Biochar-silkworm excrement co-compost alleviates Rehmannia glutinosa continuous cropping obstacles with coordinated soil, plant, and rhizosphere bacterial responses.

Continuous cropping obstacles (CCO) severely constrain the sustainable cultivation of Rehmannia glutinosa (R. glutinosa) by degrading soil quality and disturbing rhizosphere microbial communities. This study developed a rice-husk biochar-silkworm excrement co-composting system and evaluated the resulting compost in continuously cropped soil. Among six composting treatments, the compost containing 9% biochar (BSC3) showed the highest overall quality, reaching a peak temperature of 63.9 °C, a final cation exchange capacity of 39.4 cmol+ kg-1, a C/N ratio of 11.1, and a germination index above 80%. Compared with the no-biochar compost control, BSC3 also showed a 105% increase in the relative abundance of Bacillota. Application of BSC3 at 3% increased tuberous-root dry weight to 20.0 ± 2.38 g pot-1, 85.8% higher than the continuous-cropping control, and increased catalpol, verbascoside, and rehmannioside D contents by 226.7%, 107.1%, and 233.3%, respectively. The 3% BSC treatment also improved photosynthetic performance and antioxidant capacity and was associated with shifts in rhizosphere bacterial community structure. In the rhizosphere, Bacillota abundance increased by 45.6% relative to the continuous-cropping control, reflecting a shift in bacterial community composition. At a 5% application rate, plant responses were weaker than those at 3% and were accompanied by higher soil electrical conductivity and nutrient availability. Overall, 9% biochar produced the best-performing compost within the tested screening system, and 3% BSC exhibited the strongest plant response under the present pot conditions. However, long-term field validation and further assessment of heavy metals, pathogens, potential salt accumulation, and other agronomic safety indicators are required before field application.