Soil-Applied Selenium Reduces Cadmium Accumulation and Alters Selenium Enrichment in Artemisia selengensis in Cd-Contaminated Soil

Cadmium contamination threatens safe vegetable production in arable soils. This study evaluated whether soil-applied selenium (Se) could reduce Cd accumulation in Artemisia selengensis while keeping Se enrichment within a practical safety range. A field plot experiment was sampled at 75, 110, and 150 d, and two pot experiments were conducted under normal and elevated Cd backgrounds. Six sodium selenite rates (0.5–8.0 mg kg−1; Se1–Se6) plus a control were applied. In the field experiment, Se5–Se6 (6.5–8.0 mg kg−1) reduced stem Cd by 33.0–39.3% at 75 d and 34.3–36.5% at 110 d, but the reduction declined to 24.8% at 150 d. Se application increased tissue Se (leaf > stem > rhizome), while stem total Se remained within the dry-weight reference window under 12Se–Se (0.5–2.0 mg kg−1). Se5–Se6 also increased soil pH and reduced bioavailable Cd by 8.8–10.2%, whereas stem Cd reduction under an elevated Cd background was limited and non-significant. Overall, 0.5–2.0 mg kg−1 (Se1–Se2) provided a practical window for Cd mitigation, while 6.5–8.0 mg kg−1 (Se5–Se6) increased the risk of excessive Se accumulation in edible tissues.