Soil organic carbon associated in size-fractions as affected by different land uses in karst region of Guizhou, Southwest China

Land use change has a significant effect on soil organic carbon (SOC), especially on labile organic carbon (LOC) due to its rapid response to soil changes and carbon supply. The objective of the study was to assess the effects of different land uses on soil fraction size distribution, SOC and LOC concentrations and their stocks in karst region of Guizhou Province, Southwest China. Studies were based on soil sampling in paddy, dryland, abandoned cultivated land for 3-years (AC-3), and abandoned cultivated land for 15-years (AC-15) in the 0-10, 10-20, and 20-30 cm depths in karst mountain area of Guizhou Province. Three fraction-size classes [macro (250-2000 mu m), micro (53-250 mu m), silt + clay (< 53 mu m)] were fractioned, and SOC and LOC concentrations in whole soil (non-fractionated) and different fraction sizes were also determined. The results showed that paddy contained the highest SOC and LOC concentrations in whole soil and different size-fractions as compared with other land uses down to 30 cm depth. However, the proportions of LOC to SOC were higher in AC-3 than the other land uses, especially pronounced in whole soil, which ranged from 17.5 to 26.3 % in different soil depths. Paddy also contained 23.7 % (100.9 Mg ha(-1)) more SOC and 17.6 % (19.7 Mg ha(-1)) more LOC stocks than dryland, whereas the SOC and LOC stocks in AC-15 and dryland were very close to each other in each soil depth. In paddy field, we found that macro-sized fractions contributed the greatest quantities of SOC to whole soil. In addition, paddy had significantly higher SOC stock values in macro-sized fractions than the other land uses in the both 0-10 and 10-20 cm depths. In the present study, there were no obvious increases of SOC and LOC pools among whole soil and different soil size-fractions after 15 years of land abandonment. These results suggest that natural recovery of SOC may take a long time after land abandonment, and paddy could serve as an important land use type for long-term carbon sequestration in karst region of Southwest China.