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Interactive comment on “Effects of soil water content on carbon sink strength in an alpine swamp meadow of the northeastern Qinghai-Tibet Plateau” by Wei et al.

Received and published for Discussion: 24 September 2021

Response to Comments of Referee #1

Wei et al. reports that “Effects of soil water content on carbon sink strength in an alpine swamp meadow of the northeastern Qinghai-Tibet Plateau”. This study investigated the diurnal, seasonal and annual variability of CO₂ fluxes and their drivers in an alpine swamp on the northeastern Tibetan Plateau. This helps to more clearly understand the role of alpine swamp in alpine ecosystem carbon (C) cycling in Tibetan Plateau, because alpine swamp cycling in Tibetan Plateau is less focused at regional scale, compared with alpine steppe and alpine meadow. The text is well-written and clear. Nevertheless, I have reservations about the innovation of scientific questions and the reliability of some results that need to be addressed before the publication of this manuscript.  

We thank Reviewer #1 for taking the time to assess our manuscript and for providing general positive comments and main concerns. We believe the comments have helped to improve the manuscript and we carefully considered them. Here specifically we clarify the innovation of our scientific questions addressed in this study. As REF#1 pointed out, alpine swamps in Tibetan Plateau are less focused at regional scale than alpine steppes and alpine meadows. This study highlights among other things the importance of soil water availability regulating carbon sink strength of an alpine swamp, which is characterized by saturated water condition and high soil water content (SWC). The role of soil water has often been neglected or assumed to be less important relative to other factors for carbon (C) cycling. This study provides a four-year field observation dataset to characterize and quantify the importance of soil water controlling the C sink strength of an alpine swamp – one key finding is that a 15% decrease in soil water can induce 25% higher respiration and therefore weaken C sink strength by 20%, and an additional 44% increase of temperature at annual scale can also weaken the C sink strength by about 50% (see answer to comment number 4). These new insights will help us to better understand, model and predict the complex C cycle dynamics in the Tibetan Plateau driven by the almost certain future intensified climate warming.

• The experimental site is located at Haibei in the northeastern Tibetan Plateau. According to Wei et. al (2021), there are at least six eddy covariance sites at Haibei, including alpine swamp CO2 fluxes monitoring site. Haibei is the most densely distributed area of eddy covariance sites on the Tibetan Plateau. The strength of CO₂ sink and its diurnal, seasonal and interannual characteristics in alpine swamp at Haibei have been reported in previous publications, such as Zhao et al. (2005) and Zhao et al. (2010), yet it is also the first objective of this study. Thus, the innovation of the objective is not clear to me.

We appreciate REF#1 for sharing this very useful information about other existing eddy covariance sites and related references. We have now cited these previous studies in the revised manuscript see L76 and L77. According to Wei et. al (2021), there are six observational studies about C fluxes at Haibei. However, only three of the them focused on alpine swamp meadows (or wetland in Wet et. al (2021)). Among them, one study only focused on a 1-year dataset (Zhang et al., 2008), and the other two characterized the same location (Zhao et al., 2005, 2010). Moreover, these alpine swamp meadows were reported as a net C source while we our site showed a consistent C sink. The different directions of C exchange suggest that there are still uncertainties in our understanding of C exchange in this alpine swamp meadows, and further insights are obtained from studying multiple years of observations. Therefore, further efforts are still needed to improve our projection of C balance change of this ecosystem under changing climate.

Additionally, as mentioned before, previous studies focusing on C fluxes in alpine swamp meadows did not give enough consideration to the effects of soil water content on C fluxes given their nearly saturated nature. A number of previous studies have shown that temperature is an important driver of ecosystem respiration in similar alpine swamp meadows. For example, in the papers from Zhao et al. (2010) and Zhao et al. (2005), the authors showed that ecosystem respiration follows the exponential variation of soil temperature without considering soil water content. Zhu et al. (2020) also suggested that soil temperature plays the most important role in the change of monthly ecosystem respiration in the alpine wetland at Luanhaizi, northeastern Qinghai-Tibet Plateau. Therefore, in this study, we wanted to characterize and estimate the terrestrial C exchange while considering the potential effects of soil moisture. Meanwhile, it should be noted that Zhao et al. (2010) also noticed that the CO₂ emission rates decrease notably after rain events, and Zhu et al. (2020) confirmed that annual precipitation exhibits significant impact on variation of annual net C uptake. All these existing studies have suggested that C fluxes are related to water availability condition, but few studies have found that soil moisture explicitly affects respiration, and its decrease further reduces net C uptake in alpine swamp meadows, this finding could be an important factor for carbon modelling in the future.

