the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Sep 2022
Wetting and drying cycles, organic matter and gypsum play a key role in structure formation and stability of sodic Vertisols
Abstract. In the natural environment, soils undergo wetting and drying (WD) cycles due to precipitation and evapotranspiration. The WD cycles have a profound impact on soil physical, chemical, and biological properties and drive the development of structure in soils. Degraded soils are often lacking structure and the effect of organic amendments and WD cycles on structure formation of these soils is poorly understood. The aim of this study was to evaluate the role of biotic and abiotic factors on aggregate formation and stabilisation of sodic soils after the addition of gypsum and organic amendments (feedlot manure, chicken manure, lucerne pallets, and anionic poly acrylamide). Amended soils were incubated at 25 °C over four WD cycles, with assessment of soil microbial respiration, electrical conductivity, pH, sodium adsorption ratio (SAR), aggregate stability in water (ASWAT), aggregate size distribution, and mean weight diameter. Our results demonstrate that WD cycles can improve aggregate stability after the addition of amendments in sodic Vertisols, but this process depends on the type of organic amendment. Lucerne pellets resulted in highest soil microbial respiration, proportions of large macroaggregates, and mean weight diameter. In contrast, dispersion was significantly reduced when soils were treated with chicken manure, whilst PAM only had a transient effect on aggregate stability. When these organic amendments were applied together with gypsum, the stability of aggregates was further enhanced, and dispersion became negligible after the second WD cycle. The formation and stability of small macroaggregates was less dependent on the type of organic amendments and more dependent on WD cycles as the proportion of small macroaggregates also increased in control soils after four WD cycles, highlighting the role of WD cycles as one of the key factors that improves aggregation and stability of sodic Vertisols.
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-469', Manfred Sager, 02 Nov 2022
Just a comment to the presented paper:
My experience about drying and rewetting changes in alkaline nutrient-rich chernozem columns, packed in columns, are these:
Permanent wetting fixed S, and slightly mobilized P, Fe and siderophiles (Mn, V, Cr) because of slow oxygen consumption, as well as slight shrinkage of volume. Subsequent drying and rewetting lead to a flush of sulphur and partially selenium, as well as fixation of P and Fe because of intermediate aeration from drying. Aggregate stability was not analyzed. Freshly precipitated Fe(OH)3 forms colloids and micro-aggregates.
Of course, some decisions have to be taken beforehand, which might influence the result - thus literature is contradictory at the first glance. Samples were air-dried and sieved before the experiment - like in my experiment also. But the organic amendments were air-dried before the incubation experiment also, which means loss of NH3 and H2S/H2Se during the drying stage as well as aeration, in favour of aerobic microbes. Again, after the incubation experiment, drying and aeration was done also.
Unfortunately, a manuscript rating is needed, to send this text; please do not consider it too much
Differences are interpretable due to different microbes present in the organic amendments, whereas polyacrylamide means practically zero. Gypsum is rather soluble, and favours bacteria of the sulfur cycle.
Changes in micro-aggregates might be interpretable in precipitation of pedogenic oxides resp. pyrite
I wonder, why no data about mobile chemical fractions, nor soil minerals or soil amorphous phases have been done.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC1 -
AC1: 'Reply on RC1', Bernhard Wehr, 07 Dec 2022
Reviewer 1
We thank the reviewer for the positive feedback.
We agree with the reviewer that wetting and drying cycles may cause anaerobic or anoxic conditions. However, in this experiment, the soils were kept wet at field capacity only for 24 hr and then left in open air for moisture loss (to initiate drying conditions) and the soils were packed to a height of 3-5 cm in jars with 12 cm width. Hence, we do not believe our experimental conditions have anoxia in the soil. Indeed, we have not measured a decrease in nitrate in our soil solution samples (which would happen if conditions were anoxic). We can include graphs if considered necessary to show this). The organic amendments except anionic polyacrylamide (PAM) were used in dried form to overcome differences in moisture content and particle sizes. While there may have been some losses of nutrients in the organic amendments due to drying, the nutrient content was measured on the dry material. Hence, we do not believe this would affect the interpretation of our results.
We also agree that different microbial communities may be added (or favoured) by adding organic amendments. However, the focus of our paper was not on different microbial populations but on aggregation (by whatever means the aggregates are formed, be it different microbes or iron oxides). While we measured the soluble chemical fractions (soil solution composition) in our samples (these data are reported in the supplementary section of our paper), we have not analysed if there are changes in the mineral phases during the trial since this was outside the scope of our experiment.
The focus of this experiment was to understand the effect of wetting and drying cycles on improving the structure of sodic soils under the application of different organic amendments. Therefore, to correlate the changes in aggregate stability and aggregation with microbial activity, daily microbial respiration was measured. In our earlier study (Niaz et al 2022) we measured Dissolved Organic Carbon (DOC) using the same soils and amendments under continuous wet conditions and found that DOC and soil microbial respiration were strongly positively correlated with each other, hence we are confident relating microbial respiration to DOC in this paper.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC1
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AC1: 'Reply on RC1', Bernhard Wehr, 07 Dec 2022
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RC2: 'Comment on egusphere-2022-469', Anonymous Referee #2, 06 Nov 2022
General comment
This work has some scientific value, but the findings and conclusions obtained still need to be reconsolidated. Although this study focused on the physical properties of the soil, the authors' experimental design used different organic amendments with different chemical properties. It is clear that the reasons for the differences in the experimental results cannot be explained by the causes and mechanisms of the results through these soil physical property analyses conducted by the authors alone. Soil organic matter is essential for the formation of soil agglomerates, and the authors did not characterize the changes in soil chemical properties but mentioned soil organic matter and soil chemical carbon pools several times in the discussion. In addition, the characterization of microbial activity under different amendment applications by microbial respiration alone is relatively one-sided and is not sufficiently convincing for characterizing the decomposition effect of microorganisms on different amendments. The authors use unpublished results for citations several times in the discussion section, and this issue should be noted.
The authors selected four organic amendments with gypsum and used two soils, which was a multifactorial experiment that led the authors to derive 10 treatments in this study. However, the authors' conclusions do not explain well the reasons why different amendments produce different amendments in different soils. The authors also did not present the research hypothesis well. A true research hypothesis is proposed based on the purpose of the study, in conjunction with their own research design. At this point in the process, I don't think the manuscript is ready to be published. The authors should perform a high level of integration and analysis of the available data as well as the data in the supplementary material. If possible, please add the necessary soil chemical characterization data.
Specific comments
(line: comment)
- Organic matter in the title is easy to make the reader misunderstand and should be changed to organic amendments or organic material.
- Line 28: Please add the definition of PAM.
- Why did the author choose these four organic materials? What are their advantages compared with other materials? The author needs to explain this in the introduction.
- Line 111: Why did the author choose to collect only 10 cm of soil, which is not fully representative either from the perspective of soil improvement or agricultural cultivation?
- Lines 116–117: According to previous experience, the sieve with a 10-mm void does not completely remove stone and plant litter.
- The materials and methods do not mention the size of the containers in which the authors performed the incubation experiments.
- The author mentions the significance of the difference in the main text, but no significant difference can be directly observed in the chart. Authors are advised to add error bars.
- The discussion section needs to be dissected in more depth, especially 4.3.
- Lines 437–440: The author needs to list more references to prove the point.
- Lines 480–491: Conclusions should not replicate the findings but be more concise.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC2 -
AC2: 'Reply on RC2', Bernhard Wehr, 07 Dec 2022
Reviewer 2
We appreciate the time and effort the reviewer spent on our paper and for the constructive criticism. We trust that our revisions are satisfactory.
