Multidecadal persistence of organic matter in soils: investigations at the submicrometer scale

Abstract

<p><strong>Abstract.</strong> The mineral matrix, particularly clay-sized minerals, protects soil organic matter (SOM) from decomposition by microorganisms. Here we report the characterization of SOM and associated minerals over decades of biodegradation, in a French long-term bare fallow (LTBF) experiment started in 1928. The amounts of carbon (C) and nitrogen (N) declined with time for six fractions (sand, coarse silt, fine silt, coarse clays, intermediate clays and fine clays). The C : N ratios of SOM associated to silt fractions remained constant whereas they significantly decreased in clays, reaching very low values in intermediate and fine clays (C : N < 5) after 8 decades of LTBF. X-ray absorption spectroscopy revealed that (i) bulk-scale SOM chemical speciation remained almost constant, (ii) submicrometric particulate OM was present in coarse clays, even after 79 years of LTBF, (iii) illite particles became progressively SOM-free with time whereas mixed layered illite/smectite and smectites were always associated to OM throughout the bare fallow. Altogether, these results suggest that clay-sized minerals preferentially protect N-rich SOM and that smectites and mixed layered illite/smectite protect SOM more efficiently than illites.</p>

Long-term bare fallow soil fractions reveal thermo-chemical properties controlling soil organic carbon dynamics

“…Organo-mineral interaction and physico-chemical protection are thus unlikely to be important in the sand fraction, while the persistent OC content is higher than the one of the clay fraction. Considering the fractionation protocol used in this study (Lutfalla et al, 2018), preserved physical aggregates are smaller than 2 µm (Chenu et al, 2009) and thus absent from the sand fraction. This persistent OC pool must only result from the preservation of highly condensed PyOC.…”

“…Physical dispersion and particle-size fractionation were carried out on soil samples (Lutfalla et al, 2018) to separate the sand fraction (> 50 µm), the coarse and fine silt fractions ((20-50) µm and (2-20) µm) and the clay fraction (< 2 µm) using the method proposed by Balesdent et al (1998). Fifty grams of soil were shaken overnight with 20 glass beads in 180 mL deionised water to break up aggregates bigger than 50 µm.…”

“…As organo-mineral interactions occur at the submicron scale and induce variable SOC residence time depending on the size of clay subfractions (Balesdent et al, 1987), coarse, intermediate and fine clay subfractions ((0.2-2) µm, (0.05-0.2) µm and < 0.05 µm) were obtained from the clay fraction (Lutfalla et al, 2018). Two grams of clay subsamples were suspended in water and sonicated at 320 J • mL −1 .…”