Arctic warming has been linked to changes in carbon cycling in this region. Cold temperatures and anoxic conditions in the Arctic inhibit microbial activity, lowering decomposition rates. As a result mineralization rates are low, resulting in nitrogen-limited-system, further reducing biological activity. Evidence has shown that eliminating this constraint on nutrient availability results in a vegetation shift and loss of soil carbon; however, the mechanisms behind soil carbon loss are not understood. We do not know 1) which organic matter fractions are most susceptible to degradation in arctic soils, and 2) where are they located within the soil matrix. For this study, our focus was on the active mineral layer directly below the organic horizon. Soils were collected during the 2007 growing season from a long-term nutrient addition experiment in which soils had been fertilized with additional nitrogen and phosphorus since 1996 and 1988 at the Arctic LTER site at Toolik Lake, on the Alaskan North Slope. Soils have been separated into four size classes of water-stable aggregates. Sub-samples of these aggregates have been ground and analyzed for total carbon and nitrogen content. Aggregate size distribution was the same for all treatments early in the growing season, but by mid-June control soils exhibited greater macroaggregate stability/formation; however by the end of the season, all soils exhibited the same aggregate size distribution. Early in the growing season, we found fewer large macroaggregates in soils fertilized since 1988, which resulted in a lower amount of carbon associated with this fraction. In addition, because small macroaggregates constituted the greatest proportion of total soil, more carbon (>50%) was associated with this fraction than other ones under all fertilization treatments. These results indicate changes in structural dynamics during the season with nutrient addition, and that the majority of carbon is associated with macroaggregates in these soils.