article · Cleaner and Circular Bioeconomy
Desalinated seaweed biomass can be converted into biochar to enhance cattle-manure and waste-paper vermicompost. Evaluating additions of zero, three, six, and nine percent biochar over a twelve-week vermicomposting period revealed significant impacts on electrical conductivity, nutrient levels, and heavy metal concentrations. The matured compost maintained safe levels of heavy metals, well below permissible limits for soil application. Adding biochar improved germination indices for tomato, onion, Swiss chard, and carrot seeds, although cabbage germination declined at the nine percent application rate. A nine percent seaweed biochar inclusion delivered the most favourable outcomes, generating a compost with a slightly alkaline pH of 7.85, a low carbon-to-nitrogen ratio of 11.43, elevated phosphorus and potassium, and reduced sodium content. This enriched vermicompost presents a viable option for improving acidic growing media used in horticultural crop production.
Agricultural waste and marine biomass often present disposal challenges, while conventional growing media require sustainable nutrient enrichment. Converting desalinated seaweed into biochar for vermicomposting recycles organic wastes into safe, nutrient-rich soil amendments. The resulting material provides essential plant nutrients like phosphorus and potassium without hazardous heavy metal accumulation, offering a practical approach to rehabilitating acidic soils and supporting horticultural production.
The research demonstrates applied testing of desalinated seaweed biochar as a component in vermicompost, placing it at an early to applied testing stage. This amendment could enable horticultural producers and compost manufacturers to formulate nutrient-rich growing media suited for acidic soils. However, variable germination effects observed in cabbage indicate that crop-specific formulations are necessary, and real-world adoption requires further evaluation in commercial soilless media as suggested by the study.
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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.
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DOI: 10.1016/j.clcb.2026.100229
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