Changes in soil carbon stocks and related soil properties along a 50-year grassland-to-cropland conversion chronosequence in an agro-pastoral ecotone of Inner Mongolia, China

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/m(2) for C50 soil) than in the grasslands (6.32 kg C/m(2)). 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/m(2), and TN density of the cropland soil was 35 g N/m(2) 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, R-2=0.8528; P<0.0001, R-2=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.