Interactive Effect of Biochar and Potassium Enhances Copper Stress Tolerance in Hemp: Modulation of Antioxidant Defense and Mineral Homeostasis in Contaminated Mine Soils

Soil copper (Cu) contamination severely threatens sustainable hemp ( Cannabis sativa L.) production. This study investigated the interactive potential of biochar (BC) and potassium (K) supplementation to enhance hemp resilience in Cu‐contaminated mine soil (294 mg Cu kg −1 ). A greenhouse pot experiment employed a 3 × 4 factorial design: BC (0, 30, 50 g kg −1 soil) and K fertilizer (0, 100, 200, 300 kg ha −1 as K 2 SO 4 ). Results revealed that Cu stress significantly inhibited hemp growth, biomass, and photosynthetic efficiency, while elevating oxidative stress markers and antioxidant enzyme activities. Co‐application of BC and K, particularly at optimal rates (BC: 30 g kg −1 , K: 200 kg ha −1 ), effectively counteracted Cu toxicity. This interactive treatment maximized plant height (205.90 cm, +40.36% vs. control treatment group BC 0 K 0 ), stem diameter, leaf number, and biomass accumulation. Crucially, it reduced hydrogen peroxide (H 2 O 2 ) by 77.80% and malonaldehyde contents (MDA) by 70.05%, while enhancing osmoprotectants (soluble sugars, proteins, proline). The BC + K combination significantly altered the distribution of Cu in soil (bioavailable Cu—40.02%, available Cu—32.51%) and plant tissues (roots—25.96%, stems—27.00%, leaves—33.36%). These findings demonstrate that the interactive effect of BC and K alleviates Cu phytotoxicity by enhancing antioxidant defenses, improving K nutrition, and immobilizing Cu in soil, providing a sustainable strategy for hemp cultivation in contaminated lands.