Tobacco Straw Biochar Mitigates Cadmium Accumulation in Amaranth (Amaranthus tricolor L.): A Cultivar-Specific Response

Cadmium (Cd) contamination in agricultural soils poses a severe threat to food safety and human health through the food chain. This study investigated the efficacy of tobacco straw-derived biochar, applied at varying rates (0%, 1%, 2%, and 5% w/w), in mitigating Cd accumulation and modulating the growth and nutritional quality of two amaranth (Amaranthus tricolor L.) cultivars (red and green) grown in Cd-contaminated soil (initial total Cd of 2.18 mg/kg). The pot experiment revealed that biochar significantly reduced Cd uptake in both cultivars. Mechanistically, biochar elevated soil pH and drove the in-situ transformation of highly bioavailable exchangeable Cd into the more stable Fe-Mn oxide-bound fraction. Consequently, shoot Cd concentrations were notably suppressed, with the red cultivar exhibiting a superior response; the 2% biochar treatment optimally reduced its shoot Cd concentration by 37.6% compared to the control. Crucially, the amendments induced highly cultivar-specific growth responses. While biochar application simultaneously mitigated Cd toxicity and promoted biomass accumulation in red amaranth (yielding a 58.6% increase in shoot dry weight at the 2% rate), it exerted antagonistic, inhibitory effects on the growth of green amaranth. In conclusion, the incorporation of 2% tobacco straw biochar serves as a highly effective, dual-purpose strategy for significantly reduced health risks and enhancing the yield of red amaranth in Cd-contaminated fields. However, in green amaranth, biochar application induced a physiological trade-off, inhibiting growth despite successful Cd reduction. Furthermore, while Cd concentrations were significantly reduced on a dry-weight basis, future evaluations based on fresh-weight regulatory limits are required to fully confirm food safety.