Humic acid urea mitigates N2O emissions from winter wheat in a rice–wheat rotation by regulating nitrifying and denitrifying microorganisms and key genes

Maintaining winter wheat productivity while reducing nitrous oxide (N 2 O) emissions is critical for sustainable rice–wheat rotation systems in the Yangtze River Plain, China. Humic acid urea (HAU) may enhance wheat yield and mitigate N 2 O emissions, but its field performance and underlying biological mechanisms remain insufficiently understood. A two-year field experiment was conducted during the winter wheat phase of a rice–wheat rotation system, with three treatments: a zero-nitrogen control (CK), conventional urea (U, 210 kg N ha −1 ), and HAU (210 kg N·ha −1 ). Winter wheat yield, aboveground nitrogen (N) uptake, N use efficiency (NUE), and N 2 O emissions were evaluated across two growing seasons; soil carbon and N pools, bacterial communities, and N-cycling genes were examined at the jointing stage during the 2023–2024 growing season. Compared with U, HAU increased winter wheat yield by 7.3–8.7%, aboveground N uptake by 4.4–4.9%, and NUE by 3.2–3.6 percentage points. Higher yield was associated with a greater number of effective spikes. HAU also reduced cumulative N 2 O emissions by 23.3–24.1% and greenhouse gas intensity by 28.0–29.1%. At jointing, HAU increased soil NH 4 + -N and microbial biomass N but decreased NO 3 – -N, labile organic carbon, and microbial biomass carbon. These changes coincided with lower abundances of ammonia-oxidizing bacteria and the amoA , nirK , and nirS genes, and a higher abundance of the N 2 O-reducing gene nosZ . Overall, HAU improved winter wheat productivity and NUE while mitigating N 2 O emissions, supporting its potential for lower-emission N management in rice–wheat rotations.