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

Ultramafic soils, particularly those affected by mining activities, are often enriched with toxic levels of nickel (Ni), posing serious constraints to plant growth and ecosystem rehabilitation. This study evaluated the efficacy of engineered biochar–nanocomposite amendments in enhancing vetiver (Chrysopogon zizanioides) growth, biomass production, and Ni phytoextraction in Ni-elevated ultramafic soils. A pot experiment was conducted using soils collected from mined areas in Zambales, a region known for its extensive ultramafic landscapes and Ni mining operations. Seven treatment combinations were assessed: T1—No Application (Control); T2—Biochar Alone; T3—Nanocomposite Alone; T4—Biochar + Nano Silica; T5—Biochar + Nano Calcium; T6—Biochar + Nano Chitosan; and T7—Biochar + Nanocomposite. Among all treatments, T4 (Biochar + Nano Silica) resulted in the highest biomass yield (17.2 g pot⁻¹) and maximum Ni phytoextraction (31.6 mg Ni plant⁻¹), significantly outperforming all other treatments. Plants grown under T4 exhibited robust shoot and root development, and superior tolerance to Ni stress. Correspondingly, Ni accumulation in plant tissues was significantly higher in T4, suggesting enhanced metal uptake and translocation capacity. The synergistic effect of biochar and nano silica improved soil pH and nutrient availability, while also enhancing the bioavailability of Ni in the rhizosphere, promoting more effective uptake by vetiver. T7 (Biochar + Nanocomposite) and T6 (Biochar + Nano Chitosan) also contributed to improved growth and Ni uptake but were less effective than T4. The results demonstrate the potential of nano silica-engineered biochar as a low-cost, environmentally sustainable amendment for the phytoremediation of Ni-contaminated ultramafic soils. This study highlights the practical application of nanotechnology-enhanced phytoremediation strategies using locally available soil and plant resources to rehabilitate Ni-impacted mined lands in the Philippines.