The energetic and chemical fingerprints of persistent soil organic carbon. Soil 6th International Symposium on Soil Organic Matter

Pierre Barré (1) , François Baudin (2) , Sylvain Bernard (3) , Lauric Cécillon (4) , Bent T. Christensen (5) , Sabine Houot (6, 7) , Thomas Kätterer (8) , Suzanne Lutfalla (7, 6, 1) , Corentin Le Guillou (9) , Andy Macdonald (10) , Folkert van Oort (6, 7) , A. F. Plante (11) , Katell Quenea (12) , Alix Vidal (12) , Claire Chenu (6, 7)

1 CNRS - Centre National de la Recherche Scientifique
2 iSTeP - Institut des Sciences de la Terre de Paris
3 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
4 UGA [2016-2019] - Université Grenoble Alpes [2016-2019]
5 Aarhus University [Aarhus]
6 ECOSYS - Ecologie fonctionnelle et écotoxicologie des agroécosystèmes
7 Université Paris Saclay (COmUE)
8 SLU - Swedish University of Agricultural Sciences = Sveriges lantbruksuniversitet
9 UMET - Unité Matériaux et Transformations - UMR 8207
10 Rothamsted Research
11 University of Pennsylvania
12 METIS - Milieux Environnementaux, Transferts et Interactions dans les hydrosystèmes et les Sols

The absence of convincing physical or chemical procedures to characterize or isolate relatively labile versus persistent soil organic carbon (SOC) pools makes the study of persistent SOC difficult. Long-term bare fallow (BF) experiments, in which C inputs have been stopped for decades, provide a unique opportunity to study persistent SOC without the inherent artefacts induced by extraction procedures, the hypothesis being that SOC is gradually enriched in persistent C with time as labile components decompose. We determined the evolution of thermal and chemical characteristics of bulk SOC in five long-term BF experiments across Europe (Askov, Grignon, Rothamsted, Ultuna and Versailles), using a multi-technique approach involving Rock-Eval pyrolysis (RE), thermogravimetry and differential scanning calorimetry (TG-DSC), Near Edge X-Ray Absorption Fine Structure (NEXAFS) and pyrolysis gas chromatography-mass spectrometry (TMAH-Py-GC-MS). Results of RE and TG analyses showed that the temperature needed to combust the SOC increased with BF duration at all sites. Conversely, SOC energy density (in mJ mg-1 C) measured by DSC decreased with BF duration. RE results showed that hydrogen index (HI) tended to decrease with BF duration whereas the oxygen index (OI) did not show consistent trends across sites. NEXAFS signals presented little differences and were dominated by carboxyl peak. TMAH-Py-GC-MS results showed a strong relative decrease in lignin-derived compounds with BF duration and a small decline in cutin and suberin-derived compounds. Conversely, the relative intensity of alkanes increased with bare fallow duration. Our results showed that in spite of the heterogeneity of the soils at the 5 long-term BF sites, SOC that has persisted in soils for several decades have similar and defined thermal and energetic properties: persistent SOC burns at higher temperature and its combustion generates less energy. Persistent SOC in the studied temperate soils also shares some chemical properties: it has a lower HI values and is depleted in lignin-derived compounds. The increased burning temperature and lower energy density of persistent SOC suggest that SOC stability may be a function of the high energy cost and low energy gain from decomposition of this material.

Domaines: Sciences du Vivant [q-bio]

Pierre Barré, François Baudin, Sylvain Bernard, Lauric Cécillon, Bent T. Christensen, et al.. The energetic and chemical fingerprints of persistent soil organic carbon. Soil 6th International Symposium on Soil Organic Matter. 6th International Symposium on Soil Organic Matter, Sep 2017, Harpenden, United Kingdom. pp.208. ⟨hal-01607974⟩