Rhizosphere nitrogen-cycling microbial responses are associated with ammonia mitigation following chemical inhibitor application: a three-year field study.

Ammonia (NH3) volatilization from agricultural fertilization is a major source of anthropogenic reactive nitrogen pollution worldwide. Urease inhibitors (UI) and nitrification inhibitors (NI) are widely used to reduce gaseous nitrogen losses in farmland. However, their long-term effects on NH3 mitigation and associated rhizosphere microbial responses remain unclear. In this study, a three-year field experiment on the North China Plain was conducted to evaluate the effects of chemical inhibitors on NH3 volatilization and nitrogen-cycling microorganisms in rhizosphere and bulk soils. The UI significantly reduced cumulative NH3 volatilization by 53.8% and increased wheat yield by 20.5%. In addition, the inhibitor was associated with suppression of two major NH3-producing pathways in the rhizosphere, namely urea hydrolysis and dissimilatory nitrate reduction to ammonium. These responses were accompanied by lower urease activity and reduced abundances of the functional genes ureC and nrfA, suggesting a reduced potential for microbial NH3 production. Long-term inhibitor application caused little change in the overall bacterial community composition but was associated with shifts in the abundance of key nitrogen-cycling microorganisms, particularly in the rhizosphere. Overall, the findings suggest that rhizosphere microbial responses were closely associated with the observed NH3 mitigation under urease inhibitor application and provide field-based evidence to support improved nitrogen management in alkaline croplands.