Effects of Biochar-Based Fertilizers on Fenlong-Ridging Soil Physical Properties, Nutrient Activation, Enzyme Activity, Bacterial Diversity, and Sugarcane Yield

Biochar-based fertilizers can improve soil structure and fertility. However, their efficiency is affected by the raw materials of biochar. The effects of biochar-based fertilizers on the soil microenvironment under Fenlong-ridging conditions remain unclear. This study aimed to evaluate the effects of biochar-based fertilizers derived from sugarcane filter mud and rice straw on soil physicochemical properties, microbial communities, and sugarcane yield under Fenlong-ridging in Guangxi’s acidic red soil (Hapludults). A two-year field experiment (2021–2022) was conducted on a clay loam soil classified as Hapludults (USDA Soil Taxonomy) in the same experimental plots using three fertilizer applications—conventional chemical fertilization (CK), straw biochar-based fertilizer (T1), and sugar filter mud biochar-based fertilizer (T2)to determine the responses of soil physicochemical properties and bacterial community diversity to different biochar-based fertilizers and evaluate benefits to the soil environment and sugarcane yield. Soil samples (0–20 cm depth) revealed that T1 and T2 reduced bulk density by 2.31% and increased porosity by 2.00–2.31% versus CK. Notably, T2 exhibited 4.1-fold higher specific surface area than T1, driving stronger soil–bacterial interactions: it enhanced soil moisture (7.17–13.05%) and pH (17.89–24.14% in 2021; 8.68–11.57% in 2022), thereby promoting nutrient availability (N, P, K), organic matter (SOM), and sucrase activity. Microbiome analysis showed T2 enriched Gemmatimonadota and Sphingomonas (beneficial taxa) while suppressing Acidothermus. The results of RDA and Spearman correlation analysis indicated that the bacterial community structure was mainly affected by soil pH, TN, AP, and SOM. Consequently, T2 increased sugarcane yield by 5.63–11.16% over T1 through synergistic soil–microbial improvements. Future studies involving multi-site and long-term experiments are needed to confirm the broader applicability and stability of these findings. This study provides a theoretical basis for the positive regulation of sugar filter mud biochar-based fertilizers in the soil environment, bacterial community structure, and sugarcane yield.