Biochar prepared from desalinated seaweed (Gracilariopsis funicularis) has a positive potential on cattle manure-waste paper vermidegradation and plant nutrient value

Seaweed biochar-amended vermicompost (VC) has the potential to improve the physicochemical properties of growing media for horticultural crop production. This study evaluated the agronomic suitability of cattle-manure-waste paper vermicompost amended with varying proportions of desalinated seaweed ( Gracilariopsis funicularis ) biomass-derived biochar (BC) at 0, 3, 6, and 9% (w/w). BC was prepared from desalinated seaweed biomass by soaking in deionised water at 25°C for 6 h and incorporated into cattle manure prior to vermicomposting for 12 weeks, with sampling at 0, 4, 8, and 12 weeks. BC incorporation rate, vermicomposting time, and/or their interaction significantly (P < 0.05) influenced EC, nutrient dynamics, and heavy metal concentrations, whereas pH and Ca were not affected. Mature BC-amended vermicompost extracts (week 12) were evaluated for phytotoxicity using seed germination bioassays on tomato, cabbage, onion, Swiss chard, and carrot, with deionised water as the control. After 12 weeks carbon to nitrogen ratio (C/N) ranged from 11.43 to 17.60, EC (2.41-2.77 mS/cm), Olsen P (262.14-281.00 mg/kg), nitrate–nitrite nitrogen (NO 3 /NO 2 -N) (10.04-54.34 mg/kg), across treatments. All cation values increased over time, except for Ca at 9% seaweed biochar. All measured heavy metals (Cd, Cr, Cu, Ni, and Zn) were below the maximum permissible limits for biosolids soil applications after 12 weeks. For phytotoxicity, the Germination Index (GI) was improved in all selected vegetables under BC addition, except for cabbage which was reduced to 31.64 at 9% BC. Seaweed biochar at 9% rate showed most favourable results, slightly alkaline pH (7.85) and moderate EC (2.70 mS/cm), lowest C/N ratio (11.43), highest macronutrient (Olsen P; 281.00 mg/kg, exchangeable K; 8.90 g/kg, moderate N-forms), and the lowest Na concentration (3.44 g/kg). These findings suggest that this seaweed biochar-amended vermicompost may be suitable for ameliorating acidic growing media. Further evaluation of seaweed biochar-amended vermicompost in soilless media is recommended.