the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Moisture and temperature effects on the radiocarbon signature of respired carbon dioxide to assess stability of soil carbon in the Tibetan Plateau
Andres Tangarife-Escobar
Georg Guggenberger
Xiaojuan Feng
Guohua Dai
Carolina Urbina-Malo
Mina Azizi-Rad
Carlos Sierra
Abstract. Microbial release of CO2 from soils to the atmosphere reflects how environmental conditions affect the stability of soil organic matter (SOM), especially in massive organic-rich ecosystems like the peatlands and grasslands of the Qinghai-Tibetan Plateau (QTP). Radiocarbon (14C) is an important tracer of the global carbon cycle and can be used to understand SOM dynamics through the estimation of time lags between C fixation and respiration, often assessed with metrics such as age and transit time. In this study, we incubated peatland and grassland soils at four temperature (5, 10, 15 and 20 °C) and two water-filled pore space (WFPS) levels (60 and 95 %), and measured the 14C signature of bulk soil and respired CO2. We compare the relation between the Δ14C of the bulk soil and the Δ14CO2 of respired carbon as a function of temperature and WFPS for the two soils. To better interpret our results, we used a mathematical model to analyse how the calculated number of pools, decomposition rates of carbon (k), transfer (α) and partitioning (γ) coefficients affect the Δ14C -bulk and Δ14CO2 relation, with their respective mean age and mean transit time. From our incubations, we found that 14C from peatland was significantly more depleted (old) than from grassland soil. Our results showed that changes in temperature did not affect the Δ14C values of respired CO2 in either soil. However, changes in WFPS had a small effect on the 14C CO2 in grassland soils and a strong influence in peatland soils, where higher WFPS levels led to more depleted Δ14CO2. In our models, we observed large differences between slow and fast cycling systems, where low values of k modified Δ14C patterns due to the incorporation of 14C-bomb in the soil. Hence, the correspondence between Δ14C and age and transit time strongly depended on the internal dynamics of the soil (k, α, γ and number of pools) as well as on model structure. We conclude that the stability of carbon in these systems depends strongly on the direction of change in temperature and moisture and how it affects the rates of SOM decomposition. Finally, Δ14C modelling along with empirical data from SOM dynamics is a useful tool to improve predictions on interactions between terrestrial and atmospheric carbon.
Andres Tangarife-Escobar et al.
Status: open (until 28 Mar 2023)
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RC1: 'Comment on egusphere-2023-210', Anonymous Referee #1, 23 Mar 2023
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Review 2023 Tangarife-Escobar et al., Moisture and temperature effects on the radiocarbon signature of respired carbon dioxide to assess stability of soil carbon in the Tibetan Plateau
Tnagarife-Escobar et al. present a well-executed and thoughtful experiment carried out in an important geographic region. I enjoyed reading the work, and find it well argued and well composed for the most part. I would suggest that some additional data be included in the main text (which is now not given at all, or is relegated to the supplementary/appendix materials) to help clarify and strengthen arguments in the discussion. The modeling and empirical components are a bit disconnected from one another as currently presented. If the experimental results cannot be used directly in the soilR simulations, perhaps some of the data may be presented in merged figures to help the reader more directly understand the connections between these two components of the work (see detailed comments below).
Detailed comments:
Abstract line 17: The statement that temperature is a significant variable contradicts the results stated earlier in the abstract.
Lines 56-64: This is a very clear and succinct explanation of C pools and turnover. Nicely done!
Line 110: How did the presence of inorganic C potentially affect the Delta 14CO2 values?
Methods: Soil incubation times… the duration of the incubations is cited many times in the discussion as a potential confounding variable in the interpretation of the Delta 14CO2 data. However, the length of the incubations is not given in the methods. The length of the incubations should be added to the manuscript along with a discussion of how variable lengths of incubations for the individual treatments might have influenced the Delta 14CO2 data. I’m assuming different treatments were incubated for different lengths of time since the methods indicate that they were incubated until a certain amount of CO2 was produced, and given the differences in respiration rates given in the appendix the length of incubation time might have varied by an order of magnitude? Could this have an influence on the age of C being respired (i.e. longer incubation times allowed for decomposition of more structurally and/or chemically "stabilized” substrates)?
