Research was conducted to determine the rate of soil organic carbon (SOC) changes, soil carbon sequestration, and SOC decomposition of fine fescue (rain fed and irrigated), Kentucky bluegrass (irrigated), and creeping bentgrass (irrigated), and a native grass mix (rain fed) using carbon isotope techniques. Aboveground tissues were collected for biomass and C isotope ratio analysis. Soil was sampled for determination of root mass, soil bulk density, SOC, soil organic nitrogen, and C isotope ratio. Our results indicated that four years after establishment, about 11-13 percent of SOC at 0-20 cm was derived from turfgrass. Native grass mix and irrigated-fine fescue added 1.52 and 1.40 ton C/ha/yr to the 0-20 cm soil profile, which is about 30% higher than the SOC input from non-irrigated fine fescue, Kentucky bluegrass and creeping bentgrass. Decomposition rates of SOC were 0.64 and 0.61 ton per hectare per year for irrigated fine fescue and Kentucky bluegrass, respectively, which were higher than under non-irrigated fine fescue, non-irrigated native grass, and irrigated creeping bentgrass. Irrigation increased both net organic carbon input to the soil profile and SOC decomposition. Therefore the net carbon sequestration was similar for irrigated and non-irrigated fine fescue. All turfgrasses exhibited significant carbon sequestration. However, the net carbon sequestration rate is higher for native grass mix than fine fescue and creeping bentgrass. Among the tested species, Kentucky bluegrass had the lowest carbon sequestration rate.