Is Rock-Eval 6 thermal analysis a good indicator of soil organic carbon lability? - A method-comparison study in forest soils

Soil respiration tests and abundance of particulate organic matter (POM) are considered as classical indicators of the labile soil organic carbon (SOC) pool. However, there is still no widely accepted standard method to assess SOC lability and the pertinence of these two time-consuming methods to characterize SOC turnover can be questioned. Alternate ways of determining the labile SOC fraction are thus much needed. Thermal analyses, in particular Rock-Eval 6 (RE6) analysis has shown promising results in the determination of SOC biogeochemical stability. Using a large set of samples (n = 99) of French forest soils representing contrasted pedoclimatic conditions, including deep samples (up to 0.8 m depth), we compared three different methods used for SOC lability assessment. We explored whether respired-C isolated by a 10-week laboratory soil respiration test, POM-C isolated by a physical SOC fractionation scheme (particle-size > 50 pm and d < 1.6 g cm(-3)) and several RE6 parameters were comparable and how they correlated. As expected, respired-C (mg CO2-C.g(-1) SOC) and POM-C (% of total SOC) fractions strongly decreased with depth. RE6 parameters showed that SOC from deeper soil layers was also thermally less labile, more oxidized and H-depleted. Indeed, SOC from deeper soil layers had lower proportion of thermally labile SOC, higher T50-Hc-pyR (temperature at which 50% of the pyrolysable hydrocarbons were effectively pyrolyzed) and T50-CO2-OX (temperature at which 50% of the CO2 gas had evolved during the oxidation phase), larger oxygen index, and smaller hydrogen index. Surprisingly, the two classical indicators of the labile SOC pool (respired-C and POM-C) were only marginally correlated (p = 0.051) and showed layer-specific correlations. Similarly, respired-C was poorly correlated to RE6 parameters. Conversely, the POM-C fraction showed a strong negative correlation with T50-HC-PYR (rho = -0.73) and good correlations with other RE6 parameters. Our study showed that RE6 parameters were good estimates of the POM-C fraction, which represents a labile SOC pool with a residence time of ca. a couple decades that is meaningful regarding SOC stock changes upon modifications in land management. RE6 thermal analysis could therefore be a fast and cost-effective alternative to more time-consuming methods used in SOC pool determination, and may be integrated into soil monitoring networks to provide high-throughput information on SOC dynamics.