the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Apr 2025
Technical note: In situ photosynthesis-irradiance curve determination in peatlands with a modulated-light skirt-chamber
Abstract. Peatlands play a crucial role in the global carbon cycle, and among several key processes, it is essential to characterize photosynthesis-irradiance (PI) curves, which describe the relationship between light availability and carbon assimilation through photosynthetic activity. Traditional methods, such as Eddy Covariance and portable photosynthesis measurement systems, provide valuable data at the ecosystem and leaf scales, respectively. However, these approaches leave a gap in capturing carbon dynamics at intermediate scales, where complex plant assemblages and microhabitat variability influence photosynthetic activity in ways that cannot be fully resolved at broader or finer spatial resolutions. In a previous companion paper, we introduced a skirt-chamber method for measuring greenhouse gas emissions in peatlands. Building on that work, we further developed a second version, specifically designed to determine photosynthetic activity at multiple light intensities. This improved modulated-light skirt-chamber enables in situ characterization of photosynthetic responses under natural light conditions by using adjustable screens to regulate light intensity. The chamber is particularly suited for generating PI curves in peatlands and other low-stature ecosystems with diverse microhabitats. Field tests conducted in a subantarctic peatland bog demonstrated the method’s reliability. The generated PI curves fit well with existing models and closely matched measurements from an EC station at the study site, accurately capturing photosynthetic responses to light. The modulated-light skirt-chamber offers a portable, cost-effective, and flexible solution for studying carbon dynamics at an intermediate scale, bridging leaf-level measurements and ecosystem-scale observations. This method holds significant promise for advancing our understanding of carbon fluxes in complex and heterogeneous ecosystems.
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RC1: 'Comment on egusphere-2025-1357', Anonymous Referee #1, 04 May 2025
reply
General comments
Peatlands are globally important carbon (C) sinks and storage through their significant uptake of carbon dioxide (CO2) from the atmosphere and accumulation of undecomposed plant material as peat. However, peatlands also emit CO2 as well as another potent greenhouse gas methane (CH4.). These gas flux dynamics in peatlands are regulated by many environmental variables, such as temperature and water level, that are impacted by the ongoing climate change, and thus there is an urgent need for better understanding of the C cycling of the peatlands and their climate feedback under the warming climate. More studies are especially still needed about spatio-temporal variation of the fluxes in different peatland ecosystems for more accurate climate modeling. This technical note introduces a new version of the previously presented novel measurement method called “skirt-chamber” that can be used for greenhouse gas flux measurements, even in remote locations. Compared to the commonly used dynamic chamber method, the skirt-chamber is more cost-effective and non-invasive as it does not require collars for air-sealed chamber closure. The new “modulated-light skirt-chamber” is specifically designed to determine photosynthesis-irradiance (PI) curves by measuring CO2 flux rates under different light levels, to which it seems to fit well. The authors have polished the chamber design and carefully thought through the measurement set-up as well as the flux calculation method. The resulted PI-curves are showed to be comparable with previous models and eddy covariance measurements. It is also noted that the method of temperature measurements used in the current set-up still requires improvement. Thus, I find that the authors have done comprehensive work with improving their new chamber method for suiting net ecosystem exchange (NEE) measurements and are aware of the remaining limitations of their method.
The manuscript is well written, and its overall quality is good. The abstract is concise and details successfully the central background, methods and results. The introduction flows nicely and highlights the advantages and limitations of different GHG flux measurement methods. The aims of the study are clearly stated. Materials, especially the new version of the skirt-chamber design, as well as measurement methods and mathematical formulae for flux calculation are explained in detail. The authors have also tested several different shading steps to optimize their method regarding the use of different light intensity levels. Moreover, the PI curve generation is validated by comparing two different existing models. All the figures and tables are informative, clear, and include comprehensive captions. Furthermore, the supplementary material is well made giving further information about the chamber design, mass balance calculations, and result validation. I have only a few comments and questions regarding the measurement protocol, chamber design, and sampling. I recommend this manuscript to be accepted after minor revision. Please, see my more detailed comments below.