Finally, the addition and further analysis of multiple years of data from new sites is always very important also in a more regional/global context - there is a generalized sparsity of in situ observations where their temporal and spatial coverage is very limited. We believe that any effort and addition to this generalized lack of in situ data will be useful for both flux communities such as FLUXNET and ASIAFLUX and modelling community in general.

• The main drivers of NEE variation based on the different approaches are contradictory in this study. Net radiation is the leading factor affecting seasonal and annual variability of NEE based on machine learning approach (Fig. 5, Lines 231-250). However, the combined effect of temperature and soil moisture change is the main factor influencing the annual variation of NEE in Section 4.3 and Table 2. The title of this manuscript only emphasizes the effect of soil moisture. Therefore, there are three different descriptions of the dominant factor of CO₂ sink in this manuscript. The mechanism underlying NEE variation needs to be more rigorously analyzed.

Thank you for this insightful comment. Our first draft didn’t present the findings clearly and it was somewhat misleading. First of all, we should acknowledge that the original title did not fully reflect our conclusions, therefore we revised it as follows:

“Radiation, soil water content, and temperature interactions with carbon cycling in an alpine swamp meadow of the northeastern Qinghai-Tibet Plateau”.

In the S4.3, we intend to put our results into a broader context by comparing with other surrounding alpine swamp meadow sites to highlight the effects of the complex interactions between temperature and soil water content on carbon fluxes. In this section, we did not intend to explicitly disentangle the most important drivers for the NEE at annual scales among these different sites, given that detailed observations for net radiation are lacking for other sites. Such comparison highlights the importance of SWC (precipitation as a proxy for SWC as explicit SWC is not present in these either) in controlling NEE. This further validates our Random Forest (RF) findings regarding the drivers for the Re in our site (Figure 5). We should keep in mind that NEE is the difference between Re and GPP, environmental variables affecting Re and GPP could affect NEE indirectly (Song et al. 2011).We revealed that the main drivers for the GPP, Re and NEE are remarkably different. Net radiation is a key driver of seasonal and annual NEE and GPP, while soil water content is most important for Re at diurnal, seasonal and annual scales. Therefore, our findings from the RF analyses are not contradictory to the discussions regarding the difference in NEE and the potential influence factors among different sites.

However, your constructive comments point out an interesting scientific issue regarding the divergent drivers for the NEE dynamics at different time scales across different alpine swamp meadow sites. This will be an excellent point to be addressed in our future study.

• A key conclusion of this study is that ecosystem respiration (Re) increases with decreasing soil water content (SWC). This is based on the comparisons of Re and SWC observations in the late growing seasons of 2014 and 2015 (Section 4.1). However, both SWC and Re in the late growing season are largest in 2017 during the observational period (2014-2017) (Figs. 2 and 4). Thus, the conclusion of “Section 4.1 Low soil moisture is associated with enhanced ecosystem respiration” (Lines 262-312) is likely to be unreliable. All observational years (2014, 2015, 2017 and 2018) are recommended to be considered in the analyses, rather than only two years (2014 and 2015).

