The Loess Plateau is an important site for many ecological restoration studies in China. Different vegetation types change the soil environment to different extents, influencing the biogeochemical cycles of materials, such as nitrogen. Recently, a growing amount of attention has been paid to the relationship between vegetation and nitrogen forms in natural ecological systems to provide a theoretical basis for ecological restoration in these areas. Determining the relationship between soil enzymes and soil nitrogen forms under different vegetation types can provide useful information for vegetation restoration in the Loess Plateau. To investigate the transformation of soil nitrogen and its relationships with enzymes involved in nitrogen metabolism under different vegetation in the Loess Plateau, we collected 0-10 cm and 10-20 cm soil profiles under four typical vegetation types (Bothriochloa ischaemun (L.) Keng, Populus davidiana Dode, Quercus liaotungensis and Pinus tabulaeformis Carr), and from abandoned land, one of the main land use types, in the Ziwuling region. Bare fallow land, where little vegetation grows, was chosen as the control. Ziwuling (107°30'-109°40' E, 33°50'-36°50' N), is situated at 1300-1700 m above the sea, in the transitional zone between forest steppe and semiarid grasslands. The climate is mild and humid. Soil basic physicochemical properties, different nitrogen form contents, urease activity, protease activity and nitric acid reductase activity were measured. The results indicated that all forms of nitrogen were greater under forest vegetation compared with control, especially, under Q. liaotungensis and P. tabulaeformis; while organic N was markedly increased under B. ischaemun, reaching 2.18 g/kg in the topsoil and 1.52 g/kg in the subsoil. Ammonium N was the dominant inorganic N in the study area. Ammonium N content correlated with microbial biomass N, while organic N content correlated with nitrous N, mineralizable N and microbial biomass N. Vegetation type and soil layer did not influence urease activity, protease activity and nitric acid reductase activity Urease activity correlated with ammonium N (r=0.66) and organic N (r=0.69), and significantly correlated with microbial biomass N (r=0.75). Protease activity did not affect any form of nitrogen content. Nitric acid reductase activity correlated with ammonium N. Urease activity was highest under Q. liaotungensis, and it was higher in topsoil than in subsoil. By contrast, protease activity and nitric acid reductase activity did not show any significant differences among vegetation types or soil layers. These findings suggest that forest vegetation increases soil nitrogen content. Thus, this role of forest vegetation should be considered during ecological restoration projects in the Loess Plateau region. In addition, enzymes play an important role in the nitrogen transformation process. Further research is still required to explore the processes of vegetation change and nitrogen transformation and the relationship between enzymes and nitrogen forms.