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

A systematic elucidation of soil ammonia (NH3) volatilization (SAV) and the underlying drivers is imperative for evaluating NH3 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.