The characterization of microbial activity was only based on microbial respiration since we analysed the changes in dissolved organic carbon using the same soils and same organic amendments under continuous wet conditions in our earlier paper which is now published (Niaz et al 2022). The results showed that dissolved organic carbon is strongly positively correlated with microbial respiration. Likewise, the chemical characterisation of the organic amendments used in this study is published in Niaz et al., 2022 (https://doi.org/10.1016/j.geoderma.2022.116047). We agree with the reviewer that a detailed study of the microbial population can be performed, but this was outside the scope of this paper.
- We have clarified the hypothesis as follows: “We hypothesised that i) organic amendments will increase the microbial respiration and improves the formation of large macroaggregates and MWD, (ii) gypsum will improve aggregate stability due to increases Ca concentration and ionic strength, iii) organic amendments act synergistically with gypsum on aggregation, and iv) repeated WD cycles will increase the process of aggregate formation and stability”.
- We have revised the results section as suggested by this and other reviewers.
Line comments
- Organic matter in title has been replaced with organic amendments. The title has been updated as “Wetting and drying cycles, organic amendments and gypsum play a key role in structure formation and stability of sodic Vertisols”
- The full form of PAM has now been incorporated as “In contrast, dispersion was significantly reduced when soils were treated with chicken manure, whilst anionic polyacrylamide only had a transient effect on aggregate stability”.
- The four organic amendments were chosen because of two reasons: 1) they were easily available and are being used by farmers, 2) LP is used as green manure and studies have shown it is effective in ameliorating sodic soils, and 3) PAM is used in mining and construction to treat sodic dispersive soils. Furthermore, these amendments were different in terms of their chemical properties (Table 2) and C functional groups (Niaz et al., 2022) and may give a good contrast between the amendments.
- The 0-10 cm soil layer was chosen because topsoil has the greatest OM content and microbial activity. In any case, Vertisols are relatively uniform in texture and structure throughout the soil profile; using subsoil would not add much information to the study.
- The soil was sieved to 10 mm as we wanted minimise physical disturbance of the natural soil structure. The soil contained no stones or coarse fragments, and the visible plant litter was manually removed.
- The experiment was performed jars with 12 cm diameter and 15 cm height, and the soil was packed to a height of 3-5 cm (depending on the bulkiness of the organic amendments).
- As there were 10 treatments, individual error bars or asterisks would have cluttered the graphs. Therefore, we preferred to insert the Tukey HSD bar in graphs.
- The discussion has been revised as follows “the results of this experiment showed that addition of gypsum (Ca) significantly reduced soil dispersion and increased aggregate stability. This improvement in aggregate stability is because of the increased EC (ionic strength, Fig. S4) and decreased SAR (Fig. S5) after the addition of gypsum. The increased EC likely resulted in the flocculation of soil particles by reducing the diffuse double layer (van Olphen 1977, Ghosh et al., 2010, Bennett et al. 2015). Improved stability was also observed when organic amendments were applied with gypsum especially in G+PAM, G+LP, and G+FLM treated soils, which when applied alone were not able to improve aggregate stability. The PAM had an initial positive effect but led to decreased stability at completion of the second WD cycle. Although, the addition of gypsum increased aggregate stability, it was observed that addition of gypsum did not affect the proportion of large macroaggregates and MWD. However, when organic amendments were added with gypsum an improvement in proportion of large macroaggregates and MWD was observed in G+LP treated soils. This can be explained as Ca-bridging effect through which clay particles are attached to organic matter and polyvalent cations resulting in the formation of macro and micro aggregates (Wuddivira and Camps‐Roach 2007).”
- More references highlighted in blue italic colour have been included as “Although the maximum respiration rate was observed during the first WD cycle for LP and G+LP, the proportion of large macroaggregates and MWD increased at the end of fourth WD cycle. However, the proportion of large macroaggregates did not increase much as compared to the first WD cycle, likely because microbial activity was lower (Cosentino et al. 2006; Zhang et al. 2022). One of the possible reasons of increased MWD could be the accumulation of microbial binding agents over time that released continuously from microbial activity due to organic matter decomposition (Rahman et al. 2018).
- The conclusions are revised as “The stability of dispersive sodic Vertisols was improved by the application of organic amendments and gypsum, which was further enhanced by alternate WD cycles. Gypsum reduced soil dispersion but did not affect the proportion of large macroaggregates and MWD. We observed that not all organic amendments were equally beneficial in improving soil aggregation and aggregate stability. LP significantly increased the proportion of large macroaggregates compared to FLM and PAM. In contrast, CM significantly reduced soil dispersion as it had higher calcium content. It was also found that PAM only had a transient effect in controlling dispersion. In the absence of organic amendments, repeated WD cycles reduced the dispersion of sodic soils, but when organic amendments were added (with or without gypsum) soil aggregation and soil stability was improved even more. It is likely that soil microbial activity contributed to the aggregate formation. Implementation of these findings in the field would favour the use of organic amendments with gypsum to improve the physicochemical properties of sodic soils, which is further enhanced by WD cycles. The aim should be initially to prevent soil dispersion which can be achieved by the application of Ca (through application of gypsum) and then to build larger aggregates which can be achieved by the application of organic amendments”.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC2
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RC3: 'Comment on egusphere-2022-469', Anonymous Referee #3, 08 Nov 2022
Review for article "wetting and drying cycles, organic matter and gypsum play a key role in structure formation and stability of sodic Vertisols", by Niaz et al.
Dear Authors,
first of all, I think you did a nice job. The article topic fits with Soil aim and scope, and furthering the understanding of aggregate formations is paramount if we want to protect soil health and increase soil organic carbon stock. The manuscript is written in good English and it is well structured.
I have some concerns, however, about the way the study is presented: the study tries to link physical structure of the soil (aggregates) with many different factors (chemical and biological) all together. This is not wrong in itself: the soil is a complex system, and complex system cannot be fully comprehended using reductionism. However, when studying the correlations between macro-variables of complex systems, it is important to understand that we are only finding correlations and not causal links. As such, I think that re-shaping the manuscript focusing on the correlations analysed (aggregate size - WD cycle; aggregated size - amendment; Aggregates stability indices compared with respect to WD; WD - respiration) and the discussion should focus more on putting the correlations found "in context" with respect to other vertisols in real conditions.
What I suggest the most is to be laser focused in the manuscript: write down clearly all your hypotheses in the "objective" paragraph of the Introduction (at the moment there are only WD-respiration; WD - aggregate stability; number of WD cycles to improve aggregate stability). Then get back in the introduction and help us understand exactely the relationship between soil respiration and soil aggregate formation, size and stability as found in literature (which is something it is missing right now). By the way, I really felt that the manuscript was missing the link between respiration and aggregates formation/stability.
In the Materials and Methods please be a bit more rigorous: define clearly what you mean by "dispersive" (line 114); explain why the two soils, albeit very similar, have different physical behaviour (line 113); clearly define the terms in equation 1 in the text (at the moment, they are a bit confusing); clearly write down the meaning and value of 'n' for equation 2 (number of fractions = 4, but it should be written down).
Please add error bars in the figures, or at least put some error indication in supplementary materials.
The discussion should be more focused on what we see in the results; at the moment there is much speculation about possible causes - which is ok, but should not be the whole discussion. First, link the results, focus our interest on the relevant data, explain what is in the figures further, discuss limitations. Then you can go with speculation. By the way, I did not understand section 4.1: the mechanisms of microbial action are not explained in detail and are not related to existing studies, and I think the discussion says the opposite of what we see in the data! E.g. line 394 "macroaggregates break more easily during WD cycles compared to microagregates" - figure 1 shows a significant increase in macroaggregates for all treatments, all soils, between WD1 and WD4 (or am I wrong?). line 402 "the proportion of small macroaggregates did not increase after the second WD cycle" - again, this is the opposite of what I see in figure 1. The problem is that these sentences are the core of section 4.1! Please help me understand.
