Not only aboveground biomass: Soil of undisturbed Carpathian beech forests also stores substantial carbon

Primary beech-dominated forests, characterised by the dynamics of natural disturbance regimes with little or no evidence of human intervention, play a significant role in storing soil carbon (C). Their high aboveground biomass (AGB) contributes to high rates of C accumulation through the soil profile and into the deeper soil layers. However, C dynamics in deep soil horizons remain poorly documented. Hence, this study provides a quantitative assessment of soil C stocks in primary temperate beech-dominated forests with remarkably high AGB in Central and Eastern Europe. In 2023, we excavated two to three quantitative soil pits (0.5 m² each) per plot to a depth of 80 cm to estimate soil C stocks. In total, 15 plots in four primary beech-dominated forests in Slovakia and Romania were included, where aboveground biomass had been measured between 2012 and 2021. We used the quantitative soil pit method for the precise and reliable estimation of soil C, especially in deeper soil horizons with the presence of stones and deadwood. Across all studied forests, soil C stocks (105–148 Mg C ha⁻¹) were significantly lower than aboveground C stocks (291–391 Mg C ha⁻¹) but soil C stocks positively corresponded with aboveground C stocks. Soil N stocks varied from 7–9 Mg C ha⁻¹. Notably, deeper soil horizons (20–80 cm) contributed a substantial percentage share of C (61–66 %) of the total soil profile. Leaving forests intact, including primary forests and forests recently managed for timber production, will contribute to the further increases in carbon stocks in the aboveground biomass through continued biomass growth and gap-phase regeneration, and will promote soil C accumulation via sustained litter input and minimal disturbance.