Assessing the Effects of Digestates and Combinations of Digestates and Fertilizer on Yield and Nutrient Use of Brassica juncea (Kai Choy)

Round 1

Reviewer 1 Report

Review of Assessing the Effects of Digestates and Combinations of Digestates and Fertilizer on Yield and Nutrient Use of Brassica juncea (Kai Choy) by Jamison et al.

This type of work is needed. Whilst it may not be ground-breaking in terms of science, it is the kind of fundamental work that has real applied value and relevance. The growth of the AD sector worldwide will mean that understanding how to make optimal use of the resulting greater volumes of digestates generated is increasingly important. Whilst there have been a number of trials looking at the agronomic value of digestates when used as a replacement (partial or full) to inorganic fertilisers, much more such work is needed so that a greater diversity of crop and digestate types are explored. We will then be able to maximise the economic and environmental value of digestate, going forward. One of the more novel and valuable elements of this study is that possible negative impacts are also investigated. Throughout this though, one has to remember that this was a pot trial, as opposed to a field trial, but this should be a major hindrance if the trial was executed well.

The manuscript is well-written on the whole. It requires some modification  – specific comments are made below.

Abstract: well-written, informative and clear.

Introduction: again, clearly written, and provides the appropriate background and context to the study. In the last paragraph, where the objectives are stated, then the plant species studied should be stated.

Materials and Methods:

• 1: some additional detail is needed on the feedstock fed into the digester – what was the nature of the food waste added, and so on?
• Table 1:
  • Ensure consistency of expression of units – ‘per’ is expressed as ^-1 in almost all cases, but for Electrical Conductivity, it is expressed as /cm. Please check this (and for other such cases throughout the paper, e.g., Section 2.2)
  • I would prefer a space between the number, the ± and the SD number, but that decision is up to the journal team
  • The kg for the Manganese units has been formatted incorrectly as a superscript
  • It is a shame that the numbers are based on duplicates as opposed to triplicates (or more), though some values are n = 5. The reason / justification for this should be provided later
• 2: the number of replicates for the plant treatments must be provided – this is essential information
• 3:
  • Line 135 clarifies that the percentage of N derived from the digestate is based on the digestates’ ammonium N content as opposed to any other form of N. This may be an area to reflect on again later in the paper, though the explanation and justification here is clear
  • How were digestates (and indeed the fertiliser) applied, and at what point of the plant growth stage?
• 4: state at what point of their growth stage plants were when harvested – I know it was 40 days post-germination, but how close to full maturity would this be? This has implications for nutrient demand and NUE, which needs to be reflected later on in the paper (if it doesn’t already). I am not familiar with the crop that was grown, and neither will other readers be, so this kind of information will be needed (probably in the Discussion). Also on this note, was root growth compromised in a pot of this size?

Results

• Figures 1 and 3-6: the legend needs to state the number of replicates

Discussion

• This is well-written, but as well as the caveats mentioned above, I would also ask the authors for a more cautionary approach in some aspects, such as acknowledging that this was one pot trial, using a limited number of crop and digestate treatments, therefore the results should not be extrapolated too broadly – more that they re-affirm what others have shown. Similarly so for the point made about fecal coliforms – their presence would obviously be highly dependent on the feedstock (e.g. sewage sludge / biosolid – fed AD systems would be very different) and also the parameters / management of the AD process itself (though this latter point is discussed).

Author Response

Response to Reviewer #1:

We very much appreciate the constructive comments from reviewer #1. Please see the response below, which we feel have improved the manuscript.

