The authors contribute with their study to an ongoing and substantial discussion of the effect of biochar on the hydraulic properties of soils and the potential of biochar to bind and retain nitrate and ammonium in soils. While this is an important discussion for the application of biochar in agricultural soils, the submitted manuscript is not well structured and, much more importantly, it is not clearly providing a novel approach or understanding for the ongoing scientific discussion. Furthermore, the manuscript is not transparent to follow the methodological approach. It is not clear why the column retention experiment was only performed for the HSL and the described effect of additional nitrate leaching with biochar is not supported by shown data. The fairly short discussion is by far not complete. Many aspects contradicting the here reported findings are not considered (please see specific comments). This results also in a lack of new mechanistic understanding and the link to the agricultural soils. For example, the authors are not considering the effect of the two agricultural soils on the nutrient mobility or bring their findings in context of potential field applications.

I highly recommend the authors to consider a critical discussion of their findings, developing supported mechanistic understanding from these experiments, improve the transparency of the experimental approach and improve the overall manuscript structure. Given these aspects, I decided to reject the current manuscript for publication in SOIL.

Specific comments:

Abstract and introduction:

Line 9-10: specify “saturated hydraulic conductivity (Ksat)”

Line 44-46: Provide reference for this statement

Line 65: What is the mechanism for the high values found in Yin et al (2018). Please provide more details

Line 96-102: To detailed method description for an introduction. Please shorten to avoid repetition.

Material and Methods:

Line 106-108: What are the production durations of the selected chars

Line 107-108: Please provide details on the char with inoculated microbial formula

Line 113: What is the duration of the individual temperature steps?

Line: 122: The authors can avoid to mention a private company because they followed the standardized protocol

Line 123: Provide ISO number here

Line 124-128: Not clear if the authors used finally the DRIFT or FTIR. Please clarify.

Line: 137-138: Not clear to what field trials they authors are referring here. Were the soils taken from long-term field trials locations?

Line 147: What was the core volume?

Line 165: The authors should include more information about the tested models in the supplement. Which fitting parameter were considered to evaluate the goodness of fit and avoid over parameterization (e.g. AICc)

Line 176-177: Was the soil and biochar homogeneous mixed? How was this ensured?

Line 177: What would be the typical application rate on the agricultural soils used in this study?

Line 181: Why did the authors not include both soils here? Please provide a clear argument since this is substantial for the whole discussion of the manuscript.

Results: Large parts of the result section describes the biochar and soil. The author should consider to include the characterizations in the material and method section. The result section should focus on the actual findings regarding the sorption and Ksat effect of the biochar on the soils.

Line 196: Please specify “carbon, hydrogen contents and leachable DOC”

Line 196-197: Please avoid interpretation of the data and comparison to the litterateur in the result section. This is part of the discussion.

Line 203-204: This aspect should be considered in the discussion and clearly mentioned in the material and methods. The oxidation state of the biochar will also influence the surface reactivity, which may, in fact, explain the here observed findings.

Line 211: include “1410 and 1418 cm^-1”

Line 213-215: As mentioned above these differences in biochar production should be clearly presented in the material and method section and also critically discussed in the discussion

Line 237: Soil texture expressed as mass per mass (g/g) is a content and not a concentration. Furthermore, avoid digits for these values.

Table 3: Correct the number of digits for texture. Also, pH is commonly measured with on digit precision.

Section 3.3: Provide the data for the nitrate leaching. What is the order of magnitude if the nitrate release? This data needs to be shown.

Figure 2: Please show the fitted isotherms

Line 268-369: Please specify this statement and clearly indicate to which the p values correspond to.

Line: 285: “HSL at pore volume 14.3” corresponds this to the controls?

Discussion: This discussion is not complete and is not discussion available contradicting literature. A few suggestions can be found below. However, I recommend an extensive literature review to develop a structured and complete discussion.

Line 315: t is mentioned already that this char might be oxidized, the authors should clearly indicate this in the sections before.  This initial "bias" effect needs more critical discussion here. The hole paragraph provides no mechanist discussion. It is just comparing the findings with the literature. Please improve the discussion here and connect the different sportive capacities with the properties of of the chars. 

Line 318-320: Figure 2 shows actually no clear differences between SW800 and the other chars. What is the explanation? In fact, only AS800 shows the previous mentioned large binding capacities of ammonium.

Line 324-329: The whole paragraph misses to bring the findings of this study in context of studies with contradicting results which is actually in some of the already cited papers. But there is certainly more literature on this effects and higher nitrate binding capacities are reported. Only Zhang et al (2020) is cited here to support the findings of this study, which is by far not complete. Here are a few suggestions also providing contradictory findings (and literature within):

Kameyama, K., Miyamoto, T., Iwata, Y., and Shiono, T.: Influences of feedstock and pyrolysis temperature on the nitrate adsorption of biochar, Soil Science and Plant Nutrition, 62, 180–184, https://doi.org/10.1080/00380768.2015.1136553, 2016.

Cao, H., Ning, L., Xun, M., Feng, F., Li, P., Yue, S., Song, J., Zhang, W., and Yang, H.: Biochar can increase nitrogen use efficiency of Malus hupehensis by modulating nitrate reduction of soil and root, Applied Soil Ecology, 135, 25–32, https://doi.org/10.1016/j.apsoil.2018.11.002, 2019.

Yang, J., Li, H., Zhang, D., Wu, M., and Pan, B.: Limited role of biochars in nitrogen fixation through nitrate adsorption, Science of The Total Environment, 592, 758–765, https://doi.org/10.1016/j.scitotenv.2016.10.182, 2017.

