Since the 1980s, scientists have known fine roots (< 2 mm) are critical to ecosystem carbon cycling, with research long suggesting their contribution to soil carbon accrual may exceed that of aboveground parts like leaves. Yet over 40 years later, a key knowledge gap remains: the role of multi-decadal root iterative dynamics (growth, turnover, decomposition) in soil carbon accumulation—especially for "absorptive roots," the finest, most metabolically active roots (typically the distal 2–3 root orders or < 0.5 mm in diameter).

To address this gap, a research team led by Prof. KOU Liang from the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences, working with international collaborators, has revealed that due to their fast turnover and slow decomposition, absorptive roots' iterative effects generate 2.4 ± 0.1 megagrams of carbon per hectare (MgC ha^-1) in soil over two decades—surpassing the carbon contribution of leaves by 65%.

Their findings were recently published in Nature Geoscience.

In this study, the researchers compiled a comprehensive dataset of 880 field observations. These observations tracked the growth, turnover, and decomposition rates of absorptive roots across 199 woody plant species and 328 forest sites spanning the Northern Hemisphere.

Forest soils represent one of the largest terrestrial carbon reservoirs, and the research delivers critical estimates of absorptive roots' iterative effects—data essential for accurately characterizing moderately persistent forest soil carbon pools.

Furthermore, it sheds light on a longstanding scientific debate: whether forests dominated by arbuscular mycorrhizal (AM) fungi or ectomycorrhizal (EM) fungi store carbon in soil at higher rates. The team's findings offer a nuanced resolution: while EM-dominated forests overall hold more carbon in soil, roots associated with AM fungi contribute 43% more carbon to soil than those linked to EM fungi.

The study further addresses a need for global biogeochemical cycling models, which have long lacked reliable belowground metrics to integrate plant roots' influence on carbon cycles. The study identifies "specific root length"—a measure of root length per unit dry mass—as the most predictive metric for capturing absorptive roots' dynamics in the carbon cycle. This discovery provides a practical, long-sought-after belowground parameter for refining global carbon models.

Notably, the work shifts the focus of soil carbon stabilization research, which has traditionally centered on microbially dominated, highly persistent forms of soil carbon. This study confirms that absorptive roots are the primary contributors to moderately persistent forest soil carbon pools.

Against the backdrop of global biodiversity loss and shifts in forest mycorrhizal types, this study is essential for advancing integrated quantification and accurate modeling of soil carbon stocks within the Earth system, the researchers noted.

Carbon-based dynamic processes and iterative effects on soil carbon accrual of absorptive roots in Northern Hemisphere forests. (Image by Prof. KOU's team)