Specific comments
- 2.4 Study site, campaign, and flux measurements, lines 140-148: In the description of the study site and flux measurements the studied bog is said to have different microforms from hummocks to bare peat surfaces without living Sphagnum cover, and that the measurements covered random locations across different vegetation covers and topographies. Did you also measure the bare peat surfaces or other relatively wet surfaces? Even though the targeted bog is described not to be submerged, with the water table typically between 10 – 60 cm below surface, it seems that it could still have some wet spots. As I mentioned in my previous review of the author’s first technical note regarding the skirt-chamber, bare peat surfaces in bogs can especially be very wet or event water-saturated in my experience, which can make them tricky to measure with chambers. Even if not directly applicable to the studied bog here, I am still curious about how well the authors think the enhanced skirt-chamber would work for measurements in more wet peatland conditions? As one of the main advantages of the skirt-chamber compared to traditional dynamic chamber is that it is does not need any collars, it could still be difficult to conduct the measurements without causing disturbance at the measured site without boardwalks and by placing down the chamber, especially when the water table is high.
- Related to the possible disturbances and difficulties when measuring greenhouse gas fluxes in wet and intermediate peatland microforms, did you detect any ebullition during the measurements this time?
- 2.3 Measurement protocol, lines 127-138: It is said that all PI curve determinations followed the same four-stepped protocol, with the third one being the chamber closure, during which two to four light conditions were tested. Do I understand correctly that the chamber was not ventilated between each light level, but the light conditions were altered during one chamber closure? If so, how often the chamber was ventilated? What are the benefits of not ventilating the chamber between each light level? What about the possible disadvantages or issues? Was there any condensation in the chamber during a closure?
- Did you also monitor the light intensity in real time in addition to the two light/temperature data loggers (so that you could see what PAR was at any given moment) or did you only filter through it later during the data processing? On the lines 189-190 it is said that 26% of the measurements failed, e.g., due to fluctuating or limited solar irradiance. This percentage is pretty high (1/4 of the measurements), and at least for the light it can easily be improved by monitoring PAR also in-situ so that the measurement can be stopped and started from the beginning if the light intensity changes too much during the chamber closure.
- The presented modulated-light skirt-chamber seems to work well for estimating PI-curves based on CO2 measurements under different light levels. Despite the different base for flux calculation, modulated-light skirt-chamber method requires a gas analyzer for measuring CO2 and a selected tracer gas concentrations. In the example study, CH4 was selected as the tracer gas since it was also detected by the used gas analyzer. Nowadays there are quite many portable gas analyzers that commonly measure CO2 and CH4 at the same time. However, in the peatland gas exchange studies it is often interesting to be able to measure both of these gas fluxes at the same time, which saves time and effort from the “old-traditioned” measurements, when CO2 and CH4 needed to be measured in separate campaigns in the absence of the modern gas analyzers. Are there some other gases that the authors would recommend as potential tracer gas for the measurements for the scientists who wish to also use the CH4 data as it is and not to interfere with it by injecting CH4 into the chamber?
- Have you been considering the possibility to add a cooling system to the skirt-chamber? In my experience, temperature in the chamber can increase significantly already in a couple of minutes especially in sunny weather with high light intensity, which alters the conditions for the gas fluxes that are targeted with the measurements. However, many chamber systems do not have a cooling system to regulate the possible warming effect of the chamber. Did the temperature in the skirt-chamber change during the chamber closures?
