Twelve years of soil monitoring in forests and grasslands to study changes in organic carbon stocks in topsoils under control of management intensity

Grasslands and forests play an important role in the global carbon cycle and management intensity in grassland and forests influences the uptake and storage of carbon. Organic carbon stock changes in grasslands and forests depend not only on the management intensity but also on the site on which they grow. However, long term studies in real world ecosystems and along land use gradients in forests and grasslands are rare. Therefore, it is unclear how management intensity and site conditions and the interaction between the two influence soil organic carbon stock changes on the long term. Since 2011, we have been studying the mineral soil organic carbon stocks in topsoils of 150 forests and 150 grasslands in 3 German regions (Schorfheide-Chorin, Hainich-Dün and Swabian Alb). In 2011, 2014, 2017, 2021 and 2023, the organic layer (only forests) and the topsoil (0-10 cm) were sampled at 14 sampling points per plot with a split tube corer (diameter 5 cm) and a representative composite soil sample was prepared for each the 300 plots. The silvicultural management intensity index (SMI) introduced by Schall and Ammer (2013) and the land use intensity index (LUI) introduced by Blüthgen et al. (2012) are used to quantify forest and grassland management intensity, respectively, at all plots. Linear mixed effects models showed that organic carbon stock change after 12 years in 0-10 cm varied between ‑18 g C m^-2 and 61 g C m^-2 year^-1 in grasslands and ‑20 g C m^-2 and 50 g C m^-2 year^-1in forests. First results indicate that the magnitude of organic carbon stock change is related to management intensity and soil properties. Further statistical analyses will be carried out using the respective management indices for grassland and forest to study this. On average, we find a significant increase of organic carbon stocks (+23 g C m^-2) at our experimental sites. This suggests that global change might offset some of the management and site effects in forests and grasslands with CO₂ fertilization being one potential driver.