Cultivating native pasture to plant oat or some other species of grasses is generally recommended as a way to alleviate grazing pressure on native grassland and to improve winter forage levels in high altitude regions,however,its effects on soil quality have not been well understood.This study selected three types of land use,fencing pasture(FP),fencing pasture plus subsequent 25-year oat(Arrhenatherum elatius) cultivating(FP+SO),and fencing pasture plus subsequent 7-year perennial grasses (Elymus nutarrs and Bromus anermis)cultivating(FP+SPG),to investigate changes in soil organic C concentration and some soil physical properties caused by land use transition.The results show that compared with FP,FP+SO and FP+SPG significantly decreased soil organic C density in 0-30 cm depth by 27.7% and 8.5%,and mean weight diameter(MWD) by 46.9% and 42.6%,respectively.Meanwhile,FP+SO and FP+SPG significantly increased soil bulk density and particle density,and significandy decreased soil porosity.We concluded that decrease in aggregate stability and porosity,and increase in bulk density and particle density,were all related to reduction in soil organic C concentration by cultivation.