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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 18 Sep 2018

Research article | 18 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

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

Suzanne Lutfalla1,2, Pierre Barré2, Sylvain Bernard3, Corentin Le Guillou4, Julien Alléon3,a, and Claire Chenu2 Suzanne Lutfalla et al.
  • 1ECOSYS, INRA, AgroParisTech, Université Paris-Saclay, UMR 1402, Thiverval Grignon, France
  • 2Laboratoire de Géologie de l'ENS – PSL Research University – CNRS UMR 8538, Paris, France
  • 3Muséum National d'Histoire Naturelle, Sorbonne Université, CNRS UMR 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Paris, France
  • 4Unité Matériaux et Transformations, UMET, CNRS UMR 8207, Université de Lille, France
  • anow at: MIT – EAPS Department, Summons Lab, Cambridge MA, USA

Abstract. 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.

Suzanne Lutfalla et al.
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Status: final response (author comments only)
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Suzanne Lutfalla et al.
Suzanne Lutfalla et al.
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Short summary
Soils store large amounts of carbon in soil organic matter, coming from plant debris and roots. The mechanisms protecting it from biodegradation are not fully understood. Here, we carry out a size-fractionation of soil sampled at different dates in a field experiment. Thanks to carbon and nitrogen contents and spectroscopy and microscopy we conclude that organic matter enriched in nitrogen is preferentially protected from biodegradation and that clay minerals have differing protecting abilities.
Soils store large amounts of carbon in soil organic matter, coming from plant debris and roots....