EGU21-14610, updated on 04 Mar 2021
https://doi.org/10.5194/egusphere-egu21-14610
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Compound-specific δ2H using analytical pyrolysis (Py-CSIA) for assessing source and processes of soil organic matter driven by climatic changes within a Mediterranean evergreen oak forest (dehesas)

Layla M. San-Emeterio1, Ignacio Pérez-Ramos1, Maria Teresa Domínguez-Núñez2, Francisco Javier González-Vila1, and José Antonio González-Pérez1
Layla M. San-Emeterio et al.
  • 1Institute of Natural Resources and Agrobiology of Seville, Biogeochemistry and Microbial and Plant Ecology, Seville, Spain (jag@irnase.csic.es)
  • 2University of Seville, MED Soil Group. C/Prof Garcia Gonzalez 1, 41012 Seville, Spain

Soil organic matter (SOM) is composed of multiple components from the living material, such as phenolic compounds, organic acids, lipids, peptides, polyesters, etc. A relevant part of these compounds forms part of supramolecular structures or mineral associations. Non-exchangeable hydrogen in SOM compounds is worth of study as an approach to estimate dynamic processes such as stabilization, mineralization, or biodegradation. The determination of H isotopes in SOMs faces analytical challenges related with e.g., the strength of the H bond, its exchangeability with ambient H from water or the instability of the isotopic analysis [1]. Nonetheless, along with the study of C isotopes, the study of H isotopes may certainly result in a complementary to give some light in this complex system, estimate the fate of organic compounds, and to better understand the link between hydrogen and carbon cycles in SOM [2].

In this communication, we describe and validate a methodology based on analytical pyrolysis for the direct measure of compound-specific H isotope composition (δ2H) in soil samples. The technique combines Py-GC with a high-temperature conversion reactor and a continuous flow isotope ratio mass spectrometer (IRMS) (Py-GC-HTC-IRMS).

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), installed in two different habitats: under evergreen oak canopy and in the open pasture. The samples were analysed in triplicate by conventional analytical pyrolysis (Py-GC/MS) and in parallel for δ2H Py-CSIA using the same chromatographic conditions and separation column type.

Up to 32 compounds were identified by Py-GC/MS, which H isotope composition corresponded presumably to non-exchangeable H, and with origin mainly from lignin (G- and S- units) and lipids. The H isotope composition showed an estimated average of -55 ‰ ± 7.09 for G-lignin units, -64 ‰ ± 8.64 S-lignin units and lighter -112 ‰ ± 4.32 for fatty acids (-109 ‰ ± 3.65) and the n-alkane series (C-19 to C-31). Significant differences are reportedly driven by the differences in habitat: more depleted δ2H values were found in SOM produced in the open pasture than under the tree canopy. In addition, a δ2H enrichment is observed for lignin-derived compounds in SOM under the W+D treatment.

The technique used and tested is expected to bring novelty results in relation to the processes affecting the isotopic composition of non-exchangeable hydrogen exerted by climatic treatments on diverse SOM specific compounds. Besides presenting the analytical challenges that are faced, we will discuss the effects of canopy and climatic treatments to tackle potential harsh climatic conditions as predicted, especially in Mediterranean areas. 

Acknowledgement: INTERCARBON project (CGL2016-78937-R), DECAFUN (CGL2015-70123-R). MICIU for funding FPI research grants (BES-2017-07968). Mrs Desiré Monis, Mrs Alba M. Carmona & Mr Eduardo Gutiérrez González are acknowledged for technical assistance.

[1] Paul, A. et al (2016). Biogeosciences, 13, 6587–6598.

[2] Seki, O. et al (2010). Geochimica et Cosmochimica Acta, 74(2), 599-613.

How to cite: M. San-Emeterio, L., Pérez-Ramos, I., Domínguez-Núñez, M. T., González-Vila, F. J., and González-Pérez, J. A.: Compound-specific δ2H using analytical pyrolysis (Py-CSIA) for assessing source and processes of soil organic matter driven by climatic changes within a Mediterranean evergreen oak forest (dehesas), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14610, https://doi.org/10.5194/egusphere-egu21-14610, 2021.

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