Benchmarking the benchmark: comparison of methods for soil health assessment

• Five methods for assessing SOC status in agricultural soils were compared. • Significant variability in outcomes across benchmarking methodologies. • SOC benchmarks based on soil units are superior in capturing SOC variability. • Lighthouse farm networks can guide SOC targets within soil units. Soil organic carbon (SOC) plays a pivotal role in soil health, ecosystem functioning and climate change mitigation. While increasing SOC stocks in agricultural soils is a key objective for policymakers and practitioners, defining precise benchmarks that appropriately account for pedoclimatic variability remains challenging. Recently, the European Union’s Directive on Soil Monitoring and Resilience (DSMR) introduced the SOC:clay ratio as a universal indicator of soil degradation, but its applicability across diverse contexts has been questioned. As a consequence, regionally-derived benchmarks based on pedo-climatic predictors have been suggested as an alternative to SOC:clay. In this study, we evaluate three strategies for deriving SOC benchmarks in Austria: a data-driven approach, an expert-based approach and a variable-agnostic approach aligned with DSMR-defined soil units. Based on data from the Agricultural Soil Map of Austria, WORLDCLIM climate variables, and Environmental Zones in Europe, the benchmarks were evaluated on a soil health-oriented lighthouse farm survey and LUCAS points within Austria. Our findings highlight significant variability in benchmark outcomes across methodologies. While data-driven and expert-based approaches produced SOC benchmarks with higher explanatory power and management sensitivity, the variable-agnostic method yielded the most geographically cohesive clusters at the expense of specificity. Moreover, regional benchmarks proved superior to general metrics (e.g., ratios of SOC:clay or SOC:SOC exp ) in capturing SOC variability. Lighthouse farms, representing sustainable soil management practices, demonstrated the potential to guide benchmark targets within specific clusters. This study underscores the importance of regional and context-specific SOC benchmarks for soil health assessment as compared to one-size-fits-all approaches.