Valorising cinnamon crop residue: Hydrochar production for sustainable agriculture and carbon emission mitigation.

Thermal conversion of crop residue into soil amendment is a negative emission technology used in circular agriculture. However, limited research has explored the large amounts of crop residues, like wood, scrape, and leaves generated during cinnamon production in Sri Lanka for hydrochar production and its agri-environmental implications. This study used cinnamon leaf waste to produce hydrochar at 190, 220, and 260 °C for 30 and 60 min, and compared it with the raw material and its biochar counterpart produced at 550 °C for 30 min. Both char materials eliminated the phytotoxicity and reduced the electrical conductivity of cinnamon leaves, promoting seed germination. The hydrochar yield reduced with temperature and marginally increased with reaction duration (67% at 190 °C for 60 min and 49% at 260 °C for 30 min), whereas biochar yield remained below 30%. The hydrochar has a lower pH (5.1 - 6.3) than biochar (pH ∼ 7.8) and retained more nitrogen (>1.6%) than biochar (0.8%). According to the carbon persistence quantified by H/C ratios, hydrochar produced at 260 °C for 60 min had a shorter mean residence time (111 years) and a greater carbon storage capacity (442 kg CO2eq/ha) than biochar (over 600 years and 437 kg CO2eq/ha), owing to its higher yield and organic carbon content. Producing hydrochar at 260 °C for 60 min presented the best results for carbon sequestration and agricultural application of biomass, suggesting a promising approach for valorising cinnamon leaf waste for sustainable agriculture.