Engineered Biochar–Nanocomposites Enhanced Vetiver Growth and Nickel Uptake in Ni-Elevated Ultramafic Soils

Ultramafic soils, particularly those affected by mining, often contain toxic nickel (Ni) levels that hinder plant growth and ecosystem recovery. This study assessed engineered biochar–nanocomposite amendments to improve vetiver (Chrysopogon zizanioides) growth, biomass, and Ni phytoextraction in Ni-rich ultramafic soils from Santa Cruz, Zambales, the Philippines. Seven samples were tested: T1—control (no application); T2—biochar; T3—nanocomposite; T4—biochar + nano-silica; T5—biochar + nano-calcium; T6—biochar + nano-chitosan; and T7—biochar + nanocomposite. Biochar combined with nano-silica (T4) significantly enhanced vetiver growth, producing the highest root, shoot, and total biomass (469.97 g plant−1), indicating improved plant tolerance under Ni stress. The highest shoot Ni concentration (24.52 mg kg−1) and translocation factor (0.56) were observed in the biochar + nano-chitosan treatment (T6), suggesting increased Ni bioavailability and uptake. However, translocation factor values remained below unity across all treatments, indicating limited Ni transfer from roots to shoots and a dominant phytostabilization behavior. Overall, nano-silica-engineered + biochar primarily enhanced biomass production, while nano-chitosan influenced Ni uptake dynamics, highlighting the potential of engineered biochar–nanomaterial amendments for sustainable rehabilitation of Ni-contaminated ultramafic soils.