Pyrolysis-compound specific stable isotope analysis (Py-CSIA) of lignin-derived phenols (δ13C, δ2H) in soils from a typical mediterranean agro-ecosystem

Abstract de la Comunicación oral presentada en el 30th International Meeting on Organic Geochemistry (IMOG 2021), 12-17 September 2021, Montpellier, France DOI: https://doi.org/10.3997/2214-4609.202134238.- This communication describes and validates a methodology based on analytical pyrolysis for the direct measure of lignin-derived phenols δ13C and δ2H isotope composition. The technique combines Py-GC with combustion (for C) and high-temperature conversion (for H) reactors and a continuous flow isotope ratio mass spectrometer (IRMS) (Py-CSIA).Soil organic matter (SOM) is composed of multiple components from the living material, such as peptides, organic acids, lipids, phenolic compounds, etc. Accounting for the second most abundant biopolymer in nature, the dynamic of lignin in soils is worth of study. Lignin could be used as a biomarker of processes such as stabilization, mineralization or biodegradation of SOM. Apart from its wide abundance, lignin might be the most stabilized and protective compound of organic matter. As lignin can only be synthesized by plants, the isotope composition of its components can be informative about its origin and of several environment variables prevailing when it was synthesized in the plant. The study of carbon and hydrogen isotopes may certainly result in a comprehensive approach to estimate the fate of organic compounds, and to better understand the link between hydrogen and carbon cycles in SOM, especially in Mediterranean soils. This communication describes and validates a methodology based on analytical pyrolysis for the direct measure of lignin-derived phenols ¿13C and ¿2H isotope composition. The technique combines Py-GC with combustion (for C) and high-temperature conversion (for H) reactors and a continuous flow isotope ratio mass spectrometer (IRMS) (Py-CSIA). Composite dehesa surface (0-10 cm) soil samples (Pozoblanco, Córdoba, Spain) were taken from four forced climatic treatment plots representing warming (W), drought (D), its combination (W+D), and control (D), in two distinct habitats: under evergreen oak canopy and in open pasture. The samples were analysed in triplicate by analytical pyrolysis (Py-GC/MS) and in parallel for ¿13C and ¿2H Py-CSIA using the same chromatographic conditions. The ¿13C and ¿2H values were determined for a total of 10 guaiacyl (G-units: guaiacol, methylguaiacol, vinylguaiacol, vanillin, isoeugenol, acetoguaiacone) and syringyl (S-units: syringol, propenylsyringol, methoxyeugenol and acetosyringone) well resolved lignin units, identified by comparing the pyrograms from both, conventional Py-GC/MS and Py-CSIA. The ¿ 13C values ranged from -27.15 ¿ to -33.45 ¿ whilst ¿ 2H values varied from ¿ 43.92 ¿ to -137.56 ¿. These values are consistent with the isotopic composition previously found lignin residues in soil. For both ¿ 13C and ¿2H, significant differences are reportedly driven by the differences in habitat: more depleted values were found in lignin produced in the open pasture than under the tree canopy (Fig. 1), with syringyl moieties less significant than guaiacyl. In addition to the differences reported for habitats, it is observed an 13C enrichment under warming (W) treatments (up to 7 ¿; Fig. 1A and B), and under the combination of warming and dry (W+D) 2H (up to 40 ¿). No significant differences were found in the isotopic composition of S- units in open habitat, suggesting that the S- units of lignin are less sensitive to indicate substantial changes in SOM according to changes in habitat and/or climatic conditions. The results add to evidence that shifts in moisture and temperature alter lignin isotope composition in soils, mainly under the tree canopy and for G- units. Further discussion on the processes affecting the isotopic composition of lignin compounds, particularly regarding non-exchangeable hydrogen will be assessed. The technique used and tested here is promising and have the potential for the direct isotopic determinations in lignin in soils and sediments with a minimum sample preparation and handling.Ministerio de Ciencia Innovación y Universidades (MICIU) for INTERCARBON project (CGL2016-78937-R), DECAFUN (CGL2015-70123-R). L. San Emeterio also thanks MICIU for funding FPI research grants (BES-2017-07968