Substantial forest soil carbon accrual from absorptive fine roots over decadal timescales

Forest soils hold the largest terrestrial carbon pool, derived from dead plant tissues and transformed by soil biota. Current frameworks emphasize the role of soil microbes in highly persistent forms of carbon. However, moderately persistent forms of carbon also contribute substantially to forest soil carbon pools through the iterative effects of plant litter inputs and outputs over multi-decadal timescales. These sources of soil carbon are not well constrained. Here we synthesize published field data of the finest roots (absorptive roots) of mycorrhizal woody plants across major forest ecosystem types in the Northern Hemisphere. We estimate that, owing to fast turnover and slow decomposition, the iterative effects of absorptive roots on soil carbon accrual generate 2.4 ± 0.1 MgC ha⁻¹ over two decades, exceeding that of leaves by 65%. Further, roots associated with arbuscular mycorrhizal fungi contribute 43% more soil carbon than roots associated with ectomycorrhizal fungi, despite ectomycorrhizal forests dominating soil carbon storage in forest soils overall. We also find that specific root length, a readily measured trait, can be used as a proxy for iterative effects associated with root dynamics. Our findings thus provide a long-needed belowground metric for carbon modelling in the Earth system.