EFFECTS OF CONVERSION OF NATURAL AND DEGRADED GRASSLAND TO CROPLAND ON SOIL ORGANIC CARBON FRACTIONS IN NORTHEAST CHINA

Although the dynamics of soil organic carbon (SOC) during the conversion of grassland to cropland has received much attention, relatively little is known of the effects of grassland conversion to paddy soil on SOC and its fractions. This study examined eight systems: 1) natural grassland (NG); 2) NG conversion to maize for 5 years (NGM5); 3-4) NG to paddy land for 22 and 55 years (NGP22 and NGP55); 5) degraded grassland (DG) and; 6-8) DG conversion to paddy land for 4, 22, 55 years (DGP4, DGP22 and DGP55). Results showed the contents of SOC, labile organic C (LOC), recalcitrant organic C (ROC), light fraction of organic carbon (LFOC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) at 0-60 cm in NGM5 were 32.4352.30%, 44.58-66.07%, 25.33-34.24%, 11.3319.02%, 31.02-47.16% and 20.10-50.44%, respectively, lower than those of NG, indicating LOC was more sensitive than SOC to the conversion. NGP22 and NGP55 maintained higher SOC, ROC and LOC than that NG, with the highest content in NGP55. The SOC, ROC and LOC in 0-80 cm soil layer followed the order: DGP55 > DGP22 > DGP4.