Ammonium-Based Compound Fertilisers Mitigate Nitrous Oxide Emissions in Temperate Grassland

Round 1

Reviewer 1 Report

The manuscript presents results of a randomized block trial to investigate the influence of different N fertilizers on N2O emission for a grassland site. The authors are known from previous publications, which are also partially cited here and accordingly show the expertise of the authors.

The publication is very well structured, shows all the necessary information for conducting the experiments, and the results are clearly presented and discussed. Comments and recommendations for corrections are given in the list below.

The authors list several older publications in the cited literature, which should be supplemented by more recent ones. Major criticisms are the short duration of the experiment, the lack of prior history of the experimental field, and also the very limited soil sampling (lack of sampling depth) and N-analyses. Especially the short duration of the experiment allows only very limited statements. In the outlook of their conclusions, the authors confirm my opinion.

 

 

Page	Line	Remarks for the authors
1	34-37	This statement is not made in the Billen et al (2010) publication, the word “climate” does not appear once in the entire publication; cite another publication for this statement or delete.
1	40-43	This statement is of course correct, however, the publication is already nine years old, particularly to such fundamental and above all perspective statements I expect also additionally a more current literature citation. Add a more recent publication to this statement.
2	46-48	Butterbach-Bahl does speak of synthetic fertilizers, but this term should only be used in a purely chemical context. In the agricultural context and also in the environmental science field, the term "mineral fertilizer" should be used. This is because it is not an artificial substance that would not occur in nature, but this is suggested by the attribute "synthetic".
2	49-50	Butterbach-Bahl et al also uses an unprecise quantification ("dominant") in their publication and you use "primarily", it would sharpen the statement significantly if you supported it with numbers.
2	50-53	This is a "very old" publication and there has certainly been a considerable increase in scientific knowledge in the last decades. You can also read this statement in the cited publication by Butterbach-Bahl et al. (2013!!!!). A more recent publication must be used here.
2	71-73	Here the hint/information about the anaerobic processes that then take place is missing.
2	82	“ration” Do you mean “ratio”?
3	114-115	Information on reseeding is given, but much more important and necessary is the plant composition in the actual experimental year; please add here a short list of the indicator grasses and herbs.
3	115-117	This not a complete description of the soil, these are minimalistic data
3/1	127/16	CAN is common abbreviation, but please introduce this again in the text, not only in the abstract; same for NBPT;
3	table 1	I would prefer "organic" instead of "morgan" as an expression. Since in grasslands, the uppermost 30 cm are rooted and thus affected by the plants, information on soil layers to a depth of 30 cm should be provided as a minimum.
	table 1-4	The table heading is identical to the table titles/first line of the tables, redundant statement!!!
3 f.	chapter 2.2	No information is given about the history of this grassland site, except for the reseeding. Here, however, it would be very important to provide information about the previous use, the previous fertilization and possible tillage, as this also has a considerable influence on the test results. This information must be completed. What do you mean by ammonium-/nitrate-based? I cannot follow this statement. Does this refer to fertilizer production or to the quantitative proportions of the various N-forms? But for the N-forms then the statements are not true.
3	132-133	How do you define “conducive conditions for N2O production”? Please add citations for this statement, because you have not studied/tested this effect for your site.
5	chapter 2.5	The authors give a good description of the N2O sampling and analysis. However, statements about the sampling interval are missing, please add.
6	chapter 3.1	On the one hand, it is critical to see that the additional sprinkling produced very moist test conditions of the soil compared to the long-term average. On the other hand, the figures of the long-term mean for the air and soil temperature as well as the comparison of the long-term mean with the test conditions are missing here. Please add in any case.
6/9	236/255/figure 3	What is the explanation and reason to name the first fertilization period as "low N2O condition" and the second fertilization period as "high N2O condition". The different amounts of N2O emitted may also depend on the varying initial conditions, here I would choose a non-judgmental designation.
7	figure 2	Don’t plot the lines, this improves the clarity of the figure
9	263-265	The statement/phrasing of this sentence is ambiguous, as two different references can be understood. Please formulate clearly!
10	297-298	Incorrect line break
11	figure 4	Values are given for dry soil, like written in M&M? Add the information to the figure title. It would be very nice if results were also presented for the soil layers down to a depth of 30 cm. This would give a complete picture.

This paper fits into MDPI Agronomy, with a recommendation for publication, taking into account the remarks above.

 

Author Response

We are really grateful for your time and constructive comments on our manuscript.

