Abstract: This study, conducted on transitional areas of grasslands and crop fields in Gansu, China, deals with the impacts of land use on soil P dynamics. Land uses include three magnitudes of pasture degradation, lightly (LDP), moderately (MDP), and heavily degraded pasture (HDP), and cultivated fields varying 1 to 50 years of cultivation. Soil samples were collected from 18 sites at seven locations lying between 2,600 to 3,000 m above sea level. The dominant soil samples were Chernozem. The objective of this study was to understand soil P dynamics under different land uses. Phosphorus fractionation showed that proportion of soil organic phosphorus decreased sharply when pasture was put into crop production. Cultivation of grassland resulted in a decrease of organic phosphorus by 8%, 20%, and 36% after 8, 16, and 41 years cultivation, respectively. Regionally, 35% of organic phosphorus was lost within 30 to 50 years cultivation. Cultivation and fertilization significantly raised labile P and calcium associated inorganic phosphorus (Ca-Pi) levels. Mineralization of organic P, incorporation of subsoil by tillage following erosion, and fertilization were major sources of increased CaPi in topsoil of cultivated fields. In general, Fe and Alassociated Pi was higher after pasture was cultivated. Soil organic P, especially the potentially labile fraction extracted by NaOH, declined with pasture degradation and cultivation. Labile organic P, extracted by NaHCO3, is highly correlated with total organic P (R2=0.63).