This review is from the Co-Editor.
This paper describes measurements of NH3 and NO fluxes in Benin, West Africa and in June and July, 2016. The article has received one round of reviews. My primary concern is that the new version has not adequately addressed concerns around the NH3 flux measurements. Data of soil fluxes in West Africa are rare and I hope more information can be provided on the measurements so the paper can be published. I have provided other comments as well.
The application of experimental results from Vaittinen et al. (2013) is not yet convincing. The value for PFA of 13.9e12 molecules/cm2 was determined under constant conditions that do not represent the field sites: 10% sea-level atmospheric pressure, a constant temperature of 295K, constant humidity, and ambient NH3 of ~8 ppm (not 8 ppb). Vaittinen et al. (2013) themselves say the reported uptake values are minimums, rather than absolute quantities. Diurnal and day-to-day changes in these conditions could drive significant variability in air-chamber exchange, despite the low standard deviations reported in Vaittinen et al. (2013). If the chambers were frequently washed, then maybe uptake is the dominant direction of the air-chamber exchange; however, a key conclusion is that soils are largely an NH3 sink. For this reason, additional work is need to verify that the reported fluxes are between the air and soil rather than the air and chamber.
Referee 1 makes a valid point in comment 2 (below):
“The authors use the measured NO and NH3 fluxes for a stepwise linear multiple regression analysis, upscaling to country-wide soil fluxes, and comparison with soil emission estimates from the GEOSChem model. These analyses give valuable information on the importance of soil NO and NH3 exchange and our current knowledge about them. However, while the authors state that a process understanding of the NO and NH3 fluxes is not within the scope of the presented study, in my opinion it is important to understand the underlying processes of the measured fluxes. For example, the estimated emissions from soil characteristics only poorly agree with the measured fluxes in some parts, which indicates that a more detailed process understanding is necessary.”
However, the authors have made no substantive changes to the manuscript in response. Or, if they have, they have not indicated this in the formal response. Please clarify.
Rearding Referee 1’s comment:
“The assumption that the concentration in the chamber is equal to the concentration leaving the chamber to the analyzer is questionable. Due to the low flow rate required for the practical use of the closed-dynamic chamber technique, the residence time within the chamber is substantial (17-18 min). As no active mixing (e.g. with fan) is used, the chamber geometry in relation to the positioning of the ambient air inlet and sample outlet is of importance.”
Please modify the text so the reader does not need to read another paper to understand the fundamentals of the experiment. Consider this comment broadly and add a brief overview of all relevant information from Delon et al. (2017). You may refer readers to Delon et al. (2017) for additional detail, but provide the basics here.
For the new text: “This is different from Delon et al., 2017, but fluxes were anyway superior to this value.” Change the word superior to “greater than.”
Regarding Referee 1’s comment (a concern also raised by Referee 2):
“This assumption seems brave if it was not tested with a set of test experiments. Although the microbial activity is reduced due to the dry conditions, there is a chance that NH3 volatilizes with the drying of the soil sample material.”
I see you have added a reference to this phenomenon, but discussion for how this relates qualitatively and quanitaviely to your results is still needed.
New text: “Daily means of NO concentration vary from 1.28 to 5.40 ppb for all sites.” In Fig. 2 (and 4), NO concentrations appear be equal 7-9 ppb on multiple days.
As far as I can see, Figs. 2 and 4 are identical. Is this an error?
New text: “Average NH3 concentration is 6.28 ± 3.90 ppb for bare soils.” Over bare soil, it appears the NH3 concentration never exceeds ~3 ppb.
Regarding Referee 2’s comment:
“While I am sure location and resources had much to do with this, air-drying may result in large changes to ammonium concentrations. Additionally, significant changes in the amounts of ammonium can take place over prolonged storage at room temperature, even if soils are dried. It seems that the authors are aware of this issue and attempted to justify their method by citing a meta-analysis of warming experiments on N-cycle activity. (Bai et al. 2013). However, this meta-analysis found that warming and moisture reduction had no significant effect on mineralization (Bai et al, 2013: Table 1), indicating even in dried samples, pools of inorganic-N may change over time. To remedy this, the authors could have compared their ammonium concentrations to similar studies from this region; however, this was not included in the results/discussion.”
Please expand the new discussion, which at just two sentences (line 255 and line 360), is insufficient.
All plots are of poor visual quality, can the authors remake the plots with a program capable of higher resolution graphics and that allow the authors to improve the readability of the plots?
Abstract: I don’t follow the logic of the first sentence.
Line 20: change aerosols to aerosol.
Abstract should be a single paragraph.