Many thanks for this comment - we have not clearly described the comparisons in S4.1 between years in the text. Since C fluxes are affected by plant phenology and climate factors including temperature, soil moisture, and radiation simultaneously (Figure 5), in order to analyze the effects of single factor, ideally, other factors need to be identical or at least close (no significant differences). Based on this theory, we made our comparisons of specific time periods other than all the observation time. We now explicitly implemented the following text in S4.1, L288-293:

“Since C fluxes are affected by plant phenology and climate factors including temperature, soil moisture, and radiation simultaneously (Fig. 5), to analyze the effects of single factor, ideally, other factors need to be identical or at least close (no significant differences). Based on this theory and to better understand the underlying mechanisms around how SWC interacts with the C fluxes in the studied alpine swamp meadow ecosystem, we selected a specific group of data for further evaluation other than the entire observation time. ”

As described in S2.4, L177-184, we chose explicitly 2014 and 2015 for comparison in S4.1 because there was not a significant change in temperature (<1%) between these two periods, while soil water content decreased significantly more (15.4%) in 2015 (see Table S2). Therefore, within this set of conditions we can compare the influence of soil water content reduction on Re, and its influence further over NEE. Similarly, in S4.2, we also chose 2014 and 2018 for comparison, because the temperature difference between these two periods was greatest (25%) while there was no significant difference in soil water content (0.1%) (see again Table S2), so we could isolate and study the impact of temperature increase over the C fluxes.

Although the soil water content and ecosystem respiration in 2017 were at their highest, the temperature was also higher than in 2015, so we cannot compare it with 2015 to study separately the influence of soil water content change on ecosystem respiration and further net C uptake. To clarify this, we included a more detailed explanation about this comparison in the revision, see S2.4, L177-184:

“To further analyse the effect of soil moisture, radiation, and temperature on C fluxes, we selected two groups of time stamps with significant difference in SWC but almost identical Ta and Rn (i.e. late growing season of 2014 vs 2015) and significant difference in Ta but almost identical SWC and Rn (i.e. late growing season of 2014 vs 2018). Additionally, in order to analyse the effect of annual temperature on C fluxes, we selected a group of time stamps with significant difference in Ta but almost identical SWC and Rn (i.e. 2017 vs 2014, and 2018 vs 2014). We made the comparison in each group to exclude the influence of plant phenology, which can influence C fluxes significantly. The magnitude of the differences between C fluxes in the same group were analysed by the independent-sample T-test method.”

Please note also that we included a new column with net radiation to complement the comparison and expand the discussion in order to improve the manuscript’s clarity and align better with RF findings, this is including also radiation as suggested by REF#1 indirectly in the earlier point but also explicitly suggested by REF#2 later on.

Table S2. Seasonally aggregated environmental drivers and C fluxes in the late growing season of 2014, 2015, and 2018 and their relative difference between years.

Note: late GS represents late (Aug. - Sep.) growing season.

• Another key conclusion of this study is that warming leads to higher C losses rather than enhanced C uptake. This is based on the comparisons of Re and GPP observations in the late growing seasons of 2014 and 2018 (Section 4.2). Warming decreases NEE in late growing season but this does not indicate that warming decreases annual NEE. Recently, Wei et al. (2021) found that “plant uptake of CO2 outpaces losses from permafrost and plant respiration on the Tibetan Plateau” at annual scale based on 32 eddy covariance sites in the Tibetan Plateau. Thus, the authors should more rigorously examine whether warming decreases net C sink of alpine swamp on the Tibetan Plateau.

This is a great point, thanks for bringing this up. In the revised manuscript, we added a new Table S4 to characterize the effect of temperature increase on net C uptake at annual basis and added more discussion accordingly. Based on a new variance analysis, soil water content in 2015 was significantly lower than in 2014, 2017, and 2018 (p < 0.05), while there was no significant difference in soil water content between 2014, 2017, and 2018, and there was also no significant difference in net radiation between these four years either. Therefore, we chose 2014, 2017, and 2018, when there was no significant difference in moisture conditions, to study the impact of temperature increase on C fluxes at an annual scale in the alpine swamp meadow. In fact, according to our data there was a reduction of net C uptake in warmer 2017 and 2018, and an increase of GPP and Re in 2017 and 2018 compared with 2014 (the increase of GPP is relatively lower than Re in warmer 2017 and 2018). Therefore, these extended results indicate that a temperature increase can decrease the net C uptake on an annual scale in this site. We added a new Table S4 in the supplement material:

Table S4. Daily aggregated environmental drivers and C fluxes in 2014, 2017, and 2018.