Citation: https://doi.org/10.5194/egusphere-2025-1357-RC1 -
AC1: 'Reply on RC1', F. Thalasso, 13 May 2025
reply
General comments
Peatlands are globally important carbon (C) sinks and storage through their significant uptake of carbon dioxide (CO2) from the atmosphere and accumulation of undecomposed plant material as peat. However, peatlands also emit CO2 as well as another potent greenhouse gas methane (CH4.). These gas flux dynamics in peatlands are regulated by many environmental variables, such as temperature and water level, that are impacted by the ongoing climate change, and thus there is an urgent need for better understanding of the C cycling of the peatlands and their climate feedback under the warming climate. More studies are especially still needed about spatio-temporal variation of the fluxes in different peatland ecosystems for more accurate climate modeling. This technical note introduces a new version of the previously presented novel measurement method called “skirt-chamber” that can be used for greenhouse gas flux measurements, even in remote locations. Compared to the commonly used dynamic chamber method, the skirt-chamber is more cost-effective and non-invasive as it does not require collars for air-sealed chamber closure. The new “modulated-light skirt-chamber” is specifically designed to determine photosynthesis-irradiance (PI) curves by measuring CO2 flux rates under different light levels, to which it seems to fit well. The authors have polished the chamber design and carefully thought through the measurement set-up as well as the flux calculation method. The resulted PI-curves are showed to be comparable with previous models and eddy covariance measurements. It is also noted that the method of temperature measurements used in the current set-up still requires improvement. Thus, I find that the authors have done comprehensive work with improving their new chamber method for suiting net ecosystem exchange (NEE) measurements and are aware of the remaining limitations of their method.
The manuscript is well written, and its overall quality is good. The abstract is concise and details successfully the central background, methods and results. The introduction flows nicely and highlights the advantages and limitations of different GHG flux measurement methods. The aims of the study are clearly stated. Materials, especially the new version of the skirt-chamber design, as well as measurement methods and mathematical formulae for flux calculation are explained in detail. The authors have also tested several different shading steps to optimize their method regarding the use of different light intensity levels. Moreover, the PI curve generation is validated by comparing two different existing models. All the figures and tables are informative, clear, and include comprehensive captions. Furthermore, the supplementary material is well made giving further information about the chamber design, mass balance calculations, and result validation. I have only a few comments and questions regarding the measurement protocol, chamber design, and sampling. I recommend this manuscript to be accepted after minor revision. Please, see my more detailed comments below.
Our response:
We sincerely thank Reviewer 1 for the careful review and positive evaluation of our work. The comments provided are constructive and insightful, and we believe they will help us improve the quality of the manuscript in the revised version. We are particularly grateful that the reviewer evaluated both the current study and our previous publication (Thalasso et al., 2023; https://doi.org/10.5194/bg-20-3737-2023), thus providing an informed and coherent assessment of the skirt-chamber method and its development. Below, we provide detailed responses to the six specific comments raised. We are confident that the clarifications and forthcoming revisions will further enhance the clarity and robustness of the manuscript.
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Comment 1: Study site, campaign, and flux measurements, lines 140-148: In the description of the study site and flux measurements the studied bog is said to have different microforms from hummocks to bare peat surfaces without living Sphagnum cover, and that the measurements covered random locations across different vegetation covers and topographies. Did you also measure the bare peat surfaces or other relatively wet surfaces?
Even though the targeted bog is described not to be submerged, with the water table typically between 10 – 60 cm below surface, it seems that it could still have some wet spots. As I mentioned in my previous review of the author’s first technical note regarding the skirt-chamber, bare peat surfaces in bogs can especially be very wet or event water-saturated in my experience, which can make them tricky to measure with chambers. Even if not directly applicable to the studied bog here, I am still curious about how well the authors think the enhanced skirt-chamber would work for measurements in more wet peatland conditions?
As one of the main advantages of the skirt-chamber compared to traditional dynamic chamber is that it is does not need any collars, it could still be difficult to conduct the measurements without causing disturbance at the measured site without boardwalks and by placing down the chamber, especially when the water table is high.
Our response 1 – Measurement coverage across topography:
We confirm that the chamber was deployed across a representative range of topographic conditions, including hummocks, hollows, transitional zones, and with varying vegetation covers including bare peat surfaces. In response to this comment, we will include in the revised manuscript a new Table (as supplementary material) listing the 27 chamber deployment sites, along with a short description of the dominant vegetation or surface condition. A brief discussion will also be added to the main text to contextualize the diversity of sites sampled and the corresponding results observed.
Our response 2 – Applicability to wetter conditions and submerged sites:
The studied peatland section presented in this manuscript did not include any submerged area. In the revised manuscript, we will add a paragraph explicitly noting this limitation and discussing the expected performance of the chamber in submerged areas. We anticipate that in submerged or near-saturated zones, the chamber would form a seal with the peat surface, functioning similarly to a static closed chamber, with gas accumulation occurring—potentially without reaching a steady state. We agree with the reviewer that testing the chamber under such conditions would be an important future step, and we will emphasize this in the forthcoming revision of our manuscript.
