Molecular characterization of organic matter in two calcareous soils: the effects of an acid decarbonation treatment

The molecular composition of soil organic matter (SOM) of two calcareous soils highly rich in carbonates was assessed before and after decarbonation by acid washing with HCl through 13C-CPMAS-NMR spectroscopy and off-line thermochemolysis coupled with gas chromatography and mass spectrometry (THM-GC-MS). The acidic treatment promoted a considerable concentration of organic matter in both soils, thus improving the identification of molecules otherwise not easily detectable. Decarbonation induced only a slight loss of soil organic carbon (SOC), corresponding to 1.4 and 2.7% for A and B soils respectively. The acidic treatment also led to an increase in the organic carbon/total nitrogen (OC/N) ratio in soil A, while an opposite variation was found for the second soil. Moreover, variations in the concentration and molecular distribution of specific compound classes present in SOM were caused by the acid washing of soils. As confirmed by both 13C-CPMAS-NMR and thermochemolysis results, the molecules most susceptible to the acid treatment were the carbohydrates, lignin monomers (G14 and G15), fatty acids (C18 saturated and unsaturated), fatty acids of microbial origin (C15, C17, and C19), hydroxy acids (C16, C18), and dioic acids (C18) which represent the components weakly bound to the organic matrix. Our findings not only showed the efficacy of the decarbonation treatment of calcareous soils with 3 N HCl, but also indicated how the acidic washing can improve the differentiation of soils on the basis of SOM molecular characteristics.