Temporal dynamics of soil organic carbon after land-use change in the temperate zone - carbon response functions as a model approach

Land-use change (LUC) is a major driving factor for the balance of soil organic carbon (SOC) stocks and the global carbon cycle. The temporal dynamic of SOC after LUC is especially important in temperate systems with a long reaction time. On the basis of 95 compiled studies covering 322 sites in the temperate zone, carbon response functions (CRFs) were derived to model the temporal dynamic of SOC after five different LUC types (mean soil depth of 30 +/- 6 cm). Grassland establishment caused a long lasting carbon sink with a relative stock change of 128 +/- 23% and afforestation on former cropland a sink of 116 +/- 54%, 100 years after LUC (mean +/- 95% confidence interval). No new equilibrium was reached within 120 years. In contrast, there was no SOC sink following afforestation of grasslands and 75% of all observations showed SOC losses, even after 100 years. Only in the forest floor, there was carbon accumulation of 0.38 +/- 0.04 Mg ha-1 yr-1 in afforestations adding up to 38 +/- 4 Mg ha-1 labile carbon after 100 years. Carbon loss after deforestation (-32 +/- 20%) and grassland conversion to cropland (-36 +/- 5%), was rapid with a new SOC equilibrium being reached after 23 and 17 years, respectively. The change rate of SOC increased with temperature and precipitation but decreased with soil depth and clay content. Subsoil SOC changes followed the trend of the topsoil SOC changes but were smaller (25 +/- 5% of the total SOC changes) and with a high uncertainty due to a limited number of datasets. As a simple and robust model approach, the developed CRFs provide an easily applicable tool to estimate SOC stock changes after LUC to improve greenhouse gas reporting in the framework of UNFCCC.