The energetic and chemical fingerprints of persistent soil organic carbon. Soil 6th International Symposium on Soil Organic Matter
2017
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.
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