the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Sep 2021
Large Herbivores Affecting Permafrost – Impacts of Grazing on Permafrost Soil Carbon Storage in Northeastern Siberia
Abstract. The risk of carbon emissions from permafrost ground is linked to ground temperature and thus in particular to thermal insulation by vegetation and organic soil layers in summer and snow cover in winter. This ground insulation is strongly influenced by the presence of large herbivorous animals browsing for food. In this study, we examine the potential impact of large herbivore presence on the ground carbon storage in thermokarst landscapes of northeastern Siberia. Our aim is to understand how intensive animal grazing may affect permafrost thaw and hence organic matter decomposition, leading to different ground carbon storage, which is significant in the active layer. Therefore, we analysed sites with differing large herbivore grazing intensity in the Pleistocene Park near Chersky and measured maximum thaw depth, total organic carbon content and decomposition state by δ13C isotope analysis. In addition, we determined sediment grain size composition as well as ice and water content. We found the thaw depth to be shallower and carbon storage to be higher in intensively grazed areas compared to extensively and non-grazed sites in the same thermokarst basin. The intensive grazing presumably leads to a more stable thermal ground regime and thus to increased carbon storage in the thermokarst deposits and active layer. However, the high carbon content found within the upper 20 cm on intensively grazed sites could also indicate higher carbon input rather than reduced decomposition, which requires further studies. We connect our findings to more animal trampling in winter, which causes snow disturbance and cooler winter ground temperatures during the average annual 225 days below freezing. This winter cooling overcompensates ground warming due to the lower insulation associated with shorter heavily grazed vegetation during the average annual 140 thaw days. We conclude that intensive grazing influences the carbon storage capacities of permafrost areas and hence might be an actively manageable instrument to reduce net carbon emission from these sites.
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RC1: 'Comment on bg-2021-227', Anonymous Referee #1, 21 Oct 2021
This manuscript presents observations of soil cores from different landscape units and disturbance histories, and aims to answer an interesting and relevant question, “does grazing by large mammals impact permafrost carbon storage?” Unfortunately, the experimental design is fundamentally flawed, making any conclusions about the impact of herbivory on soil carbon storage impossible.
The main issue is lack of replication – the study relies on a single soil core for each combination of environment (drained lake basin or upland) and grazing (intensive or no grazing), which is insufficient given the variability of soil composition and the presence of confounding variables. We know that soil core properties are highly variable in permafrost environments due to cryoturbation, so any variation from one site to another could be due to natural spatial variability or the variable of interest, herbivory. Without replication within sites to account for spatial variability of permafrost soils there is no way to discern between those two possibilities. Additionally, soil moisture is a confounding variable that cannot be accounted for without additional samples in a wider range of environmental conditions. The authors showed that soil organic carbon varied with water/ice content and mentioned that the grazed sites in the drained lake basin flooded seasonally, while none of the other sites flood regularly. This means that patterns in soil organic carbon may be due primarily to variation in soil moisture rather than herbivory, because soil moisture and herbivory covary. Another potential confounding variable is the site history. The authors mentioned that the non-grazed drained lake site was cleared of forest a few years prior to the study while none of the other sites underwent the same treatment.
While the underlying soil core data are sound and could be used to describe some of the variability of the site, the flawed study design makes it impossible to disentangle the effects of spatial heterogeneity, soil moisture regime, site history, and herbivory. Therefore, I suggest that this manuscript be rejected and the authors reconsider the scope of question that can be answered with these data for a new submission.
Citation: https://doi.org/10.5194/bg-2021-227-RC1 - AC2: 'Reply on RC1', Torben Windirsch, 13 Dec 2021
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RC2: 'Comment on bg-2021-227', Anonymous Referee #2, 26 Nov 2021
This paper entitled “Large herbivores affecting permafrost – impacts of grazing on permafrost soil carbon storage in northeastern Siberia” by Windirsch and others reports carbon storage in the active layer of an intensively grazed thermokarst basin in Siberia. The study tests the important hypothesis of whether reintroduction of large mammals could slow permafrost degradation and greenhouse gas release with climate change. They found decreased active-layer depth and increased carbon storage in the intensively grazed location. The paper is very well written, and the hypotheses are well conceived and explained. I have some questions and concerns about the experimental design, which may simply be resolved with clearer description, or they might represent more serious issues.
