Abstract: Nitrogen is a key limiting nutrient for plant growth in arid and semi-arid regions. Asymbiotic nitrogen fixation (ANF) in the soil is a critical pathway for exogenous nitrogen input in the loess hilly region. However, both the characteristics and regulatory mechanisms of soil ANF under different types of artificial vegetation remain unclear, which limits the accurate assessments of regional nitrogen cycle. We collected soil samples from artificial forests (Robinia pseudoacacia), shrubs (Caragana korshinskii), grasslands, and farmlands in the loess hilly region, measured ANF rates, and investigated the regulatory roles of soil physicochemical properties, microbial biomass, and extracellular enzyme activities. The results showed that: (1) Compared with farmland, vegetation restoration significantly increased soil nutrient content and microbial extracellular enzyme activities. Organic carbon and total nitrogen in surface soil (0-10 cm) in restored vegetation sites were up to 3.1 and 2.2 times those of farmland, respectively, with significant differences among vegetation types and soil layers. (2) The ANF rates across different vegetation types ranged from 1.60 to 2.91 kg N·hm^-2·a^-1. Surface soils in shrublands had the highest rate (2.70±0.16 kg N·hm^-2·a^-1), being 49.88% higher than that in farmlands. The ANF rates increased with soil depth. (3) Vegetation type primarily regulated ANF by altering soil physicochemical properties, which in turn influenced extracellular enzyme activities and nutrient acquisition by soil diazotrophs, ultimately determining ANF rates. This study helps improve our understanding of how vegetation restoration affects soil nitrogen cycling in arid and semiarid ecosystems and provides insights for guiding regional restoration and nitrogen budget evaluation.

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