The Responses Of Soil Organic Carbon Components To Different Land Use Regime

Human, it is one of the biggest factors which affect carbon cycle in terrestrial ecosystems. In this study, two types of rain-fed rotated farmland (cultivated and fallowed) which at least have 50 years traditional farming history and two types grassland system which have been returned from farmland to grassland for 20 years (fenced and grazing) were chosed as the research objects in the Loess Plateau in Gansu. Based on 2 years field investigation, laboratory test and data analysis, the contents and characteristics of soil light fraction organic carbon(LFOC), soil heavy fraction organic carbon(HFOC), and soil microbial biomass carbon(MBC) were discussed in this paper.The main conclusions are as follows:1. In the two year research, LFOC and HFOC in the recuperative grazing meadow showed no significant change, indicating that the ecosystem of grazing meadow is in a stable state. Fenced grassland had significant higher soil light fraction organic carbon than grazing grassland (p<0.05). soil light fraction organic carbon increased with the prolonged time of fence. There was no significant increase in the heavy fraction organic carbon. Fencing of grazing grassland is helpful to the accumulation of soil organic carbon (p<0.05). And soil organic carbon increase gradually along with fence age. soil light fraction organic carbon is more sensitive to this kind of change.2. At 0-20cm layer, the light fraction organic carbon of fallow cropland increased significantly than that before fallow (p<0.05). The heavy fraction aslo increased significantly during this time (p<0.05), but the amplitude of variation is smaller compared with that of light fraction. Soil organic carbon content increased in fallow cultivated land, soil light fraction organic carbon is more sensitive to change of land use.3. Soil light fraction organic carbon of grassland was significantly higher than that of cropland (p<0.05). In grassland, the distribution of light fraction organic carbon and heavy fraction organic carbon decreased along soil profile. In cropland, the distribution of light fraction organic carbon showed the same pattern with that in grassland. Converting farmland into grassland is helpful to the accumulation of light fraction organic carbon.4. At 0-10cm layer, fencing of grazing grassland will increase microbial carbon content significantly (p<0.05). At both layers of 0-10cm and 10-20cm, the content of microbial carbon in five years fencing were higher than that in three years fencing, the content increased gradually along with fence age.5. The microbial carbon content increased in the fallow of cropland.6. The microbial carbon content is obviously higher in grasslang than that in cropland(p<0.05) which indicated that converting farmland into grassland could increase the microbial carbon content.