Establishing assessment criteria for soil bioindicators: insights from case studies in Europe

• EU Soil Monitoring bioindicator data needs robust assessment criteria. • Criteria development requires dedicated sampling strategies and standardized protocols. • Harmonization of soil unit classifications should further coordinated assessment. The new EU Soil Strategy aims to restore soil health across Europe by 2050, requiring policy-relevant indicators with robust assessment criteria to control and monitor its achievement. Despite the importance of biological conditions for soil health, few EU Member States have currently established criteria (e.g., target or threshold values) for evaluation of monitoring data. Our study examines the challenges and opportunities in developing such criteria based on the normal operating ranges of bioindicators in the context of soil units (i.e., homogenous soil texture and land use). Leveraging on national case studies across Europe, we find that for most bioindicators criteria could not be defined, due to data insufficiency and environmental bias. Only in France could normal operating ranges be established for earthworms, for some soil units. We identify priorities for the development of robust assessment criteria: (1) Harmonization of evaluation units and definitions, to minimize disparities in current soil texture and land use classifications between Member States, that complicate comparability of evaluation across Europe; (2) Inclusion of climatic-region specific thresholds/targets in addition to land use and soil texture; and (3) Standardization of protocols to reduce the observed variability in methods for sampling and data aggregation. As a temporary solution to optimize monitoring sampling efforts, guidelines for dealing with data deficiencies and pseudo-replication are proposed. On the longer term, our findings highlight the importance of integrated data systems and standardised frameworks for monitoring bioindicators to evaluate progress towards achievement of the EU soil health targets.