Technical Abstract: Turfgrass is a major vegetation type in the urban and suburban environment. Management practices such as species selection, irrigation, and mowing may affect carbon (C) input and storage in these systems. Research was conducted to determine the rate of soil organic carbon (SOC) changes, soil carbon sequestration, and SOC decomposition of fine fescue (Festuca spp.) (rain fed and irrigated), Kentucky bluegrass (Poa pratensis L.) (irrigated), and creeping bentgrass (Agrostis palustris Huds.) (irrigated) using carbon isotope techniques. Aboveground tissues were collected for biomass determination and carbon 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 17-24% of SOC at 0-10 cm and 1-13% from 10-20 cm was derived from turfgrass. Irrigated-fine fescue added the most SOC (3.35 ton C ha-1 yr-1) to the 0-20 cm soil profile, but also had the highest rate of SOC decomposition (2.61 ton C ha-1 yr-1). The corresponding additions and decomposition rates for non-irrigated fine fescue, Kentucky bluegrass, and creeping bentgrass in the top 20 cm soil profile were 1.39 and 0.87, 2.05 and 1.73, and 2.28 and 1.50 ton C ha-1 yr-1, respectively. Thus, the irrigated fine fescue added about 140% more SOC than did the non-irrigated fine fescue, and 55% more than irrigated-Kentucky bluegrass and creeping bentgrass. Irrigation increased both net organic carbon input to the soil profile and SOC decomposition. We found that all turfgrasses exhibited significant carbon sequestration (0.32 -0.78 ton ha-1 yr-1) during the first 4 years after turf establishment. However, the net carbon sequestration rate was higher for irrigated fine fescue and creeping bentgrass than for Kentucky bluegrass. To evaluate total carbon balance, additional work is needed to evaluate the total carbon budget and fluxes of the other greenhouse gases in turfgrass systems. [GACENet publication]