Biochar Effects on Soil Physiochemical Properties in Degraded Managed Ecosystems in Northeastern Bangladesh
Round 1
Reviewer 1 Report
Brief summary:
The authors have studied the effects of wood-derived biochars sustainably produced in northeastern Bangladesh and used as soil surface applications in three land-uses types. Effects were assessed over seven months. Results on soil physico-chemical properties (N, P, K, pH, soil moisture content, organic matter content, and bulk density) are presented and discussed. The authors conclude that biochar can improve the fertility of degraded soils in agricultural and forest systems in Bangladesh.
Broad comments on strong aspects of the manuscript:
The literature review is extensive and covers many aspects of the effects of biochars on the structure and fertility of soils including tropical soils. Lines 87-88 represent in one sentence all the great interest of reading an article which may provide new insight knowledge on biochar properties/effects on soil in three soil-ecosystems.
Broad comments on weak aspects of the manuscript:
The three hypotheses as worded offer very little interest since a large array of articles have already presented a wide variety of results to confirm or deny them.
The experimental design does not allow a reliable and rigorous comparison of the effects of sustainably produced biochar in the three soil-ecosystems since the design does not include any repetition of each of the three soil-ecosystems.
For the concept of "sustainably produced biochar" to be efficiently applied, it is imperative to provide a detailed methodology of pyrolysis conditions to prove its reliable and reproducible capacity to produce high quality biochar batches.
While the results showed that the effect of biochar on soil P is transient in the tea monoculture soil system, the authors missed an excellent opportunity to determine how biochar affects various soil P pools ( P tot, P particles, P dissolved, P org) in each soil-ecosystem. These results would prove to be more significant than determining the soil P alone.
The authors thoroughly discussed the effects of biochar treatment on the physico-chemical soil variables they measured in the context of each soil-ecosystem. However, the soil interactions with microbial diversity and plant (shoot and roots) diversity should be taken into account in each soil-ecosystem. The authors should have mentioned these important interactions into the discussion of their results.
Specific comments:
Line Comment
60 Authors should specify the range of years or a time-scale period corresponding to the lost of 20–80 tC·ha-1.
91-94 The three hypotheses should be rephrased using the sentence in lines 87-88. Thus, they would better correspond to the context of the study which compares three types of soil-ecosystems. In order to confirm or deny them, the authors would have presented results that could be considered more innovative results. Thus, they would have improved the state of knowledge on the effects of a sustainably produced biochar on soil in three soil-ecosystems.
98-146 The results come from only three plots per treatment set up in each of the three soil-ecosystems. This is insufficient given the variability in soil pedological, physico-chemical and biological characteristics, plant species diversity in two of the three ecosystems, as well as the variability of climatic conditions which prevailed during the 7 months of the experience (monsoon and winter dry season). Also, in consideration of the variability factors mentioned above, the surface application of 7.5 t ha-1 of biochar without any incorporation into the 0-10 cm soil layer, or at least superficially, can generate even more variability during the monsoon period of the 7 months experiment.
147-182 The authors must specify: (1) the duration of residence of the biomass in the furnace to complete the biochar process and also for the period that the target pyrolysis temperature is reached and especially maintained; (2) the exact conditions under which biochars are water-washed and sun-dried before their application on soil since these process will remove organic compounds which may have effects on soil biology and other soil parameters; (3) the list of quality parameters of the biochar that should be monitored, including the H / Corg and O / Corg ratios. The latter are used to determine the potential of biochar to sequester C in the soil and mitigate GHGs (Brassard et al. 2016, 2017).
Brassard, P., Godbout, S., Raghavan, V., 2016. Soil biochar amendment as a climate change mitigation tool: Key parameters and mechanisms involved. Journal of Environmental Management 181. doi:10.1016/j.jenvman.2016.06.063
Brassard, P., Godbout, S., Raghavan, V., Palacios, H.J., Grenier, M., Zegan, D., 2017. The Production of Engineered Biochars in a Vertical Auger Pyrolysis Reactor for Carbon Sequestration. Energies . doi:10.3390/en10030288
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript presents important problem - fertilization of degraded and infertile soils. The results are clearly presented, although I have a few questions and comments to the Authors.
- Why did You measure soil pH only in the H2O? pH measured in the water solution is very sensitive to weather conditions and may not reflect the actual results. This is why, You probably should measure the soil pH also in neutral salt solutions, e.g. KCl or CaCl2
- Why did You measure the total carbon concetration, not the organic carbon?
- Do You believe that the duration of the experiment (7 months) was sufficient to obtain meaningful results? Maybe the experiment time was too short and this could be the reason why You have not observed changes in some soil properties after biochar addition.
- There is too many references in the manuscript.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Broad comments on strong aspects of the revised manuscript:
The literature review was already extensive and still covers many aspects of the effects of biochar on the structure and fertility of soils including tropical soils.
The authors addressed adequately many of the weak aspects of the manuscript and most of the methodological imprecisions have been satisfactory corrected.
Broad comments on weak aspects of the revised manuscript:
The authors thoroughly discussed the effects of biochar treatment on the physio-chemical soil variables they measured in the context of each soil-ecosystem. However, the discussion section still lacks insightful discussion elements referring to the comparison of the biochar impact among the three soil-ecosystems studied. Of course, this flaw is mainly due to an experimental design that does not allow a reliable and rigorous comparison of the effects of sustainably produced biochar between the three soil-ecosystems since the design does not include repetition of each of the three soil-ecosystems. The explanations given in the coverletter only partially address this aspect.
Many lines of the revised manuscript have been highlighted in yellow. Although in too few instances, such highlighted texts indicate a modification to the original manuscript or a change of the number of the reference cited (> #57), most of the time the text of the revised manuscript highlighted was identical to the original version. This process was very irritating.
Specific comments for minor modifications:
Line Comment
284-285 The statement should stated that «As biochar residence time increased (± 150 days onwards)….» instead of ±70 days onwards.
309-310 The actual sentence should be modified: « Biochar additions substantially enhanced SOM at all sites. In the tea monoculture plots, biochar increased soil moisture content, reduced soil bulk density and increased soil P and pH. »
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
