Rock-Eval analysis of French forest soils: the influence of depth, soil and vegetation types on SOC thermal stability and bulk chemistry
Résumé
Soil organic matter (SOM) is the largest terrestrial carbon pool and SOM degradation has multiple consequences
on key ecosystem properties like nutrients cycling, soil emissions of greenhouse gases or carbon sequestration
potential. With the strong feedbacks between SOM and climate change, it becomes particularly urgent to develop
reliable routine methodologies capable of indicating the turnover time of soil organic carbon (SOC) stocks.
Thermal analyses have been used to characterize SOM and among them, Rock-Eval 6 (RE6) analysis of soil has
shown promising results in the determination of in-situ SOC biogeochemical stability. This technique combines a
phase of pyrolysis followed by a phase of oxidation to provide information on both the SOC bulk chemistry and
thermal stability.
We analyzed with RE6 a set of 495 soils samples from 102 permanent forest sites of the French national network
for the long-term monitoring of forest ecosystems (“RENECOFOR” network). Along with covering pedoclimatic
variability at a national level, these samples include a range of 5 depths up to 1 meter (0–10 cm, 10–20 cm, 20–40
cm, 40–80 cm and 80–100 cm). Using RE6 parameters that were previously shown to be correlated to short-term
(hydrogen index, HI; T50 CH pyrolysis) or long-term (T50 CO2 oxidation and HI) SOC persistence, and that char-
acterize SOM bulk chemical composition (oxygen index, OI and HI), we tested the influence of depth (n = 5), soil
class (n = 6) and vegetation type (n = 3; deciduous, coniferous-fir, coniferous-pine) on SOM thermal stability and
bulk chemistry. Results showed that depth was the dominant discriminating factor, affecting significantly all RE6
parameters. With depth, we observed a decrease of the thermally labile SOC pool and an increase of the thermally
stable SOC pool, along with an oxidation and a depletion of hydrogen-rich moieties of the SOC. Soil class and
vegetation type had contrasted effects on the RE6 parameters but both affected significantly T50 CO2 oxidation
with, for instance, entic Podzols and dystric Cambisols containing relatively more thermally stable SOC in the
deepest layer than hypereutric/calcaric Cambisols. Moreover, soils in deciduous plots contained a higher propor-
tion of thermally stable SOC than soils in coniferous plots.
This study shows that RE6 analysis constitutes a fast and cost effective way to qualitatively estimate SOM turnover
and to discuss its ecosystem drivers. It offers promising prospects towards a quantitative estimation of SOC
turnover and the development of RE6-based indicators related to the size of the different SOC kinetic pools.
Domaines
Sciences du Vivant [q-bio]
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