Enhancing Carboxyl Groups in Humic Acid with Selective Catalyst to Mitigate Active N Loss through Nitrification and Denitrification Inhibition in Low-Fertility Calcareous Fluvo-Aquic Soil

Nitrogen (N) derived from urea is easily lost through transformation in agricultural production, raising sustainability problems for resource and environmental development. Humic acid (HA) presents considerable potential for mitigating active N loss owing to its carboxyl group. However, there is a gap in targeted enrichment of carboxyl groups in HA to conquer its inherent low content. In this study, we developed oxygen vacancy-rich CuO–Fe₃O₄@BC as a novel catalyst containing Cu(I)/Cu(II) and Fe(II)/Fe(III) redox cycling. The catalyst successfully enriched the carboxyl groups of HA by 96.13%, assisted by H₂O₂. The enrichment was attributed to the targeted breakage of the C═C bonds in condensed aromatics with ≥4-benzene rings by CuO–Fe₃O₄@BC. We then prepared raw HA-enhanced urea (RHAU) and carboxyl-enriched HA-enhanced urea (CHAU) using raw HA and carboxyl-enriched HA, respectively. Subsequently, we conducted a series of incubation experiments to determine the effects of CHAU on active N loss. The results indicated that compared to RHAU, CHAU delayed the urea hydrolysis by inhibiting the urease activity and decreased the total accumulation of NH₃ volatilization by 15.07% by altering the abundance and community structure of ammonia-oxidizing bacteria. It also decreased the total accumulation of N₂O emissions by 6.69% by reducing the abundance of nirK genes and altering the community structure of nirS genes. Thus, this study demonstrated an approach for the targeted enrichment of the carboxyl group of HA to mitigate active N loss and guide the development of high-efficiency N fertilizers in agriculture and environmental sustainability.