Multidecadal persistence of organic matter in soils: Insights from STXM-NEXAFS and nano-SIMS investigations

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 insoils requires constraining the dynamics, the distribution and the chemical nature of both SOM and associatedmineral phases. Benefiting from the unique opportunity offered by a long-term bare fallow (LTBF) experimenthaving started in 1928 in Versailles (France), we report C and N decline with time for six different particle-sizefractions (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 thesubmicrometer scale of SOC dynamics using STXM-based NEXAFS and nano-SIMS. As expected, total C and Ndeclined 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 ofSOM associated to clays significantly decreased with time whereas C/N ratios of SOM associated to siltfractions 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 almostconstant with LTBF duration. STXM-based NEXAFS investigations at the submicrometer scale revealed that allsmectitic clays were associated to OM coatings at all dates whereas illite particles became progressively SOMfreewith increasing bare fallow duration. STXM-based NEXAFS investigations also revealed that particles oforganic matter can be observed within the coarse clay fractions even after 8 decades of bare fallow. Nano-SIMSinvestigations on bulk clay fractions showed that OM decline was similar for Si-, Al- and Fe-rich regions. Ourstudy provides new lines of evidence for the higher protection of N-rich SOM by minerals and is the first directobservation of the specific ability of smectites to protect SOM at a pluri-decadal timescale.