Analysis of factors involved in soil organic carbon sequestration and its relationships with the molecular composition of soil organic matter

The progress of desertification, which is very pronounced in the Mediterranean area, and the emission of greenhouse gases into the atmosphere, especially CO2, are taking the attention of the researchers. The establishment of the factors involved in these processes as well as the development of emergent technologies to solve these issues is a main objective in different scientific fields. In this perspective, soil conservation plays an important role, due to the content of soil organic matter (SOM) and its stability are important factors. Biogeochemical processes involved in the stabilization of soil organic carbon (SOC) are being the subject of study in this field. Some studies focus on the organo-mineral interactions, while others are interested in the relationship between the molecular structure or the SOM and its resilience. This thesis deals with the molecular characterization of SOM accumulated in different types of soil to establish its relationship with the potential for carbon storage in the corresponding soils, as well as the factors with a bearing on SOM quality. For this purpose, 35 soils with high variability in their organic carbon content (17–157 g·kg-1) have been selected. A detailed characterization of the organic matter has been carried out using destructive and non-destructive techniques such as analytical pyrolysis (Py-GC/MS), 13C solid-state nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, visible spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). In particular, the attention has been focused on certain types of biomarker compounds that could act as environmental indicators of soil biogeochemical processes. The families of alkanes and methoxyphenols were analyzed in detail; its molecular composition was used to distinguish between microbial synthesis and transformation of plant biomass. This study examines the utility of the Shannon diversity index (H’), calculated from the abundance of alkanes a
From a quantitative point of view, PLS regression models based exclusively on the total abundance of the 12 major methoxyphenols were especially effective in predicting carbon storage in the soil. After studying the information provided by analytical pyrolysis of SOM from different scenarios of carbon storage activity, the study was focused to the fraction traditionally considered most representative of the SOM, humic acids (HA). This fraction corresponds to a colloidal product of advanced transformation of plant and microbial biomass. The HA characterization was carried out using visible, IR and NMR spectroscopies. A PLS study using the intensities of digital IR spectra points (4000–400 cm-1) as descriptors showed that there is a relationship between IR spectral patterns and SOC content. This was also the case with E4 index (i.e., indicative of progressive humification, and based on the optical density of HAs at 465 nm). The use of principal component analysis (PCA) and MDS suggested that the bands assigned to carbonyl and amide groups were characteristic in HA of soils with low C content, while the spectra of HAs from soils with high levels of C showed a typical pattern of lignin bands, which indicates accumulation of less transformed plant residues. The IR spectral patterns were analyzed in detail by digital treatments including weighted subtraction of spectra obtained by averaging those of HAs from soils classified in the upper and lower quartiles according the SOC distribution, respectively, and calculating the statistical significance level of the differences. The results showed significant differences between the molecular composition of the HAs, according the SOC and E4 values. Peaks corresponding to aromatic, carboxyl and amide groups showed comparatively high intensity in HAs from soils with low SOC content, while peaks corresponding to lignin-derived structures were more marked in the spectra of the HAs from soils with high SOC content. In the second leve