Response of Soil Nitrogen Cycle Microbial Functions to Ecological Reconstruction in Saline‐Alkali Soils: A Dual Perspective of Natural Succession and Alfalfa Cropping

ABSTRACT Alfalfa cropping has shown significant potential in rehabilitating degraded saline‐alkali soils. However, the mechanisms linking microbial activities to soil nitrogen turnover are poorly understood. Therefore, this study examined the abundance and microbial community of functional nitrogen cycling (N‐cycling) genes ( nifH , AOB, nirK , nirS , norB , and nosZ ) in artificial alfalfa fields, naturally successional Suaeda glauca fields, and bare land. Quantitative polymerase chain reaction (qPCR) and MiSeq sequencing were employed to characterize the microbial communities. Our results revealed that soil properties, particularly pH, electrical conductivity (EC), soil organic carbon (SOC), and alkaline hydrolyzable nitrogen (AHN), played critical roles in shaping the structure of N‐cycling microbial communities. Proteobacteria emerged as the dominant phylum associated with N‐cycling functions. Among the three alfalfa varieties tested, the Zhaodong variety consistently outperformed others, exhibiting significantly higher abundances of nifH ‐, nirS ‐, norB ‐, and nosZ ‐harboring microorganisms. Notably, compared to bare land, the community assembly processes of nirS in S. glauca fields and those of nifH ‐, AOB, nirS ‐, and norB ‐ in alfalfa fields shifted from deterministic to more stochastic patterns. This transition contributed to increased microbial diversity and enhanced network stability, indicating improved ecological resilience. Alfalfa cropping and S. glauca natural succession both stimulated nitrogen fixation and nitrification, enhancing soil nitrogen retention and overall soil health. And alfalfa more effectively boosted microbial diversity, community stability, and N turnover, demonstrating greater potential for saline‐alkali soil remediation than natural S. glauca succession.