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Land use change significantly influences soil properties.There is little information available on the long-term effects of post-reclamation from grassland to cropland on soil properties.We compared soil carbon (C) and nitrogen (N) storage and related soil properties in a 50-year cultivation chronosequence of grassland in the agro-pastoral ecotone of Inner Mongolia.Field surveys on land use changes during the period of 1955-2002 were conducted to build a chronosequence of cropland of different ages since the conversion from grassland.The results showed that soil C and N storage,soil texture,and soil nutrient contents varied with land use types and cropland ages (P<0.01).In the 0-30 cm soil layer,the soil organic carbon (SOC) density was significantly lower in the croplands (3.28 kg C/m2 for C50 soil) than in the grasslands (6.32 kg C/m2).After 5,10,15,20,35,and 50 years of crop planting (years since the onset of cultivation),the SOC losses were 17%,12%,19%,47%,46%,and 48%,respectively,compared with the grasslands.The soil total nitrogen (TN) density of the grasslands was 65 g N/m2,and TN density of the cropland soil was 35 g N/m2 after 50 years of crop planting.Both the SOC and TN densities could be quantitatively determined by a negative exponential function of cropland age (P<0.0001,R2=0.8528; P<0.0001,R2=0.9637).The dissolved organic carbon (DOC) content,soil available potassium (AK) content,clay content,and pH value were decreased; and the soil bulk density and sand content were increased since the conversion of grassland into cropland during the 50-year period.Our results show soil nutrients were higher in grassland than in cropland.The conversion of grasslands to croplands induced a loss of soil C storage and changes of related soil properties.The reclamation time of cultivated soil (cropland age) had significant effects on soil properties in the study area.