Aghoghovwia, M. P., Hardie, A. G., and Rozanov, A. B.: Characterisation, adsorption and desorption of ammonium and nitrate of biochar derived from different feedstocks, Environmental Technology, 1–14, https://doi.org/10.1080/09593330.2020.1804466, 2020.

Hagemann, N., Kammann, C. I., Schmidt, H.-P., Kappler, A., and Behrens, S.: Nitrate capture and slow release in biochar amended compost and soil, PLoS ONE, 12, e0171214, https://doi.org/10.1371/journal.pone.0171214, 2017.

Section 4.2: Similar to the paragraph before, this section misses a critical discussion of the findings. The authors need to include a more mechanistic explanation of the ammonium and nitrate retention in soils. Actually, the soil effect (e.g. texture and pH) is not included at all. All these observations are also based on the experiment of the HSL. This need to be critically discussed. The effect may change drastically with different soils. Please follow also here the above mentioned literature, which is only a short list of literature on this topic.

The authors also miss to bring their findings in context of the applicability under field conditions and unsaturated soil conditions.

Section 4.3: This section also misses some aspects which need to be discussed in this context. Only one application rate of biochar was used, it is not discussed if this rate is representative for these soils and its acricultural use. Furthermore, it is known that also the application rate and particle size has an effect on the Ksat depending in the soil texture as discussed in the below listed literature.

Obia, A., Mulder, J., Hale, S. E., Nurida, N. L., and Cornelissen, G.: The potential of biochar in improving drainage, aeration and maize yields in heavy clay soils, PLoS ONE, 13, e0196794, https://doi.org/10.1371/journal.pone.0196794, 2018.

Herath, H. M. S. K., Camps-Arbestain, M., and Hedley, M.: Effect of biochar on soil physical properties in two contrasting soils: An Alfisol and an Andisol, 209–210, 188–197, https://doi.org/10.1016/j.geoderma.2013.06.016, 2013.

Barnes, R. T., Gallagher, M. E., Masiello, C. A., Liu, Z., and Dugan, B.: Biochar-induced changes in soil hydraulic conductivity and dissolved nutrient fluxes constrained by laboratory experiments, 9, https://doi.org/10.1371/journal.pone.0108340, 2014.

Line 353-354: What was the relative particle size distribution. These characteristics are not presented.

Line 354-355: How can the authors provide prove of this statement?

Line 374-376: This has not been discussed so far. But the field applications of this experiment need to be included in the critical discussion. The intention of this study was, according to the title, to consider agricultural soils. Furthermore, how can the authors draw a conclusion for flooded agricultural systems when they did not include soils from such systems?

Application of biochar to bind nutrients in soil and alter hydraulic properties of the soil is an important and relevant topic for large scale application of biochar in agricultural fields. The authors of this current paper have tried to add more insights into the existing literature in this context. Overall, after a first glance through, the reader can follow the main message of the paper. However, I have a few main points of concern regarding the manuscript:

• The title of the paper states “inhibits nutrient leaching” – the data for nitrate does not necessarily show this.
• There lacks a sense of novelty in the experimental approach of the manuscript. Experimental details are missing especially for the column studies.
• A more mechanistic insight would have been interesting. Key factors which would have been critical for achieving this and making a more impactful statement are (i) measurement of point of zero charge (for supporting any statements using electrostatic repulsion or attraction) (ii) measurements of anions and cations released during column nutrient leaching tests (iii) use of non-reactive tracers such as “deuterated water” could have been an interesting approach to understand movement of water through columns, etc.
• The term “physical and chemical interactions/affinity” is used very lightly and often in the manuscript without providing concrete proof for these interactions.
• An in-depth literature study in the discussion would have provided readers with more confidence in the conclusions that the authors wished to make.
• The entire sense of “timing of release of nitrate” and its importance needs to have been brought to light. Is it sufficient for nitrate to be captured physically for a short duration and then released?
• The entire discussion in Section 4.3 is underwhelming.

Abstract and Introduction

Line 47-48 – this is not always true. There are some biochars which have a PZC of 7.5 or higher and then they might be positively charged.

Line 85 – Suggestion is to introduce what is saturated hydraulic conductivity out here itself.

Line 91 – how is the biochar “physically altering the soil to influence Ksat?”

Materials and methods

Line 105 – From which four commercial companies?

Line 107 – What is the inoculated microbial formula?

Line 137 – Do not see the need to specify ongoing field trials if there is no connection with the current paper.

Line 156 – It makes more sense to present electrolyte concentrations on a mM or M basis, to normalize it. Why is this test done with NaCl and the column tests with CaCl2?

Line 165 – Which are the “multiple equations”?

Line 175 – How were the columns packed? Was the biochar homogeneously mixed with the soils? How was existence of preferential flow ruled out? Any tracer? What was the flow rate and the pore volume? Experimental details for reproducibility is missing in general.

Lines 175-184 – Why was ksat measured for 2 soils, whereas sorption for only 1?

Results

Line 196 – increased pyrolysis temperature usually increases carbonization.

Line 268 – what do you mean by “main effect”? p values correspondence not clear.

Figure 4-  It is very hard to discern the data and the decrease in leaching of NO3 from the control to HSL+SW500. Please consider to reduce the y axis from 100 mg/L to something smaller (4(a)) to make the graph better accessible (in regards to the data in the text) for the readers.

Discussion : In general, the discussion is not sufficient, and needs better structuring, with more references.

Line 324-329 –This explanation is a bit underwhelming. A more mechanistic approach to this would have been to also measure cations and anions in solution – if nitrate is bound to positively charged components in the ash, one should see some anions being released. PZC measurements would have been crucial in the experimental design, since a lot of the reasoning is based on “electrostatic repulsion”.

Line 340-342 – A tracer study using “deuterated water” or something similar would have been a more mechanistic way to explain the movement of water through biochar packed columns.