Sunflower and Sunn Hemp Potential as Summer Cover Crops in Southern Texas

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General Comments

The objective of the study is to assess the relevance of the strategic integration of cover crops (CC) in citrus orchards for the provision of ecosystem services such as soil cover, organic matter, nitrogen fixation and weed control, by: i) assessing the suitability of CC for the environmental conditions common to the south of Texas, ii) quantifying the potential contribution of each CC to soil health, and iii) quantifying the minimum water requirement. We assumed that certain varieties of bacteria would have greater tolerance to the soil's richness and the species' growing conditions without completely replacing their daily water.

However, there are a few elements that need to be improved, such as: references are not following the journal format, e.g., [1], [2], [3], or [4], [5]. In assessing the water needs of citrus, cover crops with active growth should be considered, as they are usually intercropped.

The study did not assess Citrus Orchard but only tested the CC and two soil and climate conditions under controlled conditions. Plants rarely repeat the same performance under field conditions.

The reason for using the reference evapotranspiration (ETo) for irrigation water management and the method used to compute ETo have not been explained or presented.

Below I have identified some specific points that need improvement or clarification.

 

Abstract

Line 18: Why was irrigation treatment based on the cumulative reference evapotranspiration (ETo)?

Line 30: AA observed only the organic mass. Is there data on soil organic matter before and after the experiment compared with other studies throughout the article?

Introduction

The introduction seems a bit detached from the study's objectives. I could not correctly associate the current research (under controlled conditions) with state-of-the-art commercial citrus orchards. For example:

Lines 92-94: AA wrote: "Incorporating CC in Texas citrus orchards is an actionable strategy that requires meticulous testing to minimize significant changes to a producer operation".

How can the evaluations under controlled conditions be proven as a strategy for reducing or minimizing the producer's operations? This has not been made clear at any point in the text.

 

Material and Methods

Lines 121-123: What was each soil's assigned gravimetric water content?

Lines 128 -129: Where is the pot size sufficient to develop each assessed CC species??

Lines 133-134? The AA says: “This methodological approach served the first step of producing relevant information to 133 a citrus producer operation in the LRGV”. Based on what methodology?

Line 144: Why are there different planting dates?

Lines 164-166: What was the reason for killing plants with a severe water shortage?

Results

Figure 1: I couldn't understand what that meant when evaluating the treatment.

Figure 3: This graph can be better displayed; now, it's hard to understand

Table 3: There must be physiological explanations and debate for the behavior with significant differences.

Figure 5: Planting or sowing? AA used to use seeds.

Discussion

There was just a short discussion about the irrigation effect on the cover plants; AA should better explain the development of these plants. It was your assessment (as opposed to others' studies and physiological phenomena explaining this development) since you failed to assess the difference in the soil after the test.

 

Conclusion

 The conclusions, like the text as a whole, leave a lot to be desired. Based on those assessments, I do not see how those plants could be included in the production process of citrus orchards. If the next step is to take the experiment into the field, the AA should wait until they get the results from the field to assess the whole ecosystem.

Author Response

The objective of the study is to assess the relevance of the strategic integration of cover crops (CC) in citrus orchards for the provision of ecosystem services such as soil cover, organic matter, nitrogen fixation and weed control, by: i) assessing the suitability of CC for the environmental conditions common to the south of Texas, ii) quantifying the potential contribution of each CC to soil health, and iii) quantifying the minimum water requirement. We assumed that certain varieties of bacteria would have greater tolerance to the soil's richness and the species' growing conditions without completely replacing their daily water. However, there are a few elements that need to be improved, such as: references are not following the journal format, e.g., [1], [2], [3], or [4], [5]. In assessing the water needs of citrus, cover crops with active growth should be considered, as they are usually intercropped.

We have corrected this oversight. We included all references following the journal format. Additionally, we agree with the reviewer that citrus water demand must be evaluated, and that is our next research step. This article provides insights into the agronomy of these three-summer annual CC, and whether they can withstand the expected water deficit conditions that are prevalent in the field. Additionally, another objective of this study, which was achieved, was to determine the minimum amount of water these CC require to reach full vegetative growth.

