|This manuscript explores the controls on CO2 emissions from peat-draining rivers, finding that pH limitation plays a central role. It is an important regional-scale analysis addressing a very interesting and understudied topic, which is relevant to the readership of Biogeosciences. However, the manuscript still requires major revisions to ensure the central findings are clearly documented and sufficient uncertainty analysis is provided for the readers. Additionally, further updates to writing and references would strengthen the paper. |
-----Provide additional information on methods, fitted parameters, and implications for results: In the current manuscript, the main findings are not sufficiently documented, leaving the reader with substantial uncertainties related to the approach and conclusions. The central conclusion that pH limitation dominates hinges on the values of the fitted parameters. Could you please provide supplemental figures to provide more insight into the methods and uncertainty analysis?
For example, in equations in Table 2 & 3, CO2 concentration will be very insensitive to O2 concentration for small Km, but will become more sensitive for higher Km. Fitted values of Km varied widely (factor of 50) between the two model formulations. The authors used this variation to rule out the linear approach in favor of the exponential approach. Please provide more information to justify the case for this interpretation. Some possible questions and ideas are below, but other information and analyses would also be welcome.
-Table 5: Could you provide a related figure showing the least squares optimization and/or model fits with these parameters? I only see the predicted vs. observed, so more information would be useful.
-Table 5 and Table 6: Could you provide the fitted parameters, and pH and O2 limitations for both the linear and exponential formulations so we can see how they differ? Both appear to have good performance in Figure 3 & 4, so it would be interesting for the reader to see both propagated throughout the manuscript. Very confusing when they are contrasted in the discussion, but the linear parameter fit values cannot be viewed in any table.
-Minor formatting: inconsistent ordering of linear and exponential is confusing (Table 2&Figure 4; Table 3 & Fig 3). Linear is missing from later tables.
-Could you provide more insight into why the fitted values were so different for the two formulations?
-Suggested figure: Plot of key equations from Table 2 and Table 3 shown with data used for fitting
-Suggested SI figure: Plot of key equations from Table 2 and Table 3 with different values of fitted parameters to give readers an idea of sensitivity to these parameters.
-How confident are you in your ability to disentangle pH and O2 effects given the noise in the data? Can you help the reader understand how different the table 2 &3 equation curves would look with different combinations of parameters? Are they similar or strongly distinguishable?
-Suggested figure: any assessment of relationships or collinearity between fitted parameters
-If O2 limitation is negligible, why is there such a strong inverse trend with peat cover vs. O2 & CO2? (Figure 2)
-Could you add a panel for pH in Figure 2, given the central role of pH?
-What is the significance of the exponential correlation lines shown in Figure 2? Do they have any relationship to the equations in other parts of the manuscript, or any other interpretation or analysis?
-Table 6: How are these limitations and uncertainties calculated? Please add more info in Methods.
-Ln 90- Provide equations used from Wannikhof (1992) within your methods to make your work easier to reproduce. Almost all other parameters needed to use eqns in Table 2 & 3 are already provided in Table 1, so please include kCO2(T) or equation to calculate it for completeness.
-Ln 218 – If fitted parameters imply no oxygen limitation, is this still able to fit data from Fig 2, or are some extremes missed entirely?
-----Other concepts requiring further discussion:
-Photomineralization -- Recent literature has suggested that CO2 emissions from peat-draining rivers may be largely driven by photomineralization, rather than microbial respiration. Would this process be expected to have the same formulation of oxygen or pH limitations? What similarities or differences would you expect? How might this change the equations you used? Would there be a strong theoretical basis for pH or oxygen limitation of photomineralization? A paragraph reviewing this issue would be helpful.
-DOM composition - Ln 31 (also 36): You make the contrast with temperate peatlands. Could you also comment somewhere on the differences between the peat and DOM composition in temperate vs. tropical peatlands, and how that might be another factor slowing decomposition?
For example, see work by Hogkins et al (2018). Also Nichols & Martin (2021).
-Ln 282: Mention and discussion of enhanced weathering is interesting. However, it does not make sense to me to have this as the final concluding paragraph, as it is not the central message of the manuscript. Perhaps it could be relocated?
-Ln 276, 282: multiple mentions of increase in pH due to carbonates. However, in Table 4, the measurement campaigns with higher concentrations of particulate carbonate (CaCO3) do not have higher pH values. Therefore, I do not understand the emphasis on this point, as it was not observed in the data.
– why do you think you captured this in Simunjan but not other sites?
- ideas for future work/speculation- if relevant, could you discuss the transition in pH as water flows from peatland drainage canals to streams to large rivers - where do you expect the pH constraint to be lifted? And could that help predict hotspots of CO2 emissions that would warrant further investigation?
-Can you please discuss further how you account for spatial and temporal variability? Fig 1- How does the distance upriver and sampling season influence the concentrations measured? How did you handle this? How might limitations related to number of sampling times and locations influence your results?
-----Data availability- The work of compiling the large dataset presented here is a major contribution to the community. Where will this dataset be made available for future research?
-Code availability – can you make any of the analyses you completed public or visible?
-Ln 33-39: awkward phrasing, hard to understand meaning without reading multiple times
-Ln 56: The Methods section would strongly benefit from a short “Overview” or “roadmap” paragraph near the beginning of the section. It is currently very difficult to follow the overall approach, and requires reading multiple times.
-Ln 19: “potential hotspot”?
-Figure 1: Can you mark the location of sampling?
-Ln 149: You mention many data sources here – please reference. Are these from others or this work? Citations or reference to data in Supplement?
-Key points on the pH limitation vs. oxygen limitation are buried within the conclusion. Perhaps you could use sub-headers or more active topic sentence to make sure that the central points are clearly communicated.
