Climate and soil properties override long-term thinning effects on soil organic carbon stability in boreal forests

Boreal forest soils store a large proportion of global terrestrial carbon (C), making their stability a critical component of climate change mitigation. While the effects of thinning on soil organic carbon (SOC) storage are well documented, its long-term influence on SOC stability remains poorly understood in boreal forests. This study assessed thinning effects 20 years after treatment on SOC stability in black spruce stands across two contrasting boreal regions of Québec, Canada: a warmer, drier region with Luvisols and a colder, more humid region with Podzols. We measured SOC stability across the forest floor and two mineral layers (0-15 and 15-30 cm) using complementary biological and physical indicators, including long-term laboratory incubation to quantify bioreactive C and recalcitrant C, and physical fractionation to isolate particulate (POC) and mineral-associated organic carbon (MAOC). Thinning effects on SOC stability were depth and region specific. In the forest floor, thinning reduced recalcitrant C stocks only in the colder region, suggesting that thinning-induced warming relieved climatic constraints on decomposition. In contrast, thinning reduced recalcitrant C in the upper mineral layer of the more productive region, likely through enhanced biological activity and priming effects. At the whole-profile scale, regional climatic and edaphic controls outweighed thinning effects, with higher POC stocks in the colder region and higher MAOC stocks in the clay-rich soils of the warmer region. Overall, SOC stability in boreal forests is regulated by climate in surface organic layers and by soil properties in mineral layers.