The manuscript reports a new process-based model for the emission of ammonia following fertiliser application, and then applies this on a global scale. The topic of agricultural ammonia emissions is an important one, and there is clearly a need for well-calibrated, process-based models as an aid to understanding the basic processes involved in ammonia volatilisation and as a means of upscaling to regional/global scales.

My understanding is that the AMCLIM model is a lightweight model that focuses on ammonia emissions in a period of 1-2 weeks following fertilisation, when the vast majority of ammonia is emitted. It uses a relatively simple Ohm's law like structure, which leads to most of the equations being linear. Non-linear contributions to N cycling, for example due to microbial activity, coupling between C and N and mineralisation of organic N compounds are ignored as being unimportant for predicting ammonia emissions shortly after fertilisation. Soil moisture and soil temperature are not explicitly modelled, but rely on measurement data. The relative simplicity makes the model a potentially valuable tool for experimental groups carrying out ammonia emission measurements. As compared to more detailed process based models (e.g. Daycent or DNDC type models) I assume the model is considerably easier to set-up, faster to run and doesn't require long spin up periods. Furthermore, the comparison to the GRAMINAE site shows that the model does a good job of capturing the variation in ammonia emissions in the days following fertilisation. 

On the other hand, I think the application of the AMCLIM model to global ammonia emissions from croplands is premature. Insufficient evidence is provided to show that the model is well calibrated. The comparison to a single grassland site in Germany suggests that the model shows promise in capturing the diurnal cycle of ammonia emissions following fertilisation. However, before applying the model to global croplands I would like to see (see also calibration comments below):

1. Improved evidence that the pH dependence of ammonia emissions is well represented. Figure 13a,b provides some information in this direction, but it is hard to conclude from this that the model is well calibrated (for example a factor 2 difference in the y axis scale is needed to show the modelled Pv values as compared to the measured values).

2. Evidence that the pH change and the impact this has on ammonia emissions following urea application is well calibrated.

3. Evidence that the total ammonia emissions are well represented across multiple crop, soil and climate conditions. For example, that the model can capture ammonia emissions from paddy rice fields in South East Asia. Figure 12 goes some way in this direction, but I find it hard to conclude from this figure that the model is performing well across multiple conditions.

My opinion is that the model description and application to the GRAMINAE site would already make a valuable paper. I would suggest to leave out the application to global croplands and publish this at a later date, once more extensive calibration can be performed using site-scale data. 

More detailed questions/comments follow below:

** Model details **

The AMCLIM model includes N uptake by plants, but ignores N uptake by microbes (immobilisation). This may be an important process, especially for fertilisation events at or before planting, when plant N uptake is low. Please discuss why/under what circumstances it is reasonable to ignore microbial N uptake.

To what extent has the sensitivity of the model to changes in temporal and spatial resolution been tested? Ideally the spatial and temporal resolution is reduced until the ammonia emissions become relatively independent of further decreases (and I see no reason why the resolution cannot be made finer than the meteorological and soil inputs, e.g. by splitting each soil layer into sub-layers). Figure 5 suggests that changing the spatial resolution of the top soil layer leads to large changes in the model behaviour (comparing circles for z1=1,2,3 cm), and thus that the model behaviour has not yet converged. 

I would expect the time-step to be important as the underlying processes have very different response times. In particular the chemical equilibrium reaction between NH3 and NH4+ is much faster than plant N uptake or nitrification. As such I think it is important to have some time-step control, especially in the minutes following broadcast fertilisation (is 15mins / 1hour short enough?). Similarly, the spatial scale will control the interaction between the concentration of NH3 and NH4+ in the top soil layer following broadcast fertilisation and the transport processes (is 4 soil layers enough?).

As a related point, it would be useful to briefly mention how the coupled differential equations are solved. Is this by a Euler method or is a higher order method used? 

I think the section 'volatilisation of NH3' could be improved, in particular the description of how the surface NH3 concentration is calculated.

It would also be useful to provide the recovery function for soil pH following urea fertilisation.

For the plant N uptake, what values are used for W_r,i (SM14 and SM17)? I couldn't find this in the supplementary information. Also it would be useful to mention how perennial crops such as grass are treated, especially since this is relevant for the GRAMINAE site (the stages in Table S1 seem to be for annual crops). 

** Calibration **

As far as I understand, model parameters are taken from the literature, and are mostly not calibrated by comparing the AMCLIM model to measurements (a small number of model variations are shown in Figure 5, but are only compared to a single 10-day measurement at 1 site). Compare, for example, to Gurung et al. Nutr Cycl Agroecosyst (2021) 119:259–273, where Bayesian techniques are used to perform a joint calibration of the 18 parameters relevant to ammonia volatilisation, comparing different levels of model complexity and by taking into account 8 different experimental sites with 42 site-year treatments. I understand that the authors cannot do everything in one paper, and am not expecting them to perform a full Bayesian calibration in this manuscript. However, I think that a comparable level of calibration is necessary before applying the model at a regional/global scale.

I would find it useful to have a table of all model parameters and their values (e.g. in the supplementary material).

** Global simulations **

I mentioned above that I believe the application to ammonia emissions from global croplands is premature, and requires additional calibration of the model. However, if the authors choose to retain the global simulations in the manuscript, it would be useful to address the following points:

As discussed in the manuscript, correct fertiliser timing is important, due to the sensitivity of ammonia emissions to meteorological conditions (especially temperature). As such, the assumption that 50% is applied at planting and 50% midway through the growing season on a global scale seems a very crude approximation. Is no better data available? If not, how much do emissions change when these assumptions are varied?

