Wavelet Analysis of GPR Data for Belowground Mass Assessment of Sorghum Hybrid for Soil Carbon Sequestration

Round 1

Reviewer 1 Report

The authors have prepared a very innovative new application of GPR in agriculture. I am very sympathetic to this type of research getting published and being in available for others to consider and apply. I do think it needs a major revision on several points. Wherever possible it would be more advantageous to use plain language to discuss the overall approach and implementation of this sensing technique. The text fluctuates between highly technical information followed by the absence of critical details that link the sensed data and the physical root data. Most of my criticism centers around the connection between how data are collected and processed, and the on the ground validation.

Airborne radar is very different than ground coupled radar. Some description of the antenna footprint is needed to better understand how the data are collected and processed. What is the smallest object sensed? How many wavelets or waveforms are collected over a given distance or surface along a transect? What is the smallest number of wavelets that can be averaged or summarized into a single observation? How many wavelets are stacked to create an observation? This is critical to using wavelet analysis to capture spatial heterogeneity which is critical to phenotyping. I was disappointed to see that the validation data was captured from very large cells or bins that are 1 to 3 meters wide figure 3 A. I think it was a missed opportunity not to use smaller bins that were ~20 or 30 centimeters wide and then combine them as necessary to find the smallest most accurate footprint for the technique.

Are you really measuring root biomass in a direct way, meaning, are you seeing reflections from an object, or is it more indirect where you are sensing changes in soil caused by the sum total of bulk roots present? For ground coupled GPR there is an effect of antenna orientation and root orientation Which results in certain angles being advantageous to detection while other angles or not. Does this come into play with airborne radar?

When I think of root phenotyping I consider more things than just biomass. A more accurate title would be: wavelet analysis of GPR data for belowground mass estimation of high biomass grasses for soil carbon sequestration. More comprehensive belowground variables such as root angle, spread of roots, root water content,  etc. would be included in a phenotyping project. Only one variety of grass was used in this experiment, it would have been advantageous to compare multiple or at least two different varieties to evaluate the potential for phenotyping.

As far as validation goes there really are only 8 observations per depth interval Figures 5 and 8. It's difficult to assess the validation efforts as there are no R square values on Figure 8. There are also no regression line equations that would allow further inference into how different the regressions are between tissue type and depth layer. Unless I am missing something there is not a comprehensive statistical analysis linking processed GPR data to the destructive sampling.

Based on this pilot project please address the following: how would soil type and moisture content affect repeated measures on a given site or between sites?  How robust are the progression equations do they transcend soil properties or are they dependent on calibration of destructive samples every time they are used?  Could the technique be used to assess plants on different sites or would all of the subject plants need to be in a common field or soil? 

fine

Author Response

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Reviewer 2 Report

This article contributes to the body of knowledge. GPR is weel considered to measure individual crop roots directly. It is improtant to mention that revious research have utilised GPR to characterise the depths of organic soil constituents and bedrock, measure the depth to and thickness of various types of soil layers, and precisely estimate soil depth in rocky forest soils. Additionally, GPR can be used to locate animal burrows, determine the depth of the water table, and track the off-site migration of agrochemicals.

Uncertain how undesirable non-informative signals were handled. Was the original signal's high-frequency content removed using wavelet thresholding algorithms? The authors ought to make this point clear.

Although it is lovely to see the steps taken, it is necessary to comprehend the lowest detectable root size for high frequency antenna employed.

It is unclear whether the suggested method may be used to identify specific fine roots; if so, please clarify and indicate whether this is a follow-up study.

Please add a note if the physical theory underlying how roots affect soil dielectric permittivity is not sufficiently explained.

There isn't a clear difference established throughout the paper as to whether the biomass included fine roots + rhizome + tissue or just fibrous roots + rhizome + tissue.

Please elaborate on the R2 discovered from the pixel counting using Pearson correlation, which is not included in the analysis.

Please clarify if necessary what the limits of this study's findings are in the conclusion section. 

 

 

Author Response

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Reviewer 3 Report

1.Regarding the limitation of sample size: It is suggested that the authors explicitly state the sample size in the paper and consider whether it is sufficient to support statistically significant conclusions. If the sample size is small, the authors can discuss the reliability and generalizability of the results and propose further research to validate the findings.

 

2.Regarding the limitation of study scope: It is recommended that the authors provide more information about the background and objectives of the study. This will help readers understand the importance and relevance of the research. Additionally, it is advised that the authors clearly state the research questions or hypotheses they aim to address and maintain consistency with them in the methods and results sections.

 

3.Regarding the limitation of data interpretation: It is suggested that the authors further analyze and discuss the correlation between GPR features and belowground biomass in the results section. The authors can explore potential mechanisms or confounding factors and provide a deeper understanding of the interpretation of the results. Specifically, the authors can consider the impact of soil conditions, plant growth patterns, or other environmental factors on the results and discuss how these factors may influence the interpretation of the study.

 

4.Regarding the limitation of generalizability: It is recommended that the authors explicitly state whether the research results are applicable to other plant species or ecosystems and discuss the limitations of generalizing the results. If the results are only applicable to sorghum or specific environmental conditions, the authors should clearly state it in the discussion section and propose possibilities and challenges for extending the study to other species or ecosystems.

 

5.Regarding the limitation of technical constraints: It is suggested that the authors provide more details about the use of GPR methods. Describing the GPR setup, antenna configuration, and data processing techniques will help readers assess the reliability and accuracy of the research methodology. If possible, the authors can provide validation of the GPR method or comparisons with other methods for measuring belowground biomass to support the selection and use of the adopted method. Additionally, since GPR captures time and amplitude information, it would be beneficial for the authors to explain how GPR signals correspond to the actual depth of belowground biomass.

Author Response

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Round 2

Reviewer 1 Report

The authors have addressed my concerns and I recommend publication

Reviewer 3 Report

The manuscript that the author has modified can be accepted.