Summertime Soil-Atmosphere Ammonia Exchange in the Colorado Rocky Mountain Front Range Pine Forest
Round 1
Reviewer 1 Report
The paper describes the case study of ammonium exchange between mountain forest and atmosphere. As the authors note, such analysis is complicated and the results could be very variable. Consequently, such study in a new ecosystem represents the interest for understanding of the NH3 exchange.
However, the analysis is based on very few measurements with very high variability (in case of soil emission potential). In principle the conclusion like “Γg is varying from 5 to 2122 and we do not know why“ seems to be insufficient. More measurements are necessary either at least to decrease the standard error or to find, which factors could determine its values. E.g., why Γg under dense canopy is higher and more variable than under open canopy (although the conditions: dense homogenous litter and no ground vegetation – seem to be more homogenous), why it fall so dramatically during measurements period etc. If it not possible, at least some hypotheses should be discussed.
· In the Introduction it would be good to describe briefly the circle of ammonium in plant-soil-atmosphere. It would be good also to explain briefly the physical meaning of emission potential.
· It would be good to show an example of equations used to calculate the NH3 flux from compensation point
· Considerable part of Discussion belongs really to the Results and even to Methods (Eq.4)
· In the Discussion the resulting pools and fluxes should be compared with the results from other publications.
· I think Eq. S1 and S7 should be briefly mentioned in the main text.
· The use of symbol c as compensation point (Eq.1) in the main text and as mixing ratio in the Supplement (Eq. S7, S9, S11) should be clarified.
Comments for author File: 
Comments.pdf
Author Response
REVIEWER 1
The paper describes the case study of ammonium exchange between mountain forest and atmosphere. As the authors note, such analysis is complicated, and the results could be very variable. Consequently, such study in a new ecosystem represents the interest for understanding of the NH3 exchange.
Point 1. However, the analysis is based on very few measurements with very high variability (in case of soil emission potential). In principle the conclusion like “Γg is varying from 5 to 2122 and we do not know why“ seems to be insufficient. More measurements are necessary either at least to decrease the standard error or to find, which factors could determine its values. E.g., why Γg under dense canopy is higher and more variable than under open canopy (although the conditions: dense homogenous litter and no ground vegetation – seem to be more homogenous), why it falls so dramatically during measurements period etc. If it not possible, at least some hypotheses should be discussed.
Response 1. We thank the reviewer for their constructive suggestions to improve the manuscript. To provide better context for the observations described in the paper, Scheme 1 was added to the Introduction outlining the major exchange processes that influence the NHx pool in the ecosystem in addition to atmosphere-ecosystem exchange. Large drops in NHx soil content could be due to nitrification and immobilization, whereas increases in NHx soil content could be due to mineralization of organic N. These processes that were not measured during this field campaign we have chosen to refrain from proposing directly what has caused the drastic NH4+ changes within the soil without the support of definitive data. All we can say, based on the large pool of NH4+ within the soil, and given that we did not observe any large emission events (from AIM-IC data), is that the observed changes in NH4+ soil content must be due to some biological processing. The Discussion section that has been updated (L513) to better highlight this.
Point 2. In the Introduction it would be good to describe briefly the circle of ammonium in plant-soil-atmosphere. It would be good also to explain briefly the physical meaning of emission potential.
Response 2. Scheme 1 was added to the Introduction describing the cycling of NHx between the atmosphere and the ecosystem (soil and vegetation), highlighting the nitrification, immobilization, and mineralization that naturally occurs in ecosystem. “The Γ is independent of meteorological conditions and represents the potential of a surface to emit NH3 based on what proportion of NHx is not protonated, which is governed by the surface acidity, i.e. [H+].” was updated, L96.
Point 3. It would be good to show an example of equations used to calculate the NH3 flux from compensation point
Response 3. Flux equation and calculation are now presented as Eq 1 in the introduction before describing compensation point derivation.
Point 4. Considerable part of Discussion belongs really to the Results and even to Methods (Eq.4)
Response 4. Moved Eq 4 (Flux calculation) to Introduction section, which reframes the discussion of NH3 compensation points and inferred fluxes.
Point 5. In the Discussion the resulting pools and fluxes should be compared with the results from other publications.
Response 5. The authors have attempted to add results from other publications, however, to calculate the soil [NH4+] pool in mg m-2, the soil density is required, which is not typically available in the referenced literature that report NH3 soil emission potentials. Literature tend to only report [NH4+] mg per kg of soil and soil sampling area, but not the soil density needed to compute the size of the [NH4+] pool. The average soil density observed at MEFO was used to estimate Stratton et al. reported values. This was added to Figure 8 and referred to in the text.
