Updating a legacy soil map to the WRB: diagnostic conflicts and taxonomic shifts in tropical volcanic soils of Réunion

Legacy soil maps based on genetically oriented systems remain the primary pedological information source across many tropical territories, yet their obsolescence hampers international data harmonisation. Despite the recognised need for updating these maps, no comprehensive procedure has been documented for the systematic reclassification of soils mapped under the French CPCS classification to the World Reference Base (WRB), particularly in tropical volcanic settings where specific diagnostic criteria pose additional challenges. This study presents a reproducible methodology for reclassifying the 1:50,000 CPCS map of Réunion Island (Indian Ocean) to WRB 2022, combining 83 newly described and analysed soil profiles with an expert reanalysis of 93 historical map units encompassing 46 soil types. The reclassification yielded 58 detailed and 15 synthetic WRB soil units across seven Reference Soil Groups: Andosols, Podzols, Cambisols, Nitisols, Gleysols, Regosols, Leptosols. Major taxonomic shifts included reclassifying Ferrallitic soils as Nitisols rather than Ferralsols, Fersiallitic and Ferruginised soils as Chromic Cambisols, and numerous Andic soils as Andic Cambisols or Nitisols (Andic). Four categories of problematic diagnostic criteria were identified: progressive formalisation of andic thresholds, unreliability of standard particle-size analysis for soils with poorly crystallised minerals, geochemical constraints on soil colour precluding mollic and umbric horizons, and incomplete expression of vertic properties. Cambisols emerged as a convergence group with markedly different qualifiers. A three-level framework distinguishing soils reclassifiable from legacy data alone, soils requiring new analyses, and soils requiring expert interpretation is proposed as a transferable tool for updating other CPCS maps across Africa, the Caribbean, and the Pacific.