Predicting Soil Nitrogen Availability for Maize Production in Brazil

Round 1

Reviewer 1 Report

This is a very well written manuscript describing methodology for predicting soil N availability for crop production. Specific comments are:

1.      The ISNT method has been subject to controversy over time. However, most methodologies function well with certain sets of conditions and not so well wit other conditions. This manuscript appears to be one where the conditions of the study provided favorable results.

2.      Temperature is the most critical factor in maintaining or promoting soil biological activity influencing soil N mineralization.  The authors should be commended for recognizing the potential effects of diurnal temperature fluctuations in designing this study. A 5 degree temperature shift can dampen biological activity in significant ways. This diurnal fluctuation can also affect plant biological activity but with a lag time or different stresses that suppress the correlation between temperature regime, N₀ and plant response to N.

This manuscript should be accepted for publication.

Author Response

Reviewer 1

1. The ISNT method has been subject to controversy over time. However, most methodologies function well with certain sets of conditions and not so well wit other conditions. This manuscript appears to be one where the conditions of the study provided favorable results.

Response: The Introduction thoroughly addresses the controversy relevant to the ISNT, with ample reference citations and clarification as to the importance of confounding factors such as plant population. See lines 72-105 of the revised manuscript.

2. Temperature is the most critical factor in maintaining or promoting soil biological activity influencing soil N mineralization. The authors should be commended for recognizing the potential effects of diurnal temperature fluctuations in designing this study. A 5 degree temperature shift can dampen biological activity in significant ways. This diurnal fluctuation can also affect plant biological activity but with a lag time or different stresses that suppress the correlation between temperature regime, N0 and plant response to N.

Response: The diurnal temperature regime adopted was based on surface soil temperature records for the region where the work was done. We tried to address this point in the Discussion. See lines 326-369 of the revised manuscript.

We thank the reviewer for his/her comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

General

This paper is suitable in principle for the journal Nitrogen. It describes a study which several methods of estimating nitrogen in soil are compared on 17 different soil types. It then shows the relevance of the findings by growing maize in pots filled with the 17 soils and measuring the early dry matter growth and N uptake in relation to the soil characteristics. The main findings are that the standard laborious, time-consuming methods are no better than some much quicker methods in estimating the nitrogen- supplying capacity of the soils for plant growth.

The paper seems geared mainly for the experience soil analyst but the results should be made of value to readers with more general interests in the N-cycle and N for plant growth by the following.

1.       The inclusion of an explanatory diagram in the Introduction showing relations between the different forms and fractions of soil N and their availability to plants.

2.       Further explanation of the reasons for the study and the selection of soil types – for example, what are the grounds for choosing a combination of native forest and various cultivated pastures, plantation crops and annual crops.

3.       To improve the visual impact of the paper, and to further illustrate the contribution of the study, some limited graphical examples of plotting one variable against another should be included (see comment below).

4.       The data on nitrogen availability, found by the various methods, should be given greater agronomic context by showing how they convert to N availability in kg/ha for a given depth of soil; as things stand, the various measures of N in the different units, and in particular the contrast among soil types, have limited meaning to the general agronomist or crop scientist.

5.       Justify more fully in the Discussion the statement “ISNT has potential to improve fertiliser N recommendation for maize production….” And explain in more detail what now needs to be done.

Further points

Abstract: The reasons for the maize pot experiment needs to be explained more fully.

Introduction is generally well argued, but could the authors give the reasons for the official policy line (line 48).

Line 68: the argument turns to maize but why not present the more general relevance for all crops. Or explain why maize was chosen.

Line 85 – argument here is unclear and needs to be re-thought: ‘response data sets’ should be clarified; and the factors cited are those the have to be accounted for in all field experiments, but they seem to be classed as the source of a problem.

 M & M

In Table 2, please explain the meaning of the heading “Resin P  K”

What is likely soil organic carbon content? It would help the general reader to have some understanding of the SOM to SOC conversion for these soils in case the reader wishes to estimate C:N rations, for example.

Note: this reviewer des not have the experience to review and comment critically on the precise methodologies used to estimate nitrogen in the chosen soils.

Results

The high correlation simply with SOM is notable: the more SOM, the more N and more plant growth. Could measures of SOM dispense with the need for the highly technical and time-consuming determinations of Nitrogen (see note below related to the Discussion)?

As requested in the general points above, there needs to be more explanation for the agronomist and general crop scientist of what all the different determinations of N mean, for example by showing how they convert to kg/ha estimated to a given depth. Also, some estimation of C:N values for the different soils would be helpful to place the soils and methods in a wider context.

