Soil Properties for Predicting Soil Mineral Nitrogen Dynamics Throughout a Wheat Growing Cycle in Calcareous Soils
Round 1
Reviewer 1 Report
General comments:
The authors did a commendable effort to improve the previous version of the manuscript, but they did not respond to the main concern raised by both reviewers, which was: taking into account plant N uptake to predict soil mineral N dynamics, which is the main objective of this study.
In order to improve this aspect, some suggestions/recommendations are provided below.
The goal is to predict Nmin dynamics during the wheat growing cycle. This means that one should be able to estimate i) N mineralized from soil organic matter (could be predicted using soil properties and/or incubation or extraction indices) ii) plant N uptake (measured data could be used for aboveground biomass, N in roots could be estimated) and iii) Nmin at the beginning of the experiment (measured = Nmin(0)).
Nmin at the different stages could thus be predicted as follows:
At stage 1 (T1), Nmin (1) = Nmin (0) + N mineralized from SOM (between T0 and T1) – N taken up by plant (between T0 and T1).
At stage X+1, Nmin (X+1) = Nmin (X) + N mineralized from SOM (between TX and TX+1) – N taken up by plant (between TX and TX+1).
NB: Main factors are considered here, but Nmin in irrigation water (if significant) could also may be considered in the balance.
The challenge here is to estimate N mineralized from soil organic matter, which can be done using soil properties and/or incubation or extraction indices to design a simple model able to predict that variable under the specific experimental conditions of this study and for each type of soil.
Once the model predicting N mineralized is obtained, Nmin in soil at each stage can be calculated with the above mentioned equations using Nmin in soil at the beginning of the experiment (i.e. Nmin (0)), measured (+ estimated for root N) plant N uptake and predicted N mineralized.
This will allow to compare predicted Nmin with observed data and to provide statistical criteria of agreement between observed and predicted values.
By doing this, an estimation of the capacity of soils to supply N could be obtained under standard conditions and further applied to field conditions (by using temperature and moisture functions) for decision support to help N management in order to obtain a target yield or GPC.
In my opinion, this is not so much additional work but would completely meet the objective of this study. The manuscript (and also decision support in N fertilization) would greatly benefit from this.
Specific comments:
Lines 130-132: I do not understand the link between routine soil physicochemical analyses and determining the soil mineralization capacity. Furthermore, as it stands, this part is not very informative and could be deleted if no additional information is provided. A few details on methods (those given at the bottom of Table 1) could be added here, while it was previously asked to shorten Material and Methods part. Otherwise, information could be let at the bottom of the table but should be deleted from the method part.
Figure 2: Please add bars for SD or SE.
Figure 2 legend: a) mg per “pot” vs b) mg per “plot”. Please add “wheat N uptake at harvest” and “values are the mean of X replicates ± SD or SE”.
Author Response
We would like to thank the reviewer for careful and thorough reading of this manuscript and for the thoughtful comments and constructive suggestions, which helped to improve the quality of this manuscript. We appreciate the positive feedback from the reviewer. We have tried to understood and to follow the recommendations from the general comments and we have changed the specific comments.
General comments:
The authors did a commendable effort to improve the previous version of the manuscript, but they did not respond to the main concern raised by both reviewers, which was: taking into account plant N uptake to predict soil mineral N dynamics, which is the main objective of this study.
In order to improve this aspect, some suggestions/recommendations are provided below.
The goal is to predict Nmin dynamics during the wheat growing cycle. This means that one should be able to estimate i) N mineralized from soil organic matter (could be predicted using soil properties and/or incubation or extraction indices) ii) plant N uptake (measured data could be used for aboveground biomass, N in roots could be estimated) and iii) Nmin at the beginning of the experiment (measured = Nmin(0)).
Nmin at the different stages could thus be predicted as follows:
At stage 1 (T1), Nmin (1) = Nmin (0) + N mineralized from SOM (between T0 and T1) – N taken up by plant (between T0 and T1).
At stage X+1, Nmin (X+1) = Nmin (X) + N mineralized from SOM (between TX and TX+1) – N taken up by plant (between TX and TX+1).
NB: Main factors are considered here, but Nmin in irrigation water (if significant) could also may be considered in the balance.
The challenge here is to estimate N mineralized from soil organic matter, which can be done using soil properties and/or incubation or extraction indices to design a simple model able to predict that variable under the specific experimental conditions of this study and for each type of soil.
