Characterizing Greenhouse Gas Emissions and Global Warming Potential of Wheat-Maize Cropping Systems in Response to Organic Amendments in Eutric Regosols, China
Round 1
Reviewer 1 Report
Atmosphere-812541
“Characterizing Greenhouse Gas Emissions and the Global Warming Potential of Wheat-Maize Cropping Systems in Response to Organic Amendments in Sloppy Upland Purplish Soil, China
Bah et al. present a study over two years of emissions of CH4 and N2O from experimental fields on a particular soil. All fields rotated winter wheat with summer maize over both years. Treatments differed in the type and quantity of fertiliser applied, comparing a control (no fertilisation) with a mineral fertiliser and two different organic fertilisers combined with mineral fertiliser (crop residue and pig manure, respectively).
The experimental design is slightly complex, with two different crops grown at particular times of the year, in both study years. This confounds season and crop, preventing full analysis of the effect of each crop on gas emissions within the four fertilisation treatments; however, this is not stated as an objective of this study, and the analysis of gas fluxes are mostly conducted at the level of cumulative annual emissions, integrating both crops for each treatment and year, and consistent with what is apparently common agricultural practice in the region.
Unfortunately, while the experimental design is mostly clearly described, the results are quite complex and are not explained well. The frequent use of undefined percentages is very confusing, because very often the relevant denominator is not mentioned. I frequently could not identify the percentage of what was being described, and this was further aggravated by occasional impossible-seeming numbers such as N2O accounting for greater than 100% of (I assume) total annual cumulative GWP. Some percentages seem to refer to a particular treatment or season, but others might be in reference to the experiment as a whole. The inclusion of negative values in places just increases the confusion, and what a negative sign means – net movement of C to atmosphere? out of the pool considered and into other pools? - clearly defined.
I recommend a complete re-write of the Results section, organising the presentation of data to emphasise clear comparisons between treatments, rather than the current structure that prioritises categories of data. Where differences between treatments are complex, such as the shift in significant differences between GWP and yield-scaled GWP, exactly which treatments were exceptional in exactly which ways needs to be clearly stated, not buried within list-type paragraphs of endless numbers. The tables are generally good, and can continue to serve as the central focus of much of the Results and Discussion, carefully positioned as close as possible to the parts of the main text that mention each table. Likewise, the Figures do not need to change very much to improve the presentation of information, and need to be located in the document very close to where they are mentioned.
The Discussion includes some highly speculative statements, unsupported by data in the current study, that are not phrased to clearly indicate the uncertainty of assigning, for example, large changes in N2O flux to an unobserved change from incomplete to complete denitrification. Other statements are hard to understand, apparently implying that some results, presented as both significant differences and informative to understanding the complex effects of season, crop, and fertiliser, are suddenly trivial or irrelevant.
The quality of English is not quite sufficient for clear interpretation. Some phrases or sentences are repeated needlessly, other sentences reference unknown previous text, and some grammatical constructions are simply baffling. Careful proofreading is needed.
Detailed comments below.
Abstract
LN 17-18: “Results showed that variation in seasonal CH4 and N2O emissions and temporal and spatial heterogeneity were reciprocally consistent.”
I'm having trouble wrapping my head around this sentence. What do you mean?
LN 26: I think the word “increase” should be included within the parentheses around 79 %
Also, is that increase (79%) statistically significantly different from the increase observed in other treatments that reached as high as 78% above... well, what treatment was taken as the baseline that other treatments increased relative to?
Introduction
The use of English is somewhat confusing. For example, LN 47 – which organic amendments are being referred to by “such”? I do not see a description of organic amendments prior to this sentence.
Experiments
LN 89: I think the units should be Kg dm-3, or Mg m-3. A little more than 1 Kg per cubic metre seems very low.
Treatments
CK – control
NPK – mineral fert
OMNPK – pig slurry + mineral
??? - crop residues + mineral
LN 96-97: does the fourth treatment, with crop residues and mineral fertiliser, have a name?
I see the name in Table 1, and it is defined well in the caption for Table 1 but needs to appear in this section.
LN 155: “in pure C.” What was the calibration gas mixture for CH4?
Results
LN 267-268 (caption for Figure 2): the description of the vertical error bars is repeated.
LN 284 – 288: were the CH4 fluxes significantly different between treatments, or not? The statements here seem to contradict each other. It seems like CK was significantly different from the others, but the other three were not different from each other. But I am very not sure about this based on these lines of text.
This is clarified in the Discussion, LN 503-504, but is very unclear here in the Results.
LN 292: why are error bars not shown? Are the treatments significantly different, or not? Large, overlapping error bars are often an indicator that differences are not significant.