We further added the following text in Section 4.2, L330-336:

“To evaluate if this finding is also consistent at an annual scale, we further analyzed annual aggregated data. An annual comparison was made between the 2014, 2017, and 2018 when SWC were found insignificantly different while temperatures in 2017 and 2018 were 44.4% higher than in 2014 (Table S4). Additionally, this 44.4% increase in Ta in 2017 and 2018 both led to stronger GPP and Re (Table S4). Although both GPP and Re increased, the intensity in Re was greater than GPP, indicating that warmer temperatures have a stronger impact on ecosystem respiration in this site, resulting in an approximately 50% decrease of the net C uptake (Table S4).”

We thank REF#1 for pointing us towards this nice paper by Wei et al., 2021 “Plant uptake of CO₂ outpaces losses from permafrost and plant respiration on the Tibetan Plateau”. We benefited a lot from this paper. The conclusion from Wei et al. (2021) are based on data of 32 eddy covariance sites during 2002 to 2020, while our study only covers 4-years of year-round observations. Differences in time and space scales may help to explain the differences we found. In fact, in the figure 4 from Wei et al. (2021) there are specific sites from Haibei where RR_NEP (Response ratio of NEP to the warming rate) is smaller than 1.0, indicating a negative effect of warming on the NEP. Potential site-specific differences together with the particularity of water condition in of alpine swamp ecosystem could be the possible reason. We have added more text in the discussion section S4.2, L347-353.

“Wei et al. (2021) also found that the uptake of C by plants will exceed the amount of C release under warmer and wetter climate conditions at annual scale based on manipulative experiments and model simulations focused on the Tibetan Plateau. Their study is based on a longer-term trend while our study only covers 4-years of year-round observations thus site-specific differences in time and space scales may explain this variability. Nevertheless, such results indicating inconsistent ecosystem responses suggest that there are still large uncertainties in the responses of C flux components to temperature variation and further work is still crucial.”

Specific comments

• Line 26, “-168.0 ±-62.5” may should be “-168.0 ± 62.5”.

Typo, thanks for picking it up. Changed accordingly.

• Lines 75-76, “only a few experiments have been conducted to specifically characterise alpine swamp meadow ecosystem C dynamics.”, but no study focusing on alpine swamp C cycling is mentioned at here. The previous studies focusing on alpine swamp CO2 fluxes in the Tibetan Plateau are recommended to be mentioned, such as alpine swamp CO2 fluxes observations at Haibei (Zhao et al., 2005, 2010), Shenzha (Qi et al., 2021), Nam Co (Liu et al., 2020), and Huanhaizi (Zhu et al., 2020).

References

Liu, Y., Geng, X., Tenzintarchen, Wei, D., Dai, D. and Xu, R., 2020. Divergence in ecosystem carbon fluxes and soil nitrogen characteristics across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone. Science of the Total Environment, 748. 10.1016/j.scitotenv.2020.142453.

Qi, Y., Wei, D., Zhao, H. and Wang, X., 2021. Carbon sink of a very high marshland on the Tibetan Plateau. Journal of Geophysical Research-Biogeosciences, 126 (4). 10.1029/2020jg006235.

Wei, D., Qi, Y., Ma, Y., Wang, X., Ma, W., Gao, T., Huang, L., Zhao, H., Zhang, J. and Wang, X., 2021. Plant uptake of CO2 outpaces losses from permafrost and plant respiration on the Tibetan Plateau. Proceedings of the National Academy of Sciences of the United States of America, 118 (33). 10.1073/pnas.2015283118.

Zhao, L., Li, J., Xu, S., Zhou, H., Li, Y., Gu, S. and Zhao, X., 2010. Seasonal variations in carbon dioxide exchange in an alpine wetland meadow on the Qinghai-Tibetan Plateau. Biogeosciences, 7 (4): 1207-1221. 10.5194/bg-7-1207-2010.