The lack of model adaption for paddy rice systems means it is likely unreliable for these systems. I would suggest to either adapt the model to paddy rice, or to leave rice out of the global simulation.

An uncertainty estimate is given in the discussion section, but no details are provided as to how this was calculated. Please provide details so that the reader can judge how seriously to take this estimate.

** Discussion **

I would find it useful to discuss:

How the model differs at a process level from other process-based models such as FAN, DLEM, Daycent or DNDC type models. What are the advantages and disadvantages of the AMCLIM model with respect to these established models and what do the authors see as the future role of the AMCLIM model? 

How does the calibration procedure compare to these other models, and what are the consequences for the level of confidence we should have in the AMCLIM model results as compared to established models?

Why has urease inhibition not been considered? This is required by many countries when broadcast spreading urea fertiliser and has important consequences for ammonia emissions. To what extent does this limit the usefulness of the model?

A dynamical process-based model AMmonia–CLIMate v1.0

(AMCLIM v1.0) for quantifying global agricultural ammonia

emissions – Part 1: Land module for simulating emissions from synthetic fertilizer use by Jiang et al.

This paper describes the ammonia emissions from synthetic fertilizer. First of all I want to compliment you with this article. It is nicely written and the results and methods are clear. However I also have some concerns.

1.    There is a nice description of the validation/calibration of the model on the GRAMINAE database. These are all observations on fertilized grassland. After that the application of the model is on a global scale with 16 crops. But none of these crops represent grassland. Is it reasonable to assume that grassland is a good representative for all 16 crops? 

2.    And from the other perspective. The GRAMINAE database shows that fertilizer is used on grassland (which is common practice in for example Europe and US). None of this fertilizer is mentioned in the global estimate of ammonia? What is the role of grassland in the global emission? Can you elaborate on this uncertainty (of not taking this load)?

3.    I have the impression that the global figures of the ammonia emissions (for example figure 6, but actually all maps) have a coverage of grassland and cropland. So I am wondering which land use database is used and whether the assumption is valid that all grassland is fertilized. Perhaps I am wrong, but I would expect that maps have more white area. Can you please elaborate on this as well?

Specific comments

Line 84: AMCLIM-Fertilizer is nowhere else mentioned. I was sometimes confused whether it should be AMCLIM or AMCLIM-Land or it should be AMCLIM-Fertilizer. Please check the document to verify that the right name is used. I think I would prefer to use AMCLIM-Fertilizer in most cases (as suggestion).

Line 85: “AMCLIM Livestock”. Not clear to me. Not mentioned in Figure 1 and not mentioned in line 92. But the contents of this must be very clear, because this determines whether a topic is described here or in the other article.

Figure 1: “chemical” fertilizers is used, but mostly in the text “synthetic” is used.

Figure 2, line 122: Why is ammonification not included? This is input TAN.

Line 158: I think here m denotes mass. But in line 152 it is meter….. Please change to make it clear.

Line 165: eq 3: Explain in the text the names in the right hand side of the equation (so s, aq and g are not explained).

Line 168: Where is H+ coming from? Can you elaborate on this?

Line 168: Reference format of Sutton

Line 187 – 190: Here square brackets are used. But why? Is [NH3(g)] not the same as M_nh3,g as mentioned in eq. 3? Please make clear what your intention is here.

Line 190: Why NH3(g) instead of TAN(g)? 

Line 210: No explanation of constants or parameters is given in the text.

Line 303: “applied to cropland” What about grassland?

Line 575: explain MAM. SON and DJF. I see  they are explained in the caption of figure 9, but this text is before the figure.

Figure 9: end of caption is text missing

Figure 9: Caption says something about percentage, but figure shows fractions. I would suggest to use everywhere the Pv (%).

Figure 9. Add per grid cell to the unit.

Figure 9: The y axes of NH3 per grid is confusing. Looks like it was for the right column. Can you put this on the left hand side of the figure or make it more clear that it belongs to the left column? This remark is for the maps figures.

Line 599: End of caption misses text.

Line 623: In figure a small f is used in F_region.

Line 628: AMLIM -> AMCLIM

Line 810: Assumptions -> assumptions

Figure A4: same remarks as for Figure 9.

Caption figure A7: Mention unit in Gg N per grid cell.

Supplementary information

Eq SM 2 and 3: in the main text (line 184: “In addition, diffusive and drainage fluxes

185 considered as losses in the soil layer above become sources of nitrogen for the layer underneath.”). I don’t see this in these equations.

Line 35: Unit of K_d?

Line 36: “fractional soil clay content”: Is this determined per grid cell. So here it is the upper soil layer? From what is this a fraction?

Line 44: K_Knitrif,opt -> K_nitrif,opt

Line 45: Is the unit of K_nitrif,opt in percentage??

Line 49: small t was reserved for time, but now it is temperature. Make the T (T_opt, T_max)

SM8: change k_nitrif,T -> K_nitrif,T

In line 50, K is used for Kelvin (correct)   Perhaps it is an idea to change all K variable into small k variables (also in main text) to avoid confusing.

Line 82: Unit of J_C,N?

Lines 87-88: What is the unit of 4 and 40?

Equation SM17: use the alfa_root and J_C,N in this formula.

Line 92 -99: “There are four ….. [end of table]” I would move this under equation SM14. Now it is coming too late.

Line 102: unit of W_uptake?

Line 110: remove one of the closing brackets

Line 114: What is 20.1 and 14.9?

Line 114: unit of pressure?

I stopped here. Please check whether the units of all parameters are given and whether they are explained in the text.