the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Feb 2024
Evaluating adsorption isotherm models for determining the partitioning of ammonium between soil and soil-pore water in environmental soil samples
Abstract. Ammonium in soil pore water is thought to participate in bidirectional exchange with the atmosphere; however, common soil nutrient analysis methods determine the bulk quantity of ammonium associated with the soil particles, rather than determining the aqueous ammonium concentration. Previous works have applied the Langmuir and Freundlich isotherm equations to ammonium-enriched soils to estimate partitioning, but this may not be representative of conditions in natural, unmanaged soils. In this work, environmental soil samples were collected from greenspaces in Toronto and used to evaluate several commonly used adsorption isotherm equations, including the Langmuir, Freundlich, Temkin and Toth equations, to determine their applicability in lightly managed and non-fertilized soils. We then compare ammonia emission potentials (a quantity predicting the propensity of ammonia to volatilize from a liquid reservoir) calculated using a conventional nutrient analysis method to that modelled using the Temkin and Langmuir equations, and demonstrate that conventional approaches may overestimate emission potentials from soils by a factor of 5 – 12.
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RC1: 'Comment on egusphere-2024-126', Anonymous Referee #1, 23 Feb 2024
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General comments
The authors present a comparative study on sorption isotherms for ammonium in soils. The manuscript is well-structured and written in a concise style. I see sound scientific work all over, with some minor issues to methodological aspects. But I am sure this could be sorted out...Specific comments
15 'conventional' I suggest to be a bit more precise at this point
26 many references of NH3 emissions from plants not from soil
29-31 I suggest to delete this motivation for sampling natural soils. The sorption mechanism (or the physico-chemistry) of soils with N fertilization, and probably higher NH4 and NH3 concentrations, is exactly the same as for natural soils. The same sorption isotherms are applied...
43 '... soil sample able to participate ...' I suggest to rewrite
110 'RSE' I strongly suggest to compute the Akaike information criterion (AIC) or the Baysian information criterion (BIC) instead of RSE for model identification. AIC relates the model error to the number of parameters.
112 parameters of non-linear functions usually do not average well, and usually using the geometric mean is better than using the arithmetic mean
117 please justify the selection of min, max and some mean value instead of a random sample. And a higher number than just 3 samples in the test data set would actually be required
119-120 points 'i.' and 'ii.'; do you intend to test the potential of CEC to predict sorption?! That's a good idea anyways, but it would be nice to mention it already in the introduction as a goal.
138 pooling is theoretically the more appropriate procedure
148 'individual' instead of 'individually'?
150 For the figure caption I suggest to replace 'standardizing the y-axis' with 'normalized'
Table 2 Use either 'SE' or 'Standard error' in the column headers for consistency; I guess the number of parameters stated for Langmuir is wrong, this is also =2; For Freundlich I would move 'Smax*0.052...' one column to the right?!
161 would be nice to have a sort of pedotransfer function to estimate Smax or other sorption parameters from CEC. I think even the small data set present in this study could be used for an initial test ...
162 well, there are differences in the model errors, however for field conditions, e.g in the context of agroecosystem models, it actually does not matter much. Simply because the sorption if NH4 is very, very high. And it does not really matter that much how high Smax is as long as the sorption at low concentrations near zero is estimated well. And in that range of concentrations I assume the four isotherms provide very similar results. Something that should probably be picked up in the discussion.
167 'an additional parameter' this is where the AIC is helpful...
155-160 I think this paragraph would benefit from additional references to literature
figure 2 I think the label of the y-axis is wrong. This probably is the non-normalized sorbed NH4 concentration, otherwise it should also vary around values <1 ...
190-194 I am not sure I understand the idea behind Eqs. 9 to 13. Eq. 7 could be rearranged to C=((mNH4-S)*p)/w and S can be computed from any of the isotherm equations given in Table 1. OK, then you need two equations, but this is easy to compute. For me it is particularly difficult to understand the assumption wC/p=0. You basically neglect the liquid phase concentration of NH4, and this is basically not required. It just makes your estimate of C worse.
196 yes, that 0.57-1.5% is probably the ratio of C/S, and just a consequence of neglecting the liquid phase NH4 concentration
226 replace 'more sound' with 'mechanistic'?
251-252 please remove the phrase '; and more research ... of both approaches.'
253-255 please delete or relate to any of your results
264 how much better? % ...Citation: https://doi.org/10.5194/egusphere-2024-126-RC1 -
CC1: 'Comment on egusphere-2024-126', John Walker, 21 Mar 2024
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This paper tackles the important question of how to properly estimate the soil NH3 compensation point for air-surface exchange modeling. To this end, the authors present an extensive and much needed examination of soil NH4+ sorption characteristics. I have a few questions about the methodology that might be considered and a suggestion to improve the usability of the results.
Line 86: Typically, the quantity of NH4+ adsorbed onto the soil is inferred from a measurement of NH4+aq in the equilibrated solution (see the papers referenced in Section 1.3). In this study, the quantity of sorbed NH4+ is inferred from measurements of Na+, Mg2+, Ca2+ and K+ in solution. A justification of this approach would be helpful. Was a comparison conducted to see if the two approaches yield similar sorption parameters? If so, it would be a useful addition to the paper.
Paragraph beginning line 143: How do the different equations compare over a narrower range of NH4+aq? The maximum NH4+aq concentration in this study (Figure 1) is larger than in the references cited (Alnsour, 2019; Venterea et al., 2015; Vogeler et al., 2011). This comparison could be informative to the applicability of the models to soils with low NH4+ concentrations.
Line 192. I would encourage the authors to avoid simplifying equation (7) to exclude the aqueous portion. Understandably, it is a small mass compared to the sorbed portion but the simplification is unnecessary.
General comment: I encourage the authors to include a table summarizing the fitting (sorption) parameters for each equation for each sorption experiment, grouped in a way that they can be related to the corresponding basic soil chemical parameters (CEC, pH, extractable NH4+). A more complete and detailed summary of the data will make the results more widely usable.
Citation: https://doi.org/10.5194/egusphere-2024-126-CC1
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