Aggregation Index and Carbon and Nitrogen Contents in Aggregates of Pasture Soils under Successive Applications of Pig Slurry in Southern Brazil
, Marisa de Cássia Piccolo 2, Eduardo Girotto 3, Marcos Paulo Ludwig 4, Julia Decarli 4, José Luiz Rodrigues Torres 5 and Gustavo Brunetto 6Round 1
Reviewer 1 Report
The article is interesting in that it methodically examines the influence of soil fertilization with pig slurry on the physical structure of the soil and its properties. A valuable article - on the practical optimal concentration of fertilization of pig slurry.xvc
Author Response
No modification to make.
Reviewer 2 Report
The manuscript “Aggregation index and carbon and nitrogen contents in aggregates of pasture soils under successive applications of pig slurry in Southern Brazil” addresses the stability of soil aggregates in different layers of the top 30 cm of a pasture soil fertilized with different amounts of pig slurry.
While the subject is interesting, the overall objective of this investigation could be presented in a clearer way. There have been many studies on the use of pig slurry and its effects on yields, aggregate stability etc., so you need to show more clearly what is new in this study.
The introduction should give a clearer overview over the relevant mechanism of aggregate stabilization in pasture soils and how this might be connected to C and N content of the aggregates. It is not simply the addition of slurry that improves soil properties. The term soil quality is normally used in relation to certain soil functions but it remains unclear what is considered good quality in aggregate stability and aggregate size distribution. The objective of the study could be phrased in a more generally relevant way instead of addressing the particular soil and grass variety used in the experiment.
Materials and methods:
The grass used was Bermuda grass, Tifton 85 only describe a variety.
Table 1 gives some meteorological data, without stating for which period. It further appears very detailed and is little used in the paper, consider whether it can be simplified or moved to supplements.
Table 2: It would be interesting to also have some information about the level of (organic) nitrogen in the soil.
Treatment description: You could improve understanding and comparability for a reader if you can give some information about (available) nitrogen contents in the different treatments in the paper, e.g., in an extra table listing the treatments. It would further be important to have some information on phosphorus and potassium contents of the pig slurry in order to be able to compare the treatments. Urea is an organic compound, not a mineral fertilizer, but it may be industrially produced. It would also be helpful to give some very brief information on analyses methods used rather than referring to “N contents, which were estimated according to the methodology proposed by the Soil Chemistry and Fertility Committee”; e.g., total nitrogen, Kjeldahl-N or similar. In addition, a description/references for the analytical methods used needs to be included and how the data was analysed statistically. It appears quite challenging as two of the treatments seem to have four replicates each while one only has one.
With respect to sampling, it is necessary to give information about how many undisturbed soil cores were taken per plot. It is not clear what sort of samples were used for the wet sieving method.
Soil aggregation indices: Rather than an analysis of soil aggregation, the wet sieving method described appears to be a method of determining aggregate size distribution and stability of different size fractions. That should be clarified. Here it would be enough to mention the principle of the method and the sieve size and refer the reader for more detailed description to the original reference. It would be useful to explain why different indices are determined.
All equations used in this paper should be numbered.
C and N in soil aggregates: Where do the undisturbed samples of macro- and microaggregates come from? Are they a result of the wet sieving procedure? Are they really undisturbed?
Results:
Figures 1 and 2: The figures are confusing because the same letters are used for the three different depths. In general, it would be easier to organize these by depth so that direct comparisons of treatments with letters indicating statistical significance are easier. You need to be more precise in the figure text I assume the y axes show grams of a 20 g subsample? Maybe that should be expressed as g 100 g-1?
Text to Table 5: You should also mention how you evaluate the aggregate stability found in the experiments. In general, ASI is high, and differences are relatively small. That is an important point that should also be discussed.
Table 6: If there are no significantly different results, it is not necessary to use letters behind the figures.