LN 307-306: again, the description of the error bars is needlessly repeated.
LN 330: “all four treatments was significant” change to “differences between all four treatments were significant” OR “all four treatments were significantly different”
Section 3.5 (LN 366 onward): what do the percentages here indicate? How can N2O emissions account for more than 100% of the GWP? I am confused about most of the percentages in this manuscript. Percentages are fractions – fractions of what?
LN 446: please remind me what the negative indicates. A net movement of carbon out of that pool? From soil to atmosphere? From atmosphere to soil?
LN 446-447: again, what do the percentages indicate? 700% of what?
LN 474: the significant relationships shown in Figure 7 d and e are curved – how was this curved model fitted to the data? How did you choose which curve to apply? What criteria did you use to evaluate competing models? What is the equation for these curves?
This also applies to the curves shown in Figure 8.
Discussion
LN 564-566: this is a surprisingly certain statement about denitrification. I see no direct evidence of a shift from incomplete denitrification – producing N2O – to complete denitrification – consuming N2O – in the data of surface-level chamber flux measurements. I agree that such a shift is likely under conditions of increasing soil moisture and carbon content, but by no means is it certain to occur, and to account for the majority of the differences in N2O fluxes.
LN 618-620: I do not understand this statement. Aerobic soil conditions “had less to do with” …? Are you implying that soil moisture was not significantly correlated with net CH4 emissions? That would be surprising. If soil water and CH4 fluxes are not important, why measure and report them?
Tables and Figures
Generally, the circles on the figures (e.g. Figure 5) could be smaller to improve clarity. They overlap considerably in some figures, with some treatments obscuring the values for other treatments. Smaller symbols would better convey the information.
In Table 2, the significance of differences between treatments sometimes changes when examining GWP vs Yield-scaled GWP. For example, annual GWP for CK is significantly different from the other treatments (letter c vs a or b), but is NOT different from NPK after Yield-scaling (letter c for both treatments). Such differences are frequent across Table 2. I do not see in the Discussion section any description or explanation for such differences apparently caused by the yield-scaling procedure.
Supplemental tables
These have some formatting issues but are otherwise acceptable.
Author Response
We thank the anonymous reviewer’s for his efforts in giving really critical and constructive comments that we strongly believe have improved the quality of the manuscript.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors have conducted a solid study but the manuscript needs to be improved. I'm also very skeptical of the SOC results and think they should not be emphasized so strongly, in particular should they not be included in the GWP calculations.
This manuscript needs language editing. Let me clearly state that English is not my native language. However, I found the authors' English usage quite jarring in some parts of the manuscript, to a point where some statements became almost wrong due to the language issues.
Some examples:
Line 45-46: The ozone depletion is caused by a catalytic reaction of nitrogen oxides with ozone and not by the radiative forcing.
Line 67: "current GWP values": These are physical quantities that don't change. The authors mean current estimates of these values.
The authors often write "between 2016 and 2017" when they mean the 2016/2017 cropping season. This is particularly confusing when they describe statistical comparisons where the comparison is not between these years.
...
I also would recommend to call the soil an Eutric Regosol in the title and throughout the manuscript and maybe mention in the introduction that apparently it's called "sloppy purplish soil" (is "sloppy" really the correct word here? I'm much more used to its second meaning.) in China.
The abstract needs to be rewritten. To me a "treatment that employed both organic manure and mineral fertilizer" and "treatments that employed a combination of organic and mineral fertilizer" are the same, so I was strongly confused. Also "Assessing impacts associated with organic amendments to cropland have not been adequately investigated." is such a trivial and subjective blanket statement that I don't think you should start from it.
I'm extremely skeptical of the results regarding SOC. Two years is simply too short to study SOC changes. A strong increase without organic amendments can only mean that the increase is due to the absence and presence of below-ground harvest residues in the measured samples. You cannot extrapolate this to a sustained increase and thus shouldn't include it in the GWP calculations.
Materials and Methods:
Why was this study conducted as a lysimeter study? When where the lysimeters established? How was agricultural management (such as ploughing and fertilization) simulated and how did it compare to local practices? What were the wheat and maize varieties? What was the preceding crop (if any)?
I don't understand the crop residue treatment. I suspect straw of the preceding crop was removed for all other treatments and incorporated for this treatment?
Please provide clay, silt and sand content of the soil.
How did you prepare samples for SOC analysis? Were they sieved? Did you remove roots?
What was the height of the chamber collar above ground?
The equations in 2.6 are badly formatted and some of them unnecessary. E.g., just write that you calculated fluxes with a linear flux model after calculating mass concentrations and add a citation. No need to provide the equation here.
