Climate-Smart Remediation: Selenium-Biochar Dual Functionality Reduces Mercury Toxicity and Improves Selenium Biofortification in Garlic under Future CO ₂ Scenarios

Elevated atmospheric CO 2 combined with mercury (Hg) contamination threatens crop safety and human health. This study investigated the dual role of selenium-enriched biochar (SeBC) in mitigating Hg bioavailability and enhancing selenium (Se) nutrition in garlic ( Allium sativum L.) under elevated CO 2 (+200 ppm). The results revealed that while CO 2 enrichment increased garlic biomass by 19.92–47.51%, it also raised Hg accumulation in bulbs (10.84%) and roots (23.18%). Conversely, SeBC application effectively reduced Hg concentrations in shoots, bulbs, and roots by 67.31%, 50.97%, and 52.97%, respectively, outperforming conventional biochar. Mechanistically, SeBC increased soil pH, promoted Hg-Se complexation, and enhanced Hg immobilization into residual forms. Additionally, SeBC increased plant Se content by 1.64% and reduced Hg translocation by 73.2%. These findings demonstrate SeBC’s potential as a climate-smart strategy to simultaneously reduce Hg contamination and improve Se biofortification in crops, offering critical insights for sustainable agriculture and food safety management.