Discussion:
The discussion is suffering from discussing the results according to the paragraphs under Results, rather than trying to combine the findings in a discussion that can see connections between the different parameters measured. At the very least, all parameters related to aggregate size distribution and stability should be discussed together, also to avoid unnecessary repetitions of comparisons with similar findings. It would be important to state why a certain aggregate size distribution is desirable.
Mechanistic explanations related to soil microbial activity are a bit vague. For instance, effects of pig slurry on mycorrhizal fungi may also depend on phosphorus availability. You should also explain the benefit of certain aggregate sizes and their stability in a pasture system where, e.g., erosion is not necessarily the biggest problem. Effects relevant in relation to the climate data you showed earlier could be discussed. Can you relate your data to effects on, e.g., infiltration? Can you come with some explanation on why PS100 and PS200 give better effects than PS300 and PS400?
The discussion on C and N in aggregates is somewhat unclear, especially the connection between protection of organic matter in microaggregates and macroaggregates. It would be beneficial to introduce some theory on aggregate hierarchy already in the introduction.
Author Response
The manuscript “Aggregation index and carbon and nitrogen contents in aggregates of pasture soils under successive applications of pig slurry in Southern Brazil” addresses the stability of soil aggregates in different layers of the top 30 cm of a pasture soil fertilized with different amounts of pig slurry .
While the subject is interesting, the overall objective of this investigation could be presented in a clearer way. There have been many studies on the use of pig slurry and its effects on yields, aggregate stability etc., so you need to show more clearly what is new in this study.
The introduction should give a clearer overview over the relevant mechanism of aggregate stabilization in pasture soils and how this might be connected to C and N content of the aggregates. It is not simply the addition of slurry that improves soil properties. The term soil quality is normally used in relation to certain soil functions but it remains unclear what is considered good quality in aggregate stability and aggregate size distribution. The objective of the study could be phrased in a more generally relevant way instead of addressing the particular soil and grass variety used in the experiment.
Author: We inserted in the text information about soil aggregation in pasture soils and its relationship with TOC and TN contents. Regarding the objective, we disagree with the reviewer's comment, which says “The objective of the study could be phrased in a more generally relevant way instead of addressing the particular soil and grass variety used in the experiment.”. Our manuscript evaluates soil aggregates, which is different from bulk soil. Therefore, we are emphasizing what happens in pasture soil aggregates when long-term fertilized with Pig Slurry.
Materials and methods:
The grass used was Bermuda grass, Tifton 85 only describe a variety.
Author: Modified as the reviewer suggested.
Table 1 gives some meteorological data, without stating for which period. It further appears very detailed and is little used in the paper, consider whether it can be simplified or moved to supplements.
Author: We have entered the data evaluation period. As for the possibility of summarizing Table 2, the authors understand that this is not necessary, as it is already well summarized, as it presents the average of data from the last 50 years. And it was a suggestion from the reviewer of the first round of corrections that suggested linking these data with the dry mass production data. And that was done in the Discussion of the Tifton 85 dry mass data.
Table 2: It would be interesting to also have some information about the level of (organic) nitrogen in the soil.
Author: Unfortunately we do not have this initial information in the soil, as in southern Brazil we use the SOC content to indicate how much nitrogen fertilizer should be used for the crops that will be planted, as stated in our fertilization manual CQFS-RS/SC - Soil Chemistry and Fertility Commission – RS/SC Manual of liming and fertilization for the States of Rio Grande do Sul and Santa Catarina Brazilian Society of Soil Science, Brazil, 2016. 376 p. )
Treatment description: You could improve understanding and comparability for a reader if you can give some information about (available) nitrogen contents in the different treatments in the paper, e.g., in an extra table listing the treatments. It would be further important to have some information on phosphorus and potassium contents of the pig slurry in order to be able to compare the treatments. Urea is an organic compound, not a mineral fertilizer, but it may be industrially produced. It would also be helpful to give some brief information on analysis methods used rather than referring to “N contents, which were estimated according to the methodology proposed by the Soil Chemistry and Fertility Committee”; e.g., total nitrogen, Kjeldahl-N or similar. In addition, the description/references for the analytical methods used needs to be included and how the data was analyzed statistically. It appears quite challenging as two of the treatments seem to have four replicates each while one only has one.
