Soil carbon benchmarks and cropland management effects: Insights from the Bavarian soil organic matter monitoring

Soil organic carbon (SOC) is a key property for soil productivity and functionality. According to the Federal Soil Protection Act in Germany, site-specific SOC contents need to be maintained in the course of agricultural land use in order to ensure soil fertility and the performance of soils as a natural resource. In this study, representative SOC benchmarks have been derived for arable mineral soils in Bavaria on the basis of a comprehensive soil organic matter (SOM) monitoring program as reference values for farmers and other stakeholders. Data from 283 representative study sites across Bavaria has been stratified according to the most influential site factors on SOC, particularly clay content and the elevation above sea level as a proxy for climate (temperature and precipitation). Due to an evaluation of the study sites in terms of agricultural management according to the principles of good agricultural practice combined with statistical exclusion of 25 % of extreme SOC contents, the benchmarks presented here can be regarded as representative reference values under specific environmental conditions. However, different management measures within the principles of good agricultural practice also influenced SOC, as organic fertilization and the cultivation of perennial forage crops positively affected SOC contents. In contrast, the cultivation of root crops negatively influenced SOC contents, while the proportion of corn within the crop rotation of up to 50 % did not affect SOC negatively. We further evaluated the suitability of the SOC/clay ratio as alternative SOC and soil structure indicator. However, the SOC/clay ratio seems to be inappropriate due to a high number of degraded sites and a systematic bias towards degradation with high clay contents. The SOC benchmarks presented here provide valuable reference values for farmers policymakers and agricultural stakeholders in optimizing cropland management practices for sustainable soil health and fertility in Bavaria and beyond. However, further research is required regarding the stratification approach and the calculation method for a widespread implementation of SOC benchmarks. Furthermore, the shifting baseline syndrome has to accounted for, so up to date SOC benchmarks remain within a range in which soils are considered healthy and fertile.