Changes of soil active organic carbon pool under difierent land use types in Karst Mountain Area

The differences of active organic carbon fractions in 0-20 cm soil layers and their relationships with soil total organic carbon（ SOC） were studied in the karst small watershed of Chenjiazhai，Puding county，Guizhou province，under different land use patterns. The results showed that the average content of total organic carbon in 0-20 cm soil layer of Chenjiazhai karst watershed in Guizhou was 42.03（± 25.08）g/kg. The organic carbon content of shrub land was the highest（ 77.44 ± 28.38 g/kg），which was 3.5 times of that of cultivated land. The content of soil water-soluble organic carbon in descending order was as follows: bushes> grassland> plantation> orchard> arable land. The content of free active organic carbon in descending order was as follows: bushes> plantation> grassland> arable land> orchard. The content of easily oxidized organic carbon in descending order were as follows: bushes> plantation>grassland>arable land > orchard. There was a significant correlation between total soil organic carbon （SOC） content and active carbon components，and the correlation coefficient between free organic carbon and free organic carbon was the highest. The content of soil organic carbon was different under different land use patterns， and the contents of active organic carbon were similar and different，which indicated the complexity of soil active organic carbon. Therefore，the characterization index of different active organic carbon can not be directly compared regardless of the value or the trend of change.