Figure 1, because of the different variances given by mixed models. However, Figure 2 and 4 reflect the uncertainties surrounding the slopes (± 95 % confidence intervals).
  
  Additional changes made by the authors:
  We have modified the X-axe name of Figure 1 and 3, to make them consistent with the statistical analysis.
  
  We are happy to answer further questions and include your recommendations to our manuscript.
  Sincerely,
  Josue De Los Rios
  
  Citation: https://doi.org/10.5194/bg-2022-6-AC1
- AC2: 'Reply on RC1', Josue De Los Rios, 23 Mar 2022
  
  Dear Referee,
  Thank you very much for your suggestions for improvement.
  Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
  
  Point 1
  Line (L) 9: Should replace "global warming" with more general "climate change" (which also includes e.g. changes in preciptitation, sea level, and even cooling at some times/locations).
  L9: "of" missing at end of line
  Response:
  The words “global warming” has been replaced by “climate change”
  
  Point 2:
  Repeated use of the term "whole profile" implies that samples were taken all the way to bedrock, which does not seem to be the case. This term should be clarified as referring specifically to "a depth of 0.9 m", and replaced with this more precise phrase wherever convenient.
  Response
  We have eliminated the word “whole”, and referred to the specific soil depth throughout the text to avoid this confusion.
  
  Point 3
  L23 SOC increases under GC indicate that the plot is still responding to historic disturbance or LUC. Implications of this need to be considered in the discussion.
  Response
  Added this in the manuscript (lines 297-303).
  “Before establishing the grassland in 2004, the study site was dominated by an arable cropping system. Since then, the field ceased to be ploughed and a multispecies perennial forage mixture with different rooting depth was established, which favours both the increase and distribution of SOC to deeper layers over time observed in GC”
  Point 4
  L76 "most likely as a result of historical erosion and sedimentation processes" - and/or downslope transport of DOC
  Response:
  We have added “and/or downslope transport after “as a result of historical erosion”
  Point 5
  L77 "To exclude these site-specific effects, this study only reports results from the upper half of the experimental site". This was an unfortunate choice (that can't be remedied now). Better would have been to include slope location as an explanatory variable in the analysis. The exclusion of lower sites introduces a new confounding effect (similar to measuring only topsoil, but excluding subsoils in the earlier studies cited). I.e. an increase in SOC on the upper slopes could be accompanied by a corresponding decrease in downslope soc, if the greater accumulation on higher ground is partly accounted for by reduced tranport processes. This possibility and how it affects potential interpretation of the results (i.e. quanitifcation of this effect was not possible within the current study and would require further research to account for differential transport processes between treatments) needs to figure in the Discussion.
  The paragraphs on L260-75 would be a relevent place to raise this issue.
  Response: Added to the manuscript (303-312[1] ).
  “These results, however, ignore the potential soil erosion effects caused by the slope gradient present on the site, excluded from our analysis by using exclusively the upper half of the site. Due to significant colluvial processes occurring at the site over time, large amounts of SOC were observed in the 30-60cm and 60-90 cm layer in several plots, with similar values to those observed in the 0-30 cm, confounding some treatment effects (not shown). These resulted in the lower half differing in SOC stocks and SOC distribution between the soil layers and treatments compared to the upper half (not shown). Slope gradients have previously shown to have an important effect on the distribution of SOC, favouring the downward movement and accumulation of SOC in the depositional area including the subsoil (Holz and Augustin, 2021; Li et al., 2019; Vos et al., 2019). The presence of a considerable slope gradient could, thus, override the treatment effects induced by the study factors tillage and N rates. Further research is required to account for the slope gradient effects and the different transport processes between the treatments.”
  
  Point 6
  Section 2.2.2: I appreciate how thoroughly the biomass inputs were assessed, and commend the authors on this. Too few studies show this level of thoroughness, which is important.
  Response:
  We are thankful for the positive feedback.
  
  Point 7
  L303 "maintain SOC stocks" should be replaced with "to mitigate losses of SOC stocks"
  Response:
  We have replaced "maintain SOC stocks" with "to mitigate losses of SOC stocks"
  
  Point 8
  Fig 1: needs error bars on the data points and a confidence interval or s.e. ribbon on the regression lines
  Response:
  We have changed Figure 1, by adding the mean ± the S.D.s as error bars for the measured SOC values. We have done the same to Figure 3 to be consistent in the whole manuscript.
  The confidence bands could not be added into the regression models of Figure 1, because of the different variances given by mixed models. However, Figure 2 and 4 reflect the uncertainties surrounding the slopes (± 95 % confidence intervals).
  
