Coastal wetlands are commonly exposed to hydrocarbon pollutants derived from extraction disasters like the Deepwater Horizon oil spill. Naturally occurring microbes can degrade oil, but the rate of oil degradation depends heavily on the key chemical and biological factors. The goal of this study was to determine the influence of interactions between marsh plants and nitrogen (N) on the degradation of oil from the Deepwater Horizon oil spill. Oil disappearance was measured with gas chromatography (GC) focusing on the change in C18 n-alkane-to-phytane ratio of oil, and instantaneous oil degradation rates were measured using an instantaneous carbon isotopic partitioning method. N addition often stimulates oil decomposition in soil slurries, but the effect of N in our mesocosms depended on plant species. N addition accelerated oil degradation in Spartina alterniflora mesocosms but slowed oil degradation in Spartina patens mesocosms. Across all plant and N treatments, oil degradation related to plant root growth. In many ecosystems including marshes, N addition has been shown to diminish root growth by reducing the need for nutrient foraging. Where N addition reduces root growth, N may ultimately exacerbate oxygen scarcity in marsh soils possibly negating or reversing the positive, direct effects that N has on oil degradation. Based on these findings, fertilization strategies that maximize marsh plant root growth will be the most effective at increasing the microbial degradation of oil and will have the greatest potential to mitigate the impacts of oil in marsh ecosystems.