Effect of climatic variability in the soil organic matter composiion studied by analytical pyrolysis

In present days there is a growing concern about the progress of desertification in different areas of the world. The unsuitable management of the soils and the change of land use may increase the desertification risk. On the other hand, desertification is typically associated with the decrease of soil organic matter (SOM) levels with the consequent loss of fertility in the soil. Presumably, all these aspects are reflected in the molecular composition of SOM. Previous studies have evidenced that a correlation exists between the carbon sequestration efficiency and the relative abundance of specific SOM constituents, e.g., alkane homologous series [1] or lignin-derived methoxyphenols [2]. This study aims to identify molecular descriptors of the SOM composition, which are responsive for the impact of climate, quantified with bioclimatic indices defining a continuous gradient between wet and dry areas. A total of 33 soil samples were collected from different areas of Spain. The studied soils presented a large variability in their chemical and physical properties, and were developed under different geological substrate and vegetation type. The sampling was carried out in the topsoil (0–10 cm) where the SOM content is higher. In order to assess desertification levels we used the De Martonne aridity index. This index was calculated from the annual average rainfall and annual average temperature for each soil sampling point. The SOM was analyzed by pyrolysis - gas chromatography mass spectrometry (Py-GC/MS) of whole soil samples. A total of 193 pyrolysis compounds were identified, and used as predictor variables in Partial Least Squares (PLS) regression models forecasting the De Martonne aridity index. In order to assess desertification levels we used the De Martonne aridity index. This index was calculated from the annual average rainfall and annual average temperature for each soil sampling point. The SOM was analyzed by pyrolysis - gas chromatography mass spectrometry (Py-GC/MS) of whole soil samples. A total of 193 pyrolysis compounds were identified, and used as predictor variables in Partial Least Squares (PLS) regression models forecasting the De Martonne aridity index. The results showed that a significant prediction of this index (R = 0.869) exclusively using the information provided by Py-GC/MS analysis of the corresponding soils is possible.A graphical-statistical method based in the classical van Krevelen diagram was used for displaying the pyrolysis results [3], representing difference values between the proportions of the 193 pyrolysis products, calculated between average pyrograms for soils in the uppermost quartile of the De Martonne index, and the average of those in the lower quartile. These values are shown as a scatterdiagram where the coordinates in the plane of the individual molecules correspond to their H/C and O/C atomic ratios, calculated from their empirical formulas. The difference values between relative abundances were represented as a density map where the green colour indicates compounds predominant in the SOM of soils formed in sites with high De Martonne index, and the red colour indicate compounds predominant in soils with low index (i.e., comparatively dry ecosystems). The Student’s t (p > 90%) was also used to evaluate the significant differences between the proportions of compounds, and was represented as a superimposed contour diagram in the Figure. The progressive desertification of the soils is associated to a molecular composition of the SOM defined by the selective accumulation of lignin-derived compounds at different stages of transformation in soil (phenols and methoxyphenols), whereas aromatic compounds and aliphatic hydrocarbons tend to be major SOM constituents in soils developed under comparatively humid climatic conditions. As a whole, the results suggests that progressive evolution towards arid climate leads to a decrease in SOM quality, as reflected by the accumulation of raw organic matter where structural units of plant macromolecules can still be easily recognized. Conversely, in soils formed in ecosystems with comparatively longer wet season, the SOM formation is carried out at expenses of the incorporation of aliphatic material of plant and microbial origin (mainly alkenes and alkanes) together with condensed aromatic structures, which are traditionally considered as typical pyrolytic products from humic substances, with high resilience or resistance to biodegradation.Referencias: [1] M.A. Jiménez-González, A.M. Álvarez, Z. Hernández, G. Almendros, Biology and Fertility of Soils 54 (2018) 617–629. [2] M.A. Jiménez-González, A.M. Álvarez, P. Carral, F.J. González-Vila, G. Almendros J. Chromatography A 1508 (2017) 130–137. [3] D.W. van Krevelen, Fuel 29 (1950) 269–284Peer Reviewe