Properties and nitrate leaching mitigation effect of thermally treated biomass-a case study of tropical and subtropical islands

This study assessed the properties of thermally treated biomass including torrefied biomass and biochar and their effect on nitrate leaching mitigation. The raw biomass materials were derived from herbaceous sugarcane residue (bagasse) and woody Alexandrian laurel, which are common feedstocks in subtropical and tropical islands. The biomass materials were treated at various temperatures ranging from 220 to 800 °C. Nitrate adsorption capacity and mitigation effect on nitrate leaching when it was applied in acidic soil were evaluated. We observed typical changes in the yield and functional groups of the thermally treated biomass with increasing treatment temperature. Nitrate adsorption tended to occur with 200-300 °C materials at a rate of approximately 3.0-7.0%. Nitrate leaching decreased only with the 800 °C material, with a mitigation rate of 30%. The experiments and multiple regression analysis revealed that the low pH and hydrophilicity of materials < 300 °C positively affected nitrate adsorption, whereas the high surface area of the 800 °C material significantly reduced nitrate leaching. Further investigation of the long-term effects of thermally treated biomass application at different treatment temperatures is necessary to reduce the nitrate leaching on subtropical and tropical islands.