Objectives In the black soil region of northeast China, maize is generally grown under continuous monoculture, which has led to a shallow black soil layer and degradation of organic matter content. To restore the microenvironment of black soil, this study evaluates the effects of biochar application and intercropping on the physicochemical properties and biological characteristics of maize rhizosphere soil.

Methods A four-year field experiment was conducted in Daqing City, Heilongjiang Province, with four treatments: maize monoculture withoutbiochar application (MB0), maize monoculture with biochar application ( MB15), maize/soybean intercropping without biochar application (MSB0), and maize/soybean intercropping with biochar application (MSB15). The yield, soil nutrient content were analyzed at maize harvest. And the diversity indices of bacteria and fungi, and the relative abundance of dominant bacteria and gungi at general and phylum levels were determined.

Results Both biochar and intercropping enhanced maize yield, and yields were in order of MSB15>MB15>MSB0>MB0 (P<0.05). Biochar application significantly increased soil organic carbon (SOC), available N (AN) and total P (TP) content, while intercropping had no significant effect. Among the four treatments, MSB15 and MB15 showed significantly higher SOC, AN and TP contents compared with MSB0 and MB0. Shannon indices of bacteria and fungi in biochar treated soils (MB15 and MSB15) were significantly higher than in MB0 treatment, while the fungal Shannon index in MSB0 treatment was significantly lower than that in the MB15 treatment. Biochar and intercropping increased the relative abundance of Actinobacteriota in the soil, with order being MB15>MB0 and MSB15>MSB0, and MSB15 treatment showing the highest abundance. Meanwhile, they decreased the relative abundance of the phylum Proteobacteria, with MSB15 treatment having the lowest abundance. Biochar increased the relative abundance of Ascomycota in soil, following the order MSB15>MSB0 and MB15>MB0, while it decreased the relative abundance of Basidiomycota, with MSB15<MSB0 and MB15<MB0. At the genus level, a two-way analysis revealed that biochar, intercropping, and their interaction all had significant effects on the bacterial genus Arthrobacter. Additionally, biochar had significant impacts on the genera Sphingomonas and Nocardioides, while intercropping significantly influenced the fungal genus Mortierella. Biochar and intercropping promoted the growth of beneficial bacterial genera, reduced the relative abundances of harmful pathogenic genera such as Cladosporium, Gibberella, and Didymella, with the MB15, MSB0, and MSB15 treatments showing lower or significantly lower abundances compared to the MB0 treatment. SOC was identified as the most critical factor influencing the community structures of soil bacteria and fungi. Crop yield exhibited positive correlation with SOC and total P (P<0.01), soil available P (P<0.05), and showed a positive correlation with the aforementioned beneficial bacterial genera, but a negative correlation with harmful pathogenic genera.

Conclusions Biochar application significantly increased the contents of soil organic carbon, alkali-hydrolyzable nitrogen, and total phosphorus, and the use of biochar under intercropping has an even better enhancing effect. Both biochar and intercropping could increase the relative abundances of beneficial bacteria and fungi while reducing that of harmful fungi in soil, consequently optimizing the soil microbial community structure, enhancing soil nutrient supply capacity, and establishing a favorable and healthy rhizosphere soil microenvironment. The combined use of biochar and intercropping is an effective measures for protecting black soil farmland environment, increasing crop yields, and promoting sustainable agriculture.