Once the model predicting N mineralized is obtained, Nmin in soil at each stage can be calculated with the above mentioned equations using Nmin in soil at the beginning of the experiment (i.e. Nmin (0)), measured (+ estimated for root N) plant N uptake and predicted N mineralized.
This will allow to compare predicted Nmin with observed data and to provide statistical criteria of agreement between observed and predicted values.
By doing this, an estimation of the capacity of soils to supply N could be obtained under standard conditions and further applied to field conditions (by using temperature and moisture functions) for decision support to help N management in order to obtain a target yield or GPC.
In my opinion, this is not so much additional work but would completely meet the objective of this study. The manuscript (and also decision support in N fertilization) would greatly benefit from this.
Following the reviewer suggestions we need to develop a model to estimate the N mineralized from soil organic matter using soil properties and incubation and extraction indices. For this purpose, we tried to create a simplified multiple regression equations and stepwise regression equations for each type of soil. However, the number of samples was too low and made that imprecise. As we thought that the reviewer suggestion was interesting we developed different equations for all soils together with the aim of estimating the N mineralized from soil organic matter. However, we think that the results obtained are not good enough because;
There are many factors in the equations and also, equations are not precise enough. We think that may not make sense analysing so many parameters (soil properties, incubations and extractions) to have an equation with so low precision. We tried to remove parameters using only soil properties or extractions, but equations were even more unprecise.
Stepwise equations for predicting N mineralized from soil organic matter:
Y (sowing-GS30) = -0,3351+97.26(Ntot)-6,94(CaCl II) +4.73 (205 Abs)
Y (GS30-GS37) = 507.52-4.97(sand)-5.10(silt)-4.6(clay)+3.95(SOM)-5.26(CaCl I)+3.45(CaCl II)
Y (GS37-GS60) = -17.51+2.99 (clay)-20.85(CaCl II)-9.13(KCl)+40.28(260Abs)
Y (GS60-harvest) = -1386.62+13.81(sand)+14.19(silt)+13.92(clay)-7.87(SOM)-7.56(CaCl I)+4.50(KCl)
2. We compared the predicted values with the observed values and R2 were low.
Furthermore, we have performed the suggested equations by the reviewer for the Nmin prediction at different stages.
Nmin (1) = Nmin (0) + N mineralized from SOM (between T0 and T1) – N taken up by plant (between T0 and T1)
3. We used the predicted values of the N mineralized for the predicted Nmin calculation. We compared the predicted Nmin with the obtained Nmin results at each stage. In these cases, the obtained R2 values were very low too.
As the results obtained following the suggestions of the reviewer were not good enough in our case, we thought that was better not adding that information in our manuscript. Even more, since we summited the manuscript we have been told to shorten that because it was classified as a communication. Our objective with this manuscript was to assess the N dynamics with simple analysis. In any case, we need to mention that we have learnt so much with this suggestion and we will be pleased to apply this recommendation in other future experiments. Anyway, if the editor or the reviewers consider that this information should be added to the manuscript we will be ready to do include that.
Specific comments:
Lines 130-132: I do not understand the link between routine soil physicochemical analyses and determining the soil mineralization capacity. Furthermore, as it stands, this part is not very informative and could be deleted if no additional information is provided. A few details on methods (those given at the bottom of Table 1) could be added here, while it was previously asked to shorten Material and Methods part. Otherwise, information could be let at the bottom of the table but should be deleted from the method part.
-Soil physicochemical analyses information was deleted from the method part.
Figure 2: Please add bars for SD or SE.
-Done (Now Figure 1)
Figure 2 legend: a) mg per “pot” vs b) mg per “plot”. Please add “wheat N uptake at harvest” and “values are the mean of X replicates ± SD or SE”.
-Done (Now Figure 1)
Author Response File: 
Author Response.docx
Reviewer 2 Report
Please consider making the following changes:
Line 13-14: Abstract:" A better understanding of the capacity of soils to supply N to wheat can enhance fertilizer recommendations." The first time you use an abbreviation you need to write out the word, so in this case nitrogen (N).
Line 44-45: "Soils available resources should be taken into account for the determination of appropriate N fertilization rates and thus enhance the efficiency of agricultural systems and ecosystem health." Consider changing this to "Soil available resources should be taken into account for the determination of appropriate N fertilization rates to enhance the efficiency of agricultural systems and ecosystem health."
