the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Effects of water table level and nitrogen deposition on methane and nitrous oxide emissions in an alpine peatland
- 1Institute of Wetland Research, Chinese Academy of Forestry, Beijing Key Laboratory of Wetland Services and Restoration, Beijing 100091, China
- 2Sino-Danish Centre for Education and Research, University of Chinese Academy of Sciences, Beijing 100049, China
- 3Sichuan Zoige Wetland Ecosystem Research Station, Tibetan Autonomous Prefecture of Aba 624500, China
- 4Department of Ecoscience and Arctic Research Centre (ARC), Aarhus University, Vejlsøvej 25, 8600 Silkeborg, Denmark
- 1Institute of Wetland Research, Chinese Academy of Forestry, Beijing Key Laboratory of Wetland Services and Restoration, Beijing 100091, China
- 2Sino-Danish Centre for Education and Research, University of Chinese Academy of Sciences, Beijing 100049, China
- 3Sichuan Zoige Wetland Ecosystem Research Station, Tibetan Autonomous Prefecture of Aba 624500, China
- 4Department of Ecoscience and Arctic Research Centre (ARC), Aarhus University, Vejlsøvej 25, 8600 Silkeborg, Denmark
Abstract. Alpine peatlands are recognized as a major natural contributor to the budgets of atmospheric methane (CH4) but as a weak nitrous oxide (N2O) source. Anthropogenic activities and climate change have put these fragile nitrogen (N)-limited peatlands under pressure by altering water table (WT) levels and enhancing N deposition. The response of greenhouse gas (GHG) emissions from these peatlands to these twin changes is uncertain. To address this knowledge gap, we conducted a mesocosm experiment in 2018 and 2019 investigating individual and interactive effects of three WT levels (WT-30, 30 cm below soil surface; WT0, soil-water interface; WT10, 10 cm above soil surface) and multiple levels of N deposition (0, 20, 40, 80 and 160 kg N ha-1 yr-1) on growing season CH4 and N2O emissions in the Zoige alpine peatland, Qinghai-Tibetan Plateau. We found that the elevated WT levels increased CH4 emission, while the N deposition had non-linear effects (stimulation at moderate levels and inhibition at higher). In contrast no clear pattern of the effect of WT levels on the cumulative N2O emission was evident, while N deposition led to a consistent and linear increase (emission factor: 2.3 %–2.8 % and 1 % in IPCC), and this was dependent on the WT levels. Across the two years, the scenario with the greatest GWP (from CH4 and N2O) was an N deposition of ca. 20 kg N ha-1 yr-1 and high WT levels (at soil surface or above). Given the current N deposition in the Zoige alpine peatland (1.08–17.81 kg N·ha-1), our results suggested that the CH4 and N2O emissions from the alpine peatlands could greatly increase in response to the possible doubling N deposition in the future. We believe that our results provide insights into how interactions between climate change and human disturbance will alter GHG emissions from this globally important habitat.
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Wantong Zhang et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-53', Anonymous Referee #1, 10 Jun 2022
The manuscript is working on an important topic with a clear objective: to examine the response of non-CO2 emissions (i.e., CH4 and N2O) to the manipulation of water table and N deposition in an alpine peatland ecosystem. Unfortunately, several major concerns from methodologies and discussions in the current draft make it unacceptable for publication by Biogeosciences.
major comments:
1. the sampling frequency in the year 2018 is too low to capture the temporal variation of the gas fluxes (only five sampling events were conducted over five months). Therefore, the cumulative emissions calculated contain high uncertainty.
2. the second hypothesis points to the altered "efficiency of utilising nutrients for CH4 and N2O production" by regulation of redox conditions through water table manipulation. However, no data in the current study can support such a hypothesis.
3. the global warming potential (GWP) in the current study simply sums up the non-CO2 emissions (based on their radiative forcing). Without including net ecosystem CO2 exchange, or change of SOC stock, critical limitations exist in the significance of this work (the effects of treatments (water table and N deposition) on the GHG budget of the studied ecosystem). The elevated CH4 emissions under a higher water table could be offset sufficiently by the depressed SOC decomposition, leaving the net effect unclear.
4. discussions are generally shallow, and some parts are inappropriate. For example, many discussions are more like introductions instead of discussions (e.g., L276-277; L316-319; L348-365, etc.). Section 4.4 is simply not implications, but background information that fits appropriately in the introduction, except for a few lines in the last paragraph. Discussions on denitrification and microbial N2O N2O production (L309-315) are weak as soil TN is the only N measured. Discussions state some findings are "quite novel" (L321-322; L344-346) but fail to justify them (what is the implication and the potential contribution/influence if these are considered novel findings?)
specific comments:
1. the units of the cumulative emissions (i.e., the main result) are confusing. Why are they "g C/N m-2 yr-1"? Based on the equation provided (L162), they should be "g C/N m-2" and calculated by integration over the growing season. Did the authors extrapolate the calculation to the non-growing seeasons?
2. related to the question above, what happens to the non-growing season? any gas sampling was conducted from the mesocosm? Due to the low temperature, probably soils are frozen and thus the microbial activities are low, but the authors are recommended to include the explanation in the methodology and justify (with proper references) that emissions from growing seasons heavily dominated the gas fluxes.
