Objectives Maize are generally continuous planting in the black soil region of northeast China, which has led to shallow black soil layer and degradation of organic matter content. To restore the microenvironment of black soil, we approached strategies of biochar application and maize-soybean intercropping.

Methods A localized field experiment, sited in Daqing City of Heilongjiang Province, was conducted for 4 years, the treatments were composed of maize monoculture without and with biochar application (MB0, MB15), and maize/soybean intercropping without or with biochar application (MSB0, 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 the yields were in order of MSB15>MB15>MSB0>MB0(P<0.05). Biochar application significantly increased soil organic carbon, available N and total P content, intercropping did not change the soil nutrient contents significantly. Among the four treatments, MSB15 and MB15 were recorded significantly higher SOC, AN and TP contents than MSB0 and MB0 did. The Shannon indices of bacteria and fungi in biochar treated soils (MB15 and MSB15) were significantly higher than those in the 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 the bacterial phylum Actinobacteriota in the soil, with the order being MB15>MB0 and MSB15>MSB0, and the MSB15 treatment showing the highest abundance. Meanwhile, they decreased the relative abundance of the phylum Proteobacteria, with the MSB15 treatment having the lowest abundance. Biochar increased the relative abundance of the fungal phylum 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. This indicated that biochar and intercropping promoted the growth of these 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. Soil organic carbon (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 availableP (P<0.05), and showed a positive correlation with the aforementioned beneficial bacterial genera, but a negative correlation with harmful pathogenic genera.

Conclusions Application of biochar could significantly increase 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 are effective measures for protecting the black soil farmland environment, increasing crop yields, and promoting sustainable agriculture.