Point 6. I think Eq. S1 and S7 should be briefly mentioned in the main text.
Response 6. A subsection on the flux calculation (Section 2.5) was added to Methods to describe the compensation point model equations used for the ground, stomatal and cuticular fluxes.
Point 7. The use of symbol χ as compensation point (Eq.1) in the main text and as mixing ratio in the Supplement (Eq. S7, S9, S11) should be clarified.
Response 7. For both the ground and the leaf flux pathways (i.e. Eq 1. and Eqs. S7, S9, S11) the symbol χ represents the compensation point given as an air concentration, i.e. the use of the symbol χ is the same in both cases. Within the framework of the two-layer compensation point resistance model used for the leaf pathway, the concentration at z0 is considered the compensation point at the canopy-atmosphere interface.
As the final presented flux unit are given in μg/m2/s we updated the flux methodology description in the supplement by replacing “mixing ratio” with “concentration”. To clarify that the compensation points are represented as an atmospheric NH3 concentration, a statement is added in the main text.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript “Summertime Soil-Atmosphere Ammonia Exchange in the Colorado Rocky Mountain Front Range Pine forest “ likes to analyse the exchange of NH3 between a forest ecosystem and atmosphere by measuring the atmosphere, soil, and vegetation components. Measurements were carried out at the Manitou Experimental Forest Observatory (MEFO) site in the Colorado Front Range, using continuous online monitoring of gas and particle phase NH3- and NH4+ , direct measurements of [NH 4+ ] and pH in soil extracts to determine ground emission potential (Γ g ), and measurements of [NH 4+ ] bulk in pine needles to derive leaf emission potential (Γ st ). Analyses revealed differences in analyses forest (land) elements (ie dense and open forest with thick and low litter layer and understorey vegetation) ranging from 5 to 2122. Γ st values ranged from 29 to 54. By accounting for the total [NH 4+] pool in each compartment, the lifetime of NH3 with respect to the surface-atmosphere exchange in the soil is on the order of years compared to much faster naturally occurring processes, i.e. mineralization and nitrification.
The paper is well written and interesting, analysing different parameters (factors) involved in the NH3 sink and emission in the Colorado Front Range forest. According to my comments, I recommend some revision before publication.
General comments
A large number of contributing sources were measured in two forest (land) elements (i.e. dense and open forest with thick and low litter layer and understorey vegetation), revealing large differences between them. However when it come to the end of the manuscript reader feels a bit lost as there is no conclusion on i) how much additional N inputs come for outside and inside forests and ii) what are conclusion of the two forest elements and iii) eventual climate effects.
A long the same lines
- Authors do not introduce the different forest elements and the need to distinguish them (eg L 90ff). What was the choice of authors and why (eg missing in L182ff)?
- would be good to have an idea on the approximate area of dense and open forest elements within the Manitou Experimental Forest Observatory (MEFO). Suggest to add a section on study sites (L146ff)
- Authors did measure in July-August, however, for reader it is difficult to understand the choice of authors and why and what is effect on annual basis (i.e. upscaling of results and conclusions)
Suggest to slightly revising the introduction to help reader to get into the subject I suggest moving some lines see specific comments
Specific comments
After L43 (prevailing wind), Synthetize L44-52 in “ Roosevelt National Forest in Northern Colorado could be positive (emission) or negative (deposition) depending on the direction of the prevailing wind. But also on anthropogenic activities, namely urban and agricultural centers, that are NH 3 -rich. They observed the NH 3 -rich air masses being depleted and NH 3 -poor air gaining NH 3 as the air passed through the forested site [14].The correlation they observed with air temperature led to the assumption the plant canopy was solely responsible forest ecosystem making it difficult to assess to what degree is the forest a source or sink of NH 3.
