Direct soil organic matter compound specific δ 13 C analysis using pyrolysis (Py-CSIA): identification of biomarkers in a dehesa from Southern Spain

In this display we show the possibility of using this particular analytical pyrolysis technique (Py-CSIA) for the direct measurement of δ 13 C in relevant specific soil organic matter components including those from polysaccharides (cellulose/hemicellulose), lignin, lipid/waxes and also peptide/protein derived compounds. According to the content showed in the following slides, we can conclude this approach can help to monitor SOM turnover rates according to the progressive transformation of compound families.


Soil organic matter in dehesas
• Mediterranean savannas are typical agro-sylvopastoral systems, characterized by the scattered presence of oak trees (Quercus ilex, Quercus suber), and the integration of livestock, forest and agricultural practices. • The particularity of these locally know "dehesas" are expected to be reflected in soil organic matter (SOM) structure. • Understanding SOM dynamic in this environment is important to forecast possible effects caused by climatic, and environmental changes and land use.

How did we proceed?
In April 2019, soil samples were taken from the 10 uppermost cm of soil in an experimental plot (under evergreen oak canopy), in collaboration with the project DECAFUN (CGL2015-70123-R). Sampling was conducted in Pozoblanco (Córdoba, Spain).
Possibilities of direct pyrolysis CSIA in bulk soil samples?
• We evaluated molecular composition of soil organic matter (SOM) from bulk soil samples, using online analytical pyrolysis coupled to gas chromatography-mass spectrometry analysis (Py-GC/MS) at 400ºC. • Furthermore, we aimed to estimate molecular turnover times of major SOM components (lignin-derived units, carbohydrates, phenols, fatty acids, etc.) using compound-specific isotope analysis (Py-CSIA), under the same chromatographic conditions. The system was externally calibrated prior to any analysis, using standard mixtures C4 for nalkanes (n-C 16 to n-C 30 ), obtained from the Stable Isotope Laboratory (Schimmelmann, Indiana University) http://arndt.schimmelmann.us/compounds.html The values obtained were accepted when fitting accordingly to a straight line (R 2 > 0.99).

Chromatographic results
• Well-resolved chromatograms using Py-GC/MS were obtained and a total of 128 pyrolysis compounds detected. • Dehesa SOM composition mainly consisted of polysaccharide, ligninderived compounds (G-and S-units), fatty acids and n-alkanes. • When coupling Py with IRMS, many chromatographic peaks (a total of 40) were well defined and with enough chromatographic separation to give accurate δ 13 C readings (n = 6). Accordingly, depleted δ 13 C values for fatty acids (-35.1 ± 2.41 ‰) and alkanes (-35.5 ± 2.20 ‰) were found, the latter with lighter isotope composition with increasing the hydrocarbon length.
ANOVA at p<0.05. Lowercase letters indicate significant differences according to Tukey posthoc analysis. "*" indicates significant differences compared between S-lignin units and the rest of compound families.

Conclusion remarks
• Compound-specific isotope analysis results in a useful technique for SOM source attribution, as SOM turnover estimation should not rely on bulk isotope values. • Our results support the suitability of Py-CSIA for the direct measurement of δ 13 C in relevant soil organic matter specific components, including those from polysaccharides (cellulose/hemicellulose), lignin, lipid/waxes and peptide/protein derived compounds. • In chromatographic terms, special attention should be driven to potential co-elution problems in IRMS, especially when analytical pyrolysis is being used in complex samples. • Our methodology described here has the potential for routine analysis and screening studies of soil samples to be complemented with conventional structural information in samples with high macromolecular complexity. • Therefore, this approach can help to monitor SOM turnover rates according to the progressive transformation of compound families.