Substantial subsoil carbon loss from beech forests since the 1980s

Soils are a major carbon (C) reservoir, with subsoils (>20 cm) storing the majority of this C. Predicting the response of subsoil C to global change remains a critical research priority, yet long-term field observations for forest ecosystems are scarce. In this study, we assess temporal C dynamics in mineral soils to 90 cm depth of 62 temperate European beech (Fagus sylvatica) stands in Austria using data from sampling campaigns in 1984, 2012 and 2022. Our results show a significant increase in C stocks between 0-20 cm and a significant decrease in C stocks between 20-50 cm and 50-90 cm depth, suggesting substantial C losses from the subsoil. These losses outweighed the C gain in topsoils, resulting in an overall soil C loss since the 1980s. Organic-rich calcareous soils appear to be particularly vulnerable to C loss, probably because they are less effective at stabilising C than soils on other substrates. We suggest that changes in climate (i.e. warmer and wetter) and factors such as changes in rooting depth or litter inputs may underlie the observed patterns of depth-dependent soil C changes. The estimated soil C loss accounted for 23% of the C accumulated in aboveground biomass, as determined by dendrochronological analysis, indicating a reduction in the ecosystem's carbon sink capacity. Our results highlight the importance of including subsoil C in forest ecosystem assessments, as it plays a key role in the overall carbon balance.