Dynamics of soil organic matter based on new Rock-Eval indices

This paper aims to develop a new approach to interpret soil organic matter (SOM) dynamics from Rock-Eval pyrolysis. Rock-Eval standard parameters (TpS2, HI, OI) are limited when applied to SOM, as they were defined for tracking the origin of sedimentary organic matter (i.e. terrestrial vs aquatic and lacustrine vs marine). This study proposes new Rock Eval based indices, projected on a new diagram plotting > 1300 samples, including litter and soil horizons, pure compounds and organic material. These new parameters reflect the thermal stability of SOM rather than its bulk chemistry. Their calculations are based on the contribution of four different areas (A1 to A4) integrated below the S2 pyrogram (amounts of released hydrocarbon compounds during the pyrolysis step). Results demonstrate that the relative values of A1 to A4 parameters can be used to survey the evolution of SOM during pedogenesis. These Rock-Eval parameters revealed a consistent thermal differentiation of SOM with depth, from thermally labile biogenic SOM in soil organic horizons to thermally stable mineral-associated SOM compounds in organo-mineral and mineral soil horizons, indicating a pedogenetic stabilization of SOM. Finally, newly defined I- and R-indices integrate the respective parts of A1 to A4 parameters into SOM dynamics, the I-index emphasizing the degree of transformation of the immature organic fraction (related to SOM stabilization), the R-index highlighting the contribution of the most refractory fraction or persistent SOM (related to pedogenic and inherited contributions). Analyses of a wide range of soils and reference samples (cellulose, lignin, humic substances, lignite, charcoal, coal, etc.) allow end-members as well as particular trends (humic, spodic, inherited) to be drawn. Specific examples are provided in order to illustrate some applications and uses for this new Rock-Eval based I/R diagram, from the study of grain size fractions to the evolution of SOM in soil chronosequences.