<p>The application of municipal sewage sludge compost (MSSC) in landscaping contributes to sustainable material recycling, the use of soil stabilizers to stabilize heavy metals and reduce associated risks. Nevertheless, a comprehensive understanding of the complex interplay between metal immobilization effects and factors such as soil stabilizer dosage, site-specific soil physicochemical properties, and plant growth is essential for practical application. The influence of varying rice husk biochar (RHB) additions on heavy metal bioavailability in MSSC-amended soil using a ryegrass pot experiment were investigated, followed by a thorough analysis of soil and plant physicochemical properties and correlation assessments. In the RHB treatment groups, most heavy - metal concentrations in ryegrass shoots were lower than those in the control group (CK), with maximum decreases from 71.3 to 13.6%. Unexpectedly, in the soil of these groups, element concentrations except Ni decreased. Morphological analysis showed that only the acid - extractable fraction of Pb in the tested group significantly decreased. Thus, biochar might increase heavy - metal leaching in the soil, probably because metals bind to fine biochar particles. The study indicates that RHB can effectively reduce the bioavailability of heavy metals to plants. However, it may also increase the leaching risk of heavy metals. This research highlights the necessity of comprehensively understanding the role of biochar size selection in its interaction with heavy metals for mitigating heavy metal pollution in soil-plant systems.</p>