Soil type matters: forest succession and soil organic matter stability in the Gorce Mountains (S Poland)

Soil organic matter (SOM) stocks in mountain ecosystems play a crucial role in carbon and nutrient cycling, thereby mitigating climate change and providing many important ecosystem services. However, their response to rapid forest succession resulting from the combined effects of land abandonment and global warming, which is particularly pronounced in mountain areas, remains strongly context-dependent. The influence of soil-forming processes, represented by soil types, is still insufficiently understood. This study investigates how SOM stocks and their stability change along forest succession in the Gorce Mountains in southern Poland, with a specific focus on the role of soil type.

We analyzed soils across different forest succession stages, ranging from grasslands through shrubs and young successional forests to permanent old-growth forests, and across different soil types (Cambisols, Gleysols, and Podzols). We assessed SOM stocks in the O horizons and in the 0–5 cm (topsoil) and 20–30 cm (subsoil) mineral soil layers. In addition, we performed soil density fractionation of the mineral topsoil and subsoil layers into free light fraction, occluded light fraction, and heavy fraction, the relative proportions of which were used as indicator of SOM stability. Our results show that both SOM quantity and stability vary significantly along the forest succession gradient; however, these patterns are strongly modified by soil type. In some soil-contexts, the transition from grassland to forest led to increased SOM stocks but also to a greater vulnerability to SOM decomposition, whereas in other soil types, the highest SOM stocks and greatest stability occurred at an intermediate forest succession stage (tall-shrub communities).

These findings highlight that soil type is a key contextual factor controlling SOM storage in mountain ecosystems. Accounting for soil-specific responses is therefore essential for predicting SOM sequestration potential under ongoing environmental change.

This project has received funding from the Central European Leuven Strategic Alliance (grant CELSA/24/002) and has been supported by a grant from the Priority Research Area Antropocene under the Strategic Programme Excellence Initiative at Jagiellonian University.