In this manuscript, the authors report on carbon isotope fractionation during formate transformation in rice paddy soils and lake sediment. They observe formate utilization mainly through homoacetogenesis to be the dominant process that outcompetes methanogenesis and sulfate reduction.

As it appears, the data were collected and evaluated correctly and there is no doubt on their reliability. Nonetheless, it does not become clear why this study was performed and what the outcome is in some few clear sentences. Why was sediment from Fuchskuhle used? Because there were still some 6 years old sediment samples in the coldroom? What are the properties of this specific (and internationally not so well-known) lake (pH, carbonate and sulfate content, trophic status etc.)? Is it representative of freshwater lakes in general? Which general conclusions can be drawn from this study?

Minor comments:

1. 111, l. 120: Omit “the”
2. 190: Sentence unclear.

Fig. 6, correct Formiat to Formate.

1. 307: Energy cannot be generated.
2. 342: equivalents.
3. 403: Where are the data on the 16SrRNA genes?
4. 404: “composition of these genes”? What do the authors mean with that?
5. 425: …are degraded in these paddy soils…
6. 434: author

General Comments

The manuscript by Conrad and Claus, entitled "Fractionation of stable carbon isotopes during formate consumption in anoxic rice paddy soils and lake sediments" presents an interesting study that analyzes isotope effects associated with formate consumption during incubation experiments. The experiments were conducted with two rice pady soils and two lake sediments, each in triplicate at an incubation temperature of 25°C and over a period of four weeks. The authors have used a straightforward experimental approach and state-of-the art analytical methods, including stable isotope analyses of methane by GC-IRMS and stable isotope analyses of formate and acetate by HPLC-IRMS. They observe formation of acetate from formate with a strong ¹³C-depletion of acetate relative to formate, pointing to homoacetogenesis via the Wood-Ljungdahl pathway.

The scientific questions are relevant and within the scope of BG. The isotopic data are novel. They are discussed in an appropriate and balanced way, and the conclusion is substantial. I couldn't detect major scientific flaws.

My major scientific concern is the amendment of fairly large amounts of formate that produce starting concentrations of more than 15 mM in the incubation experiments. How do these concentrations compare to natural formate levels in paddy soils, lake sediments or other environments? And would formate still be an excellent substrate for acetate production via homoacetogenesis at natural concentration levels? How valid are the findings at 25°C for lake sediments that are generally much colder under in situ conditions? These questions warrant some further discussion.

The manuscript is well organized overall, but the quality of the figures needs improvement. Clear and concise figure captions and legends could improve the readability of the manuscript a lot.

Specific comments and suggestions for the improvement of figures:

Figure 1: The figure legend is incomplete. The crosses representing the controls are not present. Panels c, d and h display a varying shade of red with no clarification. In my opinion, the size of the axis title font is quite small.  While the title of the x-axis is capitalized, that of the y-axis is not. In panel e, f, g, and h, it would be appropriate to add "vs VPDB" to the per mil sign on the y-axis. In panel c and d, it would be better to show molar concentrations of methane and carbon dioxide in the liquid phase rather than partial pressures in the gas phase, in particular since the conversion is explained in the methods section (L. 149-152). Partial pressures cannot be compared to the molar concentrations of acetate and formate.

Figure 2: The figure is lacking a legend. The title of the x-axis is not capitalized. Check for consistency. “The different symbols indicate three different replicates” – Well, I cannot decipher the information, and I cannot distinguish the different shades of red. The title of the y-axis gives CH₄ and acetate in mM H2. This needs explanation. The figure would benefit from a more concise figure caption.

Figure 3: The figure legend is incomplete. The crosses representing the controls are not present. For methane and carbon dioxide, it would be better to show molar concentrations in the liquid phase rather than partial pressures in the gas phase (see above).

Figure 4: See comments on Figure 2.

Figure 5: The figure is lacking a legend. It is confusing that results for Fuchskuhle are shown in two panels to distinguigh sulfidogenic and methanogenic conditions, while both data sets are combined in one panel for Vercelli. On the y-axis, “per mil vs VPDB” is missing.

Figure 6: The figure legend is unsatisfactory, because it uses abbreviations that are not self-explanatory. The figure caption is not complete, because it is lacking information on the lake sediments. In my opinion, it would be better to present the information in two panels (e.g. (a) without sulfate, (b) with sulfate). Check the spelling of formate.

Figure 7: The figure legend and caption need clarification as suggested for Fig. 1-6. Please add full information on sampling sites, such as rice paddy soil and lake sediment. The abbreviation “IRRI” in introduced in the text, but it would increase readability if the full name was given in the figure caption.

Other minor comments:

L. 111: “The bottles were the amended” seems to contain a typo. Change to “The bottles were amended”

L. 226: “In the rice paddy soils values of δ¹³C increased when formate was being consumed indicating discrimination against the heavy carbon isotope.” This sentence is not clear. Which δ¹³C-values increased? Change to “In the rice paddy soils, δ¹³C-values of formate increased when formate was consumed indicating discrimination against the heavy carbon isotope.”