Recent studies indicated a second layer of organic matter often accumulates in the lower active layer and upper permafrost of Arctic tundra soils due to cryoturbation. This portion of the soil organic matter is less humified but likely to be exposed to conditions more favorable to physicochemical and biological degradation with a warming climate resulting in a positive feedback to climate change. The objective of this study is to combine both molecular-level and modern analytical techniques to explore the pathways and mechanisms that control the transport and transformation of soil organic carbon (SOC) in Arctic tundra. The quality and quantity of SOC were characterized by using stable isotope 13C, 15N, optical properties of soluble organics, GC/MS-Pyrolysis, 13C NMR spectroscopy and 14C dating, coupled with physical, molecular level fractionation of SOC. Ultra-filtration techniques were used to separate soluble organics into high molecular weight (colloids) and low molecular weight fractions. Preliminary results show that about 81% of SOC and 71% of total N occurred in the high molecular weight (colloids) fraction with a C/N ratio ranging from 12 to 19; and the remaining in the low molecular weight fraction with a C/N ratio ranging from 5 to 16. Extractable nutrient species (NH4+, NO3-, PO43-) are predominantly in high molecular fractions and NH4+-N is the dominant species due to the cold, wet and reducing soil environment. The optical properties of soluble organic carbon show that the chemical composition of different molecular weight fractions are significantly different, indicating the heterogeneity of size-fractionated SOC.