Abstract: In this study, we carried out a maize field experiment with four treatments, including control (CK), urea application alone (U), urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) and nitrification inhibitor dicyandiamide (DCD) with urea (NBPT+DCD+U), and cyclopentanone (CCO) with urea (CCO+U), to investigate the responses of soil nitrogen transformation, the activities of nitrogen cycling-related enzymes, the abundance of nitrogen cycling-related functional genes and related microorganisms to the application of CCO, a novel bio-inhibitor. Compared with the NBPT+DCD+U treatment, the NH₄⁺-N content was increased by 12.5%-73.7% in the first 16 days, while the NO₃^--N content was reduced by 11.7%-47.4% in the CCO+U treatment from 20 to 85 days. Compared with the U treatment, the CCO+U treatment effectively delayed urea hydrolysis and significantly reduced soil urease activity by 52.5%-82.3%. CCO+U treatment significantly reduced soil nitrification potential and denitrifying enzyme activities (NR and NiR) during the maize reproductive period, suppressed the functional activities of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), and reduced the abundance of functional genes for denitrification (nir-S and nir-K). Results of thermogram and SEM analyses indicated that CCO improved crop yield by reducing the abundance of AOA amo-A, nir-S, and nir-K gene abundance. In summary, CCO improves crop yield by inhibiting hydrolysis, nitrification and denitrificationrelated enzyme activities and reducing the abundance of functional genes of nitrifying and denitrifying microorganisms.

Key words: bio-inhibitor, nitrogen transformation, cyclopentanone (CCO), soil microorganism, enzyme