Fe-modified tea waste biochar enhances short-term soil organic carbon sequestration: Mechanisms and economic benefit

Pyrolysis of tea waste to produce biochar (BC) for soil modification offers a promising reuse strategy. However, the short-term application of BC can enhance soil organic carbon (SOC) mineralization, and the economic viability of this BC across its lifecycle (from production to field application) remains unclear. In this study, Fe-modified tea waste BC (FBC) was produced at 300-900 °C to enhance SOC sequestration over 180 days, alongside an evaluation of their economic benefits. Compared with unmodified BC, FBC produced at 600 °C (FBC600) and 900 °C (FBC900) significantly increased SOC content, with increments of 1770-1800 mg/kg at 15-60 days and 660-830 mg/kg at 60-180 days. This enhancement was attributed to two factors: first, the highly aromatic structures of FBC600 and FBC900 were associated with the abundance of putative autotrophic taxa, with Fe potentially facilitating fresh SOC synthesis by these bacteria; second, dissolved organic C (DOC) decreased by 417-436 mg/kg in FBC-amended soils, which was approximately twice the cumulative mineralized C, indicating that enhanced DOC adsorption further contributed to SOC sequestration. Spectral analysis suggested preferential adsorption of small aromatic protein-like molecules in FBC600- and FBC900-amended soils. Life cycle assessment revealed significant economic benefits for both FBC600 and FBC900, with overall revenues of 674.63 USD/ton and 721.04 USD/ton, respectively, alongside net CO₂ emission reductions of 1432.12 kg/ton and 1098.16 kg/ton. This study highlights a valuable approach for tea waste reuse that offers both economic advantages and mitigation of the short-term SOC mineralization commonly induced by BC application.