Methodological investigation on different pools of organic matter in mining-impacted soils: insights from ramped thermal analysis (Rock-Eval®)

The characterization of condensed carbon like coal is currently getting a crucial issue, both in the context of agronomical use (biochar…) and assessment of environmental impact for mining-impacted soils. The existing methods are complex and give only a partial view of the quality of organic matter. For example when the Total Organic Carbon is measured, no information is obtained regarding the level of degradation of the organic matter (OM). On the contrary, ramped thermal analyses like Rock-Eval® (RE) enables to distinguish between labile organic matter (eg. originating from fresh OM) and more recalcitrant organic matter (eg. coming from already degraded OM like coal). This analysis, initially developed for the field of oil and gas, is increasingly applied on environmental samples, and provides a more detailed insight into OM composition and biogeochemical reactivity. In this work, we developed different methodologies based on RE thermal analysis to differentiate between two pools of organic matter, eg. fresh soil OM and coal OM.

Five high-elevation grassland soils were sampled in June 2022 in the Elk River coalfield of southeastern British Columbia, Canada. Horizon sequences are dominated by dark-coloured, organic matter-enriched (~5-10% total C) Ah horizons. Surface samples (Ah horizons) exhibit angular, black particles (most likely coal) which means that even in the surface layers, two pools of organic matter coexist: detrital coal particles embedded into an organo-mineral matrix.

Total carbon contents measured with RE were fully consistent (< 4% discrepancy) with measurements obtained with elemental analysis. Deeper investigations on RE results enabled to propose the use of relevant indexes derived from RE analysis, both from pyrolysis and oxidation steps (IO/IH indexes, S4CO₂…). They were exploited to estimate the carbon originating from fossil sources compared to fresh carbon pool. A relative approach (expressed in enrichment in fossil C compared to surface samples) was carried out, as well as tentative absolute quantification using IO/IH indexes from the literature as a reference. Consistent results were obtained, indicating an increase in fossil carbon content against depth, explained by an increasing contribution of deep coal rock. Mean coal contents over four profiles ranged for example from 42% to 78% from Ah1 horizon down to Ah3 horizon.

Overall, this study assesses different approaches based on RE thermal analysis to provide an estimation of the quantity of two various OM pools. This is of particular interest both in the framework of mining-polluted soils, but also for soils amended with biochars.