The study did not assess Citrus Orchard but only tested the CC and two soil and climate conditions under controlled conditions. Plants rarely repeat the same performance under field conditions.

We agree with the Reviewer’s comment. This is the primary reason that led us to start our analysis of CC fitness and survival in controlled conditions. We are currently in the early stages of planning to incorporate CC in the field. Based on the results of this experiment in controlled conditions, we have selected sunn hemp as the top performer cover crop, with potential to add nitrogen to the soil where citrus trees grow.

The reason for using the reference evapotranspiration (ETo) for irrigation water management and the method used to compute ETo have not been explained or presented.

We agree with the reviewer that reference evapotranspiration (ETo) is a misleading designation of the calculated daily water losses gravimetrically, and we have updated the term throughout the text to indicate that we call “evapotranspiration (ET)” to this water loss. We did not use grass reference evapotranspiration (ETo), and we apologize for this misnomer.

We measured the unique weight of each pot after saturation and subsequent field capacity (FC) status. The water volume required to reach FC was multiplied by its specific water deficit level (control = 100, 75, 50, and 25%), which yielded a unique target weight to maintain pots throughout the experiment. We calculated daily water losses gravimetrically and replenished to this target level by manually irrigating the pots, avoiding the walls to reduce preferential flow.

Below I have identified some specific points that need improvement or clarification.

Abstract

Line 18: Why was irrigation treatment based on the cumulative reference evapotranspiration (ETo)?

Irrigation treatments were based on replenishing a percentage (25, 50, 75, or 100%) of daily water losses measured gravimetrically, i.e. we weighted the pots daily, and since runoff was zero and drainage was negligible, we assigned the water loss to soil evaporation and plant transpiration. We called this daily water loss ‘ET’ , and we have removed “reference evapotranspiration (ETo)” from the text to avoid potential confusion with the term, which is commonly used following FAO-56 paper.

Line 30: AA observed only the organic mass. Is there data on soil organic matter before and after the experiment compared with other studies throughout the article?

We measured the above and belowground biomass production of all CC without mineral fertilization, and total reliance on native soil fertility. Table 1 presents soil organic matter values for both experimental soils, but did not quantify carbon addition from each cover crop. We are currently grinding biomass samples, and preparing a second manuscript that we hope to publish in this prestigious journal too.

Introduction

The introduction seems a bit detached from the study's objectives. I could not correctly associate the current research (under controlled conditions) with state-of-the-art commercial citrus orchards. For example: Lines 92-94: AA wrote: "Incorporating CC in Texas citrus orchards is an actionable strategy that requires meticulous testing to minimize significant changes to a producer operation". How can the evaluations under controlled conditions be proven as a strategy for reducing or minimizing the producer's operations? This has not been made clear at any point in the text.

We understand the reviewers point. However, there are currently a myriad of obstacles to successfully incorporating CC in Texas citrus orchards, the first of all is the correct identification of annual summer CC species that can withstand the harsh environmental conditions of southern Texas. Subsequently, a common question producers ask is whether those CC can survive only with rainfall water inputs, which for experimental purposes was not practical, but we tried to impose severe water deficit stress that will inform us about the sensitivity of each species when growing on soils typically found in the area of the state. Finally, we measured above and belowground biomass accumulation under two soil types and three water deficit levels, which will serve as baseline for future biomass estimation and correlations.

Material and Methods

Lines 121-123: What was each soil's assigned gravimetric water content?

Gravimetric water content was unique to each pot composed of each one of these elements

[dry_pot+dry_soil+mesh_on_pot_holes+slurryofwater_soil+tapes+water_to_clean_beaker_with_slurry] weighted over a 2500 g maximum capacity scale. Although we tried to keep weight consistent across pots, differences in length and size of tapes and meshes affected their weight. Unfortunately, it was not cost-effective to try to keep these “weight” consistent. Ultimately, we knew the dry weight of each pot filled with soil and packed to a predetermined level. Afterwards, a known volume of water was added to each pot until saturation, allowed to drain, and when drainage stopped, we recorded that weight as ‘field capacity value’, which informed irrigation volume to add based on treatment (i.e. 25, 50, 50, or 100%) replenishment.