Conclusion should clearly answer the scientific questions posed in the Introduction: if you have 3 objective, give 3 clear conclusions.
Finally, and very importantly, the literature cited is "old": the most recent article is from 2018, and the number of post-2015 articles is only 3. This is a major issue, since our conception of soil mechanisms has definitely changes as marked by Lehman 2015 "the contentious nature of soil organic matter" - and organic matter is related with microbial activity and aggregates formation and stabilization. I also suggest the reading of articles that analyse soil microbial respiration with WD cycles (Brangari 2020-2021, the latest works by Lindsay Todman, etc...). Finally, if you base some of your discussion on unpublished data, please put some of them in the supplementary materials, at least as "summary data", or, even better, include them in the article, if possible.Small corrections:
line 41 "wind, condensation, (Utomo and Dexter 1982) and evaporation"
line 49 "indirect effects on plants ecology and soil microbial..."
line 55 "soil microbial activity, since the latter influences..."
line 82 "organic matter affects aggregate stability"
line 245 "significant change in"
line 255 "after each WD cycle."
Figure 2: insert legendCitation: https://doi.org/10.5194/egusphere-2022-469-RC3 -
AC3: 'Reply on RC3', Bernhard Wehr, 07 Dec 2022
Reviewer 3
We are thankful to the reviewer for the critical and positive feedback.
- We agree that a correlation does not prove the existence of a mechanism or cause. We represented the correlations among different soil physical, chemical, and microbial characteristics as PCA biplots after each WD cycle to show how soil properties change with repeated WD cycles. We have reworded the main text making it clearer that we refer to correlations only.
- We have added more references to place our findings in a broader context and relating our results to other work done with Vertisols (Ghosh et al., 2010, Rahman et al., 2017, Rahman et al., 2018, Bennett et al., 2015, Nachimuthu et al., 2022).
- The hypotheses have been updated as shown for reviewer 2.
- The introduction have been revised by adding information about soil microbial respiration and aggregate formation as “Although, several researchers have reported that the addition of organic materials causes a rapid stimulation of microflora, which increases soil microbial respiration, in response to which extracellular polysaccharides are produced that helps in the formation of soil aggregates (Bossuyt et al., 2001), but the studies investigating the effect of organic amendments in improving the soil structure are inconclusive. For instance, the extracellular polysaccharides and large polyanions can bind clay particles into stable macroaggregates. On the other hand, organic anions can enhance dispersion by increasing the negative charge on clay particles and by complexing calcium with other polyvalent cations (such as those of aluminium), hence reducing their activity in soil solution (Ghosh et al. 2010)”.
- We have revised sections 3.1 and 4.1 to make our statements clearer. For example, the reviewer has highlighted a sentence in line 402 “the proportion of small macroaggregates did not increase after the second WD cycle”, which was changed to “at the completion of the second WD cycle”. (Samples were collected for aggregate size analysis after completion of either 1, 2 or 4 WD cycles). In our revision we will explain the data in more detail and then speculate about the reasons – for example we have included a Figure reference in the text: “The WD cycles result in rearrangement of pores and soil particles and may lead to increased rigidity and stability of soil aggregates (Horn et al. 2014). We observed a marked change in aggregate size distribution with repeated WD cycles (Fig. 1), from macroaggregates (large macroaggregates and small macroaggregates) at completion of the first WD cycle, to microaggregates at completion of the second WD cycle, and back to macroaggregates (large macroaggregates and small macroaggregates) at the fourth WD cycle. We suggest that extracellular polysaccharides formed by microbial activity (soil microbial respiration, Fig. 3) are responsible for the formation of large macroaggregates at the first WD cycle. At the second WD cycle, the microbial activity greatly decreased (Fig. 3) and macroaggregates (large macroaggregates and small macroaggregates) were broken down into microaggregates and silt+clay”
- The reviewer showed concern about the statement we made about the breakdown of large macroaggregates compared to microaggregates during WD cycles. We will rephrase that sentence as “macroaggregates are more susceptible to disintegration during wet sieving compared to microaggregates at the completion of second WD cycle. By the fourth WD cycle, some rearrangements of soil particles likely occurred, facilitated by soil drying, thereby rebuilding macroaggregates.” We could also insert a schematic to illustrate the process.
- The conclusions have been revised as shown for reviewer 2.
- The new citations have now been included at various points in manuscript some of which are Brangari et al., 2022, Freser et al., 2016, Zhang et al., 2022.
- The previously unpublished data has now been published and available online as Niaz et al., 2022. (https://doi.org/10.1016/j.geoderma.2022.116047)
- All the small corrections have been incorporated in the manuscript and the legend to Fig 2 has been inserted.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC3
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AC3: 'Reply on RC3', Bernhard Wehr, 07 Dec 2022
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RC4: 'Comment on egusphere-2022-469', Anonymous Referee #4, 08 Nov 2022
This topic is very interesting, and the authors did a lot work about it. However, in my opinion, the manuscript needs a lot revision before acceptance.
First of all, during the Introduction section, some brief information should be given about organic amendments and gypsum effects on soil structure or aggregates. The aims of the study should be rewritten, since the WD was not the only factor we concerned. Secondly, there were multifactor we need to addressed in the study, including gypsum addition or not, four organic amendments, and different runs of wetting drying. Suggest authors use two-way or three-way analysis of variance to give concise results. Please recheck the dose of PAM addition in the text and Table 3. Check some grams, for instance “Na, Ca, Mg, K” in L189, which should be in ionic forms. Also, “cahnge” in L245. In Discussion section, it was badly manner to explain the results, which made me lost. The authors should write in a clear way to explain the aim they concerned. Suggest substantial revision on the section.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC4 -
AC4: 'Reply on RC4', Bernhard Wehr, 07 Dec 2022
Reviewer 4
We thank the reviewer for their time and constructive feedback which helped us to improve our paper.
- The introduction is revised as already suggested by other reviewers. In addition, the reviewer asked to add a brief information about how gypsum and organic amendments can affect aggregation and aggregate stability which has now been incorporated in the main text. An overall brief explanation about sodic soils and their poor structure is given in the introduction from lines 52-69. The effect of organic amendments or organic matter on aggregate stability was briefly described in introduction in lines 84-86. However, the role of gypsum was not explained which has now been added in the introduction as “Traditionally the management practices used to improve the structure of sodic soils involves the displacement of Na ions from the soil exchange complex with the help of divalent cations such as Ca or increasing the ionic strength of soil solution (Ghosh et al. 2010), both of which can be achieved by the application of gypsum to these soils. The effect of gypsum for increasing ionic strength is immediate, but short lived. In contrast, the effect of gypsum for providing the counter ion (Ca to replace Na) is permanent unless additional Na is added to system (e.g., using poor quality irrigation water)”.
- The aims have been rewritten as “Thus, in the present study we used two sodic Vertisols, with the aim to: i) determine the role of gypsum and different organic amendments on aggregate formation and stability, ii) explore the combined effect of gypsum and organic amendments on soil physico-chemical and microbial properties, iii) investigate the effect of WD cycles on microbial respiration, iv) assess the effects of WD cycles on aggregate formation and stability, and v) determine how many WD cycles are needed to improve aggregate stability.
- The hypothesis has been rewritten as shown for reviewer 2.
- We have use two-way ANOVA to analyse the results which makes the description of results rather complex and repetitive. That’s why the discussion was written as the relative role of each factor (WD cycles and organic amendments or gypsum). The interaction of these factors was clearly demonstrated by presenting PCA biplots after first, second and fourth WD cycle in Fig. 5 and section 3.4 (lines 341-356). The two-way ANOVA table can be included in the supplementary data.
- The dose of PAM in Table 3 has been corrected.
- The discussions have been revised as suggested by other reviewers (see reviewer 2).
