Targeted preparation of oxygen- and aromatic-rich humic acid for decreasing urea nitrogen loss: Characterization, performance, and mechanism

Humic acids (HA) with abundant oxygen and aromatic structures possess a huge potential for improving the nitrogen utilization efficiency in agriculture. However, the conventional oxygen-enhancing methods lead to the loss of the HA aromatic structure. This study employed a novel catalyst (CuO@Fe 2 O 3 ) enriched with oxygen vacancies and redox system to target prepare the oxygen- and aromatic-rich humic acid (AHA) during a H 2 O 2 -mediated oxidation process. Compared with raw HA (RHA), AHA exhibited a 4.8 % higher oxygen and comparable condensed aromatic content, which was attributed to the selectively breaking C-O bonds. Next, urea fertilizers enhanced with RHA (RHAU) and AHA (AHAU) were prepared. Soil incubation experiments revealed that compared with RHAU, AHAU exhibited 8.6 % lower NH 3 volatilization losses by delaying urea hydrolysis (via urease inhibition) and enhancing nitrification rate (mainly by increasing the ammonia-oxidizing bacteria abundance). AHAU also decreased cumulative N₂O emissions by influencing denitrifying microbes. This targeted preparation strategy successfully overcomes the oxygen and aromatic structure trade-off in HA modification, offering a highly practical and efficient HA material for environment-friendly and high-efficiency nitrogen fertilizers.