-----Literature/references: Please update and extend referencing. Some comments and suggestions below; not exhaustive.
-Some relevant papers not yet cited include:
-Martin et al (2018) – cycling of DOM from peat-draining waters in Borneo
-Cook et al (2018) – importance of DOC export from plantations on peat
-Gandois et al (2020) – DOM in peat-draining canals and rivers
-Ln 2: “transformation into plantations” is a great simplification – many different land uses are have drained and degraded tropical peatlands. For example, in 2015 industrial plantations made up only 27.5% of peatland area in insular SE Asia (see Table 2, Miettinen et al, 2016). Same issue Line 61.
-Ln 24: Hooijer et al. (2010) regional CO2 emissions updates have since been updated by Miettinen et al (2017) and Hoyt et al (2020). Please also include most up-to-date references.
-Ln 51: Can you provide more recent references as well? For example, Gandois et al. (2020) traces peat water chemistry from drainage canals to rivers. Gandois et al (2014) compares peat porewater and river water.
- Nichols and Martin (2021) – discussion of phenol oxidase activity in tropical peat-draining waters
-Cook et al. (2018). Fluvial organic carbon fluxes from oil palm plantations on tropical peatland. Biogeosciences, 15, 7435–7450, 2018
-Gandois et al. (2014) Origin, composition, and transformation of dissolved organic matter in tropical peatlands. Geochimica et Cosmochimica Acta 137 (2014) 35–47
-Gandois et al. (2020) From canals to the coast: dissolved organic matter and trace metal composition in rivers draining degraded tropical peatlands in Indonesia. Biogeosciences, 17, 1897–1909, 2020
-Hodgkins et al (2018). Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance. Nature Communications 9:3640
-Hoyt et al. (2020) Widespread subsidence and carbon emissions across Southeast Asian peatlands. Nature Geoscience 13 435–440
-Martin et al. (2018). Distribution and cycling of terrigenous dissolved organic carbon in peatland-draining rivers and coastal waters of Sarawak, Borneo. Biogeosciences, 15, 6847–6865, 2018
-Miettinen et al. (2016) Land cover distribution in the peatlands of Peninsular Malaysia, Sumatra and Borneo in 2015 with changes since 1990. Global Ecology and Conservation 6 (2016) 67–78
-Miettinen et al (2017). From carbon sink to carbon source: extensive peat oxidation in insular Southeast Asia since 1990. Environ. Res. Lett. 12 (2017) 024014
-Nichols and Martin (2021). Low Biodegradability of Dissolved Organic Matter From Southeast Asian Peat-Draining Rivers. JGR Biogeosciences 126(6).
The manuscript by Klemme et al. presents a study explaining why tropical peat draining rivers are only a moderate source of CO2 to the atmosphere, which stands in contrast to what was assumed for global estimates. Klemme et al. test the hypothesis that decomposition and thus CO2 production in these organic C rich waters is limited by pH and O2 availability. For this, they use a comprehensive dataset of observations of DOC and CO2 concentrations, pH and other relevant physical and chemical parameters from SE Asian, peat draining rivers in combination with a conceptual model representing limitations of DOC decomposition by low pH and O2 concentrations. They find that DOC decomposition in those peat draining rivers is likely more limited by pH than by O2, and suggest that increased loads of carbonates due to agricultural liming or enhanced weathering could increase decomposition of DOC and thus CO2 emissions from those peat draining rivers.
The study is original and of great interest for the readership of Biogeosciences. The manuscript is well written, the methodology is clearly described, and results are clearly presented and support the main findings of this study. I suggest publication after minor revisions. Please, find my comments below.
L15-17 : Other studies have shown that large amounts of CO2 evading rivers are actually put in as dissolved CO2 from soil respiration (both heterotrophic and root respiration) (Abril and Borges, 2019; Lauerwald et al., 2020). Maybe you should mention that source as well.
L17-18: These are actually not model based studies that would represent peat soils. Those are more upscaling studies that lacked observations from these important systems
L42: In peat draining rivers, is there also less instream production by algae that would otherwise be a source of O2 to the water column?
L48-51: You should link these quite specific objectives here again to the more general research objective (or hypothesis to be tested): explain the moderate CO2 emissions from peat draining rivers by the effect of low pH and O2 limitation.
L95-97: I don’t understand why you have used such a projection for determining areas. For that purpose I would rather use an equal area projection, like an equal area projection after Lambert or the EckertIV projection.
L110-112: The exponential limitation factor related to pH, which is defined as negative decadic logarithm of H+ activity - would that be comparable to a linear factor relating to the H+ activity? That might be worth discussing here in one or two sentences.
L122-123: That would require that dissolved CO2 inputs via groundwater inputs and CO2 consumption by autotrophic production is negligible. These are strong assumptions that would be worth mentioning here explicitly and some discussion later on.
L140: “spatially as well as temporally”
Figure 3: The grey lines, are those regression fits or the 1:1 line, or both?
For figures 3 and 4, it would be great if you could report in addition the RMSEs.
L184: There’s a “c” missing in “concentration”.
L189-191: Do Borges et al. also report CO2 emission rates or CO2 concentrations which are comparable to those in your study?
Abril, G. and Borges, A. V: Ideas and perspectives: Carbon leaks from flooded land: Do we need to replumb the inland water active pipe?, Biogeosciences, 16(3), 769–784, doi:10.5194/bg-16-769-2019, 2019.
Lauerwald, R., Regnier, P., Guenet, B., Friedlingstein, P. and Ciais, P.: How Simulations of the Land Carbon Sink Are Biased by Ignoring Fluvial Carbon Transfers: A Case Study for the Amazon Basin, One Earth, 3(2), 226–236, doi:10.1016/j.oneear.2020.07.009, 2020.