Carbon Mineralization in a Soil Amended with Sewage Sludge-Derived Biochar

Biochar has been presented as a multifunctional material with short- and long-term agro-environmental benefits, including soil organic matter stabilization, improved nutrient cycling, and increased primary productivity. However, its turnover time, when applied to soil, varies greatly depending on feedstock and pyrolysis temperature. For sewage sludge-derived biochars, which have high N contents, there is still a major uncertainty regarding the influence of pyrolysis temperatures on soil carbon mineralization and its relationship to soil N availability. Sewage sludge and sewage sludge-derived biochars produced at 300 °C (BC300), 400 °C (BC400), and 500 °C (BC500) were added to an Oxisol in a short-term incubation experiment. Carbon mineralization and nitrogen availability (N-NH₄⁺ and N-NO₃^−) were studied using a first-order model. BC300 and BC400 showed higher soil C mineralization rates and N-NH₄⁺ contents, demonstrating their potential to be used for plant nutrition. Compared to the control, the cumulative C-CO₂ emissions increased by 60−64% when biochars BC300 and BC400 were applied to soil. On the other hand, C-CO₂ emissions decreased by 6% after the addition of BC500, indicating the predominance of recalcitrant compounds, which results in a lower supply of soil N-NH₄⁺ (83.4 mg kg^−1) in BC500, being 67% lower than BC300 (255.7 mg kg^−1). Soil N availability was strongly influenced by total N, total C, C/N ratio, H, pore volume, and specific surface area in the biochars.