Introduction

1. Plant species name inserted in the last paragraph as suggested by the reviewer (line 20).

Materials and Methods

1. Thank you for the suggestion – we have inserted more details on collection and composition of food waste feedstock p. 3 (lines 145-150): “The food waste used in the FWD was collected over a one-week period to account for the effects of slight menu changes and frozen after each collection. The final mixture consisted of approximately 40-50% starch (rice and bread), 20-30% meat, and 20-30% fruits and vegetables. The food waste was blended until homogenized, and a portion of this composite sample was used as the feedstock for anaerobic digestion to produce FWD.”
2. We have added select chemical properties of the lignocellulosic biomass and food waste feedstocks to Table 1.
3. Table 1
   1. Units in Table 1 are now consistent.
   2. kg now correctly formatted.
   3. We agree that duplicate analyses for digestate materials was not ideal, but we had to send these samples out for analysis and did not have the ability to send triplicate samples. We have changed the table to include the actual duplicate measurements, as opposed to mean and standard deviation, since these measurements were taken in duplicate. EC and pH measurements were done in house, and thus we could do 5 replicate samples.
4. Clarified number of replicates (line 164-165), in the paragraph that starts with “Percentages recorded….”, we added “(5 replicates)” after “5 blocks” to indicate the number of replicates for the plant treatments.
5. We added information about how digestates and fertilizer were applied at the end of section 2.2 (lines 136-140): “Digestates and/or fertilizer were mixed to achieve the target treatment levels with water to ensure a consistent volume of liquid was added to each plant regardless of treatment. The first treatment solutions were added immediately after seeding. In order to simulate application by fertigation, treatments were subsequently applied every 10 days throughout the growing period for a total of 4 applications.”
6. At the end of section 2.2 (lines 141-144), we added clarification on when plants were harvested (“Mature plants were harvested 40 days after seeding, corresponding to a typical harvest timeline for this vegetable.”).
7. At the end of section 2.2 (lines 143-144), we added clarification on the root status at harvest (“At harvest, roots had occupied the full volume of the rhizoboxes in the high performing treatments, but they did not appear to be compromised by the rhizobox size”).

Results

1. We added n=5 to the captions (legends) of figures 1, 3, 4, 5 and 6.

Discussion

1. Added clarification that fecal coliform counts would be dependent on both feedstock and AD management at the end of section 4.3 (lines 449-454): “Our results suggest that digestates made from lignocellulosic biomass and food waste may not pose a significant biohazard threat when applied at appropriate rates for crop growth. However, it is important to test any digestates for potential biohazards prior to land application, as their presence would likely be dependent on both the feedstock used and the conditions under which digestion took place.
2. We appreciate the Reviewer’s suggestion to qualify the extent to which results could be extrapolated. We addressed this by adding clarification regarding the limitations of this study:
   4. At the beginning of section 4.4 (Lines 456-458): “While the results obtained in this greenhouse pot study support findings in the literature as to the usefulness of digestate, we recognize that results may not be directly extrapolated to field conditions. Nonetheless, anaerobic digestion itself is of great value to many developing countries for its ability to provide a renewable and inexpensive source of energy generated from a variety of organic wastes [41].”
   5. To further address the reviewer’s suggestion, we reworked the last paragraph of the discussion (lines 472-480) to read: “In this pot study, both food waste and lignocellulosic biomass proved to be good feedstocks for the production of methane (unpublished data) while also producing fertilizers that could provide up to half of the total nitrogen needs of Kai Choy plants. Our results provide further evidence that utilizing digestate as a biofertilizer has the potential to both reduce dependence on costly imported fertilizers and open up options for agricultural production in the face of nutrient scarcity. These benefits need to be validated at field scale. However, this adds to the growing body of research showing the value of both energy and fertilizer products from anaerobic digestion of various feedstocks.”
   6. Added qualifying tone in conclusion (line 492): “Overall, the results from this pot study suggest…”

Reviewer 2 Report

The purpose of this article was to evaluate the effects of two digestates, used individually or at different combination with a fertilizer, on growth and nutrient use efficiency in Brassica juncea seedlings. Table 1 shows some selected chemical characteristics, but the flaw is that the standard deviations are higher than the correspondent means of some nutrient for FWD. For this some data are not understandable, therefore it is difficult to compare the treatments with each other, vs mineral fertilizer and point out differences in effects.
In figure 3, the legend hides the results of the last days of treatment.
line 266 RE for phosphorus was significantly different due to treatment......but the P content in FWD was 280±255, how much is ? How is calculated P Are % in fig 5?
line 283 No significant differences between the two digestates on EC value and sodium content. Explain the differences on planting media due to the digestate treatments.
High salinity is a well known detrimental effect of digestate on soil and it causes an inhibition of plant growth. However, in addition to the toxicity induced by Na and Cl, growth inhibition can also be induced by a nutritional imbalance, due to situations of nutrient deficiency or toxic concentration in the digestate used.

Author Response

We appreciate the constructive feedback from reviewer #3. Please see our response to the comments below:

1. We thank the reviewer for the remark regarding large standard deviations for the food waste digestate. In response, we have modified Table 1 to include both measurements for each of the parameters that were measured in duplicate rather than reporting a mean and standard deviation. Due to a lack of digestate material, we were unfortunately unable to run additional analyses on the FWD that showed different results for the two duplicate samples (see revised Table 1).