Line 83: "growing season on crucial stages for plant yield and GPC" this should probably be "growing season at crucial stages for plant yield and GPC"
Line 120:"Wheat aboveground biomass was sampled at GS30 (stem elongation), GS37 (leaf flag emergence)" It would be good if a description was given how the sampling took place and do the same for soil sampling- (Line 126).
Line 123-124:"N uptake was calculated, and GPC was determined by multiplying the total grain N concentration by 5.7 [19]." Please don't start a sentence with an abbreviation, but just write the word out. Figure 1: Please consider another format for this figure as it contains a lot of data that is hard to read. Please consider grouping the soils and then making a graph like that, or just present this data in a table.
Line 252-253:"N availability relies on both the initial availability of Nmin and the rate of mineralization or immobilization". Please change the N at the beginning of the sentence to Nitrogen.
Line 255: "There were no correlations at sowing or GS30 between the soil properties and Nmin values". Please consider changing the sentence to: "There were no correlations between the soil properties and Nmin values at sowing or GS30"
Be consistent with abbreviating nitrogen in sentences (not at the beginning of sentences. Please see line 279: "The mineralization of nitrogen is often affected by the clay content"
Line 344-346: "Even in a relatively small cropping area where the variability of soil properties is narrowed, the dynamics of soil nitrate and ammonium throughout the cropping season were variable and thus were the crop N necessities."- This sentence suggests that the soil nitrate and ammonium govern crop N need, because of the word "thus" in the sentence. This is not accurate. However, I would agree that crop N uptake is governed by soil nitrate and ammonium, so please consider changing the sentence a bit based on what you are trying to get across.
Author Response
Reviewer 2
We would like to thank the reviewer for careful and thorough reading of this manuscript and for the thoughtful comments and constructive suggestions, which helped to improve the quality of this manuscript.
Line 13-14: Abstract:" A better understanding of the capacity of soils to supply N to wheat can enhance fertilizer recommendations." The first time you use an abbreviation you need to write out the word, so in this case nitrogen (N).
-Done
Line 44-45: "Soils available resources should be taken into account for the determination of appropriate N fertilization rates and thus enhance the efficiency of agricultural systems and ecosystem health." Consider changing this to "Soil available resources should be taken into account for the determination of appropriate N fertilization rates to enhance the efficiency of agricultural systems and ecosystem health."
-Done
Line 83: "growing season on crucial stages for plant yield and GPC" this should probably be "growing season at crucial stages for plant yield and GPC"
-Done
Line 120:"Wheat aboveground biomass was sampled at GS30 (stem elongation), GS37 (leaf flag emergence)" It would be good if a description was given how the sampling took place and do the same for soil sampling- (Line 126).
-Done. Biomass information was added in 122-125 lines. Soil information was added in 130 lines.
Line 123-124:"N uptake was calculated, and GPC was determined by multiplying the total grain N concentration by 5.7 [19]." Please don't start a sentence with an abbreviation, but just write the word out. Figure 1: Please consider another format for this figure as it contains a lot of data that is hard to read. Please consider grouping the soils and then making a graph like that, or just present this data in a table.
-Done. Data is presented as a table now.
Line 252-253:"N availability relies on both the initial availability of Nmin and the rate of mineralization or immobilization". Please change the N at the beginning of the sentence to Nitrogen.
-Done
Line 255: "There were no correlations at sowing or GS30 between the soil properties and Nmin values". Please consider changing the sentence to: "There were no correlations between the soil properties and Nmin values at sowing or GS30"
-Done
Be consistent with abbreviating nitrogen in sentences (not at the beginning of sentences. Please see line 279: "The mineralization of nitrogen is often affected by the clay content"
- Done
Line 344-346: "Even in a relatively small cropping area where the variability of soil properties is narrowed, the dynamics of soil nitrate and ammonium throughout the cropping season were variable and thus were the crop N necessities."- This sentence suggests that the soil nitrate and ammonium govern crop N need, because of the word "thus" in the sentence. This is not accurate. However, I would agree that crop N uptake is governed by soil nitrate and ammonium, so please consider changing the sentence a bit based on what you are trying to get across.
-Done. The sentence was changed to “Even in a relatively small cropping area where the variability of soil properties is narrowed, the dynamics of soil nitrate and ammonium throughout the cropping season were variable and thus was the crop N uptake”
