How do fine root traits of fast-growing trees promote soil organic carbon stabilization?

Background and aims: 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), 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. Methods: We studied five hybrid poplar clones with different root traits located in New Liskeard, ON, Canada. We collected soil cores at depths of 0-20 and 20-40 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. Results: We found that root trait categories influencing SOC storage and stabilization varied by soil depth. At the 0-20 cm depth, root tissue density and dry mass content were negatively correlated with SOC stocks. Higher root length and mass densities were linked to greater SOC stocks and MAOC at the 20-40 cm depth. Root traits indicative of low chemical recalcitrance, such as high soluble compounds concentration and small diameter, also promoted MAOC formation. Conclusion: 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 persistence. Supplementary Information: The online version contains supplementary material available at 10.1007/s11104-025-07681-3.