Grazing under Irrigation Affects N₂O-Emissions Substantially in South Africa

Round 1

Reviewer 1 Report

This study measured N2O emissions from nitrogen fertilization on pasture in South Africa from June 2018 to June 2019. In the pasture, five fertilization treatment areas were randomly prepared in four repetitions, and N2O flux was measured in a large chamber having a diameter of 60 cm. The measurement frequency is usually once a week and within 3 days after fertilization, which is sufficient. Grazing was performed 12 times, and fertilization was then performed. Total amount of N fertilizer applied per productive year were 0, 220, 440, 660 and 880 kg N ha-1 for the treatments. The annual amount of manure-derived nitrogen is estimated to be 450 kg N ha-1.

The N2O flux increased with the increase of fertilizer application, and the annual N2O emission increased significantly. The annual flux was regressed with the amount of fertilizer applied, and both linear regression and curve regression were significant. From the EF of Tier 1 of the IPCC, EFs of chemical fertilizer origin and manure origin are estimated as 0.01 and 0.02, respectively, and compared with the measured values.

It was shown that N2O derived from manure in Tier1 is likely to be overestimated.

This paper has high originality and can be fully published.

Since the following is incomplete, it is recommended that this paper be published after correction.

205-208 It is better to add equations for IPCCmin and IPCCmin+exc and to cite them at the note of Table 4.

243-246 It is better to show the abbreviation such as Nmin, NO3-N, NH4-N etc should be shown here.

249-251 What is the frequency of the measurements of WFPS and soil temperature? It is better to plot them in figure 2 with corresponding to the time of N2O fluxe measurement.

327 Please write the unit correctly. Is it g ha-1 day-1?

332 Is N2O emission daily emission? But, the unit is strange.

333 Is soil moisture content WFPS?

337 Is the unit correct?? What is the definition of N2O-N emission?

337 Table 3 is strange. Because measurement times of soil and environmental factors and N2O-N fluxes shown in figure 2 are not coincided. Authors should show how to make the regression.

418-451 Authors should refer to Table 3 in the discussion, although Table 3 is ambiguous. Please make clear the table and reflect it in the discussion.

Looking at Figure 2, the timing of measurements of N2O flux and temperature/ WFPS is not corresponded. WFPS data are lacked at the many of N2O flux peaks. Many papers show N2O flux peaks occurred at WFPS from 0.6 to 0.8. If WFPS was measured at the peak event, the correlation may follow them.

499-500 Can you add "in grazing period" ? Because during composting also EF should be used.

Author Response

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Reviewer 2 Report

The research described is very well designed and executed with a clearly stated objectives. The manuscript is well written, but the discussion seems to be excessive and repetitive, thus can be shortened to make it more focussed.

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Reviewer 3 Report

Review of Manuscript ID: Atmosphere-857585

“Grazing under Irrigation Affects N2O-Emissions Substantially in South Africa” by Smit et al.

 

The authors provide a study aiming to quantify direct N2O emissions and associated Emission Factors of intensive pasture-based dairy systems in sub-Sahara Africa i.e. in South Africa. Field trials were conducted to evaluate fertilizer rates (0, 220, 440, 660 and 880 kg N ha-1 year-1) on N2O emissions from irrigated kikuyu-perennial ryegrass (Pennisetum clandestinum-Lolium perenne) pastures. A static chamber method was used to collect weekly N2O samples for one year. The authors observed that the relationship between N balance and annual N2O emissions was exponential, which indicated that excessive fertilization of N will add directly to N2O emissions from the pastures. However, the analysis does not speak to N2O emissions in the ‘future agricultural world’.

 

The authors have compared their results with results from IPCC (2006) for both fertilizer and manure application.

 

I recommend that the manuscript be published only after the suggested changes are made.

 

Major Comments:

 

1. The author need to explain accumulated and cumulative N2O emissions; and what is the difference between the two. This terminology is confusing.

 

2. Table 3 provides correlation between N2O-N emissions (kg N2O-N ha-1) and environmental variables under different N fertilizer regimes during the experimental period. The emission process takes place in the soil. So why the soil temperature does have good r-value with N2O emission as one would expect and demonstrated by previous biogeochemical cycling studies? On the contrary the authors observe a reasonable r-value of air temperature with N2O emission.

 

3. Figure 4 provides both a linear and nonlinear relationship between accumulated N2O-N losses (kg N2O-N ha-1 year-1) in relation to (E1, E3) increased levels of N-input (kg N ha-1 year-1) as well as (E2, E4) increased levels of N balance (kg N ha-1year-1). This is mainly a statistical result. Both the analysis will tend to suggest that as N-input continues to increase N2O emissions will also continue to increase. This will be a false conclusion since the curve should flatten at some point.

 

4. Recent important references are missing. For example, in the introduction, little mention is made to the large number of contributions to nitrogen science from the NitroEurope Project, and to the EPA’s SAB Integrated Nitrogen Study in the US. I would suggest reading through these reports, including Sutton et al. 2011, Aneja et al. 2019, Ramanantenasoa et al. (2019), and several recent such studies. I recommend much more extensive citation of the literature.

 

Comments for author File: Comments.pdf

Author Response

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Reviewer 4 Report

It is of great interest and importance to measure N2O emissions from fertilized, irrigated dairy-pasture systems. The manuscript showed very nice results on a one-year control experiment in sub-Sahara Africa with different fertilizer rates. It indicated that the relationship between N2O-N losses and N input could best be described by a nonlinear function rather than a linear function. Results from this study could benefit for updating South Africa´s greenhouse gas inventory more accurately. The manuscript is well written, and is recommended to publish after some minor revisions:

Line 169: Could variation of N2O emission be detected by weekly measurement?

Line 203: No Figure 1 to 3 before Figure 4.

Line 260: One-way ANOVA or RMANOVA?

Line 284: Was there any significant differences before the treatment?

Line 334: soil temperature…

Line 390: What kind of management?

Line 439: Negative correlation with soil temperature should be discussed?

Line 442: Have some effects on N2O emission by harvest?

Line 485: What are the special conditions for the default value? Were the monitoring conditions in this study comparable with them? Since denitrification might be the key process for N2O production, high soil water content promoted N2O emission in this study. The effect of irrigation should be discussed.

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Round 2

Reviewer 3 Report

The manuscript is much improved and presents an important air quality issue. I recommend that the manuscript be published.