Persistance à long terme des matières organiques dans les sols : caractérisation chimique et contrôle minéralogique

Abstract : Soils store three times more carbon than the atmosphere, under the form of a complex mixture of molecules called soil organic matter (SOM). Some of these molecules have been standing in the soil for hundreds to thousands of years. Three main mechanisms are invoked to explain this long term carbon persistence in soils, (i) chemical recalcitrance, (ii) physical protection in aggregates and (iii) protection by adsorption on mineral surfaces. One of the major challenges in SOM science is to better understand the relative importance of each mechanism, that is the aim of this PhD project. Here, we use samples from by long term bare fallows (5 sites across Europe). These experimental plots have been kept free of vegetation by manual or chemical weeding for several decades and have been regularly sampled and stored. As the duration of the bare fallow increases, biodegradation occurs and samples get enriched in persistent carbon.First experiments consisted in testing the efficiency of chemical oxidations (two reagent were tested, sodium hypochlorite –NaOCl- and hydrogen peroxide –H2O2) on the longest bare fallow. We concluded that oxidation methods were not able to efficiently isolate a pool of persistent carbon at the centennial timescale. In terms of mechanisms of persistence, the obtained results show that chemical recalcitrance does not seem to be the major mechanism. Indeed, over the duration of the bare fallow, the chemical composition of SOM, as seen by synchrotron based NEXAFS spectroscopy, shows little changes. There is a consistent increase in carboxylics for all sites (12% increase on average) though it is significant for 2 out of the 4 selected sites. We also studied the particular persistence of soil pyrogenic carbon, which is thought to be at least five times more persistent than bulk SOM. Results show that pyrogenic carbon lacks long term persistence. Indeed the BPCA-estimated mean residence time of pyrogenic carbon (116 years) is on average 1.6 times longer than MRT for bulk SOM (73 years). Finally, the study of mineralogical control of the persistence of SOC showed that clay minerals containing potassium (illite) seemed to protect less carbon. As seen by NEXAFS-STXM, more mineral surfaces with very little SOM appear with the duration of bare fallow. C:N ratio decreased in all clay fractions, suggesting a preferential persistence of N-rich compounds. Presence of microaggregates in the coarser clay fraction led to the coexistence of two protection mechanisms: adsorption and physical protection.
Document type :
Theses
Complete list of metadatas

Cited literature [335 references]  Display  Hide  Download

https://tel.archives-ouvertes.fr/tel-02193715
Contributor : Abes Star <>
Submitted on : Wednesday, July 24, 2019 - 4:35:11 PM
Last modification on : Friday, July 26, 2019 - 1:15:41 AM

File

49047_LUTFALLA_2015_diffusion....
Version validated by the jury (STAR)

Identifiers

  • HAL Id : tel-02193715, version 1

Citation

Suzanne Lutfalla. Persistance à long terme des matières organiques dans les sols : caractérisation chimique et contrôle minéralogique. Milieux et Changements globaux. Université Paris-Saclay, 2015. Français. ⟨NNT : 2015SACLA008⟩. ⟨tel-02193715⟩

Share

Metrics

Record views

131

Files downloads

24