Effects of Reduced N Application on Soil Ammonia Volatilization in Maize–Soybean Intercropping and Monocropping Systems

A systematic elucidation of soil ammonia (NH 3 ) volatilization (SAV) and the underlying drivers is imperative for evaluating NH 3 pollution mitigation strategies and advancing sustainable agricultural practices. Currently, no scientific consensus has been established on the effects of maize–soybean intercropping on SAV across varying nitrogen (N) application rates. A consecutive field experiment was conducted over a 2-year period from 2024 to 2025 with a split-plot design. The experiment comprised three cropping systems (maize monocropping (MM), soybean monocropping (MS), and maize–soybean intercropping (IMS)) and three N application rates (no N application (NN), 20% reduced N application (20%RN), and conventional N application (ConN)). The results demonstrated that N application markedly increased SAV. Accumulative SAV was 4.94–6.01 kg ha −1 under NN treatment, whereas it was 8.21–27.89 kg ha −1 under ConN treatment, 7.25–21.52 kg ha −1 under 20%RN treatment. Under ConN treatment, the accumulative SAV in IMS was 21.34 kg·ha −1 and 27.89 kg·ha −1 in 2024 and 2025, respectively, which were significantly higher than those in MM by 16.80% and 13.33%. Under 20% RN treatment, the accumulative SAV in IMS was 15.46 kg·ha −1 and 19.24 kg·ha −1 in 2024 and 2025, respectively, which were lower than those in MM by 3.07% and 10.59%. SAV was positively correlated with soil ammonium N concentration. Moreover, within an appropriate range, SAV increased in response to rising soil water content and temperature. Collectively, maize–soybean intercropping integrated with a 20% nitrogen reduction mitigated environmental risks associated with reactive nitrogen losses. This system constitutes a stable yield, resource-efficient, and ecologically sustainable cropping practice.