Citation: https://doi.org/10.5194/egusphere-2022-469-AC4
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AC4: 'Reply on RC4', Bernhard Wehr, 07 Dec 2022
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RC5: 'Comment on egusphere-2022-469', Anonymous Referee #5, 08 Nov 2022
Egusphere-2022-469 is an interesting work aiming to assess the impact of the combined effect of WD cycles and organic amendment and gypsum in soil stability and aggregation. The manuscript is well structured, and the objectives are identified clearly. However, the manuscript should be improved before acceptance mainly how the authors approach / addresses the research question.
Authors concluded that the results demonstrated that WD cycles can improve aggregate stability after addition of amendments – It is unclear to me if there is an overlap of the effect of the amendments over WD influence on soil stability. I would suggest review and reformulate the statements in which authors only include WD as main responsible of the increase of soil aggregation since after several WD cycles + amendment it is difficult to separate the concomitant impact of both at the same time.
I suggest to include in the introduction any explain concerning the selection of sodic Vertisols, apart from the lack of such studies in Vertisols (are these soils predominant in the region? Most of them dedicated to agriculture? Please link the information in lines 67-69 with an extra explanation about Vertisols and its characteristics. Please include a paragraph to close the introduction about the value / contribution of this study see lines 485 - 487
Did the authors carry out preincubation? Line 218 did the authors checked the effect of rapid slaking in the study soils?
Conclusions should be rewritten in the present form are a repetition of the results, please include a synthesis of key points.
Minor corrections
Line 113 physical properties instead of behaviour
Line 119 water suspension
Line 176 include the formula or unfamiliarized readers
Citation: https://doi.org/10.5194/egusphere-2022-469-RC5 -
AC5: 'Reply on RC5', Bernhard Wehr, 07 Dec 2022
Reviewer 5
We thank the reviewer for the positive feedback and hope our changes are satisfactory.
- The aims and research hypothesis have been revised as recommended by other reviewers.
- The sentences are reformulated to make it clear for readers to understand the changes driven by different amendments or WD cycles.
- The information regarding Vertisols has been incorporated in introduction as “Apart from the changes in soil structure due to the addition of different ameliorants, WD cycles can lead to more intensive changes in structure of soils dominated with smectitic clays (Vertisols). The WD cycles can directly affect aggregation of these soils through physical processes (Utomo and Dexter 1982; Denef et al. 2001). These soils are generally characterised as self-mulching soils as they exhibit shrink-swell properties imposed by the WD cycles (Pal et al. 2012). Vertisols cover a total of an estimated 340 million ha in the world (Australia, Asia, Africa, and America), out of which approximately 150 million ha is potential crop land. However, the physical properties and moisture regime of Vertisols represents serious management constraints (Pal et al. 2012). Sodic Vertisols are common in arid parts of the world. The effect of sodicity on the physical properties of Vertisols is still a subject of debate”
- We are not sure we correctly understand the question about preincubation; soils were not pre-incubated in the study, but they were collected in the field and would be pre-incubated there. We are not sure why pre-incubation would be useful – in the field, treatments are applied without pre-incubation. Our apologies if we misunderstood the question.
- The effect of rapid slaking was not checked as the soils were subjected to end over end shaking for the measurement of easily dispersible silt+clay.
- The conclusions have been revised as suggested by other reviewers.
Minor corrections
- Line 113 revised as suggested by reviewer
- Line119 corrected as suggested
- The formula used for gypsum requirement has now been incorporated in the main text from line 175 -176 “The gypsum requirement of both soil samples was calculated based on the formula given by Oster and Jayawardane (1998) as follows:
Gypsum requirement (GR)= 0.00086 x F x D x ∂b x (CEC) x (ESPi-ESPf)
Where F is exchanged efficiency of Ca-Na and for this case considered equal to 1.
Ds is the depth of soil to be reclaimed (cm)
∂b is soil bulk density (g/cm3)
CEC is cation exchange capacity (cmol+/Kg)
ESPi is initial soil exchangeable sodium percentage
ESPf is final or desired exchangeable sodium percentage.
Hence for soil 1 GR= 0.00086x1x10x1.29 (23) (14.9-6) = 2.27 Mg/ha
And for soil 2 GR= 0.00086x1x10x 1.29 (24) (15.9-6) = 2.52 Mg/ha
As an approximation, a single rate of application of gypsum, 2.5 Mg/ha was selected for both soils.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC5
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AC5: 'Reply on RC5', Bernhard Wehr, 07 Dec 2022
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RC6: 'Comment on egusphere-2022-469', Anonymous Referee #6, 09 Nov 2022
The manuscript "Wetting and drying cycles, organic matter and gypsum play a key role in structure formation and stability of sodic Vertisols" presents, as tittle says, the results of a study to evaluate the role of biotic and abiotic factors on aggregate formation and stabilization of sodic soils after the addition of gypsum and organic amendments (feedlot manure, chicken manure, lucerne pallets, and anionic poly acrylamide). Soils were amended and after 4 wetting and drying cycles (WD) different physical, chemical, and biological characteristics were measured and statistically analysed. According to results, WD cycles can improve aggregate stability of studied soils after the addition of amendments, but this process depends on the type of organic amendment. The addition of gypsum together with the amendments, further enhance the stability of aggregates, and dispersion became negligible after the second WD cycle. Authors concluded that WD is a key factor improving aggregation and stability of sodic Vertisols.
The text denotes a considerable amount of field and laboratory work. In general, the manuscript is organised and clear. It proves an effort to measure and relate a number of soil characteristics to study aggregation in sodic Vertisols. English grammar and spelling are accurate. There are some old references but they can be considered as cornerstone to support the introduction and discussion sections. Figures and tables are of good quality and necessary. Manuscript needs a MINOR revision before being accepted for publication. It needs to consider the following remarks.
General comments:
- Please explain abbreviations before using them. As PAM in line 28.
- It is important to define what you call large and small macroaggregates in the abstract and introduction (one needs to read until line 225 to find it out).
- Hypotheses are always very welcome. Thanks for doing so.
- Tables 1 and 2 are OK but probably they can be presented in the results section.
- It is not straightforward to see significant differences in the figures (vertical bars are not helpful). Why don't you simply mark where the differences are significant?
- Pay attention to the use of unpublished references to support your discussion. This can create some incertitude in your assumptions.
Specific remarks:
- Why did you choose to analyse a soil depth of 10 cm? In addition, did you sample one core from 0 to 10 cm or there were several cores depending on the size of the ring?
- Can you please include the formula given by Oster and Jayawardane (1998)? I assume you did use to calculate the 2.5 Mg ha-1 of gypsum added to soils.
- Amend “change”.
- Are these changes significant?
- In Fig. 1, explain what are LMA, SMA, MIC, and MIN in the caption.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC6 -
AC6: 'Reply on RC6', Bernhard Wehr, 07 Dec 2022
Reviewer 6
We are highly appreciative of the feedback given by the reviewer.
- The full form of PAM has now been incorporated in main text in line 28.
- Abbreviations have now been spelled out in the Introduction and Materials and methods
- The hypotheses have been revised as recommended by other reviewers.
- We presented Table 1 and 2 after the description of soils and organic amendments in the Materials and methods section, but we don’t mind to move the tables to the Results section is the editor recommends.
- As explained for reviewer 2, we prefer to use the Tukey’s HSD bar to avoid cluttering the graphs.
- The unpublished data at that time was under review and now has been published as Niaz et al., 2022 (https://doi.org/10.1016/j.geoderma.2022.116047).
Specific comments
- As explained for reviewer 2, topsoil layer has high OM and microbial activity and Vertisols are relatively uniform in texture and structure to depth
- Three core samples were collected for the measurement of bulk density for each site (Soil1 and Soil 2) while the bulk soil samples were collected with a shovel to 10 cm depth.
