Mechanistic insights into nitrogen-mediated carbon transformation during hydrothermal humification for the synthesis of nutrient-enriched artificial humic acids.

Hydrothermal humification (HTH) of biomass is a promising approach to address the depletion of soil organic matter. However, the HTH products derived from lignocellulose are limited by low nutrient availability and conversion efficiency. In this study, four nitrogen-containing additives were used to produce nitrogen-rich artificial humic acids (AHA) from crop waste. The addition of urea, ammonium chloride, ammonium sulfate, and ammonium dihydrogen phosphate at 180 °C for 1 h significantly improved both the yield and quality of AHA. Optimal conditions were achieved using ammonium sulfate at a carbon-to-nitrogen ratio of 5, resulting in an impressive AHA yield of 40.75 ± 3.99 wt% and a nitrogen content of 3.54 %, closely resembling naturally formed humic acids. This represented a 38.27 % increase in yield and a 30.44 % increase in nitrogen content compared to the control. Moreover, 5NS-AHA exhibited reduced aromaticity, increased oxygen-containing functional groups, and enhanced functionality through the regulation of aliphatic and aromatic carbon structures. The concentration of furfural in the hydrothermal aqueous phase reduced from 43.48 % to 29.85 %. Ammonium sulfate optimized the HTH pathway through synergistic mechanisms involving acid-catalyzed oxidation, nitrogenated condensation, and aromatic condensation. This work provides a visible pathway for producing nitrogen slow-release AHA fertilizers from agricultural waste.