General comment: Reviewer #1

The manuscript presents results of a randomized block trial to investigate the influence of different N fertilizers on N2O emission for a grassland site. The authors are known from previous publications, which are also partially cited here and accordingly show the expertise of the authors.

The publication is very well structured, shows all the necessary information for conducting the experiments, and the results are clearly presented and discussed. Comments and recommendations for corrections are given in the list below.

The authors list several older publications in the cited literature, which should be supplemented by more recent ones. Major criticisms are the short duration of the experiment, the lack of prior history of the experimental field, and also the very limited soil sampling (lack of sampling depth) and N-analyses. Especially the short duration of the experiment allows only very limited statements. In the outlook of their conclusions, the authors confirm my opinion.

Response: We have addressed most of the comments forwarded by the reviewer and these are addressed in detail below. Older publications are replaced by recent ones and prior land-use and management history of the experimental site is included in the revised version of the manuscript. We kindly take the recommendation that deeper soil depths need to be sampled, and we will consider that in our future studies. Based on this proof of concept experiment, a multi-year and -site experiment has been envisaged to investigate a wider effect of these fertilisers in agricultural systems.

Specific responses to specific comments by Reviewer #1

Comment: P1, L34-37, This statement is not made in the Billen et al (2010) publication, the word “climate” does not appear once in the entire publication; cite another publication for this statement or delete.

Response: Billen et al (2010) [3] is replaced by Chen et al (2018).

Chen, J.; Lü, S.; Zhang, Z.; Zhao, X.; Li, X.; Ning, P.; Liu, M., Environmentally friendly fertilizers: A review of materials used and their effects on the environment. Science of The Total Environment 2018, 613-614, 829-839.

Comment: P1, L40-43, This statement is of course correct, however, the publication is already nine years old, particularly to such fundamental and above all perspective statements I expect also additionally a more current literature citation. Add a more recent publication to this statement.

Response: [5] is replaced by a more recent citation, Tian et al (2020).

Tian, H.; Xu, R.; Canadell, J. G.; Thompson, R. L.; Winiwarter, W.; Suntharalingam, P.; Davidson, E. A.; Ciais, P.; Jackson, R. B.; Janssens-Maenhout, G.; Prather, M. J.; Regnier, P.; Pan, N.; Pan, S.; Peters, G. P.; Shi, H.; Tubiello, F. N.; Zaehle, S.; Zhou, F.; Arneth, A.; Battaglia, G.; Berthet, S.; Bopp, L.; Bouwman, A. F.; Buitenhuis, E. T.; Chang, J.; Chipperfield, M. P.; Dangal, S. R. S.; Dlugokencky, E.; Elkins, J. W.; Eyre, B. D.; Fu, B.; Hall, B.; Ito, A.; Joos, F.; Krummel, P. B.; Landolfi, A.; Laruelle, G. G.; Lauerwald, R.; Li, W.; Lienert, S.; Maavara, T.; MacLeod, M.; Millet, D. B.; Olin, S.; Patra, P. K.; Prinn, R. G.; Raymond, P. A.; Ruiz, D. J.; van der Werf, G. R.; Vuichard, N.; Wang, J.; Weiss, R. F.; Wells, K. C.; Wilson, C.; Yang, J.; Yao, Y., A comprehensive quantification of global nitrous oxide sources and sinks. Nature 2020, 586 (7828), 248-256.

Comment: P2, L46-48, Butterbach-Bahl does speak of synthetic fertilizers, but this term should only be used in a purely chemical context. In the agricultural context and also in the environmental science field, the term "mineral fertilizer" should be used. This is because it is not an artificial substance that would not occur in nature, but this is suggested by the attribute "synthetic".

Response: We agree with this. Synthetic fertiliser is replaced by mineral fertiliser.

Comment: P2, L49-50, Butterbach-Bahl et al also uses an unprecise quantification ("dominant") in their publication and you use "primarily", it would sharpen the statement significantly if you supported it with numbers.

Response: This statement is now supported with numbers. Thus, the revised statement reads:

Nitrous oxide is produced in agriculture soils primarily through microbial processes of nitrification and denitrification, which contribute to 70% of the global emissions in managed and natural soils [9].

Comment: P2, L50-53, This is a "very old" publication and there has certainly been a considerable increase in scientific knowledge in the last decades. You can also read this statement in the cited publication by Butterbach-Bahl et al. (2013!!!!). A more recent publication must be used here.

Response: [10] is replaced by a recent study Wang et al (2021).

Wang, C.; Amon, B.; Schulz, K.; Mehdi, B., Factors That Influence Nitrous Oxide Emissions from Agricultural Soils as Well as Their Representation in Simulation Models: A Review. Agronomy 2021, 11 (4), 770.