2. Figure 3 was adjusted so that the legend no longer hides the curves.

3. To address the reviewer’s questions regarding ARE calculations, we have clarified how ARE was calculated for NPK (see modifications to section 2.6, lines 196-201): “The amount of nutrient applied was calculated using the mean of the duplicate samples for the LBD and using the first value reported for FWD in Table 1. For the FWD, the analytical results from the two samples showed large differences for certain nutrients, most notably phosphorus. To address this, we utilized the replicate with the higher values because it was most similar to digestates made from food waste streams in other studies (Tampio et al, 2015; Sanchez-Rodriguez et al, 2018; Ren et al, 2020).”
Rather than taking an average of such different numbers, we chose to utilize the replicate that was most similar to parameters measured in other studies using digestates made from similar feedstock (food waste), which corresponded with the first replicate reported in Table 1. For the LBD, we did use an average, since the two replicates were close in values.

4. In response to the reviewer’s comments regarding salinity, our primary purpose was to assess the effect of digestate addition to planting media on EC and whether high sodium in the digestate affected tissue sodium concentration. We have added clarification that media EC measurements were done at the end of the growing period (lines 301-302): “The media from the treatments receiving LBD alone had the highest EC values at the end of the growing period (4.460±0.576 mS cm-1), followed by LBD50 (3.170±0.320).”

5. With regards to toxicity, LBD100 was the only treatment that showed sodium concentrations above the threshold of toxicity (0.25-0.5%). Please see lines 308-310 for adjustment in the text. With regards to the reviewer’s comments regarding nutrient imbalances, we agree that this is an important concern with digestate use. Visual analysis of the tissue at harvest did not show signs of nutrient deficiency or signs of toxicity. The only observable effects of salinity were exhibited in poor growth associated with the LBD100 treatment.

Reviewer 3 Report

The proposed paper is very interesting and well structured. The topic covered is absolutely interesting and current, even if it follows many themes already known in the bibliography. The conclusions are consistent with the evidence and  arguments presented addressing the main question that are posed.

I suggest you to implement the work in particular method and results with the following papers that are dealing the effect of digestate as alternative substrata in soilless cultivation of Salvia officinalis:

Greco, C.; Comparetti, A.; Fascella, G.; Febo, P.; La Placa, G.; Saiano, F.; Mammano, M.M.; Orlando, S.; Laudicina, V.A. Effects of Vermicompost, Compost and Digestate as Commercial Alternative Peat-Based Substrates on Qualitative Parameters of Salvia officinalis. Agronomy 2021, 11, 98. https://doi.org/10.3390/agronomy11010098

Greco, C.; Comparetti, A.; Febo, P.; La Placa, G.; Mammano, M.M.; Orlando, S. Sustainable Valorisation of Biowaste for Soilless Cultivation of Salvia Officinalis in a Circular Bioeconomy. Agronomy 2020, 10, 1158. https://doi.org/10.3390/agronomy10081158

Carlo Greco, Alessandro Agnello, Giulia La Placa, Michele Massimo Mammano, & Kestutis Navickas (2019). Biowaste in a circular bioeconomy in Mediterranean area: A case study of compost and vermicompost as growing substrates alternative to peat. RIVISTA DI STUDI SULLA SOSTENIBILITÀ(2), 345-362.

Antonio Comparetti, Carlo Greco, Michele Massimo Mammano, Kestutis Navickas, Santo Orlando, & Kestutis Venslauskas (2019). Valorisation of urban green areas for producing renewable energy and biochar as growing substrate of Sicilian aromatic and nutraceutical species in a circular economy. RIVISTA DI STUDI SULLA SOSTENIBILITÀ(2), 299-314.

Greco, C., Comparetti, A., Mammano, M., & Orlando, S. (2019). Sustainable, circular and innovative value chains using growing substrata alternative to peat for the cultivation of nutraceutical species. In Edited by prof. Asta Raupelienė (a cura di), Proceedings of the International Scientific Conference: Rural Development Research and Innovation for Bioeconomy (pp. 13-20). Vytautas Magnus University, Agriculture Academy.

Author Response

Response to Reviewer #3:

We found that the first two publications suggested by the reviewer were relevant to our study and supported findings related to EC and Na+ in the Discussion (middle of section 4.1, lines 350-354): “A previous study investigating the use of the solid portion of digestates as an alternative to peat for greenhouse production found reduced plant growth in Salvia oficianalis grown in mixtures of digestate and peat as compared with peat alone (Greco et al, 2020). A more recent study found a high EC and high plant tissue content of Na+ under digestate treatments, similar to findings from this study with the use of liquid digestates (Greco et al, 2021).”

Round 2

Reviewer 2 Report

The manuscript "Assessing the Effects of Digestates and Combinations of Digestates and Fertilizer on Yield and Nutrient Use of Brassica juncea (Kai Choy)" after revision has been significantly improved.