Our response 3 – Site disturbance:
We agree that a potential limitation of the skirt-chamber is that it requires the presence of an operator, which may lead to pressure disturbances during deployment — especially in wet or water-saturated areas where pressure is transmitted more effectively through the peat matrix. In a separate and more recent study (not part of the current manuscript), we observed that operator proximity influenced the occurrence of ebullition: gas release was triggered when the operator stepped close to the chamber but was avoided when the operator maintained a maximum distance (40–50 cm) allowing chamber operation. In the revised manuscript, we will explicitly mention this drawback and recommend mitigation strategies such as the use of snowshoes (as we did) or pressure-distributing boards. This consideration will be included in the discussion of the method’s limitations.
_________________________
Comment 2: Related to the possible disturbances and difficulties when measuring greenhouse gas fluxes in wet and intermediate peatland microforms, did you detect any ebullition during the measurements this time?
Our response – Ebullition:
We did not detect any sudden increases in CH₄ or CO₂ concentrations that would indicate ebullition events during the present study. We attribute this observation to the fact that, as previously mentioned, all measurements reported in this manuscript were conducted in non-submerged areas, where the water table was below the peat surface. In such conditions, any gas bubbles formed in deeper layers are likely to be gradually released and diluted as they pass through the unsaturated peat and vegetation, reducing the likelihood of detectable ebullition at the surface.
_________________________
Comment 3: 2.3 Measurement protocol, lines 127-138: It is said that all PI curve determinations followed the same four-stepped protocol, with the third one being the chamber closure, during which two to four light conditions were tested. Do I understand correctly that the chamber was not ventilated between each light level, but the light conditions were altered during one chamber closure? If so, how often the chamber was ventilated? What are the benefits of not ventilating the chamber between each light level? What about the possible disadvantages or issues?
Was there any condensation in the chamber during a closure?
Our response 1 – Chamber closure and light modulation without ventilation:
Yes, indeed, the reviewer understood correctly: the chamber remained closed, i.e., not ventilated, for three to four minutes, during which two to four light conditions were tested. The main reason for not ventilating between each light level was to ensure accurate determination of the chamber gas residence time (θC; Section S2) after each chamber opening/closing, achieved by injecting a methane pulse and monitoring the chamber methane concentration over the longest possible time for improved precision. Since the methane injection did not interfere with the CO2 concentration inside the chamber, θC could be determined precisely while applying different light conditions within the same closure phase. In our view, this approach increased the accuracy of θC determination and reduced the data processing effort by minimizing the number of θC determinations required. We will clarify this point explicitly in the forthcoming version of the manuscript.
Our response 2 – Condensation during measurements:
The reviewer raised a valid point. We did observe occasional condensation on the chamber window, but only under the highest light condition—i.e., when no fabric was used to reduce light intensity and under direct sun exposure. This condition was not always observed and never maintained for more than two minutes, in such manner that only slight condensation was observed during this short period. Although limited, the condensation may have slightly affected the incident light by (i) scattering direct irradiance into a more diffuse pattern and (ii) causing a minor reduction in overall transmission. However, because light intensity was measured inside the chamber, any potential effect of condensation on incident light was directly accounted for. Given the short duration and limited extent of the condensation, we consider its impact on the PI curve measurements to be negligible. We will make sure this is clearly addressed in the revised version of the manuscript.
_________________________
Comment 4: Did you also monitor the light intensity in real time in addition to the two light/temperature data loggers (so that you could see what PAR was at any given moment) or did you only filter through it later during the data processing? On the lines 189-190 it is said that 26% of the measurements failed, e.g., due to fluctuating or limited solar irradiance. This percentage is pretty high (1/4 of the measurements), and at least for the light it can easily be improved by monitoring PAR also in-situ so that the measurement can be stopped and started from the beginning if the light intensity changes too much during the chamber closure.