Results and Discussion
Please improve the graphs. It's not possible to see to which tick mark the labels on the time axis correspond. A grid would also be appreciated.
In the tables: Please clarify the flux units. Are they per period (how long was this period then?) or per year?
Please don't call CH4 fluxes "emissions". They are mostly uptake. Also, it can be confusing if you refer to lowest CH4 emissions but mean smallest CH4 uptake.
You should depict management events in Figures 2 and 4. The N2O emission peaks in 2018 seem to coincide with high nitrate concentrations and high WFPS. Were the high nitrate concentrations a result of a fertilization event?
Since this was a lysimeter study, why did you not measure nitrate leaching?
When you discuss GWP, you should also at least mention GHG emissions from fertilizer production.
Are grain yields comparable to those from local farms?
I don't believe pairwise linear correlations are useful here. We know that several parameters need to be optimal simultaneously for N2O emissions. Also, we know that the relationships are not linear (e.g., there is an optimum curve for WFPS). Personally, I like generalized additive models for exploring these relationships (i.e., the combined non-linear effect of explanatory variables).
Author Response
We thank the anonymous reviewer’s for his efforts in giving really critical and constructive comments that we strongly believe have improved the quality of the manuscript.
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper studies the green gas emissions and the global warming potential of a common crop in China, considering different kind of organic amendments in the soil. The authors focused on the emissions of CH4 and N2O over a 2 years period, covering winter and summer seasons in both years considered. The authors test 3 different amended treatments + 1 control, showing that the 3 treatments reduced CH4 emissions compared to the control. Regarding N2O the results differed highly among the 3 treatments, concluding that N2O emissions are controlled by a variety of factors.
The paper is very well structured and organized. Language is clear and concise. There are a good combination of figures and tables than help to understand better the data showed. So, I recommended the publication of the paper without many changes.
Only few comments:
Fig 3: profiles of OMNPK treatments are not clearly explained in the text. Please add a brief explanation/justification of the behavior observed.
SOC and DOC have the same meaning? They are used indistinguishly in the text but only the second is explained. Also, BD meaning are explained after using it, when it is recommended to explain the meaning always in the fist use of them. Please check that point.
When numbering the treatments, (line 95) number 2 has disappeared.
Author Response
We thank the anonymous reviewer’s for his efforts in giving really critical and constructive comments that we strongly believe have improved the quality of the manuscript.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Atmosphere-812541-V2
“Characterizing Greenhouse Gas Emissions and the Global Warming Potential of Wheat-Maize Cropping Systems in Response to Organic Amendments in Eutric Regosols, China
Bah et al. have thoroughly revised their manuscript detailing greenhouse gas emissions associated with a range of fertiliser treatments at wheat-maize agricultural fields in China. The text has been considerably clarified and my previous comments have been addressed to my satisfaction. There are a few minor spellling errors, typos, and similar trivial problems that I am confident can be easily corrected.
I have only one remaining problem, though it is not large, see below.
Results
LN 218-219: It is not correct to state differences of temperature, in degrees C, as multiples or factors; 26 C is not twice the temperature of 13 C. Zero degrees celsius is not mathematically comparable to, for example, zero cm in length. Please just state the temperatures, or the difference in temperature (simple subtraction, not division) as appropriate.
Author Response
We thank the anonymous reviewer’s for his efforts in giving really critical and constructive comments that we strongly believe have improved the quality of the manuscript. The manuscript has been well checked (spell and grammar) by a native English speaker (American English) from English editing at American Journal Experts (AJE).
Author Response File: Author Response.pdf
Reviewer 2 Report
The authors have improved the manuscript substantially and I now recommend publication after some minor improvements.
Unfortunately, there are still some language mistakes. I don't have time for details, but another round of intensive copy editing is necessary.
I don't see the downward arrows in Fig. 4.
Table 1: Please indicate the exact lengths of the seasons (in days) in the table or footnote. My group is conducting a meta analysis right now and it is often quite challenging to find this information in publications.
Line 658: C saturation is a flawed concept. What happens it that a new equilibrium between carbon inputs and mineralization is reached.
Minor remark regarding Response 6: the soil is not "sloppy" (https://www.merriam-webster.com/dictionary/sloppy) but "slopy" (https://www.merriam-webster.com/dictionary/slopy).
Author Response
We thank the anonymous reviewer’s for his efforts in giving really critical and constructive comments that we strongly believe have improved the quality of the manuscript. The manuscript has been well checked (spell and grammar) by a native English speaker (American English) from English editing at American Journal Experts (AJE).
Author Response File: Author Response.pdf
