Alfalfa (Medicago sativa) has been widely employed in the dryland region of the Loess Plateau, China to improve soil and water conservation and to develop livestock production. Our objective was to study the dynamics of plant and soil organic carbon (SOC) pools following the conversion of sloping farmland to alfalfa fields over a period of 30 years. The succession gradient is composed of seven differently aged alfalfa fields (0, 5, 9, 13, 16, 23, 30 years). The results show that soil C storage (0-100cm) dynamics were consistent with belowground biomass storage with increased planting years, but C storage always increased with the number of planting years in the 0-5cm soil layer. Planted perennial alfalfa resulted in a decline in soil C storage in the 0-100cm soil depth in the early period (nine years). During the late succession stage of alfalfa (13 years) soil C storage tends to recover, and after 16 years, storage values again dropped. However, it had recovered by 30 years at which time alfalfa productivity was very low. Vegetation C storage was mainly decided by the belowground biomass and ecosystem C storage dynamics was consistent with soil C storage. Vegetation biomass, root/shoot ratio, SOC, soil total nitrogen, and total phosphorus were the main factors affecting C storage in the entire alfalfa field ecosystem. The results suggest that C storage in vegetation is directly related to plant productivity, C storage in the soil throughout the entire alfalfa field ecosystem was not only related to plant productivity, but also to SOC and soil nutrients.