Linkage of Crop Productivity to Soil Nitrogen Dynamics under Biochar Addition: A Meta-Analysis across Field Studies

Round 1

Reviewer 1 Report

• The summary of biochars and soils information (values +SD) like physical and chemical properties such as pH, EC, CEC, Sand, Silt content, etc. should be mentioned in a table in the manuscript. 
• Although "Web of Science" and "Google Scholar" are well-known websites, both have delays in updating published papers. Why were these two websites used to choose peer-reviewed papers? Please make a note of it in the manuscript. 
• What is the data distribution by country? How many countries were included in 99 papers? Figure 1 shows that the majority of them are from China, which may confine data to a specific range of soils and climate conditions. 
• It was necessary to normalize the data based on soil and crop type. In terms of soil, plant species, and climatic conditions, the amount of yield for a certain crop differs from country to country. 
• The amount of changes that could made by biochar application for a selected crop in one soil differs from the same crop in another soil. Such aspects cannot be studied due to a lack of data. 
• The absence of a biochar dosage conclusion in the data is felt sharply. 
• The software (Get-Data Graph Digitizer 2.26) used in the paper to obtain data from figures is sensitive to image or pdf resolution, thus utilizing the software may result in a broad range of data values and tolerance of it. Furthermore, the SD for samples that were not replicated (one point on the figure refers to 2 or 3 replications, and the SD could not be calculated) was not determined preciously. Using a fixed tolerance of 10% for each set of data may result in unrealistic data. 
• The benefits of biochar on crop productivity could be influenced by a variety of factors. The effects of biochar in soils are directly related to three primary fields: biochar, soil, and selected crop. This paper only discussed crop response and nitrogen forms, and the results are clearly influenced by the lack of soil data. 
• The title of the paper includes the words "global" and "metadata," which do not appear in the document. The collecting and categorization of a diverse range of peer-reviewed papers is insufficient. 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Based on the published articles that are cited in this pre-review manuscript, it is suggested that Biochar have optimistic role in nutrient availability in plants. The meta-analysis conducted in this pre-review manuscript based on past studies’ data is quite interesting but following amendments are deemed necessary for the publication of this manuscript.

1.

Authors need to elaborate following:

1. How much nitrogen uptake and upsurge were taken into consideration since the use of Biochar solution is not clearly defined, there is an ambiguity between the PNU and already available Nitrogen in plants.
2. How much Nitrogen level increased after the usage of a specified amount of Biochar.

2

1. The meta-analysis was done, which is quite fascinating and moving towards sustainability based on BNF and TN, however, what are the consequences, if nitrogen reach a high level by the use of biochar solution, is there any recommended threshold value of usage of biochar solution and consequent nitrogen level increase?

ii.

Cm: The mean of control treatments

Csd: The standard deviation of control treatments

Cn: The sample sizes of control treatments

Tm: The mean of biochar treatments

Tsd: The standard deviation of biochar treatments

Tn: The sample sizes of biochar treatments

based on ABT analysis and N₂0 influx on plant growth and development after the addition of biochar what are the attributes for the above mentioned analysis.

3.

1. Soil TN content was positively correlated with crop productivity, but biological N2 fixation was negatively correlated with crop productivity. Please elaborate what are its implications, how to increase the amount of N uptake in the plant, considering that if N uptake by the plant is small the surplus N remains in the soil, next how to reduce it via fertilizers.

Concluding remarks:

The given data and scientific approach manifested in this manuscript are quite interesting but authors need to elaborate or clarify the above mentioned points. Moreover, please also improve the visuals given in the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

All of my comments have been addressed and I believe the manuscript is now ready for publication. Congratulations to the authors.