- The formula used for gypsum requirement has now been added in materials and methods section as advised by reviewer 5.
- Our apologies but we do not understand what is meant by “Amend “change” ”
- The significance for each parameter is indicated in the text and figures by using the p (<0.05) value and Tukey’s HSD bar in the figures. The error bars and letters to show significance were purposely removed as the line graphs got clumsier and were difficult to distinguish.
- In Fig. 1 the legends LMA, SMA, MIC and MIN have now been clearly explained in figure caption. The four different aggregate classes have been abbreviated as LMA, SMA, MIC and MIN which have been updated as “Large macro aggregates, small macroaggregates, microaggregates and silt+clay”.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC6
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-469', Manfred Sager, 02 Nov 2022
Just a comment to the presented paper:
My experience about drying and rewetting changes in alkaline nutrient-rich chernozem columns, packed in columns, are these:
Permanent wetting fixed S, and slightly mobilized P, Fe and siderophiles (Mn, V, Cr) because of slow oxygen consumption, as well as slight shrinkage of volume. Subsequent drying and rewetting lead to a flush of sulphur and partially selenium, as well as fixation of P and Fe because of intermediate aeration from drying. Aggregate stability was not analyzed. Freshly precipitated Fe(OH)3 forms colloids and micro-aggregates.
Of course, some decisions have to be taken beforehand, which might influence the result - thus literature is contradictory at the first glance. Samples were air-dried and sieved before the experiment - like in my experiment also. But the organic amendments were air-dried before the incubation experiment also, which means loss of NH3 and H2S/H2Se during the drying stage as well as aeration, in favour of aerobic microbes. Again, after the incubation experiment, drying and aeration was done also.
Unfortunately, a manuscript rating is needed, to send this text; please do not consider it too much
Differences are interpretable due to different microbes present in the organic amendments, whereas polyacrylamide means practically zero. Gypsum is rather soluble, and favours bacteria of the sulfur cycle.
Changes in micro-aggregates might be interpretable in precipitation of pedogenic oxides resp. pyrite
I wonder, why no data about mobile chemical fractions, nor soil minerals or soil amorphous phases have been done.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC1 -
AC1: 'Reply on RC1', Bernhard Wehr, 07 Dec 2022
Reviewer 1
We thank the reviewer for the positive feedback.
We agree with the reviewer that wetting and drying cycles may cause anaerobic or anoxic conditions. However, in this experiment, the soils were kept wet at field capacity only for 24 hr and then left in open air for moisture loss (to initiate drying conditions) and the soils were packed to a height of 3-5 cm in jars with 12 cm width. Hence, we do not believe our experimental conditions have anoxia in the soil. Indeed, we have not measured a decrease in nitrate in our soil solution samples (which would happen if conditions were anoxic). We can include graphs if considered necessary to show this). The organic amendments except anionic polyacrylamide (PAM) were used in dried form to overcome differences in moisture content and particle sizes. While there may have been some losses of nutrients in the organic amendments due to drying, the nutrient content was measured on the dry material. Hence, we do not believe this would affect the interpretation of our results.
We also agree that different microbial communities may be added (or favoured) by adding organic amendments. However, the focus of our paper was not on different microbial populations but on aggregation (by whatever means the aggregates are formed, be it different microbes or iron oxides). While we measured the soluble chemical fractions (soil solution composition) in our samples (these data are reported in the supplementary section of our paper), we have not analysed if there are changes in the mineral phases during the trial since this was outside the scope of our experiment.
The focus of this experiment was to understand the effect of wetting and drying cycles on improving the structure of sodic soils under the application of different organic amendments. Therefore, to correlate the changes in aggregate stability and aggregation with microbial activity, daily microbial respiration was measured. In our earlier study (Niaz et al 2022) we measured Dissolved Organic Carbon (DOC) using the same soils and amendments under continuous wet conditions and found that DOC and soil microbial respiration were strongly positively correlated with each other, hence we are confident relating microbial respiration to DOC in this paper.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC1
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AC1: 'Reply on RC1', Bernhard Wehr, 07 Dec 2022
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RC2: 'Comment on egusphere-2022-469', Anonymous Referee #2, 06 Nov 2022
General comment
This work has some scientific value, but the findings and conclusions obtained still need to be reconsolidated. Although this study focused on the physical properties of the soil, the authors' experimental design used different organic amendments with different chemical properties. It is clear that the reasons for the differences in the experimental results cannot be explained by the causes and mechanisms of the results through these soil physical property analyses conducted by the authors alone. Soil organic matter is essential for the formation of soil agglomerates, and the authors did not characterize the changes in soil chemical properties but mentioned soil organic matter and soil chemical carbon pools several times in the discussion. In addition, the characterization of microbial activity under different amendment applications by microbial respiration alone is relatively one-sided and is not sufficiently convincing for characterizing the decomposition effect of microorganisms on different amendments. The authors use unpublished results for citations several times in the discussion section, and this issue should be noted.
The authors selected four organic amendments with gypsum and used two soils, which was a multifactorial experiment that led the authors to derive 10 treatments in this study. However, the authors' conclusions do not explain well the reasons why different amendments produce different amendments in different soils. The authors also did not present the research hypothesis well. A true research hypothesis is proposed based on the purpose of the study, in conjunction with their own research design. At this point in the process, I don't think the manuscript is ready to be published. The authors should perform a high level of integration and analysis of the available data as well as the data in the supplementary material. If possible, please add the necessary soil chemical characterization data.
Specific comments
(line: comment)
- Organic matter in the title is easy to make the reader misunderstand and should be changed to organic amendments or organic material.
- Line 28: Please add the definition of PAM.
- Why did the author choose these four organic materials? What are their advantages compared with other materials? The author needs to explain this in the introduction.
- Line 111: Why did the author choose to collect only 10 cm of soil, which is not fully representative either from the perspective of soil improvement or agricultural cultivation?
- Lines 116–117: According to previous experience, the sieve with a 10-mm void does not completely remove stone and plant litter.
- The materials and methods do not mention the size of the containers in which the authors performed the incubation experiments.
- The author mentions the significance of the difference in the main text, but no significant difference can be directly observed in the chart. Authors are advised to add error bars.
- The discussion section needs to be dissected in more depth, especially 4.3.
- Lines 437–440: The author needs to list more references to prove the point.
- Lines 480–491: Conclusions should not replicate the findings but be more concise.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC2 -
AC2: 'Reply on RC2', Bernhard Wehr, 07 Dec 2022
Reviewer 2
We appreciate the time and effort the reviewer spent on our paper and for the constructive criticism. We trust that our revisions are satisfactory.
The characterization of microbial activity was only based on microbial respiration since we analysed the changes in dissolved organic carbon using the same soils and same organic amendments under continuous wet conditions in our earlier paper which is now published (Niaz et al 2022). The results showed that dissolved organic carbon is strongly positively correlated with microbial respiration. Likewise, the chemical characterisation of the organic amendments used in this study is published in Niaz et al., 2022 (https://doi.org/10.1016/j.geoderma.2022.116047). We agree with the reviewer that a detailed study of the microbial population can be performed, but this was outside the scope of this paper.
- We have clarified the hypothesis as follows: “We hypothesised that i) organic amendments will increase the microbial respiration and improves the formation of large macroaggregates and MWD, (ii) gypsum will improve aggregate stability due to increases Ca concentration and ionic strength, iii) organic amendments act synergistically with gypsum on aggregation, and iv) repeated WD cycles will increase the process of aggregate formation and stability”.
- We have revised the results section as suggested by this and other reviewers.
