Agricultural management affects the accumulation of organic carbon (OC) by influencing the amount of plant residues returned to the soil and the rate at which the residue and organic matter decomposes. Long–term agronomic studies in which carbon is being monitored are essential for determining temporal changes in Soil Organic Carbon (SOC). Soil samples were collected from 0-15cm depth from the on-going long -term experiment fields at Agricultural Research Station, Mangalore and Zonal Agricultural Research Station, Mandya after harvest of the rice crop raised during kharif 2004. The experiment plots are under mono cropping of rice and rice-pulse cropping respectively for the last 15 years. The samples were analyzed for SOC after separating the sand, silt and clay fraction. Total carbon, Light Fraction Carbon (LFC), Microbial Biomass Carbon (MBC), Water Soluble Carbon (WSC), urease and phosphatase activity and total- and available N contents of the soil were also determined before fractionation. The results revealed that at both the sites, OC concentration was highest in the clay fraction followed by silt and sand fractions of soil. At Mangalore site, clay, silt and sand fractions contained 0.43 to 0.50, 0.20 to 0.23 and 0.29 to 0.34 per cent of the SOC whereas at Mandya site it ranged from 0.18 to 0.24, 0.19 to 0.24 and 0.14 to 0.16 per cent of the SOC in the 0-15 cm depth. There was significant increase in the carbon content of all the three particle size fractions of soil due to the application of organic manures, particularly, farmyard manure and compost along with balanced dosage of NPK through fertilizers. Combined use of organic- and inorganic sources of nutrients as well as use of balanced dosage of only fertilizers caused an increase in the contents of even LFC, MBC, WSC, enzyme activities, total N and available N of soil. The LFC accounted for 9.13 to 9.49 per cent and 13.87 to 14.84 per cent of SOC at Mangalore and Mandya sites respectively. The MBC and WSC comprised a low per cent of SOC as compared to LFC.The MBC ranged from 422.9 to 549.7 mg kg-1 and 181.8 to 333.6 mg kg –1while WSC varied from 26.2 to 38.0 mg kg –1 and 19.5 to 31.2 mg kg –1 in the 0-15 cm depth at Mangalore and Mandya sites respectively. Urease activity ranged from 70.8 to 83.7 µg NH4-N g-1 soil 2hr –1 and from 81.9 to125.5 µg NH4-N g-1 soil 2hr –1 and phosphatase activity varied from 230.0 to 301.7µg PNP g-1 soil hr -1 and from 128.8 to154.8 µg PNP g-1 soil hr -1 at Mangalore and Mandya sites respectively. It could be concluded from the results that continuous incorporation of crop residues and organic manures would help in increasing the carbon content associated with even the finer fractions of soil and thus help in sequestration of C which otherwise would contribute to atmospheric pool of carbon. (Key words: organic carbon, particle size fractionation, light fraction carbon, microbial biomass carbon, water-soluble carbon, urease activity, phosphatase activity and sequestration)