Soil thermal buffering across environmental gradients is not uniform

Abstract. Large-scale ecological studies typically rely on gridded air temperature, although this may poorly represent the thermal conditions experienced by belowground organisms. Recent soil-temperature products make it possible to examine soil–air temperature differences over broad spatial scales, but it remains unclear whether soils buffer atmospheric heat and cold in the same way, and how these patterns vary among environments. We quantified soil–air thermal buffering across a continental soil-biodiversity network using ERA5-Land, treating warm and cold extremes separately. We compared these patterns with a SoilTemp–CHELSA representation and tested whether regional differences were related to broad climate, measured topsoil properties, and landscape composition. Cold extremes were buffered in 165 of 168 ERA5-Land cells (98.2 %), with a median soil–air offset of +2.14 °C. In contrast, only 41.1 % of cells buffered warm extremes, while 58.9 % showed heat amplification, with a median offset of −0.26 °C. Heat buffering also varied strongly among pedoclimatic regions. Mediterranean South (MDS) cells showed the greatest amplification, with a mean Heat offset of −4.41 °C and no positive buffering among the 16 cells. This regional contrast persisted after accounting for broad thermal climate and landscape composition. Measured topsoil properties showed little robust association with ERA5-Land buffering after correction for multiple testing. Cross-product agreement was much stronger for Cold than for Heat. The stronger edaphic associations obtained with SoilTemp–CHELSA were also retained after aggregation to ERA5-Land spatial support, indicating that the contrast between products was not explained by spatial resolution alone. Overall, soil thermal buffering was strongly asymmetric: soils generally moderated atmospheric cold, but not necessarily heat, and in some regions amplified warm extremes. Air temperature alone may therefore mischaracterize the thermal conditions experienced belowground.