2. Biomass Ash Definition and Types of Biomass Ash

Biomass ash is the inorganic, non-combustible fraction that remains after the complete or partial oxidation of plant- or animal-derived biomass (e.g., wood, straw, poultry litter, sewage sludge) and is now viewed as a secondary nutrient source in circular agriculture. It represents the non-combustible mineral fraction and is increasingly viewed as a secondary nutrient source within circular agriculture frameworks (Maj et al. in Sustainability 17(11):4925, 2025 [1]). The progressive substitution of fossil fuels by biomass has been accompanied by a steady rise in ash production. Recent energy-balance studies estimate global biomass ash output at approximately 40–60 Mt yr⁻¹, which is about one order of magnitude lower than earlier headline values (Romanowska-Duda et al. in Molecules 29(18), 2024 [2]). Although management of this by-product poses logistical challenges, the ash represents a potential nutrient resource owing to its mineral content. Historically, ash derived from biomass combustion-wood ash in particular-served as one of the earliest mineral fertilizers. Agricultural application of ash enabled partial restitution of nutrients removed during crop harvest (Barišić et al. in Materials 15(13), 2022 [3]). Contemporary circular-economy strategies advocate recycling of biomass ash to minimize disposal and close elemental loops (Tosti et al. in Waste Biomass Valorization 12(8):4703–4719, 2021 [4]).