Rate-Dependent Effects of Biochar on Soil Fertility and Bacterial–Fungal Communities in Maize Fields of the Black Soil Region: A Three-Year Field Study

Biochar can improve soil physicochemical properties and microbial habitats; however, its application rate-dependent effects in maize fields of the black soil region remain insufficiently understood under field conditions. A three-year field experiment was conducted in Jiamusi, Heilongjiang Province, China, from 2023 to 2025, with four biochar application rates: 0 (W0), 10 (W1), 20 (W2), and 40 t ha−1 (W3). Soil physicochemical properties, bacterial communities based on 16S rRNA gene sequencing, and fungal communities based on internal transcribed spacer (ITS) sequencing were analyzed to assess changes in soil fertility and microbial community composition and their relationships with environmental factors. Biochar application significantly increased soil organic matter, alkali-hydrolyzable nitrogen, available potassium, and pH. Although W3 produced the greatest nutrient enhancement, W2 exhibited a more balanced overall response across the measured soil fertility and microbial community indicators. Sequencing depth was adequate for all samples, and bacterial alpha diversity was comparatively well maintained under W2 and W3. Fungal alpha diversity exhibited pronounced interannual variation and increased under W3 in 2025. Year accounted for a greater proportion of variation in microbial community structure than did biochar treatment; however, both treatment and the year × treatment interaction also had significant effects. Among the measured soil fertility and microbial community indicators, W2 produced a comparatively balanced overall response, whereas W3 exerted stronger selective effects on microbial communities. Because crop yield, economic feasibility, labile carbon fractions, and long-term ecological outcomes were not assessed, an agronomically optimal biochar application rate cannot yet be determined.