 

Comment: P2, L71-73, Here the hint/information about the anaerobic processes that then take place is missing.

Response: The following statement is included to give more information on anaerobic process:

Several studies reported rapid and peak emissions of N₂O when the water-filled pore space (WFPS) of the soil is greater than 60%, anaerobic soil moisture condition whereby soil NO₃^- is reduced to NO₂^-, N₂O, and N₂ via facultative anaerobic bacteria [10,12,13].

Comment: P2, L80, “ration” Do you mean “ratio”?

Response: Ration is corrected to ratio.

Comment: P3, L114-115, Information on reseeding is given, but much more important and necessary is the plant composition in the actual experimental year; please add here a short list of the indicator grasses and herbs.

Response: Since reseeding in 2018, no other species than perennial ryegrass (Lolium perenne) has been identified. Negligible presence of weeds was observed in the site but not the main experimental plot.

Comment: P3, L115-117, This not a complete description of the soil, these are minimalistic data.

Response: That is right! We deleted the word “complete” from the sentence.

Comment: P3/1, L127/16, CAN is common abbreviation, but please introduce this again in the text, not only in the abstract; same for NBPT;

Response: The full forms of CAN and NBPT are now mentioned in their first appearance in the main text (Introduction).

CAN (Calcium ammonium nitrate)

NBPT (N-(n-butyl) thiophosphoric triamide)

Comment: P3, table 1, I would prefer "organic" instead of "morgan" as an expression.
Since in grasslands, the uppermost 30 cm are rooted and thus affected by the plants, information on soil layers to a depth of 30 cm should be provided as a minimum.

Response: The plant available nutrients were quantified using the morgans extraction procedure and are not organic. They represent the agronomically available nutrients as determined using this standard agronomic test. The wording has been changed to available P, available mg and available K. A footnote citing the method has also been included under the table. Morgan MF 1941. Chemical soil diagnosis by the Universal Soil Testing System. Connecticut Agricultural Experimental Station Bulletin 450 Connecticut.

Comment: Table 1-4, The table heading is identical to the table titles/first line of the tables, redundant statement!!!

Response: We have corrected the double headings in the tables presented (Table 1-4). We think that the errors in the double headings inside and outside of the table are made during formatting in the agronomy journal. Otherwise, the tables in the original manuscript we submitted contain only one heading per table written outside the table.

Comment: Chapter 2.2, No information is given about the history of this grassland site, except for the reseeding. Here, however, it would be very important to provide information about the previous use, the previous fertilization and possible tillage, as this also has a considerable influence on the test results. This information must be completed.

What do you mean by ammonium-/nitrate-based? I cannot follow this statement. Does this refer to fertilizer production or to the quantitative proportions of the various N-forms? But for the N-forms then the statements are not true.

Response: The following additional information is included in the site description section of the manuscript:

“The site is a permanent grassland site which was last reseeded in autumn 2018 with perennial ryegrass with no clover at a seeding rate of 38 kg ha^-1. After reseeding, phosphorous (16% P Superphosphate), potassium (50% K Muriate of Potash), and nitrogen fertilisers (27% N calcium ammonium nitrate) were applied at rates of 10, 40, and 40 kg ha^-1, respectively. Weeds were controlled through herbicide application in December 2018, and as a result, negligible weeds presences were detected. For the years before 2018, the plot was permanent grassland (perennial ryegrass) with grazing started in 2013 with dairy replacement stock. The last grazing occurred in September 2019, grazed by spring born calves.  Prior to the experiment being established, the site received 150-200 kg N ha^-1 per year of nitrogen fertiliser, applied in 5 equal splits between March and September.”

We used ammonium- or nitrate-based nominations to refer to the proportions of their ammonium and nitrate contents relative to the nitrate or ammonium and relative to the nitrogen content of the fertiliser. We classified NO₃^- to NH₄⁺ ratio of approximately 0.5 and less as ammonium-based and NO₃^- to NH₄⁺ ratio of well above 0.5 as nitrate-based. Classification with NO₃^- to total N was also considered, where NO₃^-: N close to 0.5 is nitrate-based and NO₃^-: N well below 0.5 is ammonium-based. CAN is generally considered as nitrate-based despite equal proportions of NH₄⁺ and NO₃^- (source: Yara fertiliser).

Comment: P3, L132-133, How do you define “conducive conditions for N2O production”? Please add citations for this statement, because you have not studied/tested this effect for your site.

