A coupled production-removal approach for Spirulina-derived biochar: Life cycle assessment and implications for carbon accounting

Value-chain emissions constitute the majority of environmental impacts across industrial systems, yet scalable, life-cycle-based strategies for reducing these impacts remain limited. Biochar has been widely studied as a functional material with climate benefits, but its contribution to supply-chain decarbonization is typically constrained by moderate inclusion rates and limited impact per unit mass. This study evaluates a coupled production-removal system in which Spirulina biomass is simultaneously valorized into biochar and a waste-derived biostimulant for soil organic carbon enhancement, thereby directly linking microalgal production with land-based carbon removal. A comprehensive life cycle assessment (LCA) with a focus on climate impact (GWP100) is conducted on a cradle-to-gate basis, with 1 kg of biochar at the factory gate as the functional unit, including Spirulina cultivation, downstream processing and drying, pyrolysis, and waste-stream valorization through field application as a biostimulant for soil carbon enhancement. Soil carbon removal is based on primary field measurements of soil organic carbon, assessed using a 100-year GWP horizon and incorporating a 20% non-permanence discount and ± 20% uncertainty for the sequestration estimate. The results show a strongly net-negative climate impact of −613,428.6 kg CO₂-eq per kg biochar (GWP100), driven primarily by biostimulant-induced soil carbon removal of −615,690.6 kg CO₂-eq per kg biochar, while the non-climate impacts are mainly linked to the upstream cultivation rather than to the biochar itself. These impacts should therefore be seen as a part of the overall system. At product level, the low inclusion rates required (<7 g·t −1 ) result in negligible contributions to other impact categories, suggesting limited risk of burden shifting under the conditions assessed. Application across four supply chains - cotton textiles, cement/construction, corrugated cardboard packaging, and virgin ABS plastics - illustrates how integrating functional ingredients into upstream materials could generate system-level mitigation potential relevant to value chain decarbonization, provided that emissions, removals, and carbon storage are transparently reported and not treated as interchangeable within corporate inventories. These findings highlight the potential of coupled production-removal systems to contribute to sustainable production, while emphasizing the importance of site-specific conditions and robust monitoring frameworks.