Phytoremediation of Heavy-Metal-Contaminated Soils: Capacity of Amaranth Plants to Extract Cadmium from Nutrient-Poor, Acidic Substrates

Round 1

Reviewer 1 Report

1. The quality of language in the paper should be improved (punctuation, sentence structure, typos, terminology etc.).
2. Low number of plants in objects. As a result, there were not enough roots, flowers, and seeds for analysis, which is necessary to determine the phytoremediation capacity.
3. It is not enough to say that the plants did not develop as expected, and that seed production was hampered. Biomass, plant height, and Cd uptake in 3 objects should be provided. You can also specify the number of leaves, their area, or the diameter of the stem. No reference to controls.
4. It is not known whether the result of the experiment is the result of an acid reaction or a lack of nutrients. Too big of a difference between 2 and 20 mg Cd kg-1 of soil.
5.Some sources say that amaranth tolerates the pH range of 5.2-7.5. Others narrow this range down to pH 6.1-6.5. Therefore, I do not think that the tested pH plays a significant role.
6. How do the authors explain the lower BAF and concentration of cadmium in stems and leaves (after 120 days) and their following increase?

Author Response

 1. The quality of language in the paper should be improved (punctuation, sentence structure, typos, terminology etc.).

The paper was thoroughly revised and screened for this type of errors and changed accordingly. Some terms that the average Challenges reader may not be familiar with were explained.

2. Low number of plants in objects. As a result, there were not enough roots, flowers, and seeds for analysis, which is necessary to determine the phytoremediation capacity.

We agree that with a higher number of plants (in a field study for example), other results on phytoremediation capacity (especially translocation patterns) would have been possible. In previous field studies (similar soil conditions), we have assessed phytoaccumulation and translocations patterns in amaranth (Haller, Jonsson et al. 2017). In this experiment however, we were not able to collect enough plant material to calculate the translocation factor. On the other hand, in this study we present data on the bioaccumulation factor at four occasions (day 90, 120, 150 and 180). Since plants were taken out at four occasions, and some plants were underdeveloped due to phytotoxicity/nutrient shortage/acidity, we were not able to collect the 0.5 g dry material that our methods required for all vegetative organs.

Reference:

Haller, H., Jonsson, A. , Lacayo Romero, M. & Jarquín Pascua, M. (2018). Bioaccumulation and translocation of field-weathered toxaphene and other persistent organic pollutants in three cultivars of amaranth (A. cruentus ‘R127 México’, A. cruentus ‘Don León’ y A. caudatus ‘CAC 48 Perú’) – A field study from former cotton fields in Chinandega, Nicaragua. Ecological Engineering, vol. 121, ss. 65-71.

3. It is not enough to say that the plants did not develop as expected, and that seed production was hampered. Biomass, plant height, and Cd uptake in 3 objects should be provided. You can also specify the number of leaves, their area, or the diameter of the stem. No reference to controls.

We have now specified that the plants grown at 20 mg kg-1 dw did not “develop according to expected plant height” on line 216-217. The growth was so severely hampered that we decided not to quantify its biomass. We see the fact that they did not “develop according to expected plant height” as a sufficient indicator that the stress resulting from the combination of Cd and low nutrients/acidity made phytoremediation with amaranth a inappropriate option in such conditions. This is now clarified on line 218-219. The plants grown at 20 mg kg-1 dw produced so little biomass that it was not possible to calculate its uptake of Cd.

4. It is not known whether the result of the experiment is the result of an acid reaction or a lack of nutrients. Too big of a difference between 2 and 20 mg Cd kg-1 of soil.

We agree it is not known which of the 3 factor; 1) low nutrients, 2) acidity and 3)the toxic effects of Cd was most determinant. We have now added acidity on line 218 and 297 to clarify this.

2 mg kg dw is a common concentration in polluted agricultural soil. 20 mg dw is clearly a less common concentration in agricultural soil but since this study addresses the use of Amaranthus hypochondriacus for phytoremeditation (and thus relevant for highly polluted soils), we decided to use this higher dose at which signs of phytotoxicity was likely to show.

5. Some sources say that amaranth tolerates the pH range of 5.2-7.5. Others narrow this range down to pH 6.1-6.5. Therefore, I do not think that the tested pH plays a significant role.

We agree that, under optimum conditions (adequate nutrition, no pollution) many cultivars of amaranth species can tolerate a pH as low as 5.2. However, a low pH makes the Cd pollution a higher concern due to increased phytoavailability (and thus phytotoxicity). In the nutrient-poor, Cd-contaminated conditions created in this experiment, we believe that the low pH constitute a significant hinder for using amaranth as for phytoremediation (which is also suggested by our results).

6. How do the authors explain the lower BAF and concentration of cadmium in stems and leaves (after 120 days) and their following increase?

There are indeed some fluctuations in the curves but the differences are not statistically significant so we can not draw any conclusions on changed concentration over time. We have now added a commented on that (line 195-196).

Reviewer 2 Report

Dear authors,

The manuscript ID challenges-2328938, entitled " Phytoremediation of Heavy Metal Contaminated Soils: Capacity of Amaranth Plants to Extract cadmium From Nutrient-Poor, Acidic Substrates" try to explain  the capacity of a hyperaccumulator to extract Cd from polluted soil in a lab experiment. The experiment results indicate that the experiments to be continued in natural environment.  

Although the paper is well written, a few remarks can be made:

- Was the experiment setup for the substrate chosen after several trials or according to other studies? Give references.

- For section 2.3, references for formulas and explanation of BAF interpretation must be added.

- Figure 1 - try to find another type of graphic representation that might be more suggestive.

- Figure 2-further explanation should be provided on the discussion of sinuous tendency depending on the stage of development of the plant .

Regards!

 

 

         

Author Response

Although the paper is well written, a few remarks can be made:

1. Was the experiment setup for the substrate chosen after several trials or according to other studies? Give references.

The aim with setup for the substrate was to mimic a nutrient-poor, acidic tropical soil (where a lot of amaranth is grown) in terms of nutrient content and pH. We have previously conducted field experiments (Haller et al 2018) with amaranths in similar soil but we didn’t do trials in different substrates prior to this experiment. This context is now clarified and a reference is provided.

Reference:

Haller, H., Jonsson, A. , Lacayo Romero, M. & Jarquín Pascua, M. (2018). Bioaccumulation and translocation of field-weathered toxaphene and other persistent organic pollutants in three cultivars of amaranth (A. cruentus ‘R127 México’, A. cruentus ‘Don León’ y A. caudatus ‘CAC 48 Perú’) – A field study from former cotton fields in Chinandega, Nicaragua. Ecological Engineering, vol. 121, ss. 65-71.

2. For section 2.3, references for formulas and explanation of BAF Interpretation must be added.

. An explanation of BAF is provided at line 113-115 but an additional clarification is now added at line 185-186 and a reference for the BAF calculation was added.

3. Figure 1 - try to find another type of graphic representation that might be more suggestive.

A new graphic representation was chosen. Since the instructions for authors for authors encourage preparation of figures in colour, we have now made the figures more coulourful.

4. Figure 2-further explanation should be provided on the discussion of sinuous tendency depending on the stage of development of the plant.

There are indeed some fluctuations in the curves but the differences are not statistically significant so we can not draw any conclusions on changed concentration over time. We have now added a commented on that (line 195-196).

Round 2

Reviewer 1 Report

The explanations and answers provided by the Authors are satisfactory.