Fine root traits mediate the long-term effects of partial cuts on soil organic carbon storage and stability in black spruce forests

<title>Abstract</title> <p> Aims Boreal forests store large amounts of soil organic carbon (SOC), to which fine roots contribute through belowground carbon inputs and stabilization mechanisms. However, the effects of partial cuts on fine root traits and their consequences for SOC dynamics remain poorly understood. We investigated long-term partial cut effects on fine root morphological, architectural and chemical traits and their link to SOC stocks and stability. Methods We studied twelve black spruce stands (Picea mariana [Mill.] B.S.P.) in two contrasting regions of Québec, Canada: Abitibi (warmer, Luvisols) and Côte-Nord (colder, Podzols) comparing control and thinned stands. SOC stability was assessed through incubation estimating bioreactive (C _bioR ) and recalcitrant carbon (C _recal ), and physical fractionation quantifying particulate organic (POC) and mineral-associated organic carbon (MAOC). Results Results showed depth-dependent thinning effects and trait–SOC relationships. In the forest floor, thinning decreased fine root cellulose and C/N, yet cellulose was positively related to C _recal , suggesting a decrease in C stability through root litter quality modification. In the 0-15 cm mineral layer, thinning reduced understory specific root length which was negatively correlated with SOC stocks. Lower C/N was associated with higher C _recal and MAOC, highlighting microbial-mediated stabilization. In the 15-30 cm mineral layer, fine root traits were unrelated to SOC stocks but predicted stability with root mass density, tissue density and dead root dry mass content controlling fresh carbon inputs, selective preservation and microbial-mineral stabilization. Conclusions These findings show that partial cuts reshape belowground carbon dynamics through depth-specific changes in fine root traits, with implications for forest management. </p>