Evaluating SOC status in agricultural soils: a comparison of approaches

Soil organic carbon (SOC) is a fundamental contributor to soil functions and health.  Since SOC is a strong predictor of many important soil properties, it is prominently featured in soil health assessments and monitoring targets.  Yet, given its strong spatial heterogeneity, specific SOC targets are highly debated. For example, the EU Directive for Soil Monitoring and Resilience proposed the ratio between SOC and clay content (SOC:clay) as a target, with 1/13 separating “degraded” from “non-degraded” mineral agricultural soils, allowing local correction factors for diverging pedo-climatic conditions. However, SOC:clay has been criticized for inherent biases. Recent publications support this perspective, calling for regionally specific SOC benchmarks. These benchmarks are typically an expected SOC value for an approximately homogeneous area, in terms of land-use and pedoclimatic conditions.  We provide an overview of three recently published protocols for deriving regionally relevant SOC targets and evaluate them on recent monitoring campaigns in Austria, using the SOC:clay ratio as a baseline.

Our results show that all regional benchmarking approaches evaluate the status of SOC in agricultural soils similarly, based on recent monitoring campaigns. Additionally, these approaches show high sensitivity to agricultural management, which the SOC:clay ratio fails to do consistently in our case study. Finally, we propose an approach for using lighthouse farms to guide SOC targets within a specific benchmark, where lighthouse farms represent the upper boundary of what is achievable in terms of soil health within a given pedoclimatic zone.