- Comparing the effect size with expected carbon release would be very helpful in interpreting the implications of this study. For example, how much of the permafrost carbon feedback could be reduced were large areas of the permafrost zone managed in this way? I know there are large uncertainties involved in this kind of analysis, but given the importance of this issue (and the hype and criticism this particular location receives) a “back of the envelope” calculation of potential importance seems justified.
- As currently written, the grazing intensity seems to have only been assessed qualitatively (indicated by park staff). This could introduce several nonrandom sources of error. For example, the animals are very likely to prefer certain ecosystem types, which could cause a strong difference in soil temperature and carbon content independent of the effect of grazing. Additionally, the assessment of the park staff could have been influenced by their knowledge of the intent of the experiment. In a non-blind assessment, this kind of implicit bias is common in qualitative assessments.
- Given the non-split-plot design, is it possible pre-existing differences in carbon stocks and soil thermal dynamics account for some of the observed difference? There can be high spatial heterogeneity in locations such as this—particularly given the fluvial and cryostratigraphical differences that are suggested by the satellite images. The differences in radiocarbon age estimates among the sites suggest to me that they were not similar to begin with—casting some doubt on the conclusion that the grazing treatment accounts for the observed differences. This could perhaps be addressed by comparing results here to a larger suite of measurements from the cited studies in this paper. For future work, using exclosures within the grazed area would allow direct testing of cause and effect with both soil and vegetation.
- How much of the observed change could be due to bulk density effects? Large herbivore presence can cause compaction, which would result in potentially a decrease in the measured active-layer thickness even if the permafrost table absolute position moves downward.
- The question of whether large herbivores could decrease the permafrost carbon feedback depends on the effect on net ecosystem carbon balance—including more than just soil carbon. When aboveground carbon stocks are taken into account, how does this influence the conclusions?
- The extensive use of acronyms to refer to treatments and sites added unnecessary complexity. I would recommend to use intuitive words rather than acronyms whenever possible.
Line edits:
39: Could you mention the percentage difference here to give readers an idea of the effect size?
54: some additional references on this, including some counterintuitive vegetation-soil interactions (Kropp et al., 2020; Loranty et al., 2018; Mekonnen et al., 2021)
Pertinent findings from another herbivore manipulation in tundra: (Min et al., 2021; Strebel et al., 2010)
174: The C13 signature is influenced by many factors besides degradation state (Abbott et al., 2016; Malone et al., 2018; Mauritz et al., 2019).
212: What influence might this recent deforestation have had on soil thermal dynamics?
287: A more informative subsection name would be helpful.
300: What do the colors in the boxplots represent?
330: This figure might be more appropriate in the introduction, since it depicts the hypothesis rather than the findings. If included here, I would recommend annotating to show the predictions that were confirmed, disproven, or inconclusive. One small note: it looks like both Wisent and American Bison are shown in this picture. Are both species present at this site?
340-375: This section seems like an extension of the results rather than a contextualization or discussion. I might recommend shortening, adding references to other work, or moving the supplementary information.
380: The pattern fits, but the study design does not seem able to establish if this was due to grazing or not.
390: All this discussion of the radiocarbon age being attributable to active-layer deepening and potentially to the grazing treatment assumes that pre-treatment SOM content and age were similar across sites. Is there evidence from other nearby sites that profiles are similar enough to assume they were the same pre-treatment?
