Abstract
Abstract. Minerals, particularly clay-sized minerals, protect soil organic matter (SOM) from decomposition by microorganisms. Here we report the characterization of SOM and the 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) in the study area declined over time for six fractions (sand, coarse silt, fine silt, coarse clays, intermediate clays, and fine clays). The C:N ratios of SOM associated with silt fractions remained constant, whereas the ratios significantly decreased in clays, reaching very low values in intermediate and fine clays (C:N < 5) after 8 decades of LTBF conditions. X-ray absorption spectroscopy revealed the following: (i) bulk-scale SOM chemical speciation remained almost constant; (ii) submicron particulate OM was present in coarse clays, even after 79 years of LTBF conditions; and (iii) illite particles became progressively SOM-free with time, whereas mixed-layer illite/smectite and smectites were always associated with OM throughout the bare fallow treatment. In summary, these results suggest that clay-sized minerals preferentially protect N-rich SOM and that smectites and mixed-layer illite/smectite seem to protect associated OM more effectively than pure illites.
Highlights
Soils represent an important carbon reservoir on the global scale: they store 3 times more carbon than the atmosphere (Batjes, 1996) and are currently considered as one of the solutions for climate change mitigation and adaptation in addition to food security as highlighted by the “4 per 1000” initiative (Soussana et al, 2017)
Apart from the coarse silt fraction which contained very low amounts of Soil organic matter (SOM), the amount of OC and N remaining after 79 years of bare fallow conditions were, as expected, higher in the clay fractions (Table 1)
The percentage of C and N remaining increased with decreasing particle-size for fine silt, coarse clays (CC), and intermediate clays (IC); the same was not observed for fine clays (FC), where much lower percentages of C and N remained, suggesting a higher labile SOM content in the FC fraction
Summary
Soils represent an important carbon reservoir on the global scale: they store 3 times more carbon than the atmosphere (Batjes, 1996) and are currently considered as one of the solutions for climate change mitigation and adaptation in addition to food security as highlighted by the “4 per 1000” initiative (Soussana et al, 2017). Soil organic matter (SOM) encompasses compounds with residence times ranging from days to millennia (Trumbore, 2000), and the mechanisms controlling SOM turnover are actively debated (Dungait et al, 2012; Lehmann and Kleber, 2015; Schmidt et al, 2011). Except to some extent for pyrogenic C (Lutfalla et al, 2017), the current thinking considers organomineral interactions (by adsorption or coprecipitation) as a dominant factor, rather than intrinsic chemical recalcitrance, for the longterm persistence of otherwise labile organic compounds in soil (Baldock and Skjemstad, 2000; Balesdent et al, 2000; Lehmann and Kleber, 2015; von Lützow and Kögel-Knabner, 2010; Schmidt et al, 2011; Sollins et al, 2006). Smectites are considered to have higher protective capabilities than illite and kaolinite, be-
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