The correlations in Table 6 are very useful – and the high point of the paper - but it would raise the impact of the paper if some limited comparisons were shown graphically with regression lines. For example, there is a potentially interesting difference between plotting DM against SOM or nitrogen and plotting N uptake against them. In the latter, soil 1 of highest SOM and N is considerably more ‘distant’ from the other soils, and it would be interesting to speculate why.

Discussion

The important conclusion that a range of other methodologies are good predictors of available N should not wait until the third paragraph (line 335). There could be a short introductory paragraph stating the main finding and then moving then to discuss the temperature regimes and then to the other methods.

Line 297 – explain why ‘a greater moisture loss’ might be the cause

Why not recommend much more explicitly that soil SOM could be a proxy. It might need to be calibrated in the field but why bother to estimate N in these soils?

Author Response

Reviewer 2

This paper is suitable in principle for the journal Nitrogen. It describes a study which several methods of estimating nitrogen in soil are compared on 17 different soil types. It then shows the relevance of the findings by growing maize in pots filled with the 17 soils and measuring the early dry matter growth and N uptake in relation to the soil characteristics. The main findings are that the standard laborious, time-consuming methods are no better than some much quicker methods in estimating the nitrogen- supplying capacity of the soils for plant growth.

The paper seems geared mainly for the experience soil analyst but the results should be made of value to readers with more general interests in the N-cycle and N for plant growth by the following.

1. The inclusion of an explanatory diagram in the Introduction showing relations between the different forms and fractions of soil N and their availability to plants.

Response: This type of diagram might seem helpful, but would be problematic in that no scientific consensus exists as to plant availability of different soil N fractions, the amino acid fraction being identified in some studies as the primary source of mineralizable N while others emphasize the importance of amino sugar N. Rather than complicating the Introduction with divergent details, we point out the contrasting nature of such findings - and the methodological implications - in the last two paragraphs of the Discussion. See lines 380-407 of the revised manuscript.

2. Further explanation of the reasons for the study and the selection of soil types – for example, what are the grounds for choosing a combination of native forest and various cultivated pastures, plantation crops and annual crops.

Response: So as to broaden the scope of inference, the soils used were from contrasting sites and ranged widely in their properties. The diverse nature of the soils used is now noted in lines 15 and 16 of the Abstract.

3. To improve the visual impact of the paper, and to further illustrate the contribution of the study, some limited graphical examples of plotting one variable against another should be included (see comment below).

Response: The manuscript now includes a multi-panel figure with this type of documentation.

4. The data on nitrogen availability, found by the various methods, should be given greater agronomic context by showing how they convert to N availability in kg/ha for a given depth of soil; as things stand, the various measures of N in the different units, and in particular the contrast among soil types, have limited meaning to the general agronomist or crop scientist.

Response: Although this suggestion is relevant to the agronomic intent of soil N testing, the purpose of the present project was to assess the predictive value of different N availability indices in relation to maize growth and N uptake. Moreover, the greenhouse data reported do not represent actual field conditions with fluctuating moisture levels, so agronomic extrapolation would be speculative at best.

5. Justify more fully in the Discussion the statement “ISNT has potential to improve fertiliser N recommendation for maize production….” And explain in more detail what now needs to be done.

Response: The next steps toward practical implementation of the ISNT are addressed by the last sentences in the Abstract and Discussion. See lines 25-27 and 403-407 of the revised manuscript.

Further points

Abstract: The reasons for the maize pot experiment needs to be explained more fully.

Response: The rationale has been clarified by revisions to the first two sentences of the Abstract. See lines 12-16 of the revised manuscript.

Introduction is generally well argued, but could the authors give the reasons for the official policy line (line 48).

Response: No reasons are given in the government documentation mandating incubation assays.

Line 68: the argument turns to maize but why not present the more general relevance for all crops. Or explain why maize was chosen.

Response: The focus on maize has been clarified in lines 38-43 of the Introduction.

Line 85 – argument here is unclear and needs to be re-thought: ‘response data sets’ should be clarified; and the factors cited are those the have to be accounted for in all field experiments, but they seem to be classed as the source of a problem.

Response: This is an important point that must be understood to make effective use of the ISNT, and we've done our best to improve clarity by rewording lines 91 and 92 of the revised manuscript. The key insight comes from recognizing that the critical ISNT level for differentiating between responsive and nonresponsive soils can be shifted by factors that affect crop N demand or the supply of soil mineral N. For example, soil acidity reduces microbial activities responsible for N mineralization, with the effect that a higher ISNT level would be required to make a soil nonresponsive to N fertilization. An upward shift in the critical test level would also result from higher planting density, which would increase crop N demand. In contrast to several studies reporting ISNT failure, there was no confounding in our work due to factors such as these.