3. another unit issue for GWP. if cumulative emissions have the unit of "g C/N m-2 yr-1", why GWP ended up with "g CO2-eq m-2" based on the equation provided (L171)? shouldn't it be "g CO2-eq m-2 yr-1"?
4. the experimental design on the levels of N deposition includes an unrealistically high dose (i.e., 160 kg N ha-1 y-1). It is fine to examine the relationships, but proper efforts should be made to justify such a design in the discussion.
5. for the calculation of cumulative emission, the authors can simply describe it like "linear interpolation between sampling events using the trapezoidal rule" instead of providing the equation and explanation for the notations (L161-166). instead, the authors are recommended to provide equations for calculating the gas fluxes rather than simply saying "calculated by the slopes of linear regression between gas concentrations" (is it corrected with temperature? atmospheric pressure?)
some minor corrections:
1. L77: highlevel -> high-level N deposition
2. L78: "aerobic conditions"? are the authors trying to mention redox conditions? similar expressions occur in several parts of the remaining text, consider rephrasing (e.g., L295, L383).
3. descriptions of the mesocosm design and treatment manipulation are not very clear, the authors are recommended to include a supplementary figure for a clear illustration. in particular, the definition of WT0 can be confusing (i.e., soil-water interface; L101), is it simply "the soil surface"?
4. L158: despite -> regardless of. Also, the description of how the GAM is applied could be oversimplified. Ask this question may help the authors to improve the description: does the current description sufficient for peer researchers to reproduce the analysis?
5. L166-168. difficult to follow. how can the heterogeneity be reduced?
6. L173-174. reference missed.
7. L175. "by applying the statistic R software" -> "using R"
8. L180. SWC has been abbreviated in L146.
9. L205-206: "the highest value occurring"-> "with the highest value observed"
10. L245: "combination" -> "interaction"
11. L267: add "CH4" before "emissions"
12. L273-274: needs rephrasing. Note that the study did not measure oxygen content, and therefore the expression like "oxygen content declined" is not appropriate. Consider: "With higher WT levels, SWC increased and likely formed more anaerobic conditions conducive to CH4 production, leading to elevated CH4 emissions (references)."
13. L276: add comma after "considerably"
14. L293-295: difficult to follow.
15. L305: reference format: Gao et al. (2014).
16. L343: from CH4 to N2O -> from N2O to CH4
17. L369: increased -> decreased
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RC2: 'Comment on bg-2022-53', Anonymous Referee #2, 18 Jun 2022
Wetland is an important source of CH4 and N2O. Global change especially changes in precipitation and N deposition could have greatly effect on them. However, how do they affect fluxes of CH4 and N2O is still unclear in wetland on the Qinghai-Tibetan Plateau. This manuscript focused on the effects of nitrogen deposition on CH4 and N2O emissions under three water table levels in the Zoige alpine peatland. Thus, it is an important and interesting topic. However, there are still minor flaws that should be revised prior possible publication by this journal.
- The present results are relying on the five levels of nitrogen deposition, but some levels (such as 160 kg N ha-1 yr-1) are extremely higher compared to the local nitrogen deposition (1.08-17.81 kg N ha-1 yr-1), could authors explain why to design the treatments?
- Authors conducted a two-year mesocosm experiment, how about the variability of soil properties and GHG emissions within the two years. Suggest you to compare the differences of SOC, TN or GHG emissions between 2018 and 2019.
- It is better to revise the second hypothesis to “The effects of N deposition on CH4 and N2O emissions would be associated with WT levels” in lines 77-79.
- Discussion should be improved, some parts are just a repeat from the Introduction.
- English in the manuscript should be improved.
Specific mistakes:
(1) delete “1% in IPCC” in the Abstract.
(2) the sentence of “the large carbon pool is nitrogen deficient and is recognized ….” in lines 32-33 is hard to understand and need to be rewritten.
(3) Delete “(mean ± SE) (n=3)” in the title of table 1.
(4) line 90: July should be revised to June.
(5) line 213: the name of Figure 1 should be changed, it is hard to see the response of GHG flux to nitrogen deposition.
(6) Line 217, “During the rowing seasons”, rowing should be revised to growing.
(7)line 274: “the exposure of CH4 production process to anaerobic conditions increased” might to be changed to “CH4 production under anaerobic conditions was increased”.
(8) Figure S1, the precipitations from the peatland in June, August and September of 2019 were extremely high, reaching more than 2500 mm in one month. You should scrutinize the raw data.
(9) line304: “show” should revised to “showed”.
(10) line 305-306: “…the study of (Gao et al. 2014)” should be revised to “…the study of Gao et al. (2014)”.
(11) line 306-307: revise the whole sentence to “ which indicated that N2O emissions was significantly increased by N addition (5.0 g N m−2 yr-1) and slightly decreased in the higher WT level in the alpine peatlands of the Qinghai-Tibetan Plateau.”
(12) line 313: “soil peat” should be revised to “soil”.
(13) line 327: (Gong et al. 2019) should be revised to Gong et al. (2019).
Wantong Zhang et al.
Wantong Zhang et al.
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