Elsevier

Pedosphere

Volume 32, Issue 4, August 2022, Pages 565-575
Pedosphere

Size fractions of organic matter pools influence their stability: Application of the Rock-Eval® analysis to beech forest soils

https://doi.org/10.1016/S1002-0160(21)60050-4Get rights and content

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.

Section snippets

INTRODUCTION

Thermal analyses are commonly used to measure the thermal stability of soil organic matter (SOM). In addition to various compositional parameters, thermal stability is considered a proxy for biogeochemical stability, defined as the bioavailability (or bioaccessibility) of SOM to decomposition by microorganisms (Plante et al., 2009). There is ample literature on this topic, and the association between “analytical” thermal stability and “functional” biogeochemical stability is much debated among

Study site and previous studies

The study site, with an area of 7 200 ha, was located in the Eawy State Forest, Normandy, France (1°18′ E, 49°44′ N). Its management in the last 200 years has been documented. The climate is oceanic temperate, with a mean annual temperature of 10 °C and mean annual precipitation of 800 mm. The soil is an Endogleyic Dystric Luvisol (FAO, 2006), developed on a > 80-cm-thick layer of loess (silty texture), overlying the clay-with-flint parent material. Soil texture was similar among stands, with

Standard Rock-Eval® parameters

As shown in Table I, TOC varied among the bulk samples (420–550 g kg-1 in litters and 30–80 g kg-1 in topsoils), O-fractions (> 410 g kg-1 in large debris, 340–560 g kg-1 in O-coarse, and 210–600 g kg-1 in O-medium), and A-fractions (380–740 g kg-1 in A-coarse, 190–400 g kg-1 in A-medium, and 15–60 g kg-1 in A-fine). The boxplots show a fairly continuous decreasing trend from the OL horizon (only beech litter) and large debris toward the O-fractions and A-coarse fraction and a drastic

Rock-Eval® parameters and SOM composition

The standard Rock-Eval® parameters (i.e., TOC, TpS2, HI, and OI) represent the general trends and signatures (Fig. 1, Fig. 2, Table I) related to the decomposition of SOM and changes in its chemical composition. The present results are consistent with the findings of previous studies using Rock-Eval® (Di-Giovanni et al., 1998; Disnar et al., 2003; Sebag et al., 2006, 2016; Saenger et al., 2013).

The TOC content (Fig. 1) decreased gradually from the large debris to A-medium fractions and

CONCLUSIONS

This study highlights some assets of the Rock-Eval® thermal analysis and its derived parameters to quantify the decomposition and stability of SOM. Various parameters, including HI, I index, and R index, which are related to the most reactive part of SOM, reflect changes in OM composition and thermal stability induced by decomposition of labile compounds as well as organo-mineral interactions in topsoils. Moreover, the combination of compositional and thermal indices assumes that at least three

ACKNOWLEDGEMENTS

Rock-Eval® is a trademark registered by the Institut Français du Pétrole Energies Nouvelles (IFPEN), France. Jean Trap's research was funded by the French “Ministère de l'Agriculture et de la Pêche”, the ”GIP ECOFOR” (No. 82005.20), and the “GRR-SER” (Environmental Sciences, Analysis and Risk Management). Mickaël Hedde's research was supported by the Haute-Normandie Region (France) in the framework of the GRR-SER and by the FR CNRS 3730 SCALE (ESTER project). The “Fondation Herbette—Université

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