 M & M

In Table 2, please explain the meaning of the heading “Resin P  K”

Response: Anion-exchange resins are commonly used in Brazil to estimate bioavailable P. This extraction technique is now noted in a footnote added to Table 2, and another footnote has been added to identify the extractant used in estimating K⁺, Ca²⁺, and Mg²⁺.

What is likely soil organic carbon content? It would help the general reader to have some understanding of the SOM to SOC conversion for these soils in case the reader wishes to estimate C:N rations, for example.

Response: Conversion of SOC to SOM has been clarified in the second footnote of Table 2.

Note: this reviewer des not have the experience to review and comment critically on the precise methodologies used to estimate nitrogen in the chosen soils.

Results

The high correlation simply with SOM is notable: the more SOM, the more N and more plant growth. Could measures of SOM dispense with the need for the highly technical and time-consuming determinations of Nitrogen (see note below related to the Discussion)?

Response: No, because SOM is often roughly estimated from soil color or by loss-on-ignition. Soil available N comes mainly from SOM mineralization, but soils differ in organic N lability, contents, fractions, and rates of mineralization. Thus, SOM levels alone are not a reliable predictor of soil N availability.

As requested in the general points above, there needs to be more explanation for the agronomist and general crop scientist of what all the different determinations of N mean, for example by showing how they convert to kg/ha estimated to a given depth. Also, some estimation of C:N values for the different soils would be helpful to place the soils and methods in a wider context.

Response: See responses to comments 2 and 4 of Reviewer 2.

The correlations in Table 6 are very useful – and the high point of the paper - but it would raise the impact of the paper if some limited comparisons were shown graphically with regression lines. For example, there is a potentially interesting difference between plotting DM against SOM or nitrogen and plotting N uptake against them. In the latter, soil 1 of highest SOM and N is considerably more ‘distant’ from the other soils, and it would be interesting to speculate why.

Response: The revised manuscript includes a multi-panel figure to graphically compare the linear relationship of plant N uptake with SOM, total soil N, net N mineralization during aerobic incubation at 18/23°C, and the ISNT.

Discussion

The important conclusion that a range of other methodologies are good predictors of available N should not wait until the third paragraph (line 335). There could be a short introductory paragraph stating the main finding and then moving then to discuss the temperature regimes and then to the other methods.

Response: An introductory paragraph has been added to the Discussion. See lines 330-335 of the revised manuscript.

Line 297 – explain why ‘a greater moisture loss’ might be the cause

Response: Higher temperatures promote evaporative loss of soil moisture, as is now clarified in line 342 of the revised manuscript.

Why not recommend much more explicitly that soil SOM could be a proxy. It might need to be calibrated in the field but why bother to estimate N in these soils?

Response: As documented by Table 6 and Figure 1, SOM was less effective for predicting plant N uptake as compared to the ISNT or incubation estimates of N₀. Also note that in our project, SOM was obtained from SOC measured by a wet chemical method that would be comparable to the ISNT in the procedure involved. In contrast, commercial soil testing labs commonly estimate SOM from soil color or by measuring loss-on-ignition, and the results would be of lower quality and thus less reliable for predicting maize N response.

We thank the reviewer for his/her comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

See the attached file.

Comments for author File: Comments.pdf

Author Response

Reviewer 3

Abstract

1. The rationale of the study is not clear, especially why the authors selected maize is uncertain. The authors should highlight the importance of maize in one sentence or just simply can say it was used as a test crop.

Response: The statement of rationale has been changed. See lines 12-14 of the revised manuscript.

2. Treatments: the authors are advised to clearly mention what are the treatments of the study

Response: Treatments consisted of the methods evaluated for assessing soil N availability.

3. Keywords: ISNT must be spelled out

Response: The list of keywords now includes the full name and abbreviation of the ISNT.

Introduction

The authors discussed mainly the incubation and mineralization assays elaborately avoiding the main issues. They should discuss the importance of predicting nitrogen mineralization i.e. the availability of nitrogen in soils and how this mineralization could be used during fertilizer recommendation. Nitrogen fertilizer is a great concern for crop production across the world providing food, feed and fiber for all living beings. On the other hand, nitrogen is considered as the godfather of environmental pollution. If N mineralization i.e. the available N could be assessed correctly the required amount of N fertilizer can be recommended for crop production. The authors are advised to rewrite the introduction section considering the above mentioned comments. They further need to include the rationale of inclusion of maize in the study.

Response: The Introduction now includes a paragraph to clarify the focus on maize as a major crop in Brazilian agriculture, and the need to account for soil variations in soil N supply. See lines 40-45 of the revised manuscript.

