Surveying the carbon pools of mountain soils: A comparison of physical fractionation and Rock-Eval pyrolysis

Résumé En

Accurate monitoring of soil organic carbon (SOC) pools with contrasted turnover times is necessary for better evaluation of the vulnerability of SOC stocks in the face of global changes. To obtain this information in heterogeneous landscapes such as mountain areas, fast and reliable approaches providing an assessment of SOC biogeochemical stability are needed. Recently, it has been shown that Rock-Eval (RE) pyrolysis enables a fast quantification of three SOC pools of different thermostabilities [Saenger, A., Cécillon, L., Sebag, D., & Brun, J.J. 2013. Soil organic carbon quantity, chemistry, and thermal stability in a mountainous landscape: A Rock-Eval pyrolysis survey. Organic Geochemistry 54, 101–114]. The aim of this study was to compare the RE pyrolysis method with the standardized and widely used technique of SOC physical fractionation for characterizing SOC pools at landscape-scale in a mountainous area of the calcareous French Alps. Physical fractionation revealed large quantities of particulate organic matter (POM, i.e. labile SOC) for all samples, suggesting an overall high vulnerability of mountain SOC stocks to environmental changes. Organic C stocks of mountain ridge topsoils (0–10 cm) may be the most vulnerable with 76% consisting of POM, while alpine and subalpine grasslands developed on Cambisols presented more stabilized forms of SOC (64% of SOC stocks being associated with minerals, MaOM fraction). RE pyrolysis gave results similar to the physical fractionation method for the identification of stability patterns of the different ecological units and for estimating their dominant SOC pools (labile vs. stable). However, some discrepancies between the two techniques were observed for the absolute quantification of SOC pools, with slightly higher correlations between the physically and thermally derived stable SOC pool than for the labile pool (R2 = 0.59 and 0.50, respectively). These differences can be explained by the fact that each method has inherent biases and isolates somewhat different pools. Additionally, RE pyrolysis provided information about the chemistry of SOC pools, suggesting a greater maturity of SOC in the MaOM than in the POM fractions. Overall, this study confirms that the RE pyrolysis technique, being both time- and cost-effective, could become a standardized proxy for assessing SOC pools at landscape-scale although it requires further comparison with other methods for several soil and land-use types.