Line comments
- Organic matter in title has been replaced with organic amendments. The title has been updated as “Wetting and drying cycles, organic amendments and gypsum play a key role in structure formation and stability of sodic Vertisols”
- The full form of PAM has now been incorporated as “In contrast, dispersion was significantly reduced when soils were treated with chicken manure, whilst anionic polyacrylamide only had a transient effect on aggregate stability”.
- The four organic amendments were chosen because of two reasons: 1) they were easily available and are being used by farmers, 2) LP is used as green manure and studies have shown it is effective in ameliorating sodic soils, and 3) PAM is used in mining and construction to treat sodic dispersive soils. Furthermore, these amendments were different in terms of their chemical properties (Table 2) and C functional groups (Niaz et al., 2022) and may give a good contrast between the amendments.
- The 0-10 cm soil layer was chosen because topsoil has the greatest OM content and microbial activity. In any case, Vertisols are relatively uniform in texture and structure throughout the soil profile; using subsoil would not add much information to the study.
- The soil was sieved to 10 mm as we wanted minimise physical disturbance of the natural soil structure. The soil contained no stones or coarse fragments, and the visible plant litter was manually removed.
- The experiment was performed jars with 12 cm diameter and 15 cm height, and the soil was packed to a height of 3-5 cm (depending on the bulkiness of the organic amendments).
- As there were 10 treatments, individual error bars or asterisks would have cluttered the graphs. Therefore, we preferred to insert the Tukey HSD bar in graphs.
- The discussion has been revised as follows “the results of this experiment showed that addition of gypsum (Ca) significantly reduced soil dispersion and increased aggregate stability. This improvement in aggregate stability is because of the increased EC (ionic strength, Fig. S4) and decreased SAR (Fig. S5) after the addition of gypsum. The increased EC likely resulted in the flocculation of soil particles by reducing the diffuse double layer (van Olphen 1977, Ghosh et al., 2010, Bennett et al. 2015). Improved stability was also observed when organic amendments were applied with gypsum especially in G+PAM, G+LP, and G+FLM treated soils, which when applied alone were not able to improve aggregate stability. The PAM had an initial positive effect but led to decreased stability at completion of the second WD cycle. Although, the addition of gypsum increased aggregate stability, it was observed that addition of gypsum did not affect the proportion of large macroaggregates and MWD. However, when organic amendments were added with gypsum an improvement in proportion of large macroaggregates and MWD was observed in G+LP treated soils. This can be explained as Ca-bridging effect through which clay particles are attached to organic matter and polyvalent cations resulting in the formation of macro and micro aggregates (Wuddivira and Camps‐Roach 2007).”
- More references highlighted in blue italic colour have been included as “Although the maximum respiration rate was observed during the first WD cycle for LP and G+LP, the proportion of large macroaggregates and MWD increased at the end of fourth WD cycle. However, the proportion of large macroaggregates did not increase much as compared to the first WD cycle, likely because microbial activity was lower (Cosentino et al. 2006; Zhang et al. 2022). One of the possible reasons of increased MWD could be the accumulation of microbial binding agents over time that released continuously from microbial activity due to organic matter decomposition (Rahman et al. 2018).
- The conclusions are revised as “The stability of dispersive sodic Vertisols was improved by the application of organic amendments and gypsum, which was further enhanced by alternate WD cycles. Gypsum reduced soil dispersion but did not affect the proportion of large macroaggregates and MWD. We observed that not all organic amendments were equally beneficial in improving soil aggregation and aggregate stability. LP significantly increased the proportion of large macroaggregates compared to FLM and PAM. In contrast, CM significantly reduced soil dispersion as it had higher calcium content. It was also found that PAM only had a transient effect in controlling dispersion. In the absence of organic amendments, repeated WD cycles reduced the dispersion of sodic soils, but when organic amendments were added (with or without gypsum) soil aggregation and soil stability was improved even more. It is likely that soil microbial activity contributed to the aggregate formation. Implementation of these findings in the field would favour the use of organic amendments with gypsum to improve the physicochemical properties of sodic soils, which is further enhanced by WD cycles. The aim should be initially to prevent soil dispersion which can be achieved by the application of Ca (through application of gypsum) and then to build larger aggregates which can be achieved by the application of organic amendments”.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC2
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RC3: 'Comment on egusphere-2022-469', Anonymous Referee #3, 08 Nov 2022
Review for article "wetting and drying cycles, organic matter and gypsum play a key role in structure formation and stability of sodic Vertisols", by Niaz et al.
Dear Authors,
first of all, I think you did a nice job. The article topic fits with Soil aim and scope, and furthering the understanding of aggregate formations is paramount if we want to protect soil health and increase soil organic carbon stock. The manuscript is written in good English and it is well structured.
I have some concerns, however, about the way the study is presented: the study tries to link physical structure of the soil (aggregates) with many different factors (chemical and biological) all together. This is not wrong in itself: the soil is a complex system, and complex system cannot be fully comprehended using reductionism. However, when studying the correlations between macro-variables of complex systems, it is important to understand that we are only finding correlations and not causal links. As such, I think that re-shaping the manuscript focusing on the correlations analysed (aggregate size - WD cycle; aggregated size - amendment; Aggregates stability indices compared with respect to WD; WD - respiration) and the discussion should focus more on putting the correlations found "in context" with respect to other vertisols in real conditions.
What I suggest the most is to be laser focused in the manuscript: write down clearly all your hypotheses in the "objective" paragraph of the Introduction (at the moment there are only WD-respiration; WD - aggregate stability; number of WD cycles to improve aggregate stability). Then get back in the introduction and help us understand exactely the relationship between soil respiration and soil aggregate formation, size and stability as found in literature (which is something it is missing right now). By the way, I really felt that the manuscript was missing the link between respiration and aggregates formation/stability.
In the Materials and Methods please be a bit more rigorous: define clearly what you mean by "dispersive" (line 114); explain why the two soils, albeit very similar, have different physical behaviour (line 113); clearly define the terms in equation 1 in the text (at the moment, they are a bit confusing); clearly write down the meaning and value of 'n' for equation 2 (number of fractions = 4, but it should be written down).
Please add error bars in the figures, or at least put some error indication in supplementary materials.
The discussion should be more focused on what we see in the results; at the moment there is much speculation about possible causes - which is ok, but should not be the whole discussion. First, link the results, focus our interest on the relevant data, explain what is in the figures further, discuss limitations. Then you can go with speculation. By the way, I did not understand section 4.1: the mechanisms of microbial action are not explained in detail and are not related to existing studies, and I think the discussion says the opposite of what we see in the data! E.g. line 394 "macroaggregates break more easily during WD cycles compared to microagregates" - figure 1 shows a significant increase in macroaggregates for all treatments, all soils, between WD1 and WD4 (or am I wrong?). line 402 "the proportion of small macroaggregates did not increase after the second WD cycle" - again, this is the opposite of what I see in figure 1. The problem is that these sentences are the core of section 4.1! Please help me understand.
Conclusion should clearly answer the scientific questions posed in the Introduction: if you have 3 objective, give 3 clear conclusions.
Finally, and very importantly, the literature cited is "old": the most recent article is from 2018, and the number of post-2015 articles is only 3. This is a major issue, since our conception of soil mechanisms has definitely changes as marked by Lehman 2015 "the contentious nature of soil organic matter" - and organic matter is related with microbial activity and aggregates formation and stabilization. I also suggest the reading of articles that analyse soil microbial respiration with WD cycles (Brangari 2020-2021, the latest works by Lindsay Todman, etc...). Finally, if you base some of your discussion on unpublished data, please put some of them in the supplementary materials, at least as "summary data", or, even better, include them in the article, if possible.Small corrections:
line 41 "wind, condensation, (Utomo and Dexter 1982) and evaporation"
line 49 "indirect effects on plants ecology and soil microbial..."
line 55 "soil microbial activity, since the latter influences..."
line 82 "organic matter affects aggregate stability"
line 245 "significant change in"
line 255 "after each WD cycle."
