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 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