To predict how climate change might affect carbon (C) storage in arctic ecosystems, a better understanding of the dynamics of C cycling in arctic soils is needed. Through 14C pulse-labeling and incubation experiments, I investigated C dynamics in tussock and wet sedge tundra from arctic Alaska. The objectives were (1) to determine pathways and rates of C transfer from roots and litter into soils, (2) to quantify transformations of C in soil organic matter (SOM), and (3) to determine how nutrient availability and living plants affected processes. This research revealed that the C cycle in arctic tundra is a dynamic system, with allocation of ~6% of recent photosynthates to soils within one day, and subsequent storage in various SOM fractions. While such root-derived material contributes ~24 g C · m-2 · yr-1, root litter appears to contribute ~53 g C · m-2 · yr-1 to SOM, and leaf litter C contributes ~38 g C · m-2 · yr-1. In tussock tundra, living plants increased decomposition rates of leaf litter, likely as a result of more stable microclimatic conditions at the soil surface. In contrast, plants inhibited the decomposition of root litter and existing SOM, perhaps by providing new labile C as a substrate to soil microbes. Soil microbes play an important role in C cycling in tussock tundra, transforming labile C compounds into more recalcitrant forms. During a freeze-thaw cycle similar to winter conditions, recalcitrant compounds may become the dominant substrate for microbes. In wet sedge tundra, microbes appear to play a less direct role in C cycling, perhaps due to high soil moisture, allowing root-derived C to diffuse more rapidly. Increasing nutrient availability via long-term fertilization resulted in greater C storage in wet sedge tundra. It appears that C turnover through the microbial biomass was faster in fertilized soils, possibly as a result of greater microbial biomass in fertilized soils and/or the production of more recalcitrant C compounds by nutrient limited plants in unfertilized soils. Overall, it appears that litter and root-derived C inputs are an important component of C cycling and storage in arctic tundra ecosystems.