： Nitrogen is critical for terrestrial ecosystems and productivity. However, the dynamics and drivers of nitrogen fractions in different years of vegetation restoration in karst areas remain to be elucidated. Thus, this investigation assessed the effectiveness of vegetation restoration over additional years on reactive nitrogen fractions, mineral-bound nitrogen, recombinant organic nitrogen, nitrogen storage and soil physicochemical indicators. The study’s results demonstrated that vegetation restoration could enhance the content of soil N fractions inside karst regions to a certain extent. In the vertical soil profile, total soil nitrogen and its component content in all years of restoration generally decreased with the increase of soil layer, and there was a Presence of surface aggregation. The annual increases in total nitrogen, microbial nitrogen, dissolved organic nitrogen and nitrogen storage were: shrub > grassland > orchard, and the yearly increases in particulate organic nitrogen, light group organic nitrogen, and recombinant organic nitrogen were: shrub > orchard > grassland. The N reserves of parks, shrubs and orchards were 1.22, 1.32 and 1.31 times higher than those of arable land. The redundancy analysis and interpretation rate of soil physicochemical indices showed that AN, AP, AK and SOC were the dominant factors controlling the distribution of soil nutrient fractions under vegetation restoration on karst sites. Soil N fractions and N storage varied with the period of vegetation restoration, and different vegetation restoration methods could improve the content of total and N fractions in karst areas to some extent. Shrub restoration favoured the accumulation of total nitrogen. Vegetation restoration promoted nitrogen fractions and storage, providing a theoretical underpinning for ecological rehabilitation and soil quality management in karst areas.

Since soil organic nitrogen (N) accounts for over 90% of total soil N, it is a vital indicator of soil fertility. However, there remains an insufficient understanding of the impacts of changes in vegetation restoration maturity on soil ON. Therefore, this study examined arable land and restored grassland, shrub-grassland, shrub, and woodland of 5 a, 15 a, 20 a, and 40 a restoration maturity, respectively to characterize changes in soil ON fractions in the 0–40 cm soil layer and how these changes are regulated by environmental factors. Large changes in soil ON fractions were observed in the 0–40 cm soil stratum of 15-a grassland. TAN, AMN, ASN, AAN, and TUN exceeded those in control (CK) by 26.70%, 22.80%, 26.08%, 31.50% and 28.77%, respectively. AIN of 20-a shrub and 15-a scrub grassland exceeded those of cultivated land by 19.8% and 5.66%, respectively. AIN of 15-a grassland exceeded other vegetation restoration maturities and facilitated soil N accumulation. The rank of the ON components according to proportion was AIN> TUN > AMN > AAN > ASN. AN, TN and SOC had the greatest effects on ON fractions of the soil, whereas powder and pH had no impacts. The results suggest that vegetation restoration can increase the ON content and N sequestration function. The results of this study can act as a scientific reference for soil quality restoration.