Material and methods

1. The name of soil orders always ended with sols. Line # 13 Entisol should be Entisols. In the table 1, all soil order’s name should be corrected accordingly. In the Soil classification column, the authors provided soil order for most of the soils, and great group for some of the soils. Eutrudox, Eutrustox, Haplustox are the great group of Oxisols. Therefore, the authors are advised to provide all order names or great groups.

Response: Soil orders are now properly identified in Table 1 for the 17 soils studied. Suborders and lower taxonomic classifications were not available for all of these soils, and space was too limited to show them in Table 1.

2. The authors did not mention how they collected soils samples. For soil bulk density (line # 119), they mentioned it was measured after soil crushing, sieving and homogenization. Core sampler should be used to collect soil samples for bulk density. The authors must explain it.

Response: The bulk density data reported in Table 1 were calculated from the mass of soil weighed into the 5-dm³ pots used in the greenhouse experiment. These values were obtained for soils that had been dried, crushed, sieved, and homogenized, and thus do not represent the bulk densities of the undisturbed soils as collected in the field.

3. Line # 119. Raij et al. 2001, it should be in a number format, which is missing. Same comments for Soil Survey Staff (2014) (line # 121), Stanford and Smith (1972) (Line # 163) and Lasdon et al. (1978) (Line # 164), and (Mulvaney et al. 2001) (line # 344).

Response: All references are now cited by number.

4. The soils after harvesting of maize was not analyzed for different parameters as mentioned in the Table 2, 3 and 4. At least, the parameters provided in the Table 3 and 4 should be analyzed to assess the differences in N mineralization with and without crop.

Response: Our focus in this project was on the value of different parameters for predicting dry matter production and N uptake by maize in the absence of N fertilization. Since roots remained in the potted soils after harvest, sampling would have been problematic to avoid plant-derived C and N enrichment and no attempt was made to repeat mineralization assays or chemical N analyses.

5. Line # 213-214: the authors calculated Nitrogen uptake by the aboveground biomass using the equation = DM x TN, which is incomplete and ambiguous for readers. The authors can use the following equation (Sarker et al. 2016):

Nitrogen uptake (kg ha-1) = Nitrogen content (%)× Dry matter (t ha-1) x 10

Sarkar, M.I.U., Rahman, M.M., Rahman, G.K.M.M., Naher, U.A. and Ahmed, M.N. 2016. Soil Test Based Inorganic Fertilizer and Integrated Plant Nutrition System for Rice (Oryza sativa L.) Cultivation in Inceptisols of Bangladesh. The Agriculturist, 14(1), 33-42.

Response: The reference by Sarkar et al. is now cited in conjunction with the calculation of N uptake. See line 228 of the revised manuscript.

6. Line # 223: The authors are advised to talk with a statistician whether LSD or DMRT is appropriate for this specific case.

Response: We discussed this matter with a professor of experimental statistics, and were assured that the analysis performed was appropriate for our research objective.

Results and Discussion

1. The results of soils analysis after harvesting of maize are required. At least, the parameters provided in the Table 3 and 4 should be analyzed to assess the differences in N mineralization with and without crop.
2. Discussion also needs to be modified based on the inclusion of new results as mentioned earlier. The authors are also requested to improve the discussion part reading the following articles of Hasnat et al. (2022), and Hossain et al. (2017):

Hasnat, M., Alam, M.A., Khanam, M., Binte, B.I., Kabir, M.H., Alam, M.S., Kamal, M.Z.U., Rahman, G.K.M.M., Haque, M.M. & Rahman, M.M. 2022. Nitrogen fertilizer and biochar on organic matter mineralization and carbon accretion in soil. Sustainability, 14, 3684. https://doi.org/10.3390/su140636843

Hossain, M.B., Rahman, M.M., Biswas, J.C., Miah, M.M.U., Akhter, S., Maniruzzaman, M. Choudhury, A.K., Ahmed, F., Shiragi, M.H.K. and Kalra, N. 2017. Carbon mineralization and carbon dioxide emission from organic matter added soil under different temperature regimes. International Journal of Recycling of Organic Wastes in Agriculture, 6, 311-319. https://doi.org/10.1007/s40093-017-0179-1

Response: See previous response to comment 4 by Reviewer 3.

Conclusions

1. The authors are advised to improve Conclusions based on the added results and discussion and objectives of the study as well.

Response: With or without the revisions made in response to reviewer comments, the key findings from this project were that (1) mineralization assays can be improved by diurnal temperature cycling, and (2) potentially mineralizable N can be estimated far more easily with the ISNT. Both findings are featured in the Conclusions, which is limited to a single paragraph for clarity and impact.

We thank the reviewer for his/her comments.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Line # 118: It should be Entisols