Long-term impact of land-use change on soil organic carbon in German agriculture and forests

Land-use change (LUC) is deep-rooted in Germany’s agricultural history. Edaphoclimatic conditions play a crucial role in determining which sites undergo LUC, while simultaneously governing soil organic carbon (SOC) dynamics. Understanding how SOC responds to these changes in land use has significant implications for carbon accounting and greenhouse gas emissions efforts. Most efforts to quantify the effects of LUC on SOC change focus on the topsoil because it is where the bulk of SOC change occurs; overlooking soils beneath 30 cm. Coupled with extensive land-use histories, soil and site data from Germany’s national agricultural and forest soil inventories were used alongside data-driven machine learning techniques to investigate the factors that govern LUC and the time and depth dependency of LUC on SOC. While the greatest change did, in fact, occur at the surface for all six LUC directions between cropland, grassland, and forest, there were significant changes all the way down to the sampled depth of about one meter, the magnitude of which was largely dependent on soil type. In fact, 35 % of the detected SOC stock change occurred below 30 cm depth. Moreover, in the topsoil, where SOC change is expected to occur most quickly, SOC continued to change for an average of 83 years (79 to 90 years) after LUC from cropland to grassland, and 180 years (151 to 223) after LUC from grassland to cropland. Land-use change its self was often governed by hydrology rather than pedological properties, with precipitation and groundwater level playing dominant roles in determining whether a site was used as cropland or grassland. This research underscores the importance of historical LUC to present day SOC dynamics, provides novel methods for modelling the historically underestimated effects of LUC on SOC, and gives insights into the edaphoclimatic considerations necessary for setting future LUC priorities.