Is the organic carbon-to-clay ratio a reliable indicator of soil health?

Climate action plans under the Paris Climate Agreement and other national commitments aimed at improving soil-based ecosystem services require the operational monitoring of soil carbon (C). The European Union is aiming to enhance soil health, and as part of the proposed Soil Monitoring Law, the European Commission recommends the monitoring of the soil C loss indicator among other soil health indicators. In this study, we evaluate the feasibility of the proposed soil C loss indicator by assessing its performance using the EU-wide 2009 LUCAS soil survey data. The proposed indicator is the soil organic carbon (SOC) to clay ratio, with a threshold value of 1:13. The results are also compared with the C stock changes reported by countries to the climate convention (UNFCCC). Our results reveal that the variation in SOC and clay content at European scale exceeds that of the data used to develop the proposed indicator. We also found that the variation in the SOC content was influenced not only by clay content but also by climate and land-use reflecting C input levels. Therefore, the defined threshold is inadequate for detecting degraded soils if the SOC and clay content are beyond the conditions used to establish the criteria. Furthermore, major discrepancies were observed between the soil carbon stock changes reported by the national greenhouse gas (GHG) inventories and the proportions of degraded soils identified by using the soil C loss indicator. We conclude that employing a single indicator such as SOC:Clay ratio with one threshold value for all soils across various land covers, management practices, and climatic conditions, as defined by the European Commission for the Soil Monitoring Law, is inappropriate for monitoring soil C loss.