L110… dominant vegetation, (ponderosa pine), to evaluate the…
Then followed by L52ff “There are many factors controlling the movement of NH 3 through the forest ecosystem making it difficult to assess to what degree is the forest a source or sink of NH 3 . an then add …”
L173-188 move to site description
Legend of Figure 2, please check if axis and legend are correct
Legend figure 3 what is MST
Figure 6 give figure titles instead of legend for colors
Add in legend that axis are in log when the case
Author Response
REVIEWER 2
The manuscript “Summertime Soil-Atmosphere Ammonia Exchange in the Colorado Rocky Mountain Front Range Pine forest “likes to analyse the exchange of NH3 between a forest ecosystem and atmosphere by measuring the atmosphere, soil, and vegetation components. Measurements were carried out at the Manitou Experimental Forest Observatory (MEFO) site in the Colorado Front Range, using continuous online monitoring of gas and particle phase NH3- and NH4+ , direct measurements of [NH 4+ ] and pH in soil extracts to determine ground emission potential (Γ g ), and measurements of [NH 4+ ] bulk in pine needles to derive leaf emission potential (Γ st ). Analyses revealed differences in analyses forest (land) elements (ie dense and open forest with thick and low litter layer and understory vegetation) ranging from 5 to 2122. Γ st values ranged from 29 to 54. By accounting for the total [NH 4+] pool in each compartment, the lifetime of NH3 with respect to the surface-atmosphere exchange in the soil is on the order of years compared to much faster naturally occurring processes, i.e. mineralization and nitrification.
The paper is well written and interesting, analyzing different parameters (factors) involved in the NH3 sink and emission in the Colorado Front Range forest. According to my comments, I recommend some revision before publication.
General comments
Point 1. A large number of contributing sources were measured in two forest (land) elements (i.e. dense and open forest with thick and low litter layer and understory vegetation), revealing large differences between them. However, when it come to the end of the manuscript reader feels a bit lost as there is no conclusion on i) how much additional N inputs come for outside and inside forests and ii) what are conclusion of the two forest elements and iii) eventual climate effects.
Response 1. We thank the reviewer for their constructive suggestions to improve the manuscript. With respect to the questions above, i) We can see from the combination of the soil and atmospheric measurements that the NH3 emission and deposition events have a fairly small impact on the budget in either reservoir on the timescale of days. Other measured inputs that were accounted for were from precipitation, which was significantly smaller than the total calculated NHx pool in the soil. The diurnal cycle in the atmospheric concentration is likely regulated by soil-atmosphere exchange but is governed by variability in temperature rather than soil composition; ii) The conclusion from measuring the two different forest elements is the very wide range of soil emission potentials for each type of soil. These differences in two forest elements highlight the variability in the local soil environment from the perspective of NHx, which in turn, has a huge impact on the NH3 exchange with the atmosphere. This makes it challenging to scale up our limited observations to say anything more general about the net flux between either forest element and the atmosphere; iii) There are many ways in which the bio-atmospheric N cycle is coupled to climate, making it difficult to draw any straightforward conclusions. Because we found that temperature is a strong driver on the compensation point, it is reasonable to assume that with increasing temperatures we would see higher emissions of NH3 from the soil, everything else being equal.
A long the same lines
Point 2. Authors do not introduce the different forest elements and the need to distinguish them (eg L 90ff). What was the choice of authors and why (eg missing in L182ff)?
Response 2. “In addition, studies have not distinguished between the variety of soil environments that exist within a forested region.” was updated, L108. “In order to fully characterize the soil-atmosphere exchange of NH3, it is necessary to investigate the dominating soil types. The MEFO field site consists of a combination of two distinct soil environments.” was added to Soil Measurements section, L194.
Point 3. would be good to have an idea on the approximate area of dense and open forest elements within the Manitou Experimental Forest Observatory (MEFO). Suggest adding a section on study sites (L146ff)
Response 3. The distribution of dense and open forest soil types is roughly 50:50, which was added to the Site description section. To upscale and infer NH3 fluxes from MEFO from each of the forest elements would require not only an estimate of the area of dense and open forest elements but also a good approximation of representative emission potentials (and change in emission potentials over time) for each element. Given the wide range of emission potentials measured, we thought this type of estimation may be misleading if trying to estimate overall forest N input and NH3 emissions. The main focus of the paper is discussing the soil-atmosphere exchange of NH3 and the challenges associated with that and what impact it has on estimating the overall ecosystem-atmosphere exchange.
“Precipitation composition data was retrieved from the National Trends Network, in part of the National Atmospheric Deposition Program, site CO21, Manitou. The site has been collecting deposition data since 1978.” Was added to Site Description, L165, to emphasize the CO21 site is for deposition data.
Point 4. Authors did measure in July-August, however, for reader it is difficult to understand the choice of authors and why and what is effect on annual basis (i.e. upscaling of results and conclusions)
Response 4. Due to the technical challenges in performing continuous measurements of this complexity, July to August was logistically and affordably the best time and length for such a study. There are, thus far, no studies that have conducted this level of analysis for a full year cycle.
