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Multivariate appraisal of soil organic carbon storage from the patterns of nitrogen-containing pyrolytic compounds

AutorJiménez González, M. A.; Almendros Martín, Gonzalo ; Álvarez, Ana María; González-Vila, Francisco Javier ; Rosa Arranz, José M. de la
Fecha de publicación3-oct-2017
EditorSociedad Española de Cromatografía y Técnicas Afines
Citación15 Jornadas de Análisis Instrumental 2017 3-5 Oct. (2017) Barcelona (Spain)
ResumenThe assessment of factors involved in soil carbon storage is crucial for developing the scientific bases of Earth¿s biogeochemical cycles. This is also the subject of current controversy about the extent to which the recalcitrance of soil organic matter depends on its molecular composition, or on external factors such as organo-mineral interactions. In any case, some studies have suggested significant correlations and possible cause-to-effect relationships between molecular-level descriptors and the recalcitrance of the soil organic matter. Assuming this background, we identify and quantify the N-compounds released by analytical pyrolysis from whole soil samples by using gas chromatography/mass spectrometry. The interest of focusing on N-compounds could be justified by: i) the fact that they amount to a reduced number of diagnostic molecules in pyrograms with more than a hundred of major compounds, ii) a progressively decreased C/N ratio is a classical index of stability and quality of soil organic matter, then changes in organic N-forms should parallel efficient soil C sequestration, iii) concentration of nonhydrolyzable ¿unknown¿ N-fractions is a general diagenetic trend in terrestrial soils, suggesting that organic matter maturity could be reflected by the pyrolytic assemblages of N-compounds. Consequently, this study explores the relationships between pyrolytic N-compounds and different soil properties with special emphasis on soil carbon storage. A total of 35 samples were collected from different Spanish soils with large variability in the concentration of organic carbon (from 17 to 157 g oxidizable C / kg soil). After identifying the major pyrolytic N-compounds (mainly indoles, pyridines, pyrazoles, benzonitriles, imidazoles, pyrroles and quinolines) its origin is discussed as the different concentration in the soils of e.g., amino acids, amino sugars, nucleotides and chlorophylls. In a second step, a series of chemometric approaches were used to unravel possible relations between the patterns of N-compounds and the soil carbon levels viz, multiple linear regression (MLR), partial least squares regression (PLS) and multidimensional scaling (MDS). In particular PLS forecasting models, exclusively using the total abundances of N-compounds, explained the concentration of organic C in the different soils (P< 0.01). These results suggest that a correlation exist between C sequestration performance and the pattern of pyrolytic N-compounds, which requires an interpretation. As a whole, the variability in the soil C content was explained as the diagenetic coevolution of C- and N-forms reflected in MDS plots, showing the ordination of pyrolytic Ncompounds in terms of soil C levels: total abundances of quinoline, methylindole, indole, dimethylpyridine and methylbenzonitrile explained most of the variability in soil C content, and could be used as molecular proxies to identify natural mechanisms of soil C sequestration.
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