Review Reports - Urea Coated with Iron and Zinc Oxide Nanoparticles Reduces Nitrogen Leaching in Sandy Soil and Improves the Performance of Young Corn Plants

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript by Oliveira et al. is well written and clear. The topic fits the journal, and the agronomic results about nitrogen leaching, dry matter, and nitrogen uptake are well presented and discussed.

However, the study has a major weakness: it does not include any physicochemical analysis of the urea granules coated with nanoparticles.

The authors used commercial nanoparticles, so it is acceptable that they did not characterize the nanoparticles themselves. But they must describe and analyze the coated urea granules. It is essential to report on the coating's uniformity, how much of the urea surface is covered, and the thickness of the coating. This data is needed to support the experimental design and the agronomic findings.

At the very least, the authors should include microscopy images (SEM or optical) of the coated granules. They should also add spectroscopic evidence (like FTIR or EDX) to prove that the ZnO and Fe₂O₃ nanoparticles are on the urea surface. Without this data, the coating process is unclear and its effectiveness cannot be verified.

Another important point is the choice of nanoparticles. The manuscript does not explain why ZnO and Fe₂O₃ were chosen. Was it only because they are micronutrients? Or were technical reasons, like particle size or surface chemistry, also important? This information would help explain differences in how well the coating worked or how the nutrients were released.

Also, the manuscript talks about coatings with sulphur and with S⁰ + nanoparticles, but it does not properly explain why these combinations were used. The authors should clarify the purpose of adding sulphur. They should also have included control treatments, such as urea coated with sulphur alone and with nanoparticles alone, to show the effect of each component.

In summary, the agronomic data is relevant, but the manuscript lacks basic experimental evidence about the coating process. This is a significant weakness. We strongly recommend adding a basic physicochemical analysis of the coated granules and a better explanation for the choice and combination of nanoparticles. This will make the study more scientifically sound and credible.

Author Response

REVIEWER 1

1) The manuscript by Oliveira et al. is well written and clear. The topic fits the journal, and the agronomic results about nitrogen leaching, dry matter, and nitrogen uptake are well presented and discussed.

However, the study has a major weakness: it does not include any physicochemical analysis of the urea granules coated with nanoparticles. The authors used commercial nanoparticles, so it is acceptable that they did not characterize the nanoparticles themselves. But they must describe and analyze the coated urea granules. It is essential to report on the coating's uniformity, how much of the urea surface is covered, and the thickness of the coating. This data is needed to support the experimental design and the agronomic findings. At the very least, the authors should include microscopy images (SEM or optical) of the coated granules. They should also add spectroscopic evidence (like FTIR or EDX) to prove that the ZnO and Fe₂O₃ nanoparticles are on the urea surface. Without this data, the coating process is unclear and its effectiveness cannot be verified.

Authors' response:

Dear reviewer, we appreciate your valuable observation. Indeed, we, the authors, acknowledge this scientific limitation of our research in this aspect. Unfortunately, we were unable to perform this characterization due to the unavailability of specific equipment for these analyses in our department. However, given the importance of the results obtained in this research, we are formalizing a partnership with the Materials Engineering department of our institution for this purpose, aiming at future research. We have included a comparative image showing the appearance of urea coated with elemental sulfur and with elemental sulfur + ZnO and Fe₂O₃ nanoparticles.

Additionally, with the aim of improving our work, we added a high-resolution image showing the overall appearance of coated and uncoated urea grains, and a scale for estimating the average grain size. We also present an estimate of the thickness of the urea coating layer with elemental sulfur and ZnO and Fe₂O₃ nanoparticles, based on the amount of material adhered to the grains and the densities of the components used in its production.

An image of the experiment with the initial growth of corn was also added. With this, we hope that the work has improved in the methodological aspect.

2) Another important point is the choice of nanoparticles. The manuscript does not explain why ZnO and Fe₂O₃ were chosen. Was it only because they are micronutrients? Or were technical reasons, like particle size or surface chemistry, also important? This information would help explain differences in how well the coating worked or how the nutrients were released.

Author's response: Thank you for this observation. A text addressing this aspect has been added to the manuscript. The choice of these sources was due to them being two plant micronutrients, with Zn being generally deficient in Brazilian soils and Fe being severely deficient in soils with high pH, such as those in the Brazilian semi-arid region. Furthermore, these micronutrients have been the subject of important biofortification studies. Fertilization with conventional sources of Fe and Zn is inefficient, especially in sandy soils with low organic matter content, due to leaching in soils with high pH caused by chemical precipitation. Soil organic matter contains chelating agents that control the release of Fe and Zn to plants; this component (organic matter) is also found in low levels in soils with high sand content. The addition of these micronutrients in fertilizers such as urea can become an important solution to this problem, promoting a more controlled release of Fe and Zn to plants.