Author: The N content for each treatment (PS10 to PS400) was calculated from the N content of the swine manure, which was estimated according to the methodology proposed by the Soil Chemistry and Fertility Committee [20]. The amount of N added was to supply the N demand for tifton 85, as proposed by the Soil Chemistry and Fertility Committee [20]. Data for P and K were entered in Table 3. The method for extracting total N was entered. To estimate the amount of swine manure to be applied based on N contents, five subsamples of manure were evaluated. This procedure was done every year. Table 3 shows the mean values of N, P and K contents. The number of repetitions of treatments was always the same, with four repetitions in all treatments.
With respect to sampling, it is necessary to give information about how many undisturbed soil colors were taken per plot. It is not clear what sort of samples were used for the wet sieving method.
Author. In each plot of each treatment, three undisturbed sub-samples were collected to compose an undisturbed sample, making up four undisturbed samples per treatment and for each depth.
Soil aggregation indices: Rather than an analysis of soil aggregation, the wet sieving method described appears to be a method of determining aggregate size distribution and stability of different size fractions. That should be clarified. Here it would be enough to mention the principle of the method and the sieve size and refer the reader for more detailed description to the original reference. It would be useful to explain why different indices are determined.
Author. We modified “Soil aggregation” by “Soil aggregate stability analysis” as the reviewer suggested. Regarding the description of the method, the authors think it is pertinent to leave it as it is, as we have shown the set of sieves we used (8.0 to 0.053 mm), and not all works with soil agreggation use this same set of sieves. This facilitates the understanding of the aggregation indices that were found, as well as the mass and distribution of aggregates in macro, meso and micro-aggregates.
Regarding the different indices used, this is done to better highlight the differences between the treatments, as a single index cannot always show all the differences between the treatments, if it has differences.
All equations used in this paper should be numbered.
Author: Okay.
C and N in soil aggregates: Where do the undisturbed samples of macro- and microaggregates come from? Are they a result of the wet sieving procedure? Are they really undisturbed?
Author: Yes, these are samples from wet sieving. And yes, they were intact, as it was the aggregates that were retained in each sieve.
Results:
Figures 1 and 2: The figures are confusing because the same letters are used for the three different depths. In general, it would be easier to organize these by depth so that direct comparisons of treatments with letters indicating statistical significance are easier. You need to be more need in the figure text I assume the y axes show grams of a 20 g subsample? Maybe that should be expressed as g 100 g-1?
Author: We disagree with the reviewer, as it is well explained in the footnote of the Figure (Bars with the same letter within the same soil layer are not different from each other by the Scott-Knott test (p<0.05).
About unit (g), it is also correct, as in the methodology we have already explained that this aggregate mass refers to 25 grams.
Text to Table 5: You should also mention how you evaluate the aggregate stability found in the experiments. In general, ASI is high, and differences are relatively small. That is an important point that should also be discussed.
Author: Yes, this was explained at the beginning of item 2.4 (Soil aggregation), as suggested by the reviewer as well. About the high ASI and the small differences, this is discussed in the text, also addressing with high ASI and high GMD and high macroaggregate mass.
Table 6: If there are no significantly different results, it is not necessary to use letters behind the figures.
Author: All tested variables were significant by the F test (ANOVA), p<0.05. However, not all showed differences by the means test (Skott knott). Therefore, we must put the letters equal, when they did not differ by means of the test.
Discussion:
The discussion is suffering from discussing the results according to the paragraphs under Results, rather than trying to combine the findings in a discussion that can see connections between the different parameters measured. At the very least, all parameters related to aggregate size distribution and stability should be discussed together, also to avoid unnecessary repetitions of comparisons with similar findings. It would be important to state why a certain aggregate size distribution is desirable.