Suggest to slightly revising the introduction to help reader to get into the subject I suggest moving some lines see specific comments
Specific comments
Point 5. After L43 (prevailing wind), Synthesize L44-52 in “Roosevelt National Forest in Northern Colorado could be positive (emission) or negative (deposition) depending on the direction of the prevailing wind. But also on anthropogenic activities, namely urban and agricultural centers, that are NH 3 -rich. They observed the NH 3 -rich air masses being depleted and NH 3 -poor air gaining NH 3 as the air passed through the forested site [14].The correlation they observed with air temperature led to the assumption the plant canopy was solely responsible forest ecosystem making it difficult to assess to what degree is the forest a source or sink of NH 3.
Response 5. This suggestion overemphasizes the impact of anthropogenic activities. The section was reworded to highlight the importance of NH3 concentration of the air mass entering the forest is what dictates the forest ecosystem exchange.
Point 6. L110… dominant vegetation, (ponderosa pine), to evaluate the…
Response 6. Parentheses were used.
Point 7. Then followed by L52ff “There are many factors controlling the movement of NH 3 through the forest ecosystem making it difficult to assess to what degree is the forest a source or sink of NH 3 and then add …”
Response 7. This sentence was moved to the beginning section of paragraph.
Point 8. L173-188 move to site description
Response 8. Section was moved.
Point 9. Legend of Figure 2, please check if axis and legend are correct
Response 9. Right Axis label updated.
Point 10. Legend figure 3 what is MST
Response 10. Mountain Standard Time is defined in Figure 2 caption.
Point 11. Figure 6 give figure titles instead of legend for colors
Response 11. Figure Titles were added.
Point 12. Add in legend that axis is in log when the case
Response 12. Noted in figure captions.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The authors considerably improved the paper. Few notes that I wanted to add.
1.The strong fall of χg in dense canopy since August 10 could be explained by the strong rain the same day leading, e.g., to the change in soil water composition - maybe it could be mentioned in Discussion.
2. I still think that it is possible and useful to compare the results with other measurements. The authors included data of Stratton et al. and [44] in the Discussion, but I think it is possible to include more references on fluxes (which are always expressed per m2), in particular from eddy covariance measurements.
Some small notes are in the attachment
Comments for author File: Comments.pdf
Author Response
1. The strong fall of χg in dense canopy since August 10 could be explained by the strong rain the same day leading, e.g., to the change in soil water composition - maybe it could be mentioned in Discussion.
Response 1. Rain fall does have the potential to change the NH4+ soil concentration. However, this would be true for both soil types. If the heavy rain event was responsible for the drop in dense canopy [NH4+], then that would also be seen in a drop in open canopy [NH4+]. However, [NH4+] content in open canopy continues to increase.
2. I still think that it is possible and useful to compare the results with other measurements. The authors included data of Stratton et al. and [44] in the Discussion, but I think it is possible to include more references on fluxes (which are always expressed per m2), in particular from eddy covariance measurements.
Response 2. The authors agree comparison to more literature values would be useful. However, based on the current literature review, there are no values published that can provide a [NH4+] chemical pool concentration in relation to a compartment specific flux, e.g. soil fluxes or plant fluxes. Reported eddy covariance data from forests, unfortunately, only report the total ecosystem flux. To the author's knowledge, there is no published eddy covariance data on NH3 fluxes from forest ecosystems to date.
(From comments in attachment)
3. There is no any distinguishing between wet, dry deposition and emission at Fig.6 - it is only resulting flux. How did you partition it? There is no wet deposition in dry days, but what about emission?
Response 3. The inferred fluxed presented in Figure 6 is the predicted dry exchange of NH3, in which positive fluxes represent emission and negative fluxes represent deposition. The authors have expanded the text in this section to highlight this point. The compensation point modeling accounts for predicted changes in the equilibrium between the surface and the atmosphere. Therefore, wet deposition (rain fall) is not included. Wet deposition is easily measured separately and provided by the National Trends Network.
4. You compare dry deposition in one plot 31.07-03.08 with dry deposition in other plot 10-13.08. Why not the same dates?
Response 4. The reason for discussing the dry deposition for these periods is because they are the only periods that show continuous negative fluxes (dry deposition). This provides an estimate in the total atmospheric input of dry NH3 to the soil. Due to the differences in the two soil plots, the period of 31 July to 3 Aug shows strong emissions (positive flux) from dense canopy, therefore, there is no dry deposition predicted for this soil type for the same period. This is also true for 10 to 13 August, in which the open canopy only predicts positive fluxes (emission).