Zhao, L., Li, Y.N., Zhao, X.Q., Xu, S.X., Tang, Y.H., Yu, G.R., Gu, S., Du, M.Y. and Wang, Q.X., 2005. Comparative study of the net exchange of CO2 in 3 types of vegetation ecosystems on the Qinghai-Tibetan Plateau. Chinese Science Bulletin, 50 (16): 1767-1774. 10.1360/04wd0316.

Zhu, J., Zhang, F., Li, H., He, H., Li, Y., Yang, Y., Zhang, G., Wang, C. and Luo, F., 2020. Seasonal and interannual variations of CO2 fluxes over 10 years in an alpine wetland on the Qinghai-Tibetan Plateau. Journal of Geophysical Research-Biogeosciences, 125 (11). 10.1029/2020jg006011.

Thank you very much for this very useful information concerning previous studies including sites focusing on alpine swamp CO₂ fluxes. Following the recommendation from REF#1 we have cited these previous studies and updated the description in the revised manuscript to further highlight the novelty of this study, S1, L75-78:

“Although many studies concerning ecosystem C dynamics on the QTP have focused on alpine meadow ecosystems (Saito et al., 2009; Zhao et al., 2005, 2010; Zhu et al., 2015b) and alpine swamp meadow ecosystems (Zhao et al., 2005, 2010; Qi et al., 2021; Liu et al., 2020; Zhu et al., 2020), the effect of SWC on the C uptake is still unclear as compared to that of temperature for alpine swamp meadow ecosystems. ”

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Interactive comment on Biogeosciences Discuss., https://doi.org/10.5194/bg-2021-193

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Interactive comment on “Effects of soil water content on carbon sink strength in an alpine swamp meadow of the northeastern Qinghai-Tibet Plateau” by Wei et al.

Received and published for Discussion: 10 October 2021

Response to Comments of Referee #2

Wei et al. did an interesting job about carbon balance of an alpine swamp meadow on the Qinghai-Tibet plateau, enriching the dataset of carbon fluxes in alpine wetland. This study highlights that decreasing in soil water content in this wetland ecosystem can stimulate ecosystem respiration significantly and weaken ecosystem carbon sink strength as a result. The writing is good and data processing is reasonable. Some major concerns in data interpretation should be addressed.

We are thankful for the reviewer’s insightful comments that have improved the manuscript. We have carefully considered the reviewer’s remarks and clarified our manuscript accordingly.

Major Comments:

1. In section 3.3, radiation has been identified as the main driver for the variation of GPP and NEE, and air temperature as the second force. However, in section 4.2 and 4.3, only the role of air temperature is discussed through the comparison of different study periods and different studies. This may confuse the reader because the main driver for NEE seems inconsistent in the Result and Discussion section. The authors may want to highlight the role of warming on carbon fluxes, but a statement or discussion on radiation is necessary to make the manuscript with good clarity.

Thanks for your useful comments. We therefore added the following text accordingly at the beginning of the Discussion section, L264-274:

“Since NEE is the difference between Re and GPP, environmental variables affecting Re and GPP could affect NEE indirectly (Song et al. 2011). Radiation affects the magnitude of plant photosynthesis and controls temperature, which is one of the key factors related to C fluxes. This suggests that abundant radiation benefits photosynthesis and respiration and thus directly affects the C sink strength in the alpine wetland ecosystem of the Qinghai Lake (Cao et al., 2017). Niu et al. (2017) shows that 99% of the interannual variation of NEE in an alpine swamp meadow can be well explained by temperature conditions, precipitation and radiation. The results of this study demonstrate that ecosystem C sequestration is regulated not only by radiation and temperature but also by soil moisture in the alpine swamp meadow site studied herein. Given there is no significant difference in net radiation between the four years we studied, the effects of soil water content and temperature on C fluxes on diurnal, seasonal, and annual scales are therefore discussed in detail below.”

