A baseline projection of soil organic carbon stocks in German mineral croplands

European policy aims to enhance soil organic carbon stocks in order to improve soil fertility and resilience and for mitigation of climate change. A sound impact assessment of measures requires robust baseline estimates of soil organic carbon (SOC) trends in a changing climate.

For the German greenhouse gas emission projections, we modelled organic carbon in mineral topsoils (0-30 cm) of German croplands on a 100m*100m grid for the time-period between 1990-2075. Our interdisciplinary high-resolution modelling approach took into account the projected impacts of recent agricultural policy on agricultural management as assessed by agro-economic models. Crop rotations for each field across Germany were created using a Bayesian approach combining agricultural statistics (past and projected), remote sensing data and information from the Integrated Administration and Control System (IACS). Climate change was included based on local weather projections derived by the German Weather Service from the RCP4.5 and RCP8.5 climate scenarios. Soil carbon and texture data were obtained from maps based on the German Agricultural Soil Inventory and the European LUCAS survey. Using these management, climate and soil data, we modelled the SOC dynamics with the SOC decomposition model RothC, which has been validated with data from German agricultural long-term observation sites.

Our simulations indicate a trend of declining SOC stocks. We attribute this to impacts of climate change and management changes towards lower livestock densities, which cause decreasing organic fertilizer application rates. Increased cover cropping area partly counterbalances this trend.

The projected SOC trends can be used as a business-as-usual or reference scenario to quantify the climate effect of carbon-enhancing agricultural measures.