Organic