Pyrolysis-compound specific isotope analysis for the direct characterization of lignin in soils. Implications for tracing climate change effects

Lignin in soils is a result of organic matter input and depletion, which is regulated by various factors, such as environmental factors like climate change (temperature, moisture, etc.) [1]. Therefore, lignin is considered an indicator of soil organic carbon storage and dynamics. Given its chemical structure, hence recalcitrance, lignin could be used as a biomarker of processes such as stabilization, mineralization, or biodegradation of soil organic matter (SOM) [2]. The study of C and H isotopes may certainly result in a comprehensive approach to estimate the fate of organic compounds, and to better understand the link between H and C cycles in the soil. The use of C stable isotopes is widely used to investigate sources, evolution, and dynamics of SOM, whilst H stable isotopes provide geographical information and insight into the water dynamics in soils [3]. This communication describes a methodology based on analytical pyrolysis for the direct measure of lignin-derived phenolic compounds specific ¿13C and ¿2H isotope composition (Py-CSIA). Based on this methodology, we aim to evaluate the ¿13C and ¿2H isotope composition of lignin-derived phenols as biomarkers of changes in SOM dynamics driven by climatic factors. 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 for chemical structural characterization by analytical pyrolysis (Py-GC/MS) and in parallel for ¿13C and ¿2H (Py-CSIA) using the same chromatographic conditions. The ¿13C average CSIA values ranged from -27.15 ¿ to -33.45 ¿. Significant differences are reportedly driven by the differences in habitat: more depleted values were found in lignin produced in the open pasture than in tree habitat. In addition to the differences reported for habitats, it is observed an 13C enrichment (up to 7 ¿) associated to the warming (W) treatments. Also remarkable is a gradually 13C enrichment trend observed in open pasture for siringyl lignin units, that may be attributable to climatic stress. The ¿2H average values of lignin methoxyphenols varied from ¿ 43.92 ¿ to -137.56 ¿. Again, a significant depletion is observed in the open pasture samples. Climatic changes are more pronounced in open pasture, especially in the lignin guaiacyl units. Lignin methoxyl groups from W+D treatment were found significantly enriched in 2H relative to other climatic treatments. This study demonstrates that methoxyphenols ¿13C and ¿2H values provides a useful tool for inferring climatic processes in soil organic matter in Mediterranean ecosystems. Further discussion on the climatic, environmental processes affecting the isotopic composition of lignin compounds, particularly regarding non-exchangeable hydrogen will be assessed. References [1] Hofmann et al. European Journal of Soil Science 60 (2009) 250-257. [2] Seki et al. Geochimica et Cosmochimica Acta 74 (2010). 599-613. [3] Paul et al. Biogeosciences 13 (2016) 6587¿6598.This work was supported in part by the UE and FEDER-Junta de Andalucía Research Project "MarkFire" (PAIDI2020, PY20_01073). L. San Emeterio also thanks Ministerio de Ciencia Innovación y Universidades (MICIU) for funding FPI research grant (BES-2017-07968)