Please note that further implementations including radiation numbers and discussion were made in the following points. But please also note that also the original title has been revised to reflect REFS#2 and #1 points:

“Radiation, soil water content, and temperature interactions with carbon cycling in an alpine swamp meadow of the northeastern Qinghai-Tibet Plateau”.

2. In section 4.1, two groups of data (the late growing season of 2014 and 2015) were used to analyze the effects of soil moisture on carbon fluxes. Although the authors declared that air temperature and phenology of these two periods were comparable, I could not find information about radiation of these two periods. As radiation has already been concluded as a main driver for the variation of GPP and NEE in the Result section, it is critical to build the comparison based on comparable radiation, or the results can be pointless. The same issue goes for the comparison between the late growing season of 2014 and 2018 in section 4.2 (L322-324).

We appreciate this comment about the comparisons in S4.1 and S4.2. The REF#2 is right, we did not present the values of net radiation (Rn) in the original comparison because there was no significant difference in Rn between these compared periods. However, following the referees’ advice, we have now added radiation number to Table S2 that are addressed and discussed in S4.1 and S4.2.

Table S2. Seasonally aggregated environmental drivers and C fluxes in the late growing season of 2014, 2015, and 2018 and their relative difference between years.

Note: late GS represents late (Aug. - Sep.) growing season.

We further edited the text in S2.4, L177-184 to include radiation not only in this comparison, but also in an additional test looking specifically at annual data following REF#1 comment, number 4 (covered above). REF#1 was not convinced about the conclusion addressed in S4.2 regarding “Temperature increase leads to higher C losses rather than enhanced C uptake” - the referee suspected that our claim about warming decreasing NEE in late growing season do not necessarily indicate that warming can decrease annual NEE. Therefore, in order to address this point, we also added a new test to confirm our previous finding:

“To further analyse the effect of soil moisture, radiation, and temperature on C fluxes, we selected two groups of time stamps with significant difference in SWC but almost identical Ta and Rn (i.e. late growing season of 2014 vs 2015) and significant difference in Ta but almost identical SWC and Rn (i.e. late growing season of 2014 vs 2018). Additionally, in order to analyse the effect of annual temperature on C fluxes, we selected a group of time stamps with significant difference in Ta but almost identical SWC and Rn (i.e. 2017 vs 2014, and 2018 vs 2014). We made the comparison in each group to exclude the influence of plant phenology, which can influence C fluxes significantly. The magnitude of the differences between C fluxes in the same group were analysed by the independent-sample T-test method.”

And in S4.2, L330-336:

“To evaluate if this finding is also consistent at an annual scale, we further analyzed annual aggregated data. An annual comparison was made between the 2014, 2017, and 2018 when SWC were found insignificantly different while temperatures in 2017 and 2018 were 44.4% higher than in 2014 (Table S4). Additionally, this 44.4% increase in Ta in 2017 and 2018 both led to stronger GPP and Re (Table S4). Although both GPP and Re increased, the intensity in Re was greater than GPP, indicating that warmer temperatures have a stronger impact on ecosystem respiration in this site, resulting in an approximately 50% decrease of the net C uptake (Table S4). ”

3. The comparison between the late growing season of 2014 and 2015 shows that drought in 2015 stimulated Re noticeably (L279-282). However, the differences in GPP between these two periods are not significant (L290), the authors should be more careful to make the statement that high soil water content would suppress GPP (L287-295). Figure 5 shows that the contribution of soil water content to the variation of GPP is small on all the time scales.

The referee is fully right, thanks for pointing to this issue. We agree that our data do not support the potential suppressing effect of SWC on GPP, as since this paragraph is not central to the overall storyline, we have removed the text below as well as the references from the revised manuscript to improve clarity:

“There is evidence that excessive soil water can negatively affect plant physiological and ecological processes by, for example, insufficient supply of metabolic substrates and the production of toxic substances (Jackson and Colmer, 2005), which may reduce the overall plant photosynthetic efficiency (Xu and Zhou, 2011). Although there was no significant difference between the late growing season GPP of 2015 and 2014 in terms of both daily accumulated GPP and diurnal rates of GPP, the decline observed in SWC during September 2015 (the driest month with 65.1% SWC) led to a 11% increase of daily accumulated GPP (Fig. S1; Table S3). The excess of SWC in 2014 caused an inundation of the aboveground plant domain, which also likely contributed to the lower value in GPP by reducing plant photosynthetic efficiency (Cronk et al., 2001; Hirota et al., 2006). Since the increase of GPP could not offset the increase in Re, September 2015 and the late growing season of 2015 experienced a lower C sink strength.”

