I would like to recognize first the effort you have done in order to improve figures and text of your manuscript. Overall, the paper reads much better now. Thank you.
However, still, please allow me to be a bit critical to the overall methodology that your analysis applies:
If I am not mistaken, your analysis uses the concentration differences ("in"-"out") of the non-conservative Mg to calculate a dilution/preferential flow effect, which is then used to calculate CO2 uptake. (If I am mistaken, please change the manuscript so that others don't get the same impression is I do.)
This approach warrants a much more careful treatment and discussion, than present in the current version of the manuscript.
p 16, l 26-28: Please explain exactly why "it is unlikely" and also include "adsorption" in your listing in the end of the sentence. Also, very low volumes of water actually comes through the mesocosms (or am I mistaken here too?), so the phrase 'large quantities of water bypassing ...' should be exchanged for something else.
This brings me to an alternative explanation for the Mg difference: The high Mg concentrations at 1.5 cm depth could be due to evaporation as the removal of water may leave Mg in solution at elevated concentrations (cf. Thaysen et al., 2014a (doi.org/10.5194/bg-11-7179-2014) and 2014b (doi.org/10.2136/vzj2014.07.0083)). Very low water volumes actually passes through the column despite continuous irrigation, and no water balance which can account for the residence time was or could be made. Who knows, maybe water moves upwards in large parts of the column? Therefore, please elaborately discuss this alternative explanation for the Mg concentration differences, and soften your statements accordingly throughout the manuscript. For example, the first time the dilution is mentioned is in bi-sentence (..., recognizing processes that lead to dilution, ...; p 13 l 6-7) by which time the reader apparently has to just accept this as a fact?
I probably just have overlooked a point in your work - if so, please argue fully and carefully in the text of the diminutive effects of the non-conservative Mg behavior and evaporation.
Several of the authors are highly experienced researchers, who I am sure are able to help avoid 'circular conclusions' and 'overly speculative conclusions' in the final version. (Excuse my direct address here, please.)
An example of what I see as a circular conclusion appears in the abstract (p 1 l 21-23): "Calculations that explicitly factor out the dilution effect of bypassed water, lead to relative fluxes about a magnitude higher, confirming that preferential flow paths and surface runoff in the field must be included in assessments for the CO2 consumption potential of Enhanced Weathering in general." The statement appears to assume that the previous CO2 sequestration rate is correct and that a rate ten times higher cannot be correct?
An example of what I see as an 'overly speculative conclusion' appears also in the abstract: Here I agree and find it correct to conclude that "Porewater Mg/Si molar ratios suggest that dissolved Si from the added minerals stays in the system over the observation period,..." but the reason that follows is not warranted: "...because a cation depleted Si layer forms
on the reactive mineral surface of freshly ground rocks.". And the anticipation of such a layer does not prove its actual existence, which is strongly inferred in the sentence that then follows: "This layer has not reached equilibrium thickness within the first two years."
For a reader of the abstract it must be made clear that no direct observation of the Si-enriched layer was observed, but only inferred from water chemistry. Again, perhaps I missed a point. If so, when you correct the text, please state the method by which layer was observed.
Finally, it goes without saying that I still find this research very valuable and important for climate mitigation. I certainly hope the authors will address the issues I experienced when reading the paper to enable its final publication in the near future.
I know, for typical authors, at least myself, criticism like the above may seem to change a lot and challenge the publication (again - that is, if I haven't overlooked some important points). The hardest part is if one has to abandon a conclusion, perhaps even replace one with the opposite. Yet, I hope one realizes that even in that case the paper is actually really close to being finalized.