Thermal stability of soil carbon pools: Inferences on soil nature and evolution

•The application of DIN 19539 allows the discrimination of three soil carbon pools.•Two organic (TOC400 and ROC) and one inorganic (TIC900) carbon pools.•TOC400 at 300−400 °C, ROC at 510−600 °C, TIC900 at 700−900 °C.•TOC400/ROC ratio vsδ13CTOC links SOM thermal and compositional features.•Temperature of carbonate breakdown vsδ13CTIC reflects mineralogical differences. The quantification of soil carbon pools is a pressing topic both for the agriculture productivity and to evaluate the Greenhouse Gases (GHG) sequestration potential, therefore a rapid and precise analytical protocol for carbon speciation is needed. Temperature-dependent differentiation of soil carbon in compliance with the DIN (German Institute for Standardization) 19539 standard has been applied for the first time on 24 agricultural soil samples from the Po River Plain (Italy), with the aim of investigate their thermal behavior in the 50−900 °C interval. The results invariably show the existence of three soil carbon pools having different thermal stabilities, namely, thermally labile organic carbon (TOC400), residual oxidizable carbon (ROC) and total inorganic carbon (TIC900), in the intervals of 300−400 °C, 510−600 °C and 700−900 °C, respectively. Significant relationships have been observed between the above mentioned organic and inorganic carbon pools and the associated isotopic composition: 1) inverse correlation between TOC400/ROC and δ13C links thermal stability and soil organic matter (SOM) composition; 2) direct correlation between carbonate breakdown temperature and δ13C denotes the mineralogical association of the inorganic pool. The results give clues regarding the nature and evolution of soil carbon pools.