Dear Reviewer,

First of all, the authors want to thank you for your nice words and the insightful comment.

The authors are aware that the type of biochar is one that degrades relatively quickly, and as such we included the full information on its properties, in this case, specifically the H/C ratio of 0.5. The fact that this biochar is not representative of the materials used in the European biochar market can be certainly pointed out in the discussion, and we will include a sentence about it. We can also include more detailed information about the biochar in the manuscript; however, an extensive description of the material is already contained in the article by Ventura et al. 2015.

However, we want to point out some facts in the article that should support the findings of the study:

1. The Biochar left in the field after 8 years is anyway much lower than that expected from the literature for a H/C 0.5 Biochar.
2. The H/C 0.5 Biochar degradation time estimates in the literature come from laboratory analysis, while our results come from the field – as such, other processes that could increase the speed of mineralization of the biochar cannot be ruled out, as pointed out in section 4.2. We are working in the direction of better understanding the biochar mineralization processes in the field that are not yet accounted for, and to do that, we are actually implementing the Long Term Experiment Platform to test and validate processes assumptions (https://site.unibo.it/environmental-management-research-group/en/long-term-platform ).
3. The biochar mineralization calculated after 8 years is in line with the mineralization rate estimated after 3 years, 3 years during which gas chambers were operated in the site and, thus, the gaseous losses were measured continuously.
4. Biochar leaching cannot be ruled out completely, however it is not very likely, since our measurements over the soil profile did not show an increase in SOC or TOC in the deeper (20-40 cm) profile in time. The soil texture was very fine, and this makes the leaching less likely. We are actually designing an experiment to verify whether there was any biochar leaching in the soil. Finally, we recognize that this is something that our study is not accounting implicitly, and as such is identified as a study limitation in section 4.3, line numbers 420-425.

We hope we have been able to address the comment to the reviewer satisfaction, and remain open to further discussion.

Sincerely,

The Authors.

Dear Gabriel, 

First of all, thank you for your comments, which highlight some interesting open points of our study, and help us discuss them more in detail.  

About the first major comment, we must admit we did not measure leaching directly in the field experiment. However, during the first three years of the experiment, we measured both soil respiration and soil content of both naturally occurring SOM and added biochar. The results of the model, together with these measurements, point to the fact that if there was any leaching, it was negligible – at least in the first 3 years of experiment. The soil of the study area has texture 12% clay, 34% silt, e 54% sand, very similar (but with less sand) to a sandy loam soil used in  a recent columnar experiment (Schiedung 2020); in that study, only 1% of the biochar was reported as being leached, most of it leached during the first flushing of the column. Finally, we measured the biochar in the soil at different depths, and the concentration of biochar at different depths did not change in time, i.e. we did not measure any downward migration of the biochar in the soil profile. 

What happened after the first 3 years of experiment, however, is more debatable, since no soil respiration measurements were available for years 4 to 8. However, we believe the situation would not change much, since the biochar was already aged during the first 3 years. Anyway, we admit that the experiment and the model would benefit from a direct measure of the downward migration; we will enlist this issue in the discussion as a study gap and study need, to be filled in the future. We propose the reviewer that we include all this discussion above in the article discussion. 

In a second major comment, you suggest to elaborate on the aggregation of biochar and clay minerals. Indeed, we visually directly observed interaction between them, with the biochar particles completely covered by adsorbed clay particles. However, this visual observation was not quantified. Thus, we cannot say much about a possible stabilizing effect of the clay particles; more so considering our biochar degraded in the soil much faster than we expected. We propose the reviewer we will include this observation and this limitation in the discussion of the article.  

Finally, we want to thank you for your suggestions, which we will address fully in the manuscript revision.   

Sincerely, 

The Authors