Biochar Enhances Soil Fertility Through Bacterial Community Modulation in Karst Slopes

Biochar can markedly modify the activity of soil bacterial communities and enhance soil quality. However, in karst regions, evidence remains limited on whether—and by what mechanisms—biochar enhances soil fertility by reshaping bacterial diversity and co‐occurrence network structure. This study focused on karst yellow soil, conducting a two‐year natural monitoring experiment on runoff plots (2 × 1 m). The experimental design evaluated multiple factors: slope gradient (15°, 25°), biochar application time (1, 2 years), and application rates (B30 = 30 t/ha, B60 = 60 t/ha) to assess changes in soil fertility and bacterial community characteristics. In all treatments, biochar application improved soil fertility. The segmented structural equation model showed that under the 15° condition, fertility enhancement was mainly achieved by increasing bacterial diversity, while under the 25° condition, it depended on the bacterial network structure. During the initial application period (1 year), bacterial diversity regulated the effect, while with long‐term application (2 years), fertility improvement was unrelated to the microbial community. Interaction analysis showed that the amount of biochar added was the main factor for improving soil fertility in different treatments ( F = 58.34). Increasing the amount showed a dose‐related improvement in fertility ( R 2 = 0.706, p < 0.01), and different mechanisms were observed between different amount levels: at B30, bacterial diversity, network structure, and key species ( Actinobacteria ) had a significant impact on soil fertility, while at B60, biochar application mainly increased bacterial diversity index to improve soil fertility. These results established a functional chain connecting biochar application, microbial community regulation, and enhanced soil fertility in slope ecosystems. The demonstrated efficacy supported biochar implementation as a viable strategy for improving soil quality on sloping lands in karst regions.