Lines 128 -129: Where is the pot size sufficient to develop each assessed CC species??

Yes, the pot size was enough to support plant vegetative growth under water stress conditions.

Lines 133-134? The AA says: “This methodological approach served the first step of producing relevant information to a citrus producer operation in the LRGV”. Based on what methodology?

Based on the selection of cover crop species with potential to be incorporated in the field. We deliberately selected these three species that could offer a concrete benefit once they are planted in the field. Moreover, we had limited information on the drought tolerance level of these crops when planted on these soils, which we were trying to elucidate.

Line 144: Why are there different planting dates?

The primary goal of this approach was to plant the cover crops in different times of the summer season, and evaluate their performance under prolonged arid conditions. Additionally, since the target crash crop is perennial, we were also investigating when could be adequate to plant the cover crops out in the field.

Lines 164-166: What was the reason for killing plants with a severe water shortage?

Plants were severely dehydrated. This result was rapidly noticeable when plants received either 25 or 50% ET treatments. Therefore, when we observed the stem lying on the surface of the soil, and unresponsive to irrigation after two consecutive days. We interpreted this stress level as permanent wilting point, and harvested the plant.

Results

Figure 1: I couldn't understand what that meant when evaluating the treatment.

On figure 1 we are presenting the daily average air temperature and relative humidity cover crops experienced during the life cycles. This information is important to determine the range of environmental conditions where these species can thrive and survive.

Figure 3: This graph can be better displayed; now, it's hard to understand

This was a mistake during the formatting process of the manuscript. It has been resolved.

Table 3: There must be physiological explanations and debate for the behavior with significant differences.

Yes, we agree with the reviewer. We tried to explain these differences on current lines 366-368: “Sunn hemp produced long roots and when irrigated > 75% ET also produced roots with high surface area and diameter values, which suggest an adequate allocation of re-sources to support construction costs of both above and belowground biomass.” And lines 374-376: “Sunflower invested in root length and surface area construction costs, which may have improved plant performance. Plant’s resource allocation to root costs was primarily driven by water availability and not soil type.” Unfortunately, we are constrained by the data collected and cannot speculate on other physiological parameters.

Figure 5: Planting or sowing? AA used to use seeds.

Planting. We corrected this oversight.

Discussion

There was just a short discussion about the irrigation effect on the cover plants; AA should better explain the development of these plants. It was your assessment (as opposed to others' studies and physiological phenomena explaining this development) since you failed to assess the difference in the soil after the test.

We did not elaborate extensively on the irrigation effects because plants above and belowground responses to water availability through supplementary irrigation has been reported previously in the literature. We added a few sentences to strengthen the discussion on water effects on plant growth.

Current lines 375-376: “Plant’s resource allocation to root costs was primarily driven by water availability and not soil type.”

Lines 377-378: “Aboveground vegetative growth was severely impaired by deficit irrigation treatments (i.e. 25, 50, and 75% ET).”

Conclusion

The conclusions, like the text as a whole, leave a lot to be desired. Based on those assessments, I do not see how those plants could be included in the production process of citrus orchards. If the next step is to take the experiment into the field, the AA should wait until they get the results from the field to assess the whole ecosystem.

We conclude the manuscript with identification of suitable CC species with high potential to be incorporated in the field. Additionally, our results indicate that all three CC are able to germinate and establish in both soils without mineral fertilization. This result is very important since most producer will not fertilize the cover crop, thereby these plants will have to forage available soil resources to produce biomass and subsequently elicit the desirable ecosystem service.

