the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jan 2024
Nitrous oxide as second most important greenhouse gas in tropical peatlands
Abstract. Earth’s climate largely depends on carbon and nitrogen exchange between the atmosphere and tropical peatland ecosystems. Permanently wet peatlands take up carbon dioxide in plants and accumulate organic carbon in soil but release methane. Man-made drainage releases carbon dioxide from peat soils. Carbon and nitrous gas exchange and their relationships with tropical peatland conditions are poorly understood. We analysed natural peat swamp forests and fens, moderately drained and dry peatlands under a wide variety of land uses. The tropical peat swamp forests were large greenhouse gas sinks while tropical peatlands under moderate and low soil moisture levels emitted carbon dioxide and nitrous oxide. Carbon dioxide uptake of 160 mg m–2 h–1 dominated the net greenhouse gas budgets overall, while nitrous oxide emission of 90 mg CO2-equivalent m–2 h–1 on average was the second most important contributor (ahead of average methane emissions of 36 mg CO2-equivalent m–2 h–1) across the whole tropical peat moisture range.
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-24', Anonymous Referee #1, 31 Jan 2024
On the face of it, this is an interesting paper. It provides some data on an important and understudied topic and includes a wide range of field sites.
However, the methodology, in so much as it has been described, is extremely poor. So much so that the results and conclusions are, at best, worthless and, at worst, misleading.
The measurements span a few days per site over the course of 11 years with individual sites typically having only 3-6 days of measurements in total.
The authors derive GPP from satellite-derived NDVI, this is in itself questionable given this approach has not been tested in tree-covered tropical peatlands and the lack of verification of the data provided or cited. The presence of tree cover on peatlands causes NDVI to be related in a different way to several nutrient and catchment characteristics that will affect GPP, see the following recent paper: https://doi.org/10.1016/j.scitotenv.2023.165132
This questionable GPP data, at 1km2 resolution, is then paired with a few days of chamber measurements, i.e. totally different spatial scales, to get NEE, discounting both spatial and temporal heterogeneity in fluxes. This process is poorly described, with no mention of the spatial or temporal overlap in these measurements or how they were reconciled. Important details are missing such as the number of replicates, chamber size, vegetation within the chambers, when the collars were installed, the time of day the measurements were taken
I hoped to find some value in the N20 and CH4 flux data. Sadly, we now know N20 is an extremely ‘spikey’ flux, sensitive to rain events and the rise and fall of the water table, see https://doi.org/10.1016/j.agrformet.2020.108280 . A 3-6 day measurement campaign will not capture these spikes. Likewise, CH4 while not as spikey as N20 is seasonally variable and a single annual point measurement is simply a waste of time and research funds, the same can be said for the other fluxes.
I had also hoped to find something worthy of considering for publication within the correlation matrix that is described in the methods but missing from the manuscript and the acknowledgements. Indeed the authors mention samples were taken back to the laboratory in Tartu, however, no further information or data is given regarding this.
In summary, this manuscript has severe methodological flaws. I consider the data so flawed as to have no value.
Additionally, there are problems with how the paper is written for instance with the correlation matrix described and then omitted. The discussion is also cursory with barely a few sentences dedicated to CO2 and CH4, perhaps justifiable given the poor quality of this data. Sadly the discussion of N20 is little better with no consideration of how N20 fluxes vary over time and how representative a few days of chamber measurements may be.
I urge the authors to think long and hard about the value of this type of research. Had the resources been used to study a single location, with regular and synchronous measurements, ideally supported by EC, something useful and interesting could have been achieved. While the bar for publication is lower in ecosystems and areas that are poorly studied, it has not been acceptable to publish only a few days of chamber measurements per site since ~1970. It is also important that we are intellectually honest about the limitations of our data which I do not see evidence of. As it is this data is a mess and I cannot see how it can be used.
Citation: https://doi.org/10.5194/egusphere-2024-24-RC1 -
RC2: 'Comment on egusphere-2024-24', Anonymous Referee #2, 27 Mar 2024
Tropical peatlands contain large amounts of carbon accumulated over time in partially decomposed debris (e.g. wood, litter) and develop where high rainfall or poor drainage leads to waterlogging, limiting oxygen availability and inhibiting decomposition. This is the result of a fine balance between ecology, climate and geology that can be profoundly altered by climate, e.g. more frequent and severe droughts, and other environmental changes, e.g. deforestation, land conversion, currently occurring in the tropics. As a result of these changes, previously accumulated peat organic matter is exposed to oxygen, promoting aerobic decomposition of organic carbon and releasing carbon dioxide (CO2) into the atmosphere, as well as other potent greenhouse gases, methane (CH4) and nitrous oxide (N2O). However, existing estimates of these greenhouse gas emissions from tropical peatlands vary widely, leading to uncertainty.
