Elevated CO2 Modulates Selenium Bioaccumulation in Garlic (Allium sativum L.): Contrasting Effects of Biochar and Phosphate Fertilizer Amendments

Elevated CO2 (eCO2) influences crop nutrition, but the impact of its interaction with soil amendments on selenium (Se) bioavailability is unclear. This study investigated how eCO2 (+200 ppm), biochar (BC, 1% w/w), and phosphate fertilizer (PF, 1 g kg−1) affect Se uptake in garlic—a model crop chosen for its efficiency in accumulating and transforming Se into bioactive forms. The results showed that eCO2 significantly enhanced garlic biomass by 19.1–34.2% and decreased soil pH by 0.05–0.13 units. Concurrently, eCO2 increased Se concentration in garlic tissues by 2.9–13.3% compared to ambient CO2 (aCO2). Biochar amendment reduced soil Se bioavailability, leading to a 15.2–22.8% decrease in garlic Se concentration under eCO2. In contrast, phosphate fertilizer enhanced Se bioavailability via competitive ligand exchange, increasing Se uptake by 18.7–31.4%. These findings demonstrate that PF can be strategically co-managed with eCO2 to optimize Se biofortification in garlic, providing a practical strategy to safeguard nutritional security under future climate scenarios.