Multidecadal persistence of organic matter in soils: Insights from STXM-NEXAFS and nano-SIMS investigations
Résumé
The mineral matrix and clays in particular protect soil organic matter (SOM) from decomposition by microorganisms.
Better understanding the mechanisms responsible for the pluri-decadal persistence of carbon in
soils requires constraining the dynamics, the distribution and the chemical nature of both SOM and associated
mineral phases. Benefiting from the unique opportunity offered by a long-term bare fallow (LTBF) experiment
having started in 1928 in Versailles (France), we report C and N decline with time for six different particle-size
fractions (sand [>50μm], coarse silt [20-50μm], fine silt [2-20μm], coarse clays [0.2-2μm], intermediate clays
[0.05-0.2μm] and fine clays [0-0.05μm]). We also report spatially resolved characterization at the
submicrometer scale of SOC dynamics using STXM-based NEXAFS and nano-SIMS. As expected, total C and N
declined with time in all fractions. The lowest declines were observed for intermediate size clays [0.05-0.2μm]
in which only 53 % of C and 63% of N were lost between the initiation of the bare fallow and 2008. C/N ratios of
SOM associated to clays significantly decreased with time whereas C/N ratios of SOM associated to silt
fractions remained constant. SOM associated to intermediate and fine clays were notably very low (below 5)
after 8 decades of bare fallow. Bulk-scale C-NEXAFS data reveals that the speciation of SOC remained almost
constant with LTBF duration. STXM-based NEXAFS investigations at the submicrometer scale revealed that all
smectitic clays were associated to OM coatings at all dates whereas illite particles became progressively SOMfree
with increasing bare fallow duration. STXM-based NEXAFS investigations also revealed that particles of
organic matter can be observed within the coarse clay fractions even after 8 decades of bare fallow. Nano-SIMS
investigations on bulk clay fractions showed that OM decline was similar for Si-, Al- and Fe-rich regions. Our
study provides new lines of evidence for the higher protection of N-rich SOM by minerals and is the first direct
observation of the specific ability of smectites to protect SOM at a pluri-decadal timescale.
Domaines
Sciences du Vivant [q-bio]
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