Soil contamination by microplastics (MPs) and heavy metals (HMs), particularly cadmium (Cd), poses an emerging threat to agricultural sustainability, food safety, and human health. Although the individual effects of MPs and Cd on crop performance have been widely investigated, their interactive impacts on rice remain poorly understood. Biochar (BC) and melatonin (MT) have recently attracted attention for their capacity to alleviate HM toxicity and abiotic stress in plants; however, their combine