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