Soil represents a larger reservoir of soil organic carbon (SOC) than terrestrial vegetation, offering a great potential for reducing the widespread adverse consequences of climate change. In forests and tree plantations, fine roots significantly impact SOC stabilization through their functional traits. However, it is not obvious which fine root traits between those related to chemistry (easily decomposable or recalcitrant), to architecture or morphology are the most conducive to SOC stabilization in phylogenetically related fast-growing trees. We assessed the effects of root functional traits on SOC storage and stabilization by studying five hybrid poplar clones (Populus spp.) with different root traits in plantations located in New Liskeard, ON, Canada. We collected soil cores at depths of 0-20, 20-40 and 40-60 cm, and determined bulk soil organic carbon, particulate organic carbon (> 53 μm, POC) and mineral-associated organic carbon (< 53 μm, MAOC) fractions and fine root (< 2 mm diameter) traits. We found that root length density (RLD) was the best predictor of increased SOC stocks and MAOC among all root traits. Soil organic C stocks and MAOC were also positively correlated with root traits indicative of low chemical recalcitrance (i.e. high N and soluble compounds concentrations and low lignin/N). Such easily decomposed root matter could be readily consumed by soil microorganisms and promote adsorption of microbial by-products onto mineral surfaces. Thus, root traits that increase the soil volume explored by fine roots and are associated with easily decomposed organic compounds play a key role in SOC accumulation and persistence.