Reduced Degradation of the Herbicide 4-Chloro-2-Methylphenoxyacetic Acid (MCPA) in Soil Induced by the Fungicide Mixture Mancozeb, Metalaxyl-M, and Chlorothalonil Used in Tank Mixtures and Spray Series

Round 1

Reviewer 1 Report

The subject of the manuscript is consistent with the scope of the Journal. The topic of research is interesting. Some revisions are needed for improving the manuscript.

 1. List the homogeneous groups in Figs. 1-3. This will facilitate the interpretation of the results.

2. Please, be sure that all the references cited in the manuscript are also included in the reference list and vice versa with matching spellings and dates.

3. I would also request you to kindly remove some of the very old citations in the manuscript.

4. The literature is incorrectly cited in the current version of the manuscript. Please check the instructions for the authors. Use the Microsoft Word template.

Author Response

Dear Reviewer 1,

thank you very much for your positive evaluation of our manuscript and for your comments. Enclosed you will find our response letter (please see the attachment).

Yours sincerely,

Kim Thu Nguyen

Author Response File: Author Response.pdf

Reviewer 2 Report

I would like to thank the authors for their comprehensive work. The manuscript is very well prepared and contains useful information. 

Author Response

Dear Reviewer 2,

Thank you very much for your positive evaluation of our manuscript.

Yours sincerely,

Kim Thu Nguyen

Reviewer 3 Report

The authors are correct in stating that pesticide mixtures are currently more the norm than the exception.

Any research aimed at assisting to make pesticide use more effective and safer for the environment and living organisms in it is laudable and valuable. However, the casting of aspersions on pesticides and their impacts simply for the sake of aiding the banning of pesticide use or restricting their use for spurious reasons cannot and should not be supported. I am not saying that the authors in this case had such intentions, I simply make a general statement on the need for judicious assessment of scientific method and interpretation of results of studies assessing the impacts of pesticides. 

In the present study the herbicide MCPA was singled out as a "model" pesticide and selected fungicides were used as partners in a mixture. There are more than 150 different herbicides in existence, and probably a couple of hundred fungicides plus about the same number of insecticides; not to mention bactericides and nematicides. Imagine the number of permutations that are theoretically possible for tank mixtures. How can this particular study of very limited scope hope to sway registration authorities to make procedural changes to pesticide registration protocols?

NB: A single soil from a specific location was employed to represent the soil environment -- hopelessly inadequate in terms of representing soil types where these pesticides are typically used.

NB: A serious shortcoming is that the authors, under Materials and Methods, do not mention the equivalent field rates for the pesticide concentrations employed in this study. In other words, does the study simulate field conditions, or to what extent do treatment concentrations deviate from field-applied rates?

Are the differences in MCPA dissipation rate really of practical and toxicological significance given that the Weed Science Society of America Herbicide Handbook (2014 edn) states the half-life of MCPA in soil as "typical half-life is 5 to 6 days". 

The speculation in lines 282 to 294 to the effect that the fungicide products inhibited soil fungi, and consequently stimulated bacteria, that degraded MCPA is unproven since no attempt was made to identify and quantify bacteria and fungi species in the soil used, neither pre-trial, mid-trial, nor post-trial. 

In summary, the questionable research method and limited scope of the study simply does not fit (support) the conclusion reached, namely that pesticide registration protocols should be based on pesticide product mixtures and not on sole products. If this stance can be modified to a more realistic level, I could consider supporting publication of this research.

Author Response

Dear Reviewer 3, 

thank you very much for your comments. Enclosed you will find our response letter (please see the attachment).

Yours sincerely,

Kim Thu Nguyen

Author Response File: Author Response.pdf

Reviewer 4 Report

The authors present a manuscript dealing with the effect of fungicide addition on the soil persistence and degradation of MCPA. The article is sound from the point of view of evaluating, under semi-commercial conditions, the impact of combined agrochemicals frequently used together. 

Experimental design is adequate, even sterilized soil samples should be included as different conditions may alter previous observations of this effect. 

Concerning the results, I recommend the authors use a logarithmic axis and absolute values (at least as supplementary data) to ease the comparison among treatments.

Statistical methods should use a more robust analysis like Duncan's test.

Finally, as this kind of research seeks to evaluate the combined effect of agrochemicals on living beings, it would be necessary to assess the impact of treatments on the microbiota. The soil is assumed to have microorganisms, but to what extent are they affected by fungicide addition?  You should provide evidence of the viability of microorganisms at time -14 (before fungicidal addition) and further samplings (at least 0 and 56). A  qPCR from tfdA or tfdA-like genes would be appropriate.

Author Response

Dear Reviewer 4,

thank you very much for your positive evaluation of our manuscript and for your comments. Enclosed you will find our response letter (please see the attachment).

Yours sincerely,

Kim Thu Nguyen

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Based on the author's response to the initial review, I can now recommend publication.

Reviewer 4 Report

The authors have corrected the document giving appropriate responses to suggestions.  I endorse the manuscript's publication.