Biochar from poplar sawdust for digestate nutrient recovery and potential for long-term carbon sequestration

Abstract Poplar sawdust-derived biochar was studied as a multifunctional material for nutrient recovery from digestate and for its potential for long-term carbon sequestration in soil. Biochar was produced via pyrolysis at 300–900 °C for 10–40 min. Considering solid yield, carbon sequestration, and phosphate removal, 500 °C for 20 min was selected as the optimal pyrolysis condition. Biochar was further characterized and tested in slurry adsorption experiments. Adsorption kinetics followed a pseudo-second-order model, while phosphate uptake followed the Langmuir isotherm, indicating monolayer chemisorption. Under slurry conditions, adsorption capacities for phosphate, ammonium, and potassium were 4.44, 116.83, and 157.37 mg·g⁻¹, respectively. Biochar stability improved significantly, with the recalcitrance index (R 50 ) increasing from 0.37 in raw biomass to 0.58 after pyrolysis. This index remained largely unchanged following nutrient adsorption, indicating that nutrient loading did not substantially alter the thermal oxidative stability of the biochar. A practical application was proposed by mixing biochar with digestate (10 g.kg⁻¹ fresh matter) during on-farm transport to the field, achieving nutrient loading within 3 h under ambient conditions. Life cycle assessment showed a net reduction of 698.7 kg CO₂e·Mg⁻¹ of biochar, confirming its climate-mitigation potential. The findings support a sustainable strategy using biomass and residues from forests and livestock. Furthermore, the present study proposes a conceptual integration of pyrolysis and anaerobic digestion systems, in which digestate is used as a nutrient source for biochar enrichment, thereby improving manure management and advancing circular bioeconomy goals. Further research should evaluate field performance, long-term agronomic impacts, and environmental safety.