Targeted fortification of phenolic hydroxyl groups in humic acid to further enhance urea use efficiency in maize seedling

Phenolic hydroxyl group (ArOH) plays an important role in humic acid (HA) enhancing nitrogen (N) fertilizer efficiency. However, the content of ArOH in HA is usually low, while its traditional enriching modification are inefficient or even showed a potential toxicity risk. In this work, we developed a novel catalyst named CuO@Al 2 O 3 enriched with mesopores and oxygen vacancies, and target-modified HA to increase its ArOH using a feasible mechanical catalytic method. The results showed that phenolic-rich HA (CuO@Al 2 O 3 -HA) were successfully prepared with 40 % higher ArOH content than HA, which was consistent with the 43.13 % increase in C-O/Ar-O content in XPS C1s spectra. Compared to RHA, CuO@Al 2 O 3 -HA had less tannin attributed to the Ar-C bond breakage catalyzed by CuO@Al 2 O 3 to form ArOH. Subsequently, we prepared CuO@Al 2 O 3 -HA-enhanced urea (CuO@Al 2 O 3 -HAU), and found that CuO@Al 2 O 3 -HAU improved maize seedling growth by the elevation of leaf chlorophyll content (by 8.78 %) and lowering N loss by NH 3 volatilization inhibition (by 12.43 %), thus enhanced the biomass and N uptake of maize seedling by 11.29 % and 21.22 %, respectively, compared to RHA-enhanced urea (RHAU). Therefore, this study provided a method to enrich the ArOH content of HA, and a green approach for the upgrading of high-efficiency urea products. • CuO@Al 2 O 3 with mesopores and oxygen vacancies was mechanochemically synthesized. • CuO@Al 2 O 3 selectively catalyzed the cleavage of Ar-C bonds in humic acid. • Phe-group content in humic acid was sharply improved by 40 %. • Pot experiment confirmed the effectiveness in promoting root growth and N uptake. • Soil incubation experiment revealed mechanism on optimized N utilization.