Pedosphere 32(4): 565--575, 2022
ISSN 1002-0160/CN 32-1315/P
©2022 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Size fractions of organic matter pools influence their stability: Application of the Rock-Eval® analysis to beech forest soils
David SEBAG1,2,3, Eric P. VERRECCHIA2, Thierry ADATTE4, Michaël AUBERT5, Guillaume CAILLEAU6, Thibaud DECAËNS7, Isabelle KOWALEWSKI3, Jean TRAP8, Fabrice BUREAU5, Mickaël HEDDE8
1Normandie Univ, Université de Rouen Normandie (UNIROUEN), Centre National de la Recherche Scientifique (CNRS), M2C, Rouen 76000 (France)
2Institute of Earth Surface Dynamics (IDYST), Geopolis, University of Lausanne, Lausanne 1015 (Switzerland)
3Institut Français du Pétrole Energies Nouvelles (IFPEN), Earth Sciences and Environmental Technologies Division, Rueil-Malmaison 92852 (France)
4Institute of Earth Sciences (ISTE), Geopolis, University of Lausanne, Lausanne 1015 (Switzerland)
5Normandie Univ, Université de Rouen Normandie (UNIROUEN), Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement (INRAE), Laboratoire Étude et COmpréhension de la bioDIVersité (ECODIV), Rouen 76000 (France)
6Laboratory of Microbiology, Institute of Biology, University of Neuchâtel, Neuchâtel 2000 (Switzerland)
7Centre d'Ecologie Fonctionnelle et Evolutive (CEFE), Université de Montpellier, CNRS, Ecole Pratique des Hautes Etudes (EPHE), Institut de Recherche pour le Développement (IRD), Montpellier 34000 (France)
8Eco&Sols, INRAE, IRD, Université de Montpellier, Montpellier 34000 (France)
Corresponding Author:David SEBAG
ABSTRACT
      Soil organic matter (SOM) is a complex heterogeneous mixture formed through decomposition and organo-mineral interactions, and characterization of its composition and biogeochemical stability is challenging. From this perspective, Rock-Eval® is a rapid and efficient thermal analytical method that combines the quantitative and qualitative information of SOM, including several parameters related to thermal stability. This approach has already been used to monitor changes in organic matter (OM) properties at the landscape, cropland, and soil profile scales. This study was aimed to assess the stability of SOM pools by characterizing the grain size fractions from forest litters and topsoils using Rock-Eval® thermal analysis. Litter (organic) and topsoil samples were collected from a beech forest in Normandy (France), whose management in the last 200 years has been documented. Fractionation by wet sieving was used to separate large debris (> 2 000 μm) and coarse (200-2 000 μm) and fine particulate OM (POM) (50-200 μm) in the organic samples as well as coarse (200-2 000 μm), medium (50-200 μm), and fine (< 50 μm) fractions of the topsoil samples. Rock-Eval® was able to provide thermal parameters sensitive enough to study fine-scale soil processes. In the organic layers, quantitative and qualitative changes were explained by the progressive decomposition of labile organic compounds from plant debris to the finest organic particles. Meanwhile, the grain size fractions of topsoils presented different characteristics. The coarse organo-mineral fractions showed higher C contents, albeit with a different composition, higher thermal stability, and greater decomposition degree than the plant debris forming the organic layer. These results are consistent with those of previous studies that microbial activity is more effective in this fraction. The finest fractions of topsoils showed low C contents, the highest thermal stability, and low decomposition degree, which can be explained by the stronger interactions with the mineral matrix. Therefore, it is suggested that the dynamics of OM in the different size fractions be interpreted in the light of a plant-microbe-soil continuum. Finally, three distinct thermostable C pools were highlighted through the grain size heterogeneity of SOM:free coarse OM (large debris and coarse and fine particles), weakly protected OM in (bio)aggregates (coarse fraction of topsoil), and stabilized OM in the fine fractions of topsoil, which resulted from the interactions within organo-mineral complexes. Therefore, Rock-Eval® thermal parameters can be used to empirically illustrate the conceptual models emphasizing the roles of drivers played by the gradual decomposition and protection of the most thermally labile organic constituents.
Key Words:  aggregate,decomposition,litter,organo-mineral interaction,plant-microbe-soil continuum,soil organic matter,thermal analysis,topsoil
Citation: Sebag D, Verrecchia E P, Adatte T, Aubert M, Cailleau G, Decaëns T, Kowalewski I, Trap J, Bureau F, Hedde M. 2022. Size fractions of organic matter pools influence their stability: Application of the Rock-Eval® analysis to beech forest soils. Pedosphere. 32(4): 565-575.
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