Accounting for soil class variability in Mediterranean Europe using legacy soil maps and the topsoil LUCAS survey

Abstract

Mediterranean soils exhibit high pedological diversity, shaped by complex climatic, geomorphological, and anthropogenic drivers. Assessing this variability at the continental scale requires harmonized sampling frameworks. The LUCAS topsoil module provides standardized EU-wide topsoil physico-chemical and biological data, but its capacity to represent soil type diversity, particularly in heterogeneous Mediterranean landscapes, has not yet been evaluated. To perform this evaluation, this study compared LUCAS topsoil sampling points with legacy soil maps (LSMs) across multiple spatial scales (continental, national, sub-national) in Mediterranean Europe. We employed two complementary metrics: (i) the Average Absolute Coverage Difference (AACD), (ii) and a proportional mean index. Results show that continental-scale maps (e.g., WRB-FULL, WRB-LEV1) achieved AACD values below 1%, showing that sampling point distribution reflects well the soil type variability at this spatial scale and taxonomic levels. The performance was good also when sub-national maps with higher level of pedodiversity (Soil Map of Sicily) were used, with AACD between 1.2% and 1.7%. Overall, higher capacity to capture pedological diversity was observed in maps featuring higher soil diversity, larger spatial extent, and a greater number of soil map units (SMUs). However, some specific RSG, Histosols and Leptosols were underrepresented, whereas Calcisols were systematically oversampled. Overall, the LUCAS sampling design captures soil type variability at multiple scales, and incorporating local pedodiversity could reduce biases and improve compliance with the European Soil Monitoring Law. Legacy soil maps remain essential for optimising sampling design and extending harmonised soil monitoring to data-scarce regions such as the Near East and North Africa (NENA).