Microorganisms perform a crucial function in the biogeochemical processes that maintain soil quality. Nonetheless, the influence of biochar on soil microbial communities and their ecological functions remains poorly understood in black soils. To investigate this, a 2-year field experiment was conducted with four biochar application treatments: 0 t ha− 1 (CK), 6 t ha− 1 (BC6), 12 t ha− 1 (BC12), and 24 t ha− 1 (BC24). Microbial diversity and community composition under each treatment were assessed using high-throughput sequencing techniques. PICRUSt2 and FUNGuild were employed to predict microbial functional profiles. Compared to the control (CK), biochar addition led to notable shifts in both bacterial and fungal community structures. It also significantly enhanced bacterial α-diversity, as reflected by increased Shannon index, OTU counts, and Chao1 richness. However, a gradual decline in bacterial diversity was observed with rising biochar application rates. Taxonomic analysis revealed that biochar treatment significantly elevated the relative abundances of specific bacterial groups—such as Acidobacteria, Chloroflexi, Candidatus_Solibacter, and Bryobacter—as well as fungal groups such as Ascomycota, Zygomycota, Mortierella, Penicillium, and Fusarium. These effects were most evident under the moderate application rate (BC6). These microbial community changes may contribute to maintaining agroecological functions and soil health in biochar-amended soils. Regarding ecological functions, biochar application enhanced soil bacterial metabolic potential and saprotrophic fungal abundance, with more significant effects in the BC6 treatment, while reducing plant pathogenic fungi. This suggests beneficial effects on soil health maintenance and elemental cycling. Therefore, from the perspective of soil microbial community characteristics, biochar application positively influences black soil quality improvement. Considering environmental and economic benefits, a lower biochar application rate (6 t ha⁻1) may represent an optimal strategy for carbon sequestration, soil quality enhancement, and agroecological function maintenance in the studied system.

Optimal rate of biochar application has positive effects on soil functional microbial abundance and agroecological function in a black soil of Northeast China
    Abstract
        Graphical abstract
    1 Introduction
    2 Materials and methods
        2.1 Site description
        2.2 Preparation and properties of biochar
        2.3 Experimental design, collecting soil samples, and figuring out the physical and chemical properties
        2.4 High-throughput sequencing, data analysis, and soil DNA extraction
        2.5 Statistical analysis
    3 Results and discussion
        3.1 The chemical and physical properties of soil
        3.2 Diversity of microbial communities
        3.3 Species composition of microbial communities
        3.4 Association of microbial communities with soil physicochemical parameters
        3.5 Potential functions of microbial communities
        3.6 Optimization strategies for biochar application rates in soils
    4 Conclusions
    References

• Sen Dou(Key Laboratory of Soil Resource Sustainable Utilization for Commodity Grain Bases of Jilin Province, College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China)
• Bowen Zhang(Key Laboratory of Soil Resource Sustainable Utilization for Commodity Grain Bases of Jilin Province, College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China)
• Wenjing Zhu(Key Laboratory of Soil Resource Sustainable Utilization for Commodity Grain Bases of Jilin Province, College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China)
• Yuhan Xia(Key Laboratory of Soil Resource Sustainable Utilization for Commodity Grain Bases of Jilin Province, College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China, Key Laboratory of Straw Comprehensive Utilization and Black Soil Conservation, The Ministry of Education, Jilin Agricultural University, Changchun 130118, China)
• Song Guan(Key Laboratory of Soil Resource Sustainable Utilization for Commodity Grain Bases of Jilin Province, College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China, Key Laboratory of Straw Comprehensive Utilization and Black Soil Conservation, The Ministry of Education, Jilin Agricultural University, Changchun 130118, China)
• Fangni Zhu(Technology Innovation Institute of Jilin Province, Changchun 130052, China)