Conventional subsoil irrigation techniques do not lower carbon emissions from drained peat meadows

Weideveld, Stefan Theodorus Johannes; Liu, Weier; van den Berg, Merit; Lamers, Leon Peter Maria; Fritz, Christian

doi:https://doi.org/10.5194/bg-18-3881-2021

Articles | Volume 18, issue 12

https://doi.org/10.5194/bg-18-3881-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-18-3881-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 12

Research article

|

29 Jun 2021

Research article |

| 29 Jun 2021

Conventional subsoil irrigation techniques do not lower carbon emissions from drained peat meadows

Stefan Theodorus Johannes Weideveld, Weier Liu, Merit van den Berg, Leon Peter Maria Lamers, and Christian Fritz

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Final revised paper (published on 29 Jun 2021)
Preprint (discussion started on 02 Jul 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'remarks on latest data en conclusions', Jos Schouwenaars, 20 Jul 2020
- AC3: 'Author - Response - SC#1', Stefan Weideveld, 06 Sep 2020
RC1: 'referee comment on "Sub-soil irrigation does not lower greenhouse gas emission from drained peat meadows"', Anonymous Referee #1, 01 Aug 2020
- AC1: 'Author - Response - Ref#1', Stefan Weideveld, 06 Sep 2020
RC2: 'Review Weideveld et al.', Anonymous Referee #2, 16 Aug 2020
- AC2: 'Author - Response - Ref#2', Stefan Weideveld, 06 Sep 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (26 Sep 2020) by Kees Jan van Groenigen

AR by Stefan Weideveld on behalf of the Authors (29 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Dec 2020) by Kees Jan van Groenigen

RR by Anonymous Referee #1 (20 Dec 2020)

Suggestions for revision or reasons for rejection

The authors improved their manuscript. They now describe their reason for this study adequately. However, I have still several concerns that should be addressed before reconsideration for publication in Biogeosciences. Especially the discussion part needs to be revised as some discussion points are not well-thought-out and references are imprecise or wrongly cited.

N2O data
Hypothesis 3 cannot be answered with this data set. With a low frequency of sampling, it cannot be excluded that peak emission were missed. In order to answer this question I would suggest to have at least weekly N2O measurement (with higher resolution in situation with high N2O emission risk or continuous data from Eddy-Covariance systems. As the authors stated in their response, it was intended to measure N2O at a higher frequency. Unfortunately, it failed. I would suggest deleting hypothesis 3 instead of trying it to answer it with in inadequate data set. I suggest considering to change ghg to carbon emissions in the title. In the abstract the frequency for N2O measurements are missing, but the authors stated, that the N2O emission were lower in 2017 than in 2018. In addition, N2O data are included in the ghg budget (241, 455). This implies that there is a sufficient data base for calculation of N2O budget. In addition, N2O emissions could also be underestimated by missing peak emission in summer (466). Therefore, it is not easy to estimate the contribution of N2O to ghg budget. Please add number of N2O (and CH4) measurements in Table 4. According to the method section site D 17 measurement campaigns took place. However, I count 13 data points for SII.
L462:The resolution of N2O data is too low to detect peak emissions. For N2O peaks not only fertilization events but also soil humidity is essential. The soil humidity is changed by treatments and could differ. The soil moisture in one treatment could be high enough for N2O emissions, but not in the control. The N2O emisions could also occur one different days due to different moisture regimes in soil. Therefore, it is not possible to state that no peak emission were missed.

Experimental set up.
My second concern is the frequency of grass mowing within the chambers in 2017. The grass was cut 8 times, which is considerably higher than in 2018 and for the whole pasture (4-5 times) and as generally applied for intensively used pastures. For me it is not clear why the grass was cut in 2017 at this high frequency. The frequency of mowing affect the grass development and thus yield. Therefore, comparison of yield may be biased by different management and not only by climatic conditions, as the authors interpret it. I still wonder why a considerably CO2 uptake can be observed after grass cuts, as the CO2 uptake is generally considerably reduced after mowing not only for organic soils, but also for mineral soils (Beetz et al. 2013, Poyada et al. 2016, Eickenscheidt et al. 2015, Schmitt et al., 2010). This question was also addressed in the first review. One reason may be that the modelling/gap-filing is of too low quality to capture the management events of the grassland or the grass was only cut slightly, so considerably amount of CO2 can still be taken up after harvest. However, other grassland with 4-5 cuts per year show the reduction of CO2 uptake after mowing.

discussion
Especially, the whole discussion session needs to be carefully revised. The argumentation and wording are not always straightforward or even false. It is not clear, what the land use, peatland type, management and origin of the cited sites are. In addition, it is often not distinguished between field experiment or laboratory studies.

