Consequences of planned afforestation versus natural forest regrowth after disturbance for soil C stocks in Eastern European mountains

- We studied 12C and 14C content in three Bulgarian forest soil profile time series.
- We reconstructed C dynamics after land use changes (LUC) and disturbances in a forest.
- Disturbances did not affect C dynamics, LUC affected them in topsoil and subsoil.
- LUC are representative of changes happening in Eastern Europe in the last decades.
- LUC in Eastern European countries might represent an unexpected C sink.

Westudiedcarbon and radiocarbon content in three historical time series of soil profiles from sites with different histories in the Parangalitsa biosphere, Rila mountains (Bulgaria).Our aim was to understand the consequences of forest regrowth in Eastern European forests for C stocks in the medium and long term.Disturbance history of the area has been studied in detail in recent decades, and we were therefore able to assess the impact of natural events such as windthrows and insect outbreaks on soil C stocks.By combining unique historical profile data with radiocarbon measurements, we obtained a picture of the kinetics at three different sites. These sites were 1) no land use change but an old (>120 years) forest subjected to decades old disturbances from the secondary consequences of a windthrow, 2) a forest site subjected to decades old disturbances from the direct consequences of a windthrow and 3) a grassland converted to forest.While the difference between direct and indirect disturbance did not have a relevant effect on C stocks, the ecological successions triggered either by afforestation or by disturbances caused similar dynamics of SOC decrease followed by accumulation, according to the pattern of primary productivity. The accumulation was marked in the conversion, and clearly involved the subsoil layers as well, leading to a C sequestration considered durable.The huge economic and social changes happening in Eastern Europe in the last two decades have generated a clear trend towards afforestation of lands that were formerly cultivated. The SOC changes from grassland to forest use in these areas (¿50 t ha −1) represented a doubling in stocks in less than 40 years, and their impact on a global scale could represent an unexpected C sink.