References
Abbott, B. W., Baranov, V., Mendoza-Lera, C., Nikolakopoulou, M., Harjung, A., Kolbe, T., et al. (2016). Using multi-tracer inference to move beyond single-catchment ecohydrology. Earth-Science Reviews, 160(Supplement C), 19–42. https://doi.org/10.1016/j.earscirev.2016.06.014
Kropp, H., Loranty, M. M., Natali, S. M., Kholodov, A. L., Rocha, A. V., Myers-Smith, I., et al. (2020). Shallow soils are warmer under trees and tall shrubs across Arctic and Boreal ecosystems. Environmental Research Letters, 16(1), 015001. https://doi.org/10.1088/1748-9326/abc994
Loranty, M. M., Abbott, B. W., Blok, D., Douglas, T. A., Epstein, H. E., Forbes, B. C., et al. (2018). Reviews and syntheses: Changing ecosystem influences on soil thermal regimes in northern high-latitude permafrost regions. Biogeosciences, 15(17), 5287–5313. https://doi.org/10.5194/bg-15-5287-2018
Malone, E. T., Abbott, B. W., Klaar, M. J., Kidd, C., Sebilo, M., Milner, A. M., & Pinay, G. (2018). Decline in Ecosystem δ13C and Mid-Successional Nitrogen Loss in a Two-Century Postglacial Chronosequence. Ecosystems, 21(8), 1659–1675. https://doi.org/10.1007/s10021-018-0245-1
Mauritz, M., Celis, G., Ebert, C., Hutchings, J., Ledman, J., Natali, S. M., et al. (2019). Using Stable Carbon Isotopes of Seasonal Ecosystem Respiration to Determine Permafrost Carbon Loss. Journal of Geophysical Research: Biogeosciences, 124(1), 46–60. https://doi.org/10.1029/2018JG004619
Mekonnen, Z. A., Riley, W. J., Berner, L. T., Bouskill, N. J., Torn, M. S., Iwahana, G., et al. (2021). Arctic tundra shrubification: a review of mechanisms and impacts on ecosystem carbon balance. Environ. Res. Lett., 29.
Min, E., Wilcots, M. E., Naeem, S., Gough, L., McLaren, J. R., Rowe, R. J., et al. (2021). Herbivore absence can shift dry heath tundra from carbon source to sink during peak growing season. Environmental Research Letters, 16(2), 024027. https://doi.org/10.1088/1748-9326/abd3d0
Strebel, D., Elberling, B., Morgner, E., Knicker, H. E., & Cooper, E. J. (2010). Cold-season soil respiration in response to grazing and warming in High-Arctic Svalbard. Polar Research, 29(1), 46–57. https://doi.org/10.1111/j.1751-8369.2010.00154.x
Citation: https://doi.org/10.5194/bg-2021-227-RC2 - AC1: 'Reply on RC2', Torben Windirsch, 13 Dec 2021
Status: closed
-
RC1: 'Comment on bg-2021-227', Anonymous Referee #1, 21 Oct 2021
This manuscript presents observations of soil cores from different landscape units and disturbance histories, and aims to answer an interesting and relevant question, “does grazing by large mammals impact permafrost carbon storage?” Unfortunately, the experimental design is fundamentally flawed, making any conclusions about the impact of herbivory on soil carbon storage impossible.
The main issue is lack of replication – the study relies on a single soil core for each combination of environment (drained lake basin or upland) and grazing (intensive or no grazing), which is insufficient given the variability of soil composition and the presence of confounding variables. We know that soil core properties are highly variable in permafrost environments due to cryoturbation, so any variation from one site to another could be due to natural spatial variability or the variable of interest, herbivory. Without replication within sites to account for spatial variability of permafrost soils there is no way to discern between those two possibilities. Additionally, soil moisture is a confounding variable that cannot be accounted for without additional samples in a wider range of environmental conditions. The authors showed that soil organic carbon varied with water/ice content and mentioned that the grazed sites in the drained lake basin flooded seasonally, while none of the other sites flood regularly. This means that patterns in soil organic carbon may be due primarily to variation in soil moisture rather than herbivory, because soil moisture and herbivory covary. Another potential confounding variable is the site history. The authors mentioned that the non-grazed drained lake site was cleared of forest a few years prior to the study while none of the other sites underwent the same treatment.
