Combined use of bio-nano-selenium and soil amendments mitigates cadmium toxicity and enriches selenium in barley under multi-level contamination

Cadmium (Cd), a ubiquitous environmental contaminant, poses severe threats to ecological and agricultural systems due to its pronounced toxicity in both plants and animals. This study investigated the combined effects of soil amendments (sodium metasilicate or biochar) and foliar bio-nano-Se application on barley grown under three soil Cd contamination levels (5, 10, and 20 mg·kg⁻¹), with a focus on grain Cd accumulation, Se content, and agronomic traits. While the combined amendments generally alleviated the Cd-induced decreases in TGW and YP, treatments T3 (5 mg·kg −1 Cd + biochar + bio-nano-Se), T10 (10 mg·kg −1 Cd + biochar + sodium metasilicate + bio-nano-Se), and T13 (20 mg·kg −1 Cd + biochar + bio-nano-Se) were particularly effective, increasing TGW by 47.96%, 62.31%, and 69.28%, respectively, and enhancing YP relative to their controls. The combined application of bio-nano-Se, sodium metasilicate, and biochar (as in treatments T5, T10, and T15) was among the effective treatments that reduced grain Cd content to meet the food safety standard while also enabling Se enrichment (20–50 μg·(100 g) −1 ) in barley grains. Under the tested conditions, grain Cd and Se showed divergent responses to increasing soil Cd stress, suggesting that Se accumulation does not simply follow the same pattern as Cd uptake. These findings provide preliminary insights for the development of integrated amendment approaches that may contribute to the safe cultivation of Se-fortified barley in Cd-contaminated soils, though field validation is needed.