Decadal trends of organic carbon in German agricultural soils – Preliminary findings from the German Agricultural Soil Inventory

Accurate monitoring of soil organic carbon (SOC) is essential for the development of national greenhouse gas inventories as well as for the assessment of strategies implemented to maintain and increase SOC stocks in agricultural systems. Limited access to high quality SOC data across large spatial and temporal scales remains the main barrier for scientists, policy makers and practitioners to make informed decisions about soil management. To address this knowledge gap, the Thünen Institute of Climate-Smart Agriculture conducted the German Agricultural Soil Inventory (BZE-LW), with the first campaign carried out between 2010 and 2018 and the second campaign initiated in 2022. During the first campaign, 3,104 arable, grassland, and permanent crop sites were sampled on an 8×8 km grid and analyzed for SOC contents and stocks, along with additional soil parameters, down to a depth of 1 m. In addition to soil sampling, annual farm management data were collected via questionnaires. The second campaign, which involves resampling of the same sites, enables to quantify and explain changes in SOC contents and stocks at the decadal scale. Based on the most recent results (n=587), minor changes in SOC contents were observed in cropland soils. Average SOC stocks declined significantly in 0–30 cm (−1.6%) and 0–50 cm (−2.7%) with a statistically insignificant increase of 0.9% in the top 10 cm. Grassland soils showed more pronounced SOC losses, with significant declines in both 0–30 cm (−5.9%) and 0–50 cm (−5.1%). Current hypotheses attribute SOC losses to land use history, soil type, rapid climate warming characterized by a 2.1°C rise in the mean air temperature in Germany over 50 years, and farm management activities such as declining nitrogen fertilization. Conversely, increased adoption of cover cropping and reductions in tillage intensity might have partially mitigated against SOC stock losses in croplands. Disentangling these drivers while accounting for several methodological adjustments to the first soil sampling campaign are key tasks, which will serve to inform future soil monitoring efforts in Germany and beyond. To further highlight the importance of data quality, we will emphasize the methodological challenges associated with detecting SOC changes across spatial scales ranging from individual sites to regional levels.