Figure 2: insert legendCitation: https://doi.org/10.5194/egusphere-2022-469-RC3 -
AC3: 'Reply on RC3', Bernhard Wehr, 07 Dec 2022
Reviewer 3
We are thankful to the reviewer for the critical and positive feedback.
- We agree that a correlation does not prove the existence of a mechanism or cause. We represented the correlations among different soil physical, chemical, and microbial characteristics as PCA biplots after each WD cycle to show how soil properties change with repeated WD cycles. We have reworded the main text making it clearer that we refer to correlations only.
- We have added more references to place our findings in a broader context and relating our results to other work done with Vertisols (Ghosh et al., 2010, Rahman et al., 2017, Rahman et al., 2018, Bennett et al., 2015, Nachimuthu et al., 2022).
- The hypotheses have been updated as shown for reviewer 2.
- The introduction have been revised by adding information about soil microbial respiration and aggregate formation as “Although, several researchers have reported that the addition of organic materials causes a rapid stimulation of microflora, which increases soil microbial respiration, in response to which extracellular polysaccharides are produced that helps in the formation of soil aggregates (Bossuyt et al., 2001), but the studies investigating the effect of organic amendments in improving the soil structure are inconclusive. For instance, the extracellular polysaccharides and large polyanions can bind clay particles into stable macroaggregates. On the other hand, organic anions can enhance dispersion by increasing the negative charge on clay particles and by complexing calcium with other polyvalent cations (such as those of aluminium), hence reducing their activity in soil solution (Ghosh et al. 2010)”.
- We have revised sections 3.1 and 4.1 to make our statements clearer. For example, the reviewer has highlighted a sentence in line 402 “the proportion of small macroaggregates did not increase after the second WD cycle”, which was changed to “at the completion of the second WD cycle”. (Samples were collected for aggregate size analysis after completion of either 1, 2 or 4 WD cycles). In our revision we will explain the data in more detail and then speculate about the reasons – for example we have included a Figure reference in the text: “The WD cycles result in rearrangement of pores and soil particles and may lead to increased rigidity and stability of soil aggregates (Horn et al. 2014). We observed a marked change in aggregate size distribution with repeated WD cycles (Fig. 1), from macroaggregates (large macroaggregates and small macroaggregates) at completion of the first WD cycle, to microaggregates at completion of the second WD cycle, and back to macroaggregates (large macroaggregates and small macroaggregates) at the fourth WD cycle. We suggest that extracellular polysaccharides formed by microbial activity (soil microbial respiration, Fig. 3) are responsible for the formation of large macroaggregates at the first WD cycle. At the second WD cycle, the microbial activity greatly decreased (Fig. 3) and macroaggregates (large macroaggregates and small macroaggregates) were broken down into microaggregates and silt+clay”
- The reviewer showed concern about the statement we made about the breakdown of large macroaggregates compared to microaggregates during WD cycles. We will rephrase that sentence as “macroaggregates are more susceptible to disintegration during wet sieving compared to microaggregates at the completion of second WD cycle. By the fourth WD cycle, some rearrangements of soil particles likely occurred, facilitated by soil drying, thereby rebuilding macroaggregates.” We could also insert a schematic to illustrate the process.
- The conclusions have been revised as shown for reviewer 2.
- The new citations have now been included at various points in manuscript some of which are Brangari et al., 2022, Freser et al., 2016, Zhang et al., 2022.
- The previously unpublished data has now been published and available online as Niaz et al., 2022. (https://doi.org/10.1016/j.geoderma.2022.116047)
- All the small corrections have been incorporated in the manuscript and the legend to Fig 2 has been inserted.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC3
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AC3: 'Reply on RC3', Bernhard Wehr, 07 Dec 2022
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RC4: 'Comment on egusphere-2022-469', Anonymous Referee #4, 08 Nov 2022
This topic is very interesting, and the authors did a lot work about it. However, in my opinion, the manuscript needs a lot revision before acceptance.
First of all, during the Introduction section, some brief information should be given about organic amendments and gypsum effects on soil structure or aggregates. The aims of the study should be rewritten, since the WD was not the only factor we concerned. Secondly, there were multifactor we need to addressed in the study, including gypsum addition or not, four organic amendments, and different runs of wetting drying. Suggest authors use two-way or three-way analysis of variance to give concise results. Please recheck the dose of PAM addition in the text and Table 3. Check some grams, for instance “Na, Ca, Mg, K” in L189, which should be in ionic forms. Also, “cahnge” in L245. In Discussion section, it was badly manner to explain the results, which made me lost. The authors should write in a clear way to explain the aim they concerned. Suggest substantial revision on the section.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC4 -
AC4: 'Reply on RC4', Bernhard Wehr, 07 Dec 2022
Reviewer 4
We thank the reviewer for their time and constructive feedback which helped us to improve our paper.
- The introduction is revised as already suggested by other reviewers. In addition, the reviewer asked to add a brief information about how gypsum and organic amendments can affect aggregation and aggregate stability which has now been incorporated in the main text. An overall brief explanation about sodic soils and their poor structure is given in the introduction from lines 52-69. The effect of organic amendments or organic matter on aggregate stability was briefly described in introduction in lines 84-86. However, the role of gypsum was not explained which has now been added in the introduction as “Traditionally the management practices used to improve the structure of sodic soils involves the displacement of Na ions from the soil exchange complex with the help of divalent cations such as Ca or increasing the ionic strength of soil solution (Ghosh et al. 2010), both of which can be achieved by the application of gypsum to these soils. The effect of gypsum for increasing ionic strength is immediate, but short lived. In contrast, the effect of gypsum for providing the counter ion (Ca to replace Na) is permanent unless additional Na is added to system (e.g., using poor quality irrigation water)”.
- The aims have been rewritten as “Thus, in the present study we used two sodic Vertisols, with the aim to: i) determine the role of gypsum and different organic amendments on aggregate formation and stability, ii) explore the combined effect of gypsum and organic amendments on soil physico-chemical and microbial properties, iii) investigate the effect of WD cycles on microbial respiration, iv) assess the effects of WD cycles on aggregate formation and stability, and v) determine how many WD cycles are needed to improve aggregate stability.
- The hypothesis has been rewritten as shown for reviewer 2.
- We have use two-way ANOVA to analyse the results which makes the description of results rather complex and repetitive. That’s why the discussion was written as the relative role of each factor (WD cycles and organic amendments or gypsum). The interaction of these factors was clearly demonstrated by presenting PCA biplots after first, second and fourth WD cycle in Fig. 5 and section 3.4 (lines 341-356). The two-way ANOVA table can be included in the supplementary data.
- The dose of PAM in Table 3 has been corrected.
- The discussions have been revised as suggested by other reviewers (see reviewer 2).
Citation: https://doi.org/10.5194/egusphere-2022-469-AC4
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AC4: 'Reply on RC4', Bernhard Wehr, 07 Dec 2022
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RC5: 'Comment on egusphere-2022-469', Anonymous Referee #5, 08 Nov 2022
Egusphere-2022-469 is an interesting work aiming to assess the impact of the combined effect of WD cycles and organic amendment and gypsum in soil stability and aggregation. The manuscript is well structured, and the objectives are identified clearly. However, the manuscript should be improved before acceptance mainly how the authors approach / addresses the research question.
Authors concluded that the results demonstrated that WD cycles can improve aggregate stability after addition of amendments – It is unclear to me if there is an overlap of the effect of the amendments over WD influence on soil stability. I would suggest review and reformulate the statements in which authors only include WD as main responsible of the increase of soil aggregation since after several WD cycles + amendment it is difficult to separate the concomitant impact of both at the same time.
