Interim report of the repeated German agricultural soil inventory

The dynamics of organic soil carbon (SOC) play an important role in atmospheric CO2 concentrations and are therefore included in national greenhouse gas inventories. SOC is also essential for soil fertility. On behalf of the Federal Ministry of Food and Agriculture, the Thünen Institute of Climate-Smart Agriculture therefore conducted the first nationwide representative inventory of agricultural soils (BZE-LW) between 2010 and 2018. A total of 3,104 arable, grassland and permanent crop sites were sampled in an 8x8 km grid and analysed for SOC contents and stocks, as well as other parameters, down to a depth of 1 m. The BZE- LW repeat inventory project began in 2022, and since the beginning of 2023, resampling of the sites identified at that time has been in full swing. In addition to soil sampling, annual management data is also being collected. The main objective of this project is to quantify and explain potential changes in SOC contents and stocks over the past decade. This interim report presents the initial results of the ongoing repeat inventory. Compared to the initial BZE-LW, there were some deviations in the implementation of the repeat inventory. Only the top 50 cm are sampled and organic soils are not resampled. Instead of a central profile pit and eight additional core drillings, four small pits are now excavated for sampling. Some parameters are not recorded again (e.g. soil type, grain size distribution, stone content), while others have been added (e.g. aggregate stability, air capacity, cation exchange capacity). By October 2025, approximately 1,350 sites in eight federal states had been resampled and almost 1,000 had been analysed for bulk density and SOC content in order to calculate mass-corrected SOC stock changes. During the initial evaluation, it was noticed that the initial SOC content of the topsoil from the profile pit was systematically slightly too high, leading to an overestimation of SOC losses. For this reason, the SOC contents from additional core samples of the initial BZE-LW were used instead. However, analysis of those has not been completed yet, which is why only 587 sites have been included in the evaluation at this stage. Slight changes in SOC content have been observed in arable soils over the past decade. While a slightly positive trend in SOC content was observed on average in 0-10 and 10-30 cm, the change in SOC stocks in 0-30 cm (-1.6%) and 0-50 cm (-2.7%) was significantly negative due to a slight decrease in bulk density despite mass correction. At a depth of 0-10 cm, however, the change in SOC stocks was also slightly positive (0.9%), which can possibly be explained by a nationwide decline in tillage intensity and the resulting redistribution of SOC in the soil profile. However, the evaluation of the management data from the questionnaire was so far focused on one parameter: the frequency of cover cropping. In this respect, the BZE-LW data correspond well with national data, which show approximately a doubling of the annual cover crop area in the period under review. However, this gradual increase in SOC input into German arable soils was apparently not sufficient to compensate for potential negative influences on SOC. For grassland, there was a more pronounced decrease in SOC stocks, which was most pronounced at a depth of 0-10 cm (-8.1%). At depths of 0-30 cm and 0-50 cm, the significant relative decreases were -5.9% and -5.1%. Negative trends were also observed on average at all depth levels for the 14 permanent crop sites to date. The investigation of the causes of these SOC losses is still ongoing. According to the hypotheses developed here, it is land use history and soil genesis, rapidly advancing climate change, and recent changes in cultivation practices that are affecting the SOC dynamics currently being observed: The historically wet and often SOC-rich sandy soils of north-western Germany tend to suffer particularly severe SOC losses under current land use, which is consistent with the results of long-term soil observations in Lower Saxony and the neighboring Netherlands. Over the last 50 years, there has been an average air temperature increase of 2.1êC at the BZE-LW sites, about half of which has occurred in the last 1-2 decades. According to modelling and experimental work, warming alone is sufficient to explain the II magnitude of average SOC losses. Finally, national statistics clearly indicate a reduction in livestock farming and nitrogen fertilisation, which is also likely to have a negative impact on SOC stocks. Separating the various factors influencing SOC stocks requires complex methodology and will be a central part of the next project phase, alongside the completion of resampling and the remeasurement of the initial core samples. Another challenge will be the preparation and implementation of new reporting requirements, in particular the EU Soil Monitoring Law. The trends observed to date only apply to part of the Federal Republic of Germany. The average rates of change presented here should therefore not be extrapolated.