Thesis (Ph.D., Natural Resources) -- University of Idaho, 2016 | Elevated atmospheric nitrogen deposition caused by human activity induces a forest carbon sink across broad parts of the Northern hemisphere. In addition to more rapid tree growth, this increase in carbon sequestration could be due to soil carbon accumulation caused by slower organic matter decomposition. The objective of this dissertation was to understand and compare how elevated nitrogen deposition affects decomposition of two major tree litter sources: leaf litter and fine roots. A long-term (>15 years) nitrogen deposition experiment enabled a three-year decomposition study across the span of the Northern Hardwood Biome in Michigan. Fine root and leaf litter biochemical composition and the contribution of leaves and roots to ecosystem biochemical fluxes was quantified. Fine roots were more chemically recalcitrant than leaf litter. At the ecosystem scale, fine roots dominated litter fluxes of acid-insoluble fraction (AIF, also known as Klason lignin) and condensed tannins to soil. Decomposition was estimated using a double-exponential model to describe litter mass loss. Annual litter production was combined with decomposition patterns to estimate how plant litters contribute to soil organic matter. Nitrogen additions increased the initial decomposition of leaf litter, but inhibited the later stages of fine root decomposition. Slower fine root decomposition caused a 23.8 % additional retention of root mass (g m -2) after six years of decomposition. Wet chemistry and Fourier-transform infrared spectroscopy (FTIR) were used to quantify chemical changes of both litter types. Both gravimetrically-defined AIF and lignin/carbohydrate characteristic IR peak ratios indicated that lignin was selectively preserved under simulated nitrogen deposition. The slower degradation of AIF contributed 73.9 ± 5.2 % of additional root mass retention under simulated nitrogen deposition. Although nitrogen deposition studies have focused on leaf litter, these results highlight the dominant role of fine roots in plant-soil carbon fluxes and suggest that slower fine root decomposition is a major driver of soil organic mass accumulation under elevated nitrogen deposition.