The topic addressed by Pärn et al. is scientifically significant because it sheds light on the still uncertain budget estimates of CO2, CH4 and N2O fluxes in tropical peatlands. As the title suggests, the study focuses mainly on N2O fluxes, which is particularly relevant as tropical agricultural peatlands have been identified as global hotspots for N2O emissions, with implications for nitrogen management.
However, the study has some major drawbacks:
Overall, the study is poorly written, despite being on a highly relevant topic. The text lacks justification and accurate information, which should be supported by references and not reduced to a list of names/dates.
For example, the state of the art in the introduction is superficial, listing too many key aspects of ecosystem functioning and not describing important mechanisms. Why / how can climate change influence ecosystem respiration (from soil CO2 efflux measurements in peatlands?) via more extreme events (severe drought, inundation) in the tropics? Compared to CH4 and N2O, the CO2 cycle is relatively well understood and can be detailed.
Methodologically, the study is based on a too small sample size of measured soils (n = 3-4 opaque chambers), a short measurement period (3-6 days) and a “rough” upscaling to ecosystem scale based on the small sample size and temporal resolution. Several omissions in the explanation of the methodology and numerous inconsistencies in the use of terminology / acronyms throughout the text make the study difficult to understand and follow. For example, there is no explanation of what is meant by “dry season” and “wet period” (season?), although this seems to be one way of analysing the soil flux data. Also, the information provided on the measurement frequency of soil greenhouse gas fluxes needs further clarification for better understanding. Consider providing more specific details about how often these measurements were taken, including the time intervals between gas sampling and measurements to clarify the experimental design and data collection process across so many different study sites. Calculation of gross primary productivity and change of scale with remote sensing data is extremely risky given the small sample size and temporal resolution.
The Results and Discussion section is too short. More results are needed, e.g. relationships between ancillary measurements such as soil temperature and water content with soil greenhouse gas fluxes. Many variables are listed in the Methods section, but only a few are discussed in the Results and Discussion section.
Overall, more data is needed to strengthen the study and make it reliable. In addition, a table with descriptive statistics is necessary, it is the first piece of information to provide to understand and present all the data used for the study. In addition, a lot of work needs to be done on the writing in order to justify the study through a state of the art well linked to the objectives, to present and interpret the results properly and to discuss them.
Citation: https://doi.org/10.5194/egusphere-2024-24-RC2
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-24', Anonymous Referee #1, 31 Jan 2024
On the face of it, this is an interesting paper. It provides some data on an important and understudied topic and includes a wide range of field sites.
However, the methodology, in so much as it has been described, is extremely poor. So much so that the results and conclusions are, at best, worthless and, at worst, misleading.
The measurements span a few days per site over the course of 11 years with individual sites typically having only 3-6 days of measurements in total.
The authors derive GPP from satellite-derived NDVI, this is in itself questionable given this approach has not been tested in tree-covered tropical peatlands and the lack of verification of the data provided or cited. The presence of tree cover on peatlands causes NDVI to be related in a different way to several nutrient and catchment characteristics that will affect GPP, see the following recent paper: https://doi.org/10.1016/j.scitotenv.2023.165132
This questionable GPP data, at 1km2 resolution, is then paired with a few days of chamber measurements, i.e. totally different spatial scales, to get NEE, discounting both spatial and temporal heterogeneity in fluxes. This process is poorly described, with no mention of the spatial or temporal overlap in these measurements or how they were reconciled. Important details are missing such as the number of replicates, chamber size, vegetation within the chambers, when the collars were installed, the time of day the measurements were taken
I hoped to find some value in the N20 and CH4 flux data. Sadly, we now know N20 is an extremely ‘spikey’ flux, sensitive to rain events and the rise and fall of the water table, see https://doi.org/10.1016/j.agrformet.2020.108280 . A 3-6 day measurement campaign will not capture these spikes. Likewise, CH4 while not as spikey as N20 is seasonally variable and a single annual point measurement is simply a waste of time and research funds, the same can be said for the other fluxes.
