Effects of cultivation and abandonment on soil carbon content of subalpine meadows, northwest China

Land use changes have a significant impact on soil carbon emission and sequestration worldwide. Accurate evaluation of the effect of land use change (cultivation and abandonment) on soil carbon content of subalpine meadows is required to monitor the soil carbon dynamics of rangeland ecosystems in China. Based on collection of soil cores and vegetation, investigations of four types of land use (undisturbed natural meadow, land cultivated for 20 years, land abandoned for 3 years following cultivation, and land abandoned for 10 years following cultivation) were undertaken in the headwater area of the Heihe River in northwest China. Three soil carbon fractions [soil organic carbon (SOC), light fraction organic carbon (LFOC), and microorganism biomass carbon (MBC)] were determined in the laboratory, and the relative abundances of LFOC/SOC and MBC/SOC were calculated. Repeated cultivation by ploughing reduced the carbon content of the top soil layer, resulting in more uniform vertical distribution of soil organic matter. Ten years after cessation of cultivation, the organic carbon content within the top 10-cm soil layer (0-10 cm) had reached 90 % of the content in native meadows, equivalent to a mean annual sequestration rate of 1.73 t C ha(-1). The rate of LFOC restoration was faster than that of SOC restoration. The variation in the ratio of MBC to SOC (0.91-1.07 %) was small. The activity of cultivation reduced all indicators of soil carbon status, which were not completely restored to the level of natural meadow, even after abandonment of cultivation for 10 years. Nevertheless, abandonment of cultivation is a practical, even if long-term, means of improving carbon sequestration in subalpine meadow of China.