Mechanistic explanations related to soil microbial activity are a bit vague. For instance, effects of pig slurry on mycorrhizal fungi may also depend on phosphorus availability. You should also explain the benefit of certain aggregate sizes and their stability in a pasture system where, e.g., erosion is not necessarily the biggest problem. Effects relevant in relation to the climate data you have shown earlier could be discussed. Can you report your data to effects on, e.g., infiltration? Can you come with some explanation on why PS100 and PS200 give better effects than PS300 and PS400?
The discussion on C and N in aggregates is somewhat unclear, especially the connection between protection of organic matter in microaggregates and macroaggregates. It would be beneficial to introduce some theory on aggregate hierarchy already in the introduction.
Author: Corrected as per reviewer 3 suggestions.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
Please see the attached file.
Comments for author File: Comments.pdf
Author Response
General comments
This manuscript evaluated effects of PS continuous applications on grass biomass, soil aggregation and aggregate C and N contents at different soil depths and help changes in soil fertility.
This work is also valuable for rational PS management in pasture soil. It is worth to publish. I am not a native speaker, generally English expression is good, but some is still not easy to understand.
So, I suggest authors need further linguistic improvement.
Additionally, this paper needs more statistical analysis and a review discussion section and conclusions. The detailed comments are provided as follows:
Specific comments:
L 54-55: concept of soil quality should be greater than soil fertility. Here should not be repeated soil fertility. Similar problem in L82-83.
Author: Ok. We modified as suggested by the reviewer.
L 115-116: PS and mineral N rate described not clear, based on my understanding these rates mean N?
Author: Yes, PS and mineral N rate mean the rates of N. We've included it in the text.
L 127: “The application of the treatments started in November 2012”, it is given in above lines, not repeat it again.
Author: Ok. We excluded the repeated part.
L 142, unit needs to be revised as kg not Kg.
Author: Fixed in all text.
L 178 statistical analysis should be performed by two-way ANOVA to check the interaction between treatments and soil depths for the relevant variables.
Author: We do not agree with the reviewer, as the experiment is not a factorial. The experiment was set up in a randomized block design, with only the treatment factor that we want to test. We do not want to see the effect of depth, as we already know that both soil aggregation and organic matter contents are higher in the topsoil compared to the deeper layers.
L193: uppercase letter revised as lowercase
Author: Corrected.
L193-196, very complicated description, refine these.
Author: Okay, we modified.
L 200: citation of Figure 1 should be stand just before the stop, not end of the paragraph. The same for other citations.
Author: We made the changes throughout the text.
L238-239, should be in discussion section, not results section.
Author: Okay.
L 250 why table 5 had no statistical analysis?
Author: Because they are dimensionless indices and in works that use these measures, they do not perform statistical analyses. Therefore, throughout the text we present it in terms of proportional values ​​(how much was greater or less in relation to any treatment).
L 295-297 moves this to discussion.
Author: Okay, we modified.
L 300-307, it is wordy, and to some extent it repeats same meaning.
L 308-313, similar meaning as previous paragraph, it should be combined together for L300-313, and briefly and logically write.
Author: Regarding the reviewer's comments (L300-313), we disagree. In this part we are informing and discussing that the use of PS increases the dry mass production of tifton 85, being an alternative for disposal in an area with high swine production. And we also show other works with similar results.
L350-361, this part just listed the results of other studies, it should be compared with its results, so called discussion. I suggest authors should review 4.2.2. The same problem also applies for section 4.2.3. I suggest authors combined these two sections together named as soil aggregation
Author: Okay. Has been modified.
L 398-443, this part is relatively better than others, but authors should clearly discuss one point by one point to prevent from repetition.
Author: Ok
L 444, conclusion or conclusions?
Author: Conclusions
The conclusions’ section should not be a summary of your study or an extension of the
discussion. The conclusions’ section should illustrate the mechanistic links of findings obtained under applied treatments. The authors should avoid repeating what has already been presented in results and discussion.
Author: Has been modified.
Author Response File: Author Response.pdf
