Biochar and melatonin alleviate microplastic-cadmium (MP-Cd) stress in rice by enhancing antioxidant activity and regulating Cd transport.

BACKGROUND

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 combined potential to mitigate MP-Cd co-stress has not yet been explored. This study aimed to evaluate the individual and enhanced effects of BC and MT on rice growth, physiological and molecular responses, Cd bioavailability, and soil properties under MP-Cd co-contamination.

RESULTS

Exposure to Cd (20 mg kg⁻¹) and MPs (1%) significantly inhibited rice growth and productivity by inducing oxidative stress, enhancing Cd uptake and accumulation, suppressing chlorophyll biosynthesis, and impairing water and nutrient acquisition. In contrast, the combined application of BC (2%) and MT (100 µM) markedly alleviated these adverse effects and outperformed individual amendments. Co-application substantially increased chlorophyll content (82%), leaf relative water content (48.47%), antioxidant enzyme activities (57.84-99.42%), proline accumulation (49.21%), and endogenous melatonin (EM) levels (48.35%). At the molecular level, BC + MT treatment upregulated antioxidant-related genes (OsAPx6, OsCAT, OsPOD, and OsSOD), the proline biosynthesis gene OsP5CS, and the MT biosynthesis gene OsCOMT, while significantly downregulating Cd transporter genes (OsNRAMP1 and OsHMA3). Furthermore, this combined treatment reduced soil Cd bioavailability and Cd accumulation in rice tissues, while improving soil fertility by increasing nitrogen (N), phosphorus (P), potassium (K), and soil organic carbon (SOC).

CONCLUSIONS

This study provides the first evidence that the combined application of BC and MT effectively mitigates the detrimental effects of simultaneous MP and Cd contamination in rice. The enhanced physiological, molecular, and soil-level improvements induced by BC and MT collectively enhance rice growth and productivity under MP-Cd stress. These findings highlight a promising, integrated remediation strategy to manage co-pollution of MPs and HMs in agricultural soils, with important implications for sustainable crop production and food security.