  Additional changes made by the authors:
  We have modified the X-axe name of Figure 1 and 3, to make them consistent with the statistical analysis.
  
  We are happy to answer further questions and include your recommendations to our manuscript.
  Sincerely,
  Josue De Los Rios
  
  Citation: https://doi.org/10.5194/bg-2022-6-AC2
RC2:
'Comment on bg-2022-6', Anonymous Referee #2, 13 Mar 2022

The manuscript reports annual SOC stocks over a period of 6 years comparing different management systems. However, bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative. Further, SOC stocks must be calculated on equivalent soil mass basis based on measured bulk densities. It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting. Further, total C is reported as SOC without indicating test for carbonates. More comments and suggestions in an annotated manuscript attached.

Citation: https://doi.org/10.5194/bg-2022-6-RC2
- AC4: 'Reply on RC2', Josue De Los Rios, 23 Mar 2022
  
  Dear Referee,
  
  Thank you very much for your suggestions for improvement.
  
  Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
  
  We are happy to answer further questions and include your recommendations to our manuscript.
  
  Sincerely,
  
  Josue De Los Rios
  
  General comment
  
  The manuscript reports annual SOC stocks over a period of 6 years comparing different management systems. However, bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative. It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting. Further, total C is reported as SOC without indicating test for carbonates. More comments and suggestions in an annotated manuscript attached.
  
  (1) Regarding: “bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative.”
  
  Response:
  
  The dynamics in SOC stocks we are reporting are only linked to changes in SOC concentrations, not to changes in bulk density.
  
  (2) Regarding: “It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting”
  
  Response:
  
  The SOC stocks in the control (GC) are not in equilibrium at 30-60 cm depth. Before grassland establishment in 2004, the study site was dominated by an arable cropping system with a 3-year rotation. It is probable that since then, the SOC stocks increase is ongoing. We are addressing that point in the new version.
  
  (3) Regarding: “total C is reported as SOC without indicating test for carbonates”
  
  Response:
  
  We did not find carbonate content within the samples after using HCl. Therefore total C is referred as total SOC.
  
  Further observations added to the manuscript are documented in the attached document.
  
  Citation: https://doi.org/10.5194/bg-2022-6-AC4
AC3: 'Comment on bg-2022-6', Josue De Los Rios, 23 Mar 2022

Dear Referee,
Thank you very much for your suggestions for improvement.
Following, we would like to share a point-by-point report of the changes made to the manuscript based on your review.
We are happy to answer further questions and include your recommendations to our manuscript.
Sincerely,
Josue De Los Rios

General comment
The manuscript reports annual SOC stocks over a period of 6 years comparing different management systems. However, bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative. It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting. Further, total C is reported as SOC without indicating test for carbonates. More comments and suggestions in an annotated manuscript attached.

(1) Regarding: “bulk density was only determined once at the end of the study in 0-30 cm and estimated for 30-60 and 60-90 cm and, thus, the SOC stock changes are highly speculative.”
Response:
The dynamics in SOC stocks we are reporting are only linked to changes in SOC concentrations, not to changes in bulk density.

(2) Regarding: “It is unclear whether SOC stocks at the control were in equilibrium or the baseline was drifting”
Response:
The SOC stocks in the control (GC) are not in equilibrium at 30-60 cm depth. Before grassland establishment in 2004, the study site was dominated by an arable cropping system with a 3-year rotation. It is probable that since then, the SOC stocks increase is ongoing. We are addressing that point in the new version.

(3) Regarding: “total C is reported as SOC without indicating test for carbonates”
Response:
We did not find carbonate content within the samples after using HCl. Therefore total C is referred as total SOC.

Further observations added to the manuscript are documented in the attached document.

Citation: https://doi.org/10.5194/bg-2022-6-AC3

Josue De Los Rios, Arne Poyda, Thorsten Reinsch, Christof Kluß, Ralf Loges, and Friedhelm Taube

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https://doi.org/10.5194/bg-2022-6-supplement

Josue De Los Rios, Arne Poyda, Thorsten Reinsch, Christof Kluß, Ralf Loges, and Friedhelm Taube

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Short summary

Land use change (LUC) and conventional tillage (CT) are resulting in the reduction of the high soil organic carbon (SOC) stocks stored in grassland ecosystems during their conversion and renovation, contributing thus to global warming. Using no-tillage (NT) is seen as an avenue to minimize or even conserve SOC stocks during these events. Our results show that SOC losses are greatly reduced after grassland conversion to arable land, whereas during renovation it contributes to conserve them.


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