Changes in soil organic carbon stocks and quality on a national scale – Decadal trends of the German Agricultural Soil Inventory

Understanding changes of soil organic carbon (SOC) contents is important to estimate the role of soils as emission sinks or sources. Agricultural land use has led to substantial losses of SOC and recent projections indicate continuous decreases on a European scale, while main drivers remain elusive. The German Agricultural Soil Inventory is currently resampling around 3,000 sites to determine decadal SOC changes. Here we present the trends in SOC stocks of the first 800 analysed sites. We identified significant losses of SOC stocks in cropland and grassland soils by approximately 4% in the upper half meter. Most SOC was lost from overall carbon rich soils. Our analysis will extend to the role of past land use changes and management to identify key drivers of SOC dynamics. In addition, mid-infrared spectroscopy will be used to explore the role of SOC quality and composition for determining the decadal SOC changes. Recently, we used compositional information, for example the relative composition of aliphatic to aromatic compounds, to identify SOC change direction at land-use change sites. A large spectral library is being built to extend this approach to the national Soil Inventory and thereby improve our biogeochemical understanding of bulk SOC trends and establish new indicators of such.