Field application of biochar-based amendments for safe crop production in karst paddy with a high geological background: Unraveling Cd-Se dynamics

The global prevalence of geogenically derived cadmium (Cd) enrichment poses significant risks to agricultural sustainability and food security. Micronutrients such as selenium (Se) usually co-occur with Cd in such regions but exhibit low bioavailability in soils, further causing a “hidden hunger” of nutrient depletion in the food supply chain. Through controlled field experiments, we tested strategically designed biochar-based amendments (clay-, manure-, and iron-enriched variants) at 5–20 t ha −1 on limestone-derived paddy soils to synchronously mitigate Cd mobility while enhancing grain Se accumulation. Biochar amendments decreased grain Cd bioaccumulation by 42.3–85.4% and increased grain Se bioaccumulation by 91.6–357.5% across both growing seasons, with a better effect observed in synergy with yield increase in the second growing season, reflecting longer-term soil conditioning and progressive weathering of the amendments. Among the three amendments, BC-ZVI at 10–20 t ha −1 achieved the fastest and most consistent Cd immobilization, lowering grain Cd below the national safety threshold (0.2 mg kg −1 ) in both growing seasons while increasing grain Se by 91.6–357.5%. BC-Clay and BC-Manure required the second season to reach comparable Cd performance across all application rates. Elevations in soil pH, CEC, and available phosphorus may have accounted for the (im)mobilization effect. Field and literature data further indicated a decoupling between soil total Cd-Se co-enrichment and their mobile or plant-accumulated pools. Findings from this study demonstrate a practical and effective strategy for producing Se-enriched, low-Cd rice in regions with elevated geogenic Cd-Se background.