For our next step, we are confident that data collection would suffice to produce a different publication with different objectives and experimental design.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is well written, with clear, unambiguous and professional English throughout the text. The introduction and background provide informative and relevant context. The references are well referenced and relevant, but some references are missing from the list (lines 39, 270-272). The figures are relevant, of high quality, well labelled and well described, but Figures 3 and 4 are partially visible, making it impossible to follow irrigation at 100% ETo. This is an original primary study that falls within the scope of the journal. The research questions are clearly defined, relevant and meaningful. It is clear from the manuscript how the research fills an identified knowledge gap. Rigorous research was conducted to high technical and ethical standards. The methods described in the study provide sufficient detail and information for replication. All primary data were provided; they are robust, statistically sound and well controlled. The conclusions are well formulated, linked to the original research question and limited to the supporting results.

Comments for author File: Comments.pdf

Author Response

The manuscript is well written, with clear, unambiguous and professional English throughout the text. The introduction and background provide informative and relevant context. The references are well referenced and relevant, but some references are missing from the list (lines 39, 270-272). The figures are relevant, of high quality, well labelled and well described, but Figures 3 and 4 are partially visible, making it impossible to follow irrigation at 100% ETo. This is an original primary study that falls within the scope of the journal. The research questions are clearly defined, relevant and meaningful. It is clear from the manuscript how the research fills an identified knowledge gap. Rigorous research was conducted to high technical and ethical standards. The methods described in the study provide sufficient detail and information for replication. All primary data were provided; they are robust, statistically sound and well controlled. The conclusions are well formulated, linked to the original research question and limited to the supporting results.

We thank the reviewer for his/her kind words. We have incorporated the missing references. Similarly, we apologize for figures 3 and 4 not being fully visible. This was a formatting mistake when the corresponding author uploaded the manuscript. We have fixed these issues now.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript concern important topic, evaluation of cover crops fitness to environmental conditions and some plants growth in southern Texas. The problem with this work is the careless manuscript preparation.

Fig. 1 - no data (air temperature and relative humidity) for the first days after planting.

Moreover Fig. 1 does not show which graphs refer to which year of research (2022, 2023). The graph title does not work for this either.

In Fig. 2 it is not clear in what units plant mortality is expressed? From what surface/object?

The method of determining plant mortality was also not presented in the Material and Methods chapters.

Table 2 also requires correction: e.g. "Root traits" - does it only refer to "length" or rather also to "surface area" and "diameter"?

Table 2 title - the term "Summary of significance" is not very precise.

In my opinion the results for dry mass (Fig.3) and/or for dry mass of roots (Fig. 4) are problematic. In 2022, the dry mass results (Fig. 3) were about 300 times greater than the dry mass of roots shown in Fig. 4; while in 2023, the dry mass was more than 20,000 times greater than the dry mass of roots shown in Fig. 4. This is impossible.

The manuscript is not prepared in accordance with the guide for authors. This applies in particular to the manner of citing and the list of cited literature.

In my opinion, this manuscript needs Major revision. Authors should carefully revise the manuscript.

Comments for author File: Comments.pdf

Author Response

The manuscript concern important topic, evaluation of cover crops fitness to environmental conditions and some plants growth in southern Texas. The problem with this work is the careless manuscript preparation.

Fig. 1 - no data (air temperature and relative humidity) for the first days after planting. Moreover Fig. 1 does not show which graphs refer to which year of research (2022, 2023). The graph title does not work for this either.

Unfortunately, the graduate student responsible for data collection did not check that the sensors were not working properly the first 15 days of the study, and we lost that data. However, during 2022, the first nine days were similar to all plants (line 144), and this initial data gap does not severely impact plant traits measured, and/or treatments imposed. Additionally, we have added the year on top of the panels to clarify any doubt.

In Fig. 2 it is not clear in what units plant mortality is expressed? From what surface/object?

We added a left Y axis indicating the number of individuals counted per sampling date for both emergence (left side of the panel), and mortality (right side of the panel).

The method of determining plant mortality was also not presented in the Material and Methods chapters.