Methods: What is the reasoning behind the choice of WFPS values? Ninety-five percent is very high. Doesn’t this value inhibit evolution of gases from the soil matrix? How/why were 65% and 95% chosen?
Table 2: Unclear what is being compared here. Is the anova between grasslands and peatlands at each treatment level of temp/moisture? Or is it comparing different levels of temp within each soil category? It's confusing because the soils weren't radiocarbon dated *after* incubation, correct?
Table 3: This is really a lot of different conditions... and on top of that you discuss the type I, type II or type III systems. How do these three things relate to one another ("fast/slow", "parallel/series", "type I/II/II")? Also, please add to the "System" column "grassland" and "peatland" in addition to "fast" and "slow". I know it's in the text directly below, but it would help the reader keep on top of all the modeling approaches.
Figure 5: I feel that it is important to have an additional two panels in this figure showing the total amount of C respired by each of the treatments for a given length of time or the respiration rates. This information is referenced in the discussion, but I don't see it anywhere. In the discussion, the manuscript makes a point about the relative importance of the age vs. the amount of respired C, so the amounts should be shown. See additional comment regarding appendix table A1 below.
Figures 4 and 5: The 10 deg C thing… something unique seems to be happening at this temperature in the peatland soils. Do you have some explanatory hypotheses? This temperature also has strong outliers in both soil types (Figure 4), would you please comment on this?
Figures 6-9: Please label all these panels of figures as "grass vs peat" and "fast vs slow". Preferably in the figure itself, but at least in the caption. This will help the reader more easily keep track of what they're looking at.
Model/data fusion: Can soilR not use the Delta 14C of the respired C to constrain alpha/gamma and k1/k2 values? Or is that too computationally intensive at this point? I find the paper to be well written, but there is not a lot of integration of the incubation data with the modeling exercise. What would help me understand the connections would be plotting (some? All?) the data from figure 4 onto figures 6-9. This would directly show me how the experimental results map onto the different modeled scenarios. This would really the reader more quickly understand the connections between the type I/II/III systems and the model parameters (gamma/alpha and decomposition rate constants).
Table A1: Ok. Here is where the significant temperature effect is... this looks like a pretty linear response of respiration rate to increasing temperatures. Why not include this result in the main text? I think it's alluded to in the abstract, but there's no actual evidence of a temperature effect included in the main text at this point (maybe add to figure 5).
Conclusions: The conclusions lack punch. What are the broader implications of this work? How do the experimental treatments relate to current climate projects for the QTP? Will the peatlands dry out and change this from a type X to a type X system? Will the grasslands get hotter and therefore respire X more gigatons of C on an annual basis? The introduction states that these soils are being studied because of they hold vast stores of C. What do your experiments suggest for the fate of these C stores under future climate scenarios?
Citation: https://doi.org/10.5194/egusphere-2023-210-RC1
Andres Tangarife-Escobar et al.
Data sets
Moisture and temperature effects on the radiocarbon signature of respired carbon dioxide to assess stability of soil carbon in the Tibetan Plateau Andres Tangarife-Escobar, Georg Guggenberger, Xiaojuan Feng, Guohua Dai, Carolina Urbina-Malo, Mina Azizi-Rad, and Carlos Sierra https://doi.org/10.5281/zenodo.7620008
Model code and software
Moisture and temperature effects on the radiocarbon signature of respired carbon dioxide to assess stability of soil carbon in the Tibetan Plateau Andres Tangarife-Escobar, Georg Guggenberger, Xiaojuan Feng, Guohua Dai, Carolina Urbina-Malo, Mina Azizi-Rad, and Carlos Sierra https://doi.org/10.5281/zenodo.7620008
Andres Tangarife-Escobar et al.
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