Digestate Not Only Affects Nutrient Availability but Also Soil Quality Indicators
Jeff Schoenau
Round 1
Reviewer 1 Report
Very interesting and usuful work which may help to understand the importance of using digestate.
However the authors used very rich in nitrogen and OM (5.4 % !!!). This composition of soil is certainly a limiting factor to show the positive impacts of digestate on the soil and crop.
1. What is the main question addressed by the research?
It's How the addition of digestate May improve soil composition and yields crop. Very important issue.
2. Do you consider the topic original or relevant in the field? Does it
address a specific gap in the field?
Yes
3. What does it add to the subject area compared with other published material?
There is an add value
4. What specific improvements should the authors consider regarding the methodology? What further controls should be considered?
They can not change the methodology now.
But for further investigations they have to choose soil which poor in organic matter and nitrogen.
5. Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed?
Yes
6. Are the references appropriate?
Yes
7. Please include any additional comments on the tables and figures.
Author Response
The authors are grateful to the reviewer for the comments to our manuscript “Digestate does not only affect nutrient availability but also soil quality indicators”. These comments are valuable and helpful to improve our future works.
Reviewer comment:
….However, the authors used very rich in nitrogen and OM (5.4 % !!!). This composition of soil is certainly a limiting factor to show the positive impacts of digestate on the soil and crop.
Answer to comment: We fully agree that SOM is high, and according to our data, there is a significant N mineralization which means a significant supply of N to crops. In the studied crops, this mineralization accounts for most of crop needs, but in other crops with higher requirements of N, this means only a part of the N requirement. The effect on crops was studied in a previous publication, and we put the focus here on the effects on soil properties and in particular on biochemical properties related to nutrient cycling and microbial traits. High mineral N in soil reduces the assessment of the efficiency of N fertilizer in terms of crop recovery. However, we think that our results provide interesting information with a view of assessing the effects of the use of digestate in a representative soil for horticultural production, sometimes rich in nutrients as a consequence of traditional overfertilization.
The following phrase was added to the conclusions:
Future works with digestate application in soils with different properties will be needed for deeper understanding about its effects on soil functionality.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Review of Agronomy Digestate does not only affect nutrient availability but also soil quality indicators
General Comments:
An interesting paper that provides useful information on the impact of biodigestate application on soil properties. Some additional data: lettuce and kale crop data N and P uptake and removal, would aid in more rigorous interpretation of the effects on processes like mineralization, soil N and P balance, N and P loading. Paper could be condensed by selecting or or the other heatmap or PCA analysis to indicate controlling parameters, but not both. Also the NIR analyses seems like an add on that provides little further insight. Paper could use a go through on grammar and syntax.
Specific Comments:
Abstract:
Concise, give location of study.
Introduction:
Good background and justification
Materials and Methods:
L127: Any estimates of how much N would be supplied by mineralization of organic matter in this soil? It would likely be significant given the high content of organic matter, organic N. Some insight into this needs to be provided as it comes up later in results and discussion.
L138: gram negative
L154: determined
Results:
L202: May do rough estimate of mineralization from native soil organic matter if you have preplant soil available N, post-harvest soil available N and plant N uptake in control. Alternatively may make approximation based on a percentage of soil organic N that is mineralized.
Tables: Readability may be improved by highlighting the mean values such as using bolding or different font size, or else removal of the standard error (deviation?) values entirely and just report mean value followed by letter.
Discussion:
L 304-315: What about the production of nitrate from nitrification of ammonium in these soils? This seems to not be considered. Nitrate that is produced may be taken up by crop or lost by leaching, denitrification. This may explain lack of digestate effect on ammonium content? I wonder since 35 % of added N was inorganic N in digest added to lettuce and 20% of total N added to Kale, the authors may consider where this inorganic N added went: crop uptake and removal, lost? Is crop N uptake and removal data available?
L 320: Apparent N recovery is referred to. Please clarify if you mean the soil or plant or both. Since you do not appear to have plant N data, assume it is the soil.
L348: Much of added P remains in extractable available form (Olsen extractable). This is perhaps expected given the high levels of extractable P to begin with (>100) in this soil, which indicate that adsorption sites in soil may already be close to saturated.
L401: Toxic substances are referred to digestate, but is not clear what this may be. Potassium is mentioned, but am wondering about salinity. The EC at end of season is not affected but what about in the time period following application to harvest. Also wonder about ammonium/ammonia toxicity given the very high contents of ammonium in these soils and the appreciable amount of inorganic N (presumably ammonium) that is added in the biodigestate at the high rates.
Conclusion:
Some more specific recommendations regarding optimal rate, mineral N-digestate combination would add.
A few style, grammar errors that can be quite easily addressed.
Author Response
The authors are grateful to the reviewer for the comments to our manuscript “Digestate does not only affect nutrient availability but also soil quality indicators”. These comments are valuable and helpful to revise and improve our paper. We carefully read the comments and revised them, hoping to get everyone 's recognition. The manuscript was revised for language and grammar. The following are the main corrections in the paper and the responses to the reviewer. The modified parts in the text have been marked with track changes.
Reviewer 2
Reviewer comments
An interesting paper that provides useful information on the impact of biodigestate application on soil properties.
Answer to comment: we appreciate the positive comments by reviewer.