We have corrected this oversight. Current lines 162-164 now read: “Plants were harvested when visual symptoms of water deficit were severe, and aboveground tissue did not respond to irrigation for two consecutive days. These data were recorded periodically to quantify individuals’ mortality through time.”

Table 2 also requires correction: e.g. "Root traits" - does it only refer to "length" or rather also to "surface area" and "diameter"?

We agree with the reviewer. This error was also produced during the formatting of the manuscript. We fixed it, and now “root traits” clearly indicates all the root parameters measured.

Table 2 title - the term "Summary of significance" is not very precise.

We changed the Table 2 title. It now reads “Summary of analysis of variance…”. This title is more specific than the title previously used.

In my opinion the results for dry mass (Fig.3) and/or for dry mass of roots (Fig. 4) are problematic. In 2022, the dry mass results (Fig. 3) were about 300 times greater than the dry mass of roots shown in Fig. 4; while in 2023, the dry mass was more than 20,000 times greater than the dry mass of roots shown in Fig. 4. This is impossible.

We double checked these data, and we agree and apologize to the reviewer. We made a mistake when labeling the right axis of figure 3. It was supposed to be in mg, and not in g. We have corrected this issue. Additionally, we agree with the reviewer that there is a significant variability in the data, but we attributed it to the different environmental conditions between the two repetitions, and slightly lower air temperatures in 2023 compared with 2022 that were conducive to high aboveground biomass accumulation.

The manuscript is not prepared in accordance with the guide for authors. This applies in particular to the manner of citing and the list of cited literature. In my opinion, this manuscript needs Major revision. Authors should carefully revise the manuscript.

We corrected this oversight, and have included missing references in the journal format. We carefully revised the entire manuscript prior to resubmission.

 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Comments on the improved version of the manuscript:

The manuscript has been substantially improved and adapted to the format used in the journal. However, it still contains weaknesses and errors that require correction.

The title of Table 1 is "Physicochemical properties of experimental soils used in the study".

However the table also includes physical and chemical properties. This is not the same.

Table 1.: It is not clear to me in what units SOM is expressed? %? - this needs to be completed in table 1.

It is not clear to me whether Total or absorbable forms of K, P and Mg are shown in Table 1?

Lines 149-150: nine and 19 days after planting? - maybe better 9 and 19 days after planting??

The title of chapter 2.2 is: "Aboveground plant traits"? But what about roots? - see lines 175-177 ("Roots were removed from the fridge, allowed to reach room temperature, carefully placed in plastic trays with enough water to cover them")

Figure 2:

Dear author, It is still not clear to me what the percentage of plant mortality was in each date. I suggest adding information about the total number of plants in each species or converting this data to percentages.

The description for Table 2 should be placed next to this table.

Chapter 3.3 contains discussion items (lines 280-285) that should be placed in the Discussion section.

Table 3 contains "Surface area". However, there is no information whether this is the surface of the roots? or the above-ground parts?

Figure 4: The authors marked individual charts as: 25% ETo, 50% ETo, etc. Meanwhile, it should be 25% ET, 50% ET, etc. - just like in the text.

Figure 5 does not present research results. It should be placed in the Materials and Methods section.

The caption under Figure 5 is incorrect - it does not match the legend in the figure.

The conclusions are overly general. They should be more specific to the results presented in the manuscript.

Authors should carefully revise the manuscript.

Author Response

Comments on the improved version of the manuscript:

The manuscript has been substantially improved and adapted to the format used in the journal. However, it still contains weaknesses and errors that require correction.

The title of Table 1 is "Physicochemical properties of experimental soils used in the study". However the table also includes physical and chemical properties. This is not the same.

We appreciate the reviewer comment. We have updated the title of the table to indicate Physical and chemical properties of the soils used in the study.

Table 1.: It is not clear to me in what units SOM is expressed? %? - this needs to be completed in table 1.

We added (%) to SOM in Table 1. Thanks for this correction.

It is not clear to me whether Total or absorbable forms of K, P and Mg are shown in Table 1?