I had also hoped to find something worthy of considering for publication within the correlation matrix that is described in the methods but missing from the manuscript and the acknowledgements. Indeed the authors mention samples were taken back to the laboratory in Tartu, however, no further information or data is given regarding this.
In summary, this manuscript has severe methodological flaws. I consider the data so flawed as to have no value.
Additionally, there are problems with how the paper is written for instance with the correlation matrix described and then omitted. The discussion is also cursory with barely a few sentences dedicated to CO2 and CH4, perhaps justifiable given the poor quality of this data. Sadly the discussion of N20 is little better with no consideration of how N20 fluxes vary over time and how representative a few days of chamber measurements may be.
I urge the authors to think long and hard about the value of this type of research. Had the resources been used to study a single location, with regular and synchronous measurements, ideally supported by EC, something useful and interesting could have been achieved. While the bar for publication is lower in ecosystems and areas that are poorly studied, it has not been acceptable to publish only a few days of chamber measurements per site since ~1970. It is also important that we are intellectually honest about the limitations of our data which I do not see evidence of. As it is this data is a mess and I cannot see how it can be used.
Citation: https://doi.org/10.5194/egusphere-2024-24-RC1 -
RC2: 'Comment on egusphere-2024-24', Anonymous Referee #2, 27 Mar 2024
Tropical peatlands contain large amounts of carbon accumulated over time in partially decomposed debris (e.g. wood, litter) and develop where high rainfall or poor drainage leads to waterlogging, limiting oxygen availability and inhibiting decomposition. This is the result of a fine balance between ecology, climate and geology that can be profoundly altered by climate, e.g. more frequent and severe droughts, and other environmental changes, e.g. deforestation, land conversion, currently occurring in the tropics. As a result of these changes, previously accumulated peat organic matter is exposed to oxygen, promoting aerobic decomposition of organic carbon and releasing carbon dioxide (CO2) into the atmosphere, as well as other potent greenhouse gases, methane (CH4) and nitrous oxide (N2O). However, existing estimates of these greenhouse gas emissions from tropical peatlands vary widely, leading to uncertainty.
The topic addressed by Pärn et al. is scientifically significant because it sheds light on the still uncertain budget estimates of CO2, CH4 and N2O fluxes in tropical peatlands. As the title suggests, the study focuses mainly on N2O fluxes, which is particularly relevant as tropical agricultural peatlands have been identified as global hotspots for N2O emissions, with implications for nitrogen management.
However, the study has some major drawbacks:
Overall, the study is poorly written, despite being on a highly relevant topic. The text lacks justification and accurate information, which should be supported by references and not reduced to a list of names/dates.
For example, the state of the art in the introduction is superficial, listing too many key aspects of ecosystem functioning and not describing important mechanisms. Why / how can climate change influence ecosystem respiration (from soil CO2 efflux measurements in peatlands?) via more extreme events (severe drought, inundation) in the tropics? Compared to CH4 and N2O, the CO2 cycle is relatively well understood and can be detailed.
Methodologically, the study is based on a too small sample size of measured soils (n = 3-4 opaque chambers), a short measurement period (3-6 days) and a “rough” upscaling to ecosystem scale based on the small sample size and temporal resolution. Several omissions in the explanation of the methodology and numerous inconsistencies in the use of terminology / acronyms throughout the text make the study difficult to understand and follow. For example, there is no explanation of what is meant by “dry season” and “wet period” (season?), although this seems to be one way of analysing the soil flux data. Also, the information provided on the measurement frequency of soil greenhouse gas fluxes needs further clarification for better understanding. Consider providing more specific details about how often these measurements were taken, including the time intervals between gas sampling and measurements to clarify the experimental design and data collection process across so many different study sites. Calculation of gross primary productivity and change of scale with remote sensing data is extremely risky given the small sample size and temporal resolution.
The Results and Discussion section is too short. More results are needed, e.g. relationships between ancillary measurements such as soil temperature and water content with soil greenhouse gas fluxes. Many variables are listed in the Methods section, but only a few are discussed in the Results and Discussion section.
Overall, more data is needed to strengthen the study and make it reliable. In addition, a table with descriptive statistics is necessary, it is the first piece of information to provide to understand and present all the data used for the study. In addition, a lot of work needs to be done on the writing in order to justify the study through a state of the art well linked to the objectives, to present and interpret the results properly and to discuss them.
Citation: https://doi.org/10.5194/egusphere-2024-24-RC2
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