Abstract
Answer to comment:The location of the field experiment was added in. the abstract: “..This work aimed at assessing the fertilizing effect of digestate on chemical and biological soil properties in a field experiment at eastern Portugal with two horticultural crops involving nine treatments:…”
Some additional data: lettuce and kale crop data N and P uptake and removal, would aid in more rigorous interpretation of the effects on processes like mineralization, soil N and P balance, N and P loading.
Answer to comment:
We fully agree that these additional data is interesting. Nutrient uptake by crop was studied in a previous paper cited in this manuscript (Horta and Carneiro 2021). Crop extraction was consistent with soil data present in this manuscript, where however the focus was put on how digestate affects soil function, and how this functionality can be affected by particular traits of the product.
Paper could be condensed by selecting or the other heatmap or PCA analysis to indicate controlling parameters, but not both. Also, the NIR analyses seems like an add on that provides little further insight.
Answer to comment: We think that all the analysis mentioned are complementary; in particular, heat map and PCA does not provide redundant information since the PCA deals with spectral information which is affected by the effects of treatments on soil organic matter traits, while the heat map is a two-dimensional representation of soil data which provides a graphical description of main effects of treatments on studied variables.
Paper could use a go through on grammar and syntax.
Answer to comment: a revision in depth of grammar has been done to improve the manuscript
L127: Any estimates of how much N would be supplied by mineralization of organic matter in this soil? It would likely be significant given the high content of organic matter, organic N. Some insight into this needs to be provided as it comes up later in results and discussion.
Answer to comment: We agree with the comment and this has been explained in results and discussion. The high content of organic matter provides enough N to cover the needs of the crops, so the response of crop to fertilization with the digestates can be diminished, however, we can observe the behaviour and functionality of soil.
L138: gram negative
Answer to comment: accepted as suggested.
L154: determined
Answer to comment: accepted as suggested.
L202: May do rough estimate of mineralization from native soil organic matter if you have preplant soil available N, post-harvest soil available N and plant N uptake in control. Alternatively may make approximation based on a percentage of soil organic N that is mineralized.
Answer to comment: we agree with the need of some additional information about mineralization of OM should be considered, due to the high value of organic material present in the soil. Considering the N uptake, the inorganic N content in the soil before and after cultivation, we have estimated that OM contributes approximately at 85 kg N ha-1. This has been incorporated into the manuscript.
Tables: Readability may be improved by highlighting the mean values such as using bolding or different font size, or else removal of the standard error (deviation?) values entirely and just report mean value followed by letter.
Answer to comment: Tables has been improved according the comment.
L 304-315: What about the production of nitrate from nitrification of ammonium in these soils? This seems to not be considered. Nitrate that is produced may be taken up by crop or lost by leaching, denitrification. This may explain lack of digestate effect on ammonium content? I wonder since 35 % of added N was inorganic N in digest added to lettuce and 20% of total N added to Kale, the authors may consider where this inorganic N added went: crop uptake and removal, lost? Is crop N uptake and removal data available?
Answer to comment: There is not an effect of DG on ammonium in lettuce, but there was in Kale. Perhaps this means an accumulative effect as mentioned in the manuscript. However, there is not an effect on ammonium after lettuce that cannot be explained by an increased crop uptake according to Horta and Carneiro (2021) since apparent recoveries are lower with DG than with mineral fertilizer. This may be perhaps an effect of increased nitrification with DG since this product increases pH, and this may enhance nitrate loss through leaching. In a second crop, this effect should be less marked since initial pH values were higher.
L 320: Apparent N recovery is referred to. Please clarify if you mean the soil or plant or both. Since you do not appear to have plant N data, assume it is the soil.
Answer to comment: we agree in the need of further clarification. The recovery N is in plant referring to the work published by Horta and Carneiro 2021, we have clarified this in the manuscript
L348: Much of added P remains in extractable available form (Olsen extractable). This is perhaps expected given the high levels of extractable P to begin with (>100) in this soil, which indicate that adsorption sites in soil may already be close to saturated.
Answer to comment: we agree, with such a high available P content, there is a significant saturation of sorbent sites, thus contributing to explain the increased Olsen P values with DG supply. In addition, the joint application of P and organic matter reduce P sorption and precipitation reactions in soil (Delgado et al., 2002). Now it has been included in the discussion.
L401: Toxic substances are referred to digestate, but is not clear what this may be. Potassium is mentioned, but am wondering about salinity. The EC at end of season is not affected but what about in the time period following application to harvest. Also wonder about ammonium/ammonia toxicity given the very high contents of ammonium in these soils and the appreciable amount of inorganic N (presumably ammonium) that is added in the biodigestate at the high rates.
Answer to comment: the comment was included to explain the decrease in microbial biomass at high DG rates. We found that reference for potassium. However, it is known that volatile organic compounds can be derived from the application of DG, and this may have a negative effect on microorganisms. Now, we have included this comment supported by references in the discussion.
We fully agree that high ammonium content in the soil can be phytotoxic, also due to having an acidic pH, nitrification processes can be affected, accumulating high amounts of ammonium, however, no symptoms of toxicity in plants were shown. Regarding the EC, we do not believe that it could be a problem because the EC of the digestates were below 1.5 dS m-1, its salinity diluting when mixed with the soil considering that 3 dS m−1 is the threshold value for affecting crops.
Some more specific recommendations regarding optimal rate, mineral N-digestate combination would add.
Answer to comment: A recommendation has been added to conclusions.
Author Response File: Author Response.docx