While the underlying soil core data are sound and could be used to describe some of the variability of the site, the flawed study design makes it impossible to disentangle the effects of spatial heterogeneity, soil moisture regime, site history, and herbivory. Therefore, I suggest that this manuscript be rejected and the authors reconsider the scope of question that can be answered with these data for a new submission.
Citation: https://doi.org/10.5194/bg-2021-227-RC1 - AC2: 'Reply on RC1', Torben Windirsch, 13 Dec 2021
-
RC2: 'Comment on bg-2021-227', Anonymous Referee #2, 26 Nov 2021
This paper entitled “Large herbivores affecting permafrost – impacts of grazing on permafrost soil carbon storage in northeastern Siberia” by Windirsch and others reports carbon storage in the active layer of an intensively grazed thermokarst basin in Siberia. The study tests the important hypothesis of whether reintroduction of large mammals could slow permafrost degradation and greenhouse gas release with climate change. They found decreased active-layer depth and increased carbon storage in the intensively grazed location. The paper is very well written, and the hypotheses are well conceived and explained. I have some questions and concerns about the experimental design, which may simply be resolved with clearer description, or they might represent more serious issues.
- Comparing the effect size with expected carbon release would be very helpful in interpreting the implications of this study. For example, how much of the permafrost carbon feedback could be reduced were large areas of the permafrost zone managed in this way? I know there are large uncertainties involved in this kind of analysis, but given the importance of this issue (and the hype and criticism this particular location receives) a “back of the envelope” calculation of potential importance seems justified.
- As currently written, the grazing intensity seems to have only been assessed qualitatively (indicated by park staff). This could introduce several nonrandom sources of error. For example, the animals are very likely to prefer certain ecosystem types, which could cause a strong difference in soil temperature and carbon content independent of the effect of grazing. Additionally, the assessment of the park staff could have been influenced by their knowledge of the intent of the experiment. In a non-blind assessment, this kind of implicit bias is common in qualitative assessments.
- Given the non-split-plot design, is it possible pre-existing differences in carbon stocks and soil thermal dynamics account for some of the observed difference? There can be high spatial heterogeneity in locations such as this—particularly given the fluvial and cryostratigraphical differences that are suggested by the satellite images. The differences in radiocarbon age estimates among the sites suggest to me that they were not similar to begin with—casting some doubt on the conclusion that the grazing treatment accounts for the observed differences. This could perhaps be addressed by comparing results here to a larger suite of measurements from the cited studies in this paper. For future work, using exclosures within the grazed area would allow direct testing of cause and effect with both soil and vegetation.
- How much of the observed change could be due to bulk density effects? Large herbivore presence can cause compaction, which would result in potentially a decrease in the measured active-layer thickness even if the permafrost table absolute position moves downward.
- The question of whether large herbivores could decrease the permafrost carbon feedback depends on the effect on net ecosystem carbon balance—including more than just soil carbon. When aboveground carbon stocks are taken into account, how does this influence the conclusions?
- The extensive use of acronyms to refer to treatments and sites added unnecessary complexity. I would recommend to use intuitive words rather than acronyms whenever possible.
Line edits:
39: Could you mention the percentage difference here to give readers an idea of the effect size?
54: some additional references on this, including some counterintuitive vegetation-soil interactions (Kropp et al., 2020; Loranty et al., 2018; Mekonnen et al., 2021)
Pertinent findings from another herbivore manipulation in tundra: (Min et al., 2021; Strebel et al., 2010)
174: The C13 signature is influenced by many factors besides degradation state (Abbott et al., 2016; Malone et al., 2018; Mauritz et al., 2019).
212: What influence might this recent deforestation have had on soil thermal dynamics?
287: A more informative subsection name would be helpful.
300: What do the colors in the boxplots represent?
