We appreciate the reviewer very much for the positive and constructive comments and suggestions concerning our manuscript. The comments are very helpful for revising and improving our paper. The main corrections in the paper and the response to the comments are as following.

The authors evaluated the effects of different biochar gradients combined with nitrogen fertilizer on the size, proportion, stability, and carbon content of Mollisols soil aggregates. In addition, they also examine how biochar affects the structure of microbial populations and identify the main factors influencing changes in microbial composition and recommend an optimal biochar application ratio to improve soil quality by modifying aggregates and the soil microbial community structure of Chinese black soils. They concluded that microbial abundance increased significantly when biochar and fertilizer were combined, and soil aggregation and SOC of black soil were significantly improved. The graphic description in the paper is relatively clear and intuitive, which well summarizes the conclusions expressed by the author.

1. Field experiments are a more objective reflection of actual production issues, and it is a pity that we did not see the author's research on crop yields, but it is still an impressive work, and the findings are essential if we want to improve our understanding of the improvement of soil quality through biochar. However, in my opinion, it is very necessary to make proper revisions before acceptance.

Response: Thank you very much for your careful review and constructive suggestions regarding our manuscript. The improvement of soil structure and quality is an important prerequisite for improving crop yield and quality. Therefore, this paper focuses on the analysis of the effect of biochar on soil improvement. However, we have not ignored the trends in crop yield and quality in the improved soils, which have been analysed in detail in subsequent manuscript.

1. Line 16-23, The summary of the Abstract section, the conclusions seem ambiguous. I suggest that the author provide a clear and reliable conclusion in concise language.

Response: Thanks for the advice. We have revised the “Abstract” part as you suggested. We combed through sentences that were relatively vague in meaning to make them clearer and more logical:" Field trials showed that organic carbon accumulation in large aggregates was higher, averaging 28.14%. Biochar application with N fertilizer significantly increased the proportion of aggregates (> 0.25mm), effectively increasing soil carbon storage and agglomerate stability. The phospholipid fatty acids analysis revealed that the accumulation of exogenous carbon and nitrogen effectively improved the microbial community structure (P < 0.05). The C3N treatment increased the F/B ratio by 36.46%, whereas the C3 treatment increased the gram-positive (Gm)/gram-negative (Gm⁺⁻) ratio by 19.67%."

1. Line 44-47, There is large room for improvement in description and expression. Some sentences are long and contain a lot of repetitive information.

Response: Thanks for the advice. The sentence has been amended to: "The principal manifestations of the soil degradation were a significant loss of soil organic carbon (SOC), a decrease in soil aggregation (Zhang et al. 2018), and degradation of soil structure (Luo et al. 2020; Zhang et al. 2019). "

1. Line 60-61, awkward sentence with controversial expression. Try to explain the limited time for straw decomposition.

Response: Thank you for the valuable suggestion. We further explain the constraints of straw decomposition and add references to the literature. The sentence has been amended to: "This is especially likely in high-latitude Chinese Mollisols, where decomposition rate of returning straw is very limited due to long cold time and low temperature. The decomposition time of straw is from 3 months to one year, and the effect is poor. (Wang et al. 2011)."

1. Line 87, "Principal ecological activities including…", double check the term or phrase.

Response: Thanks for catching this. The phrase was modified as: "Principal ecological activities in soil".

1. Line 215, Please explain original profiled soil?

Response: The "original profiled soil" means "the soil without adding any foreign substances". The phrase was modified as: "The surface soil (0–10 cm) had the highest moisture content in the CK treatment".

1. Line 227, please define "bottom soil".

Response: The "bottom soil" means "the 0-40cm soil layer". To make the sentence clear and understandable. The phrase was modified as: "First, the number of macroaggregate components was lower in the 10–40 cm soil layer than in the surface soil".

1. Line 230, "combined application"? unclear expression.

Response: "combined application" means "Biochar combined with nitrogen fertilizer"

1. Line 288-290, Not necessary to simply repeat the numbers or values in the figures.

Response: The interpretation of the diagram has been simplified as requested.

1. Line 342, macroaggregates, small macroaggregates? Please be uniform in the text.

Response: Made the change as suggested.

1. Line 410, Among the effects of biochar combined with N fertilizer on microbial communities, the authors suggest the effectiveness of reduced N fertilizer application to improve microbial richness, and the potential negative environmental effects after high doses of N fertilizer application should be further briefly discussed.

Response: Thank you for the valuable suggestion. Based on your suggestions, the potential negative environmental effects have been supplemented:" In this study, C2N1/2 treatment had the best improvement effect on microbial community structure. Nitrogen source always an important source of microbial nutrients. However, when nitrogen fertilizer application exceeds crop absorption capacity and soil retention capacity, the excess nitrogen may be leachated to deep soil and pollute groundwater. Meanwhile, the unbalanced C/N ratio also became the main factor for the significant decline of soil biodiversity (Yuan et al. 2017)."

1. Line 418, Please explain the difference between TOC and SOC. Both parameters are used.

Response: For TOC in soil (measured in the experiment), it consists of soil organic carbon (SOC) and soil inorganic carbon (SIC). SIC is generally present in the form of carbonate, which was negligible in the soil samples in this experiment, so TOC was used in this paper to represent the amount of organic matter in the samples. However, since microorganisms are unable to use SIC and mainly use SOC as carbon source. It is therefore generally more accurate to use the term SOC for the interpretation and justification of conclusions.

1. Line 426-428, Repeated statement.

Response: Thanks for your careful checks, we are very sorry for our carelessness. It has been corrected

1. Line 430-432, the meaning of this sentence is unclear, rephrase the sentences to make the logic smooth and the meaning clear.

Response: Thank you for the valuable suggestion. The phrase was modified as: " The increased fungal abundance has been proposed as an important biological factor in soil aggregate formation (Yuan et al. 2015; Zheng et al. 2020). Previous research has shown that the improvement of aggregate stability and carbon storage is an important condition to improve the microbial communities (Zhang et al. 2021)."

1. Line 454, delete "The PCA showed that".

Response: Made the change as suggested.