We included information indicating that “macro and micronutrients are Mehlich-3 extractable levels, expressed in mg of the element by kg of soil. Current lines 141-142

Lines 149-150: nine and 19 days after planting? - maybe better 9 and 19 days after planting??

We changed the text to read “9 and 19 days after planting” as suggested by the reviewer. Line 145.

The title of chapter 2.2 is: "Aboveground plant traits"? But what about roots? - see lines 175-177 ("Roots were removed from the fridge, allowed to reach room temperature, carefully placed in plastic trays with enough water to cover them")

We thank the reviewer for this very important suggestion. We have updated the title of the section 2.2 to better reflect its content. It now reads: “Above and belowground plant traits”. Line 153.

Figure 2: Dear author, it is still not clear to me what the percentage of plant mortality was in each date. I suggest adding information about the total number of plants in each species or converting this data to percentages.

Dear reviewer, Figure 2 does not indicate percentage of plant mortality. We are indicating number of individuals dead at specific days after planting (DAP). Each cover crop is represented by a unique shape: Buckwheat (▲), sunflower (●), and sunn hemp (■), and on the left we present emergence at 3, 6, and 9 DAP. In other words, at 3 DAP, only 3 buckwheat and 4 sunflower had emerged, whereas 13 sunn hemp plants emerged.

On the right side we present mortality at 30, 31-60, and >61 DAP. In other words, at 30 DAP, 9, 8, and 6 plants died for buckwheat, sunflower, and sunn hemp, respectively. There were at total of 72 experimental units, composed of 24 individuals for each cover crop (n=24). Based on reviewers’ prior suggestions, we added a Y-axis indicating number of individuals. Additionally, we also updated the figure to indicate n=24 for emergence data.

The description for Table 2 should be placed next to this table.

We have added a description right below the table: Lines 206-207 now read: “Results of ANOVA test indicate significant effects on plant traits due to single treatment or treatments interaction when P-value was < 0.05.”

Chapter 3.3 contains discussion items (lines 280-285) that should be placed in the Discussion section.

We agree with the reviewer, and apologize for this oversight. We have added this paragraph to a more suitable place in the discussion section. Lines 452-457 now read: “Additionally, water stress has different effects on SPAD (i.e. leaf chlorophyll content). Some researchers report an increase [39, 40] while others report a decrease [41], and still, others observe no change [42] in the chlorophyll levels under water stress. The triple interaction of the treatments evaluated on SPAD suggests high variability of this plant trait with the legume (sunn hemp) trending above the other two CC studied.”

In addition, we updated all the references numbers in accordance with the journal guideline after this modification.

Table 3 contains "Surface area". However, there is no information whether this is the surface of the roots? or the above-ground parts?

We have specified “root surface area” to avoid potential confusion to the reader. Line 303 now reads: “root length (cm), root surface area (cm²)…”

Figure 4: The authors marked individual charts as: 25% ETo, 50% ETo, etc. Meanwhile, it should be 25% ET, 50% ET, etc. - just like in the text.

Figure 4 was updated.

Figure 5 does not present research results. It should be placed in the Materials and Methods section. The caption under Figure 5 is incorrect - it does not match the legend in the figure.

We corrected the legend on Figure 5. However, we disagree with the reviewer suggestion. This figure presents irrigation volume results based on the ET treatments and cover crop species, thereby we would appreciate if Figure 5 remains in the same place.

The conclusions are overly general. They should be more specific to the results presented in the manuscript.

We updated the conclusion section of the manuscript to reflect the connection between the objectives and results obtained in the study. We made concluding remarks on fitness (i.e. biomass production and adaptability to harsh environmental conditions), response to drought treatments, and minimum water requirements for the cover crops evaluated.

Authors should carefully revise the manuscript.

Done.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript has been substantially revised in accordance with my suggestions. In one case, the authors have provided satisfactory justification for the location of one of the figures. In my opinion, the manuscript may be published in the journal Agronomy.