Our response – Real-time light monitoring:
We greatly appreciate the reviewer’s suggestion, which would indeed represent a valuable improvement to the current implementation of the skirt-chamber method for PI curve determination. As mentioned in the manuscript, light intensity was monitored using sensors placed inside the chamber, and the data were processed afterward during analysis. We fully agree that real-time PAR monitoring would provide the operator with immediate feedback on irradiance conditions, enabling the repetition of measurements in case of sudden fluctuations. We will include this point as a recommendation for future applications of the method in the revised version of the manuscript.
_________________________
Comment 5: The presented modulated-light skirt-chamber seems to work well for estimating PI-curves based on CO2 measurements under different light levels. Despite the different base for flux calculation, modulated-light skirt-chamber method requires a gas analyzer for measuring CO2 and a selected tracer gas concentrations. In the example study, CH4 was selected as the tracer gas since it was also detected by the used gas analyzer. Nowadays there are quite many portable gas analyzers that commonly measure CO2 and CH4 at the same time. However, in the peatland gas exchange studies it is often interesting to be able to measure both of these gas fluxes at the same time, which saves time and effort from the “old-traditioned” measurements, when CO2 and CH4 needed to be measured in separate campaigns in the absence of the modern gas analyzers. Are there some other gases that the authors would recommend as potential tracer gas for the measurements for the scientists who wish to also use the CH4 data as it is and not to interfere with it by injecting CH4 into the chamber?
Our response – Other tracer gas:
The reviewer raised an important point regarding the possibility of using an alternative tracer gas, particularly when simultaneous measurement of CH4 and CO2 emissions is of interest. We acknowledge that this aspect was not clearly explained in the current manuscript. Importantly, the use of CH4 as a tracer in our method does not prevent the determination of CH4 emissions. In our previous study (Thalasso et al., 2023; https://doi.org/10.5194/bg-20-3737-2023), we showed that modifying light conditions for PI curve construction had no effect on CH4 fluxes. Therefore, any segment of the CH4 concentration data recorded prior to the CH4 pulse injection can be used to calculate CH4 emissions. In the present study, following each chamber closure, CH4 concentrations were monitored for approximately one minute before the CH4 pulse injection. This time window is sufficient to estimate CH4 fluxes, although extending it by an additional 30 seconds in future applications would improve the accuracy of CH4 flux estimation when CH4 emissions are a core objective. It is also worth noting that each chamber closure provides an independent opportunity to estimate CH4 fluxes, allowing for multiple emission values to be derived from a single chamber deployment. From our perspective, this approach is more practical than switching to an alternative tracer gas, which would inevitably increase cost and equipment weight, potentially compromising the portability of the method. We will clarify this point in the revised manuscript and include a recommendation for adjusting the protocol when simultaneous CH4 and CO2 flux measurement is desired.
_________________________
Comment 6: Have you been considering the possibility to add a cooling system to the skirt-chamber? In my experience, temperature in the chamber can increase significantly already in a couple of minutes especially in sunny weather with high light intensity, which alters the conditions for the gas fluxes that are targeted with the measurements. However, many chamber systems do not have a cooling system to regulate the possible warming effect of the chamber. Did the temperature in the skirt-chamber change during the chamber closures?
Our response – Impact on temperature, cooling system
We fully agree with the reviewer’s concern regarding potential temperature increases inside the chamber during closure. In the current manuscript, we acknowledge (around L285) that one limitation of our study is the use of a suboptimal temperature sensor inside the chamber. As a result, we are unable to provide a reliable assessment of temperature dynamics during chamber closure. In response to this comment, we will expand the discussion in the revised manuscript to include the importance of using a more appropriate temperature sensor for future applications. We will also discuss possible strategies to limit temperature increases, including the use of active cooling systems such as Peltier elements, as previously suggested by Jentzsch et al. (2024; https://doi.org/10.5194/bg-21-3761-2024). These improvements would enhance the ability of the skirt-chamber to maintain near-ambient conditions during measurements. We will make sure this is clearly addressed in the revised version of the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-1357-AC1
Data sets
Photosynthesis-irradiance curve in peatlands with a modulated-light skirt-chamber Frederic Thalasso, Julio A. Salas-Rabaza, and Brenda Riquelme del Río https://data.mendeley.com/datasets/crhk97t7cy/1
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