Chemometric prediction of soil organic matter quality and quantity from the pyrolytic patterns of N-bearing compounds

Póster presentado en la XXXII Reunión Nacional de Suelos (RENS2019) 10-13 de septiembre de 2019 en Sevilla

There is a worldwide concern to predict and control the effects of climatic change on the soils. In fact, soil organic matter (SOM) is the largest reservoir of C in Earth’s surface and one of the most dynamic pool. Apart from this, the structure and composition of the SOM are largely responsive to environmental changes. For this reason, the most current research focuses on the study of the mechanism of C stabilization in soils that generally depends on both natural and anthropogenic factors, including soil use and management practices. In particular, a controversial aspect about SOM formation processes is to understand how the C from the biomass is stabilized in the soil by either physical protection based on organo-mineral interactions or after a structural rearrangement the SOM macromolecular structure. Organic N-compounds in soil are of particular interest, to the extent that its chemical structure and speciation status in the SOM can play an important role in its N bioavailability. In fact, the accumulation of recalcitrant N-compounds can be an indicator of soil quality and could be related with the potential of C sequestration in certain soils. In this study, 35 soils from different areas of Spain with a wide range of SOM content were selected. Analytical pyrolysis (Py-GC/MS) was used to identify N moieties and pattern in the SOM of whole soil samples. A general characterization of SOM quality was performed using solid state 13C NMR and UVvis spectroscopy. The N-compounds identified by Py-GC/MS corresponded to seven principal chemical structures: indoles, pyridines, pyrazoles, benzonitriles, imidazoles, pyrroles and quinolones. These compounds are considered as pyrolytic markers of well-defined classes of biomass constituents, viz., protein derivatives, amino sugars and chlorophylls. Partial least squares (PLS) regression was used to explore the possible prediction of SOM quality and content using the percentages of pyrolytic N-compounds as descriptors. This approach lead to a significant forecasting model for the soil C content, and suggests that soils with different C levels also have SOM with characteristic molecular composition as regards the pattern of nitrogen compounds. Multidimensional scaling (MDS) and principal components analysis (PCA) showed the extent to which these N-compounds are correlated with different indices informative of SOM status and quality. The above results suggest that soil N-compounds pattern as seen by analytical pyrolysis encompasses relevant information as regards to SOM stabilization processes and quality.

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