Response: We usually denote WFPS > 60% as conducive in Irish conditions as many studies in the region so far reported WFPS > 60% as a favourable condition for production of high N₂O. We have included a citation here.

Comment: P5, Chapter 2.5. The authors give a good description of the N2O sampling and analysis. However, statements about the sampling interval are missing, please add.

Response: We added the following sentence to describe sampling frequency:

“Gas sampling took place 5 days a week in the first two weeks of fertiliser applications, and frequency of sampling was reduced to three days a week in the subsequent weeks.”

Comment: P6, Chapter 3.1. On the one hand, it is critical to see that the additional sprinkling produced very moist test conditions of the soil compared to the long-term average. On the other hand, the figures of the long-term mean for the air and soil temperature as well as the comparison of the long-term mean with the test conditions are missing here. Please add in any case.

Response: We have modified the statements in chapter 3.1 by including data on long-term air and soil temperature. The modified statements read as follow:

“Average air (soil) daily temperature ranged between 10.1 (11.0) and 19.3 (28) ⁰C with average temperature 14.4 (18.1) and 16.0 (17.8) ⁰C during the first and second fertiliser applications, respectively. The long-term (2010-2019) average air and soil temperature was 14.7 and 17.1 ⁰C, respectively, during June-August.”

Comment: P6/9, 236/255/figure 3, What is the explanation and reason to name the first fertilization period as "low N2O condition" and the second fertilization period as "high N2O condition". The different amounts of N2O emitted may also depend on the varying initial conditions, here I would choose a non-judgmental designation.

Response: We removed low N2O condition in L236, P6, high N2O condition in L255, P9, and low and high N2O condition in Figure 3.

Comment: P7, figure 2, Don’t plot the lines, this improves the clarity of the figure

Response: We removed the dotted lines from figure 2.

Comment: P9, 263-265, The statement/phrasing of this sentence is ambiguous, as two different references can be understood. Please formulate clearly!

Response: Yes, this sentence doesn’t read well! We modified the sentence as follow:

“In comparison with the first application, all fertilisers in the second application event produced greater cumulative N₂O emissions, with emissions from nitrate and ammonium-based fertilisers resulting in 225 to 417% and 55 to 338% increase, respectively (Fig. 3a).”

Comment: P10, 297-298 Incorrect line break

Response: Line break removed.

Comment: P 11, figure 4, Values are given for dry soil, like written in M&M? Add the information to the figure title. It would be very nice if results were also presented for the soil layers down to a depth of 30 cm. This would give a complete picture.

Response: Yes, values are given for dry soil. We have added dry soil in the y-axis of figure 4.

We sampled soils for 10 cm depth, which is the main rooting depth for grassland soils in contrast to arable soils where the top 30cm best represents the agronomic rooting zone. We will take this comment into consideration in our future studies.

Reviewer 2 Report

The manuscript present interesting results concerning the use of different fertilisers and their implication in nitrous oxide emissions in temperate grassland. The subject of this work is interesting but there are some points that need to revised.

 

• In the table 1, delete the first line “Soil characteristics (0-10 cm) of the experimental site with results expressed as mean ± SE (n = 8)”, since it is signalled in the legend of the table.
• Explain in material and method why authors have chosen 10 cm soil depth for sampling.
• It is better if authors showed an image of the experimentation in field (like image of collar areas where they have applied sampling), it is better to the reader to understand the experimental design.
• Change the Fig 2 to table with the values means and the standard error. There is a lot of data in the Figure and the patterns of treatments are not distinguished between them.

 

• Authors indicated that statistical analysis where significant between treatment and they have presented P values in the text. But it is also better if the authors add the p values in the figure.
• In the Table 3, the same comment of the Table 1, delete the first line in he table since it signalled in the legend.
• Line 297-298, correct the title.
• Add p values in the table 3, and adjusted the format of the table.
• Finally, agricultural practices, like fertilization, can help a lot in reduction of N₂O emission and in the protection of the environment. We suggest the authors to give to this advantage more details in their study (discussion–conclusion) concerning environment and soil protection.

 

 

 

Author Response

We are really grateful for your time and constructive comments on our manuscript.

General comment and suggestion: Reviewer #2

The manuscript present interesting results concerning the use of different fertilisers and their implication in nitrous oxide emissions in temperate grassland. The subject of this work is interesting but there are some points that need to revised.

Comment: In the table 1, delete the first line “Soil characteristics (0-10 cm) of the experimental site with results expressed as mean ± SE (n = 8)”, since it is signalled in the legend of the table.

Response: We have corrected the double headings in the tables presented (Table 1-4).