3) Also, the manuscript talks about coatings with sulphur and with S⁰ + nanoparticles, but it does not properly explain why these combinations were used. The authors should clarify the purpose of adding sulphur. They should also have included control treatments, such as urea coated with sulphur alone and with nanoparticles alone, to show the effect of each component.

Authors' response: Thank you for this observation. A text addressing some of these aspects has been included in the manuscript. Indeed, the use of elemental sulfur or nanoparticles (NPs) as a urea coating has already been explored in other research (Dikimpa et al., 2020; Alam et al., 2021), but in isolation. Elemental sulfur is already used commercially as a urea coating. In this work, elemental sulfur is added to improve emulsification, along with vegetable oil, in the "paste" formation process. Furthermore, during the release process in the soil, elemental sulfur forms sulfuric acid, which not only reduces nitrogen volatilization but also helps maintain the Fe and Zn released from the grains in soluble forms due to the acidification of the microsites where these reactions occur.

4) In summary, the agronomic data is relevant, but the manuscript lacks basic experimental evidence about the coating process. This is a significant weakness. We strongly recommend adding a basic physicochemical analysis of the coated granules and a better explanation for the choice and combination of nanoparticles. This will make the study more scientifically sound and credible.

Authors' response: We sincerely appreciate your suggestions for improving our work.

Reviewer 2 Report

Comments and Suggestions for Authors

Nitrogen is the most important yield-forming element and has a significant impact on plant yield. However, if used improperly, it can pose a risk of soil, groundwater, and air pollution with nitrogen compounds. The risk of nitrogen leaching is greater in light soils with low organic matter and clay content. Therefore, research testing solutions aimed at reducing nitrogen leaching and losses and increasing its utilization by plants should be considered interesting and of not only cognitive but also practical importance. A certain limitation of the practical significance of these studies is that they were conducted under controlled conditions, in small vases. However, the authors emphasized this in the discussion and conclusion, pointing out the need for field research

The experiment was well planned and conducted. The research methods are well described. The results were statistically analyzed, and the tables and graphs are generally clear and carefully prepared. After reading the manuscript, I have several comments that may positively impact its value.

The experiment was well planned and conducted. The 'Introduction' chapter contains no information regarding the effect of sulfur on nitrogen metabolism. Has such research already been conducted? Is there any information available?

line [94-96] is essentially a repetition of the information provided above, in line [89-92]. However, it lacks a precisely formulated research hypothesis that the authors want to verify (confirm or reject) in their research

Table 1- in the explanations below the table there is 'Fe and Zn - Mehlich 1 extractant’ - but the table does not provide Fe or Zn content. The same applies to the following parameters: FC - field capacity, PWP - permanent wilting point, AWC - available water content. It seems that the unit for physical properties, i.e. sand and clay content, is incorrect. Instead of g kg-1, it should be %. Or the values given are incorrect.

Figure 1b - the Y axis has an incorrect unit and RUmax or RHmax, RUmin or RHmin

Line [250] – is “…and urea coated with iron oxide NPs + zinc oxide NPs (U+S+NP)’ should be “… and urea coated with sulfur and iron oxide NPs + zinc oxide NPs (U+S+NP)’

Line [270] – is ‘Different letters above columns indicate significant differences between means at the 5% level by Tukey’s test” - there are no columns or letters in the chart.

Figure 3 - in the title of the chart there is ‘shoot dry matter-SDM (b), root dry matter-RDM (c), shoot dry matter-SDM (d). and the title of the Y-axis in graph 3b is ‘CDM (g per plant)

Author Response

1) The experiment was well planned and conducted. The 'Introduction' chapter contains no information regarding the effect of sulfur on nitrogen metabolism. Has such research already been conducted? Is there any information available?

Authors' response: Indeed, nitrogen and sulfur are closely related in nitrogen metabolism, since both are constituents of amino acids and proteins. A mention of the role of sulfur was included in the "Discussion" section.

2) line [94-96] is essentially a repetition of the information provided above, in line [89-92]. However, it lacks a precisely formulated research hypothesis that the authors want to verify (confirm or reject) in their research.

Authors' response: The authors provided a modification to this passage (lines 94-96) to improve the formulation of the hypothesis.

3) Table 1- in the explanations below the table there is 'Fe and Zn - Mehlich 1 extractant’ - but the table does not provide Fe or Zn content. The same applies to the following parameters: FC - field capacity, PWP - permanent wilting point, AWC - available water content. It seems that the unit for physical properties, i.e. sand and clay content, is incorrect. Instead of g kg-1, it should be %. Or the values given are incorrect.

Authors' response: Thank you. This has been corrected in Table 1.

4)Figure 1b - the Y axis has an incorrect unit and RUmax or RHmax, RUmin or RHmin