Recent energy-balance studies put worldwide biomass ash production at roughly 40–60 million tons per year, not hundreds of millions (Vassilev et al. in Fuel 129:292–313, 2014 [1]; Romanowska-Duda et al. in Molecules 29(18):4397, 2024 [2]). Because wood- or straw-derived ash is rich in K, Ca, Mg, P, and micronutrients, it can be reused as a nutrient input in circular agriculture; yet trace Cd, Pb and Hg may still occur, so every batch should be tested to confirm it meets fertilizer-quality limits (Buss et al. in J. Clean. Prod. 208:960–967, 2019 [3]). Biochar is a highly carbonized material obtained in controlled pyrolysis of biomass (with limited oxygen). It is characterized by high porosity, high specific surface area, and an alkaline reaction (Antonangelo et al. in Biochar 7(1):1–28, 2025 [4]).