Comment: Explain in material and method why authors have chosen 10 cm soil depth for sampling.

Response: Top 10 cm is selected because this depth contains a maximum amount of nutrients and major root densities although depths below also play important role in nutrient cycling. This statement is included in section 2.4.

Comment: It is better if authors showed an image of the experimentation in field (like image of collar areas where they have applied sampling), it is better to the reader to understand the experimental design.

Response: A photo of the experimental site is included in the revised manuscript.

Comment: Change the Fig 2 to table with the values means and the standard error. There is a lot of data in the Figure and the patterns of treatments are not distinguished between them.

Response: Another reviewer (Reviewer #1) suggested removing only the lines in this figure (figure 2) to increase the clarity of the figure. We agreed with his/her comment and removed only the lines from the figure. These types of results are often presented in a figure format because readers would like to see the emission trends following fertiliser applications, which otherwise not possible if is presented in a table.

Comment: Authors indicated that statistical analysis where significant between treatment and they have presented P values in the text. But it is also better if the authors add the p values in the figure.

Response: P values, P<0.05 is added in figure 3.

Comment: Authors indicated that statistical analysis where significant between treatment and they have presented P values in the text. But it is also better if the authors add the p values in the figure.

Response: We have corrected the double headings in the tables presented (Table 1-4).

Comment: Line 297-298, correct the title.

Response: Line break removed and the title is corrected.

Comment: Add p values in the table 3, and adjusted the format of the table.

Response: P values added in table 3, and its format is also adjusted.

Comment:  Finally, agricultural practices, like fertilization, can help a lot in reduction of N₂O emission and in the protection of the environment. We suggest the authors to give to this advantage more details in their study (discussion–conclusion) concerning environment and soil protection.

Response: The main focus of the paper is to investigate the effect of fertiliser type on greenhouse gas emissions and combating climate change. We have discussed the effects of fertilisation on N2O emissions (climate aspect) in our discussion within the context of the study objectives. However, our study didn’t focus on the effect of fertilisation on soil protection and thus this is outside the scope of the paper.

We are really grateful for your time and constructive comments on our manuscript.

General comment and suggestion: Reviewer #2

The manuscript present interesting results concerning the use of different fertilisers and their implication in nitrous oxide emissions in temperate grassland. The subject of this work is interesting but there are some points that need to revised.

Comment: In the table 1, delete the first line “Soil characteristics (0-10 cm) of the experimental site with results expressed as mean ± SE (n = 8)”, since it is signalled in the legend of the table.

Response: We have corrected the double headings in the tables presented (Table 1-4).

Comment: Explain in material and method why authors have chosen 10 cm soil depth for sampling.

Response: Top 10 cm is selected because this depth contains a maximum amount of nutrients and major root densities although depths below also play important role in nutrient cycling. This statement is included in section 2.4.

Comment: It is better if authors showed an image of the experimentation in field (like image of collar areas where they have applied sampling), it is better to the reader to understand the experimental design.

Response: A photo of the experimental site is included in the revised manuscript.

Comment: Change the Fig 2 to table with the values means and the standard error. There is a lot of data in the Figure and the patterns of treatments are not distinguished between them.

Response: Another reviewer (Reviewer #1) suggested removing only the lines in this figure (figure 2) to increase the clarity of the figure. We agreed with his/her comment and removed only the lines from the figure. These types of results are often presented in a figure format because readers would like to see the emission trends following fertiliser applications, which otherwise not possible if is presented in a table.

Comment: Authors indicated that statistical analysis where significant between treatment and they have presented P values in the text. But it is also better if the authors add the p values in the figure.

Response: P values, P<0.05 is added in figure 3.

Comment: Authors indicated that statistical analysis where significant between treatment and they have presented P values in the text. But it is also better if the authors add the p values in the figure.

Response: We have corrected the double headings in the tables presented (Table 1-4).

Comment: Line 297-298, correct the title.

Response: Line break removed and the title is corrected.

Comment: Add p values in the table 3, and adjusted the format of the table.

Response: P values added in table 3, and its format is also adjusted.

Comment:  Finally, agricultural practices, like fertilization, can help a lot in reduction of N₂O emission and in the protection of the environment. We suggest the authors to give to this advantage more details in their study (discussion–conclusion) concerning environment and soil protection.

Response: The main focus of the paper is to investigate the effect of fertiliser type on greenhouse gas emissions and combating climate change. We have discussed the effects of fertilisation on N2O emissions (climate aspect) in our discussion within the context of the study objectives. However, our study didn’t focus on the effect of fertilisation on soil protection and thus this is outside the scope of the pap