I suggest to include in the introduction any explain concerning the selection of sodic Vertisols, apart from the lack of such studies in Vertisols (are these soils predominant in the region? Most of them dedicated to agriculture? Please link the information in lines 67-69 with an extra explanation about Vertisols and its characteristics. Please include a paragraph to close the introduction about the value / contribution of this study see lines 485 - 487
Did the authors carry out preincubation? Line 218 did the authors checked the effect of rapid slaking in the study soils?
Conclusions should be rewritten in the present form are a repetition of the results, please include a synthesis of key points.
Minor corrections
Line 113 physical properties instead of behaviour
Line 119 water suspension
Line 176 include the formula or unfamiliarized readers
Citation: https://doi.org/10.5194/egusphere-2022-469-RC5 -
AC5: 'Reply on RC5', Bernhard Wehr, 07 Dec 2022
Reviewer 5
We thank the reviewer for the positive feedback and hope our changes are satisfactory.
- The aims and research hypothesis have been revised as recommended by other reviewers.
- The sentences are reformulated to make it clear for readers to understand the changes driven by different amendments or WD cycles.
- The information regarding Vertisols has been incorporated in introduction as “Apart from the changes in soil structure due to the addition of different ameliorants, WD cycles can lead to more intensive changes in structure of soils dominated with smectitic clays (Vertisols). The WD cycles can directly affect aggregation of these soils through physical processes (Utomo and Dexter 1982; Denef et al. 2001). These soils are generally characterised as self-mulching soils as they exhibit shrink-swell properties imposed by the WD cycles (Pal et al. 2012). Vertisols cover a total of an estimated 340 million ha in the world (Australia, Asia, Africa, and America), out of which approximately 150 million ha is potential crop land. However, the physical properties and moisture regime of Vertisols represents serious management constraints (Pal et al. 2012). Sodic Vertisols are common in arid parts of the world. The effect of sodicity on the physical properties of Vertisols is still a subject of debate”
- We are not sure we correctly understand the question about preincubation; soils were not pre-incubated in the study, but they were collected in the field and would be pre-incubated there. We are not sure why pre-incubation would be useful – in the field, treatments are applied without pre-incubation. Our apologies if we misunderstood the question.
- The effect of rapid slaking was not checked as the soils were subjected to end over end shaking for the measurement of easily dispersible silt+clay.
- The conclusions have been revised as suggested by other reviewers.
Minor corrections
- Line 113 revised as suggested by reviewer
- Line119 corrected as suggested
- The formula used for gypsum requirement has now been incorporated in the main text from line 175 -176 “The gypsum requirement of both soil samples was calculated based on the formula given by Oster and Jayawardane (1998) as follows:
Gypsum requirement (GR)= 0.00086 x F x D x ∂b x (CEC) x (ESPi-ESPf)
Where F is exchanged efficiency of Ca-Na and for this case considered equal to 1.
Ds is the depth of soil to be reclaimed (cm)
∂b is soil bulk density (g/cm3)
CEC is cation exchange capacity (cmol+/Kg)
ESPi is initial soil exchangeable sodium percentage
ESPf is final or desired exchangeable sodium percentage.
Hence for soil 1 GR= 0.00086x1x10x1.29 (23) (14.9-6) = 2.27 Mg/ha
And for soil 2 GR= 0.00086x1x10x 1.29 (24) (15.9-6) = 2.52 Mg/ha
As an approximation, a single rate of application of gypsum, 2.5 Mg/ha was selected for both soils.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC5
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AC5: 'Reply on RC5', Bernhard Wehr, 07 Dec 2022
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RC6: 'Comment on egusphere-2022-469', Anonymous Referee #6, 09 Nov 2022
The manuscript "Wetting and drying cycles, organic matter and gypsum play a key role in structure formation and stability of sodic Vertisols" presents, as tittle says, the results of a study to evaluate the role of biotic and abiotic factors on aggregate formation and stabilization of sodic soils after the addition of gypsum and organic amendments (feedlot manure, chicken manure, lucerne pallets, and anionic poly acrylamide). Soils were amended and after 4 wetting and drying cycles (WD) different physical, chemical, and biological characteristics were measured and statistically analysed. According to results, WD cycles can improve aggregate stability of studied soils after the addition of amendments, but this process depends on the type of organic amendment. The addition of gypsum together with the amendments, further enhance the stability of aggregates, and dispersion became negligible after the second WD cycle. Authors concluded that WD is a key factor improving aggregation and stability of sodic Vertisols.
The text denotes a considerable amount of field and laboratory work. In general, the manuscript is organised and clear. It proves an effort to measure and relate a number of soil characteristics to study aggregation in sodic Vertisols. English grammar and spelling are accurate. There are some old references but they can be considered as cornerstone to support the introduction and discussion sections. Figures and tables are of good quality and necessary. Manuscript needs a MINOR revision before being accepted for publication. It needs to consider the following remarks.
General comments:
- Please explain abbreviations before using them. As PAM in line 28.
- It is important to define what you call large and small macroaggregates in the abstract and introduction (one needs to read until line 225 to find it out).
- Hypotheses are always very welcome. Thanks for doing so.
- Tables 1 and 2 are OK but probably they can be presented in the results section.
- It is not straightforward to see significant differences in the figures (vertical bars are not helpful). Why don't you simply mark where the differences are significant?
- Pay attention to the use of unpublished references to support your discussion. This can create some incertitude in your assumptions.
Specific remarks:
- Why did you choose to analyse a soil depth of 10 cm? In addition, did you sample one core from 0 to 10 cm or there were several cores depending on the size of the ring?
- Can you please include the formula given by Oster and Jayawardane (1998)? I assume you did use to calculate the 2.5 Mg ha-1 of gypsum added to soils.
- Amend “change”.
- Are these changes significant?
- In Fig. 1, explain what are LMA, SMA, MIC, and MIN in the caption.
Citation: https://doi.org/10.5194/egusphere-2022-469-RC6 -
AC6: 'Reply on RC6', Bernhard Wehr, 07 Dec 2022
Reviewer 6
We are highly appreciative of the feedback given by the reviewer.
- The full form of PAM has now been incorporated in main text in line 28.
- Abbreviations have now been spelled out in the Introduction and Materials and methods
- The hypotheses have been revised as recommended by other reviewers.
- We presented Table 1 and 2 after the description of soils and organic amendments in the Materials and methods section, but we don’t mind to move the tables to the Results section is the editor recommends.
- As explained for reviewer 2, we prefer to use the Tukey’s HSD bar to avoid cluttering the graphs.
- The unpublished data at that time was under review and now has been published as Niaz et al., 2022 (https://doi.org/10.1016/j.geoderma.2022.116047).
Specific comments
- As explained for reviewer 2, topsoil layer has high OM and microbial activity and Vertisols are relatively uniform in texture and structure to depth
- Three core samples were collected for the measurement of bulk density for each site (Soil1 and Soil 2) while the bulk soil samples were collected with a shovel to 10 cm depth.
- The formula used for gypsum requirement has now been added in materials and methods section as advised by reviewer 5.
- Our apologies but we do not understand what is meant by “Amend “change” ”
- The significance for each parameter is indicated in the text and figures by using the p (<0.05) value and Tukey’s HSD bar in the figures. The error bars and letters to show significance were purposely removed as the line graphs got clumsier and were difficult to distinguish.
- In Fig. 1 the legends LMA, SMA, MIC and MIN have now been clearly explained in figure caption. The four different aggregate classes have been abbreviated as LMA, SMA, MIC and MIN which have been updated as “Large macro aggregates, small macroaggregates, microaggregates and silt+clay”.
Citation: https://doi.org/10.5194/egusphere-2022-469-AC6
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Cited
Sara Niaz
J. Bernhard Wehr
Ram C. Dalal
Peter M. Kopittke
Neal W. Menzies
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