Here are some examples:
Line 127ff:
I guess that the site had been drained for long-term (highly decomposed material line 444). Thus, the top 30-40 cm have under oxidized condition for long-term and easily decomposable organic material may have been already decomposed. Accordingly, relative stable organic matter may have been accumulated at the top 30-40 cm which is in contrast to the argumentation in line 127ff. In addition, it is not clear if the water content of deeper layers is saturated and how the O2 saturation is, as these parameters are site-specific. The annual averaged soil temperature at deeper layers may be the same as at top layers.

Line117ff:
After Tiemeyer et al. 2020 there is a general dependency of groundwater table and CO2-emissions rates for all land use classes, most strongly at water levels between -20 and -50. The dependency of deeper groundwater level on CO2 emissions is less clear. The findings of the authors are in agreement with recent literature.

L 405-409: here are results presented not discussion

L422-423:
The soil moisture data shows differences between 33 and 90% (Appendix). Not sure, if I would classify these differences as small variations.

L423-425:
I can only guess the meaning of this sentence. Please reformulate.

L 427: Please reformulate the sentence. What is effect size

L 473:
In Tiemeyer et al. 2016 it can be clearly seen that there are two sites with NEE > 60 t CO2 (Fig 1), so there have been higher values measured before. The cited values (Tiemeyer at al 2020) are the new German EF based on published data. The German EF are self-evident lower than the highest measured values.

Soil moisture
The authors now include soil moisture from one sampling day in August 2017. Unfortunately, the groundwater was quite similar between treatments in 2017. I asked for the soil moisture data from sensors. If these data are not included in the manuscript, than they can be deleted from the material and method section. T soil is used for the modeling. How is the data coverage of the 5 cm soil data, as it can be expected that malfunction of sensors was both for soil moisture and soil temperature? When was the Hobo sensor installed? Which data was used for modelling 5 cm or 10 cm Hobo data? How much uncertainty add the missing soil data to the NEE? Which data was used, when there was a data gap?

Soil properties are now included in the Appendix. I would suggest to incorporat the information about C content (g/kg) of different layers in Table 1. The unit g/l is not often used. It is rather carbon stock than carbon content?

Please provide also errors of NECB and c-export and display them in Table 3

Eickenscheidt, T., Heinichen, J., Drösler, M., 2015. The greenhouse gas balance of a rained fen peatland is mainly controlled by land-use rather than soil organic carbon content. Biogeosciences 12, 5161–5184. https://doi.org/10.5194/bg-12-5161-2015
Schmitt, M., M. Bahn, G. Wohlfahrt, U. Tappeiner & A. Cernusca. 2010: Land use affects the net ecosystem CO2 exchange and its components in mountain grasslands, Biogeosciences, 7, 2297-2309.

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ED: Publish subject to minor revisions (review by editor) (20 Feb 2021) by Kees Jan van Groenigen

AR by Stefan Weideveld on behalf of the Authors (03 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (20 May 2021) by Kees Jan van Groenigen

AR by Stefan Weideveld on behalf of the Authors (26 May 2021) Author's response Manuscript

Short summary

Raising the groundwater table (GWT) trough subsoil irrigation does not lead to a reduction of carbon emissions from drained peat meadows, even though there was a clear increase in the GWT during summer. Most likely, the largest part of the peat oxidation takes place in the top 70 cm of the soil, which stays above the GWT with the use of subsoil irrigation. We conclude that the use of subsoil irrigation is ineffective as a mitigation measure to sufficiently lower peat oxidation rates.