Legacy soil organic carbon stocks in central Spain from whole soil profiles and standardized depths: Influence of land cover and parent material

Inventorying soil organic carbon (SOC) and understanding its controlling factors is crucial for climate change mitigation. Sampling cost often limits inventories to the topmost layer, which may leave a substantial part of the SOC stock unaccounted for. The aim of this work was to assess how much SOC was left unrecorded by using standardized depths (0-30, 0-50 and 0-100 cm) versus the whole profile, as well as to assess relationships with soil taxa and the influence of the most widely mapped soil-forming factors at detailed scales: parent material and land cover, as a basis for detailed SOC stock quantifications. For this purpose, we used a legacy dataset containing 3,044 soil profiles from Madrid Region in central Spain. Once the SOC stocks for such depths and the whole profile were quantified, we found that measurements for the 0-30 cm depth represented above 80 % of the total SOC stock in shallow soils, particularly in highland land cover types, gneiss-formed soils, and taxa with melanization process. However, 0-30 cm stocks leave out almost 60 % of the total stock in alluvial soils, soils developed on decalcification clays and colluvial deposits. For these soils and under olive groves and vineyards, SOC inventories should include 0-100 cm measurements to provide proper representativeness. In other cases, we advise to extend sampling at least to 0-50 cm for a better assessment of the total SOC stock. This work also highlights the aptitude of soil taxa to provide relevant insights for the SOC evaluation in areas with available soil maps.