Abstract: Saline–alkali soils represent a critical land resource with untapped potential for improving arable land quality, boosting agricultural productivity, and expanding the effective cultivated area in China. However, persistent saline-alkali stress in these agroecosystems keeps nitrogen use efficiency (NUE) at a notably low level, ranging from only 19% to 33%, while driving substantial reactive nitrogen losses that have emerged as a major bottleneck restricting the long-term sustainability of saline-alkali agriculture. This review systematically synthesizes recent research advances in nitrogen cycling processes and targeted regulation strategies in saline-alkali croplands. Current evidence confirms that elevated soil salinity exerts strong inhibitory effects on core nitrogen transformation pathways — including nitrogen mineralization, nitrification, and denitrification — and this suppression follows a distinct threshold-dependent pattern. For example, significant inhibition of nitrogen mineralization and nitrification is only observed when soil salinity exceeds 3‰. These altered nitrogen transformation processes, coupled with poor soil structural stability, seasonal rainfall pulses, and region-specific irrigation and drainage practices, collectively trigger massive nitrogen losses via three dominant pathways: ammonia volatilization, nitrate leaching, and nitrous oxide emissions, all of which show clear regional heterogeneity. In the inland northwest and Yellow River irrigation areas, prominent surface salt accumulation and high-frequency flood irrigation substantially elevate the risk of nitrate leaching and gaseous nitrogen emissions. In the soda saline-alkali regions of northeast China, severe soil alkalization and elevated pH drastically amplify ammonia volatilization losses. In coastal saline zones, dynamic water-salt interactions, combined with seasonal rainfall events and shallow groundwater level fluctuations, lead to severe nitrate leaching that can exceed 30% of total applied nitrogen. To mitigate these constraints, the adoption of optimized tillage regimes, precision irrigation-drainage management, and rational surface mulching practices can effectively modulate soil water-salt dynamics and alleviate saline-alkali stress in the root zone. Building on these foundational measures, further optimization of nitrogen fertilizer application methods, product types, and application timings, paired with integrated use of organic and inorganic amendments — such as acidified organic materials, biochar, straw residues, and mineral conditioners — can significantly improve crop nitrogen uptake efficiency and minimize undesirable nitrogen losses. Drawing on the gaps identified in the current body of research, this review outlines priority scientific directions for future investigation, including the construction of distributed regional monitoring networks, the development of novel functional amendment materials and tailored fertilizer products, the design of site-specific ecological farming systems, the application of smart digital regulation technologies, and the establishment of regionally standardized fertilization recommendation frameworks. These coordinated efforts are expected to provide robust theoretical and technical support for precision nitrogen management and the sustainable development of saline-alkali agroecosystems.