Optimizing the pre-treatment of marine biomass (Laminaria pallida and Gracilariopsis funicularis) for enhanced production of climate-smart agricultural biochar

Marine biomass has gained attention as a sustainable feedstock for biochar, but its high salt content limits its agricultural application. This study investigated the water-soaking pre-treatment of Laminaria pallida and Gracilariopsis funicularis to reduce salinity and improve biochar quality for agricultural applications. The experiment included three factors: water temperature (25 °C and 100 °C), soaking duration (0, 0.5, 2, and 6 h), and seaweed species, resulting in 16 treatment combinations. Significant effects (P < 0.05) of soaking conditions were observed on biochar pH, electrical conductivity (EC), macronutrient concentrations (C, N, P), cations (Na, K, Ca, Mg), and heavy metals. Cold-water soaking of G. funicularis for 6 h produced the most favorable biochar, with a reduced sodium concentration (from 23.9 g/kg to 12.2 g/kg), lower EC (29.3 mS/cm), moderate pH (8.6), and high nutrient content (N: 5.17%; P: 127 mg/kg). In contrast, L. pallida biochar retained higher sodium (46.5 g/kg to 62.6 g/kg) and strong alkalinity (pH 11.3), limiting its agronomic suitability. Hot water soaking reduced N and P but increased Zn concentrations (up to 5.5 mg/kg and 9.52 mg/kg in L. pallida and G. funicularis, respectfully), indicating trade-offs between nutrient retention and metal enrichment. Importantly, all heavy metals measured (Cd, Cr, Cu, Ni, and Zn) remained below permissible limits for organic soil amendments. Unlike previous studies, the current study systematically optimised a simple, low-cost pre-treatment that reduces salinity while preserving nutrient value in marine biomass-based biochar. These findings demonstrate that, compared to L. pallida, pre-treated G. funicularis biochar has strong potential as an eco-friendly soil amendment in arid regions, where salinity management and nutrient efficiency are critical. Future studies should integrate this approach with vermicomposting or microbial inoculation to further leach excess cations and stabilise biochar quality.