Review of “Above- and Belowground Plant Mercury Dynamics in a Salt Marsh Estuary in Massachusetts, USA” by Wang et al. The authors carried out a systematic investigation on vegetation and soil in a salt marsh. It is important work to improve the understanding of Hg cycling in salt marsh area. However, the Hg isotopic mixing model exists critical mistakes. Firstly, some endmember isotopic signatures are lacking. The authors cited the global mean results, but I think this is not feasible. Citing results nearby the field site seems to be more convincing. Secondly, present model was built based on MDF and even-MIF. Plant uptake Hg from atmosphere or soil could cause significant negative MDF, so present model results are speculative. Thirdly, the authors cannot offer QA/QC about Hg isotopic measurements, and the error bars by measurement are lacking. In addition, the uncertainties of model estimations are not fully calculated.

In detail:

Introduction: I suggest the authors should add one paragraph about Hg isotope, which was used to quantify the source of plant.

Line 20-21: Here peak aboveground Hg pool is 9.0 ug m-2, which is below that in November with 16.2 ug m-2. What differences?

Line 39: largest. How much is the global input flux?

Line 43-45: How about the anthropogenic pollution at present salt marshes?

Line 48-53: Cite more research papers, not only review. 

Line 53: It is overestimated.

Line 53-59: I don’t think it is relative to the Hg cycling. 

Line 102-103: Were the soil samples ground by coffee grinders? I think such operation could cause cross contamination. How much mesh were sieved with? The soil homogeneity is very important.

Line 117-121: How about the Hg concentration in solution to isotopic measurement?

Line 127-130: Report the isotopic results of standard samples, i.e. NIST 1515 and MESS-4. I also cannot find those values in SI. QA/QC is important for Hg isotope measurement. 

Line 231-234: It is a good discussion about foliage accumulation by C3 and C4 plants. But it is not enough. I hope to get more discussion about the mechanism. 

Line 235-243: Such comparison in Hg concentration in different sites is tedious as too many factors controlled the Hg concentration.

Line 259: How to identify the finer roots? <1mm or <2mm? Need more data to support it.

Line 277: Recent study has demonstrated the inconsistent MIF between foliage and atmosphere.

Line 281-282: The authors should compare the isotopic data with the reported grass isotopic values, rather than all the plant values. As you said, the Hg assimilation may exist great difference between C3 and C4 plant. 

Line 283-285: It is speculatively. I do not agree with it.

Line 288: How to build the model based on MDF? As we know, the plant uptake Hg could induce significant MDF. I do not believe this model results if it is based on MDF.

Line 291-292: How about the uncertainty of this estimation?

Line 291-303: I do not believe the model results. Briefly, all the MDF values are more positive than that in measured vegetation results in Figure 6 a). I cannot think this mixing model can be solved in present pattern. Further, I suggest to add error bars (2SD) in Figure 6.

Line 304-312: This paragraph is speculatively. Not all the slope of ~1 suggested the inorganic Hg photoreduction.

Line 414-422: Turnover flux is an interesting discussion. But I think such flux is overestimated.

Line 745-749: Please cite the original research papers, not the review paper.

Line 752: Dry HgII deposition flux with 2.1. How to estimate it? Add the references.