330: This figure might be more appropriate in the introduction, since it depicts the hypothesis rather than the findings. If included here, I would recommend annotating to show the predictions that were confirmed, disproven, or inconclusive. One small note: it looks like both Wisent and American Bison are shown in this picture. Are both species present at this site?
340-375: This section seems like an extension of the results rather than a contextualization or discussion. I might recommend shortening, adding references to other work, or moving the supplementary information.
380: The pattern fits, but the study design does not seem able to establish if this was due to grazing or not.
390: All this discussion of the radiocarbon age being attributable to active-layer deepening and potentially to the grazing treatment assumes that pre-treatment SOM content and age were similar across sites. Is there evidence from other nearby sites that profiles are similar enough to assume they were the same pre-treatment?
References
Abbott, B. W., Baranov, V., Mendoza-Lera, C., Nikolakopoulou, M., Harjung, A., Kolbe, T., et al. (2016). Using multi-tracer inference to move beyond single-catchment ecohydrology. Earth-Science Reviews, 160(Supplement C), 19–42. https://doi.org/10.1016/j.earscirev.2016.06.014
Kropp, H., Loranty, M. M., Natali, S. M., Kholodov, A. L., Rocha, A. V., Myers-Smith, I., et al. (2020). Shallow soils are warmer under trees and tall shrubs across Arctic and Boreal ecosystems. Environmental Research Letters, 16(1), 015001. https://doi.org/10.1088/1748-9326/abc994
Loranty, M. M., Abbott, B. W., Blok, D., Douglas, T. A., Epstein, H. E., Forbes, B. C., et al. (2018). Reviews and syntheses: Changing ecosystem influences on soil thermal regimes in northern high-latitude permafrost regions. Biogeosciences, 15(17), 5287–5313. https://doi.org/10.5194/bg-15-5287-2018
Malone, E. T., Abbott, B. W., Klaar, M. J., Kidd, C., Sebilo, M., Milner, A. M., & Pinay, G. (2018). Decline in Ecosystem δ13C and Mid-Successional Nitrogen Loss in a Two-Century Postglacial Chronosequence. Ecosystems, 21(8), 1659–1675. https://doi.org/10.1007/s10021-018-0245-1
Mauritz, M., Celis, G., Ebert, C., Hutchings, J., Ledman, J., Natali, S. M., et al. (2019). Using Stable Carbon Isotopes of Seasonal Ecosystem Respiration to Determine Permafrost Carbon Loss. Journal of Geophysical Research: Biogeosciences, 124(1), 46–60. https://doi.org/10.1029/2018JG004619
Mekonnen, Z. A., Riley, W. J., Berner, L. T., Bouskill, N. J., Torn, M. S., Iwahana, G., et al. (2021). Arctic tundra shrubification: a review of mechanisms and impacts on ecosystem carbon balance. Environ. Res. Lett., 29.
Min, E., Wilcots, M. E., Naeem, S., Gough, L., McLaren, J. R., Rowe, R. J., et al. (2021). Herbivore absence can shift dry heath tundra from carbon source to sink during peak growing season. Environmental Research Letters, 16(2), 024027. https://doi.org/10.1088/1748-9326/abd3d0
Strebel, D., Elberling, B., Morgner, E., Knicker, H. E., & Cooper, E. J. (2010). Cold-season soil respiration in response to grazing and warming in High-Arctic Svalbard. Polar Research, 29(1), 46–57. https://doi.org/10.1111/j.1751-8369.2010.00154.x
Citation: https://doi.org/10.5194/bg-2021-227-RC2 - AC1: 'Reply on RC2', Torben Windirsch, 13 Dec 2021
Data sets
Large herbivores affecting terrestrial permafrost in northeastern Siberia: biogeochemical and sediment characteristics under different grazing intensities Windirsch, T., Grosse, G., Ulrich, M., Forbes, B. C., Göckede, M., Zimov, N., Macias-Fauria, M., Olofsson, J., Wolter, J., and Strauss, J. https://doi.pangaea.de/10.1594/PANGAEA.933446
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