4. The authors concluded that warming would weaken carbon sink strength in this alpine swamp meadow ecosystem because it would increase Re more than GPP. The authors also pointed out that other studies have reported opposite results that warming would stimulate GPP more than Re. However, these studies were conducted in different ecosystems, such as Arctic marshlands and Arctic tundra (L334-341). A discussion that focuses on alpine swamp meadow would be more worthwhile for understanding the effects of climate warming on carbon balance of alpine wetland ecosystem.

Thank you very much for comment. Now we have now removed the discussion around different ecosystems and replaced it with the following paragraph in S4.2, L345-353:

“Liu et al. (2018) concluded that warming has a significant inhibitory effect on GPP and minor effect on Re, resulting in a weaker carbon sequestration capacity of their studied alpine wetland ecosystem. Wei et al. (2021) also found that the uptake of C by plants will exceed the amount of C release under warmer and wetter climate conditions at annual scale based on manipulative experiments and model simulations focused on the Tibetan Plateau. Their study is based on a longer-term trend while our study only covers 4-years of year-round observations thus site-specific differences in time and space scales may explain this variability. Nevertheless, such results indicating inconsistent ecosystem responses suggest that there are still large uncertainties regulating the responses of C fluxes to temperature variation and further work is still crucial. ”

Specific Comments:

1. The font of ‘CO2’ in the draft should be the same as others.

Corrected accordingly.

2. Potentilla in Line 96 and Kobresia in L121 should be italic.

Thanks! Corrected accordingly.

3. L174-175: what do you mean by ‘this is one random forest per hour of the day, day of the year and year, respectively’?

We apologize for not been clear. In order to run random forest (RF) diurnally, seasonally and annually, we needed to aggregate the data as revised below in S2.4, L175-176:

“we run multiple random forests with growing season data binned per hour of the day, day of the year and year, respectively (Table S1).”

4. Figure 4 and Figure 5 should have tags.

Changed accordingly.

5. L373-379: The stronger C sink strength is first attribute to saturated soil condition rather than lower temperature, but then to higher precipitation and lower temperature.

Thank you very much for pointing this out. As the rest of sites included in Table 2 do not have all available SWC data, in S4.3 we used precipitation as a proxy for soil moisture (see L371-372: “we examined the temperature and precipitation (as a proxy for SWC) impacts on NEE (Liu et al., 2016).”). This reasoning is consistent with our data; for example, in Figure 2b it is clear that when precipitation is low in 2014 in e.g. August and October compared to other years, SWC decreased significantly.

We further clarified in the text S4.3, L380-385:

“This is likely a result of the inhibiting effects of the nearly-saturated soil condition over soil respiration rather than by the lower temperatures (Sun et al., 2021). Therefore, in permanently or seasonally inundated swamp meadows, high SWC may have triggered lower C loss rates further benefiting C preservation. At our site the higher C sink strength was likely attributed to higher precipitation (and therefore higher SWC) and lower temperature, which created colder and more humid conditions than other sites (Table 2).”

6. The data supports the opinion that soil water content is a key control on ecosystem respiration, but soil water content does not affect the overall C sink strength (i.e., NEE) directly.

Agreed, we deleted the words “and the overall C sink strength” from the conclusion. Now the sentence was changed to “Soil moisture, however, has the largest influence over Re variability on diurnal, seasonal, and interannual scales, suggesting that soil water content is a key control on ecosystem respiration.”-------------------------------------------------------------------------------------------------------

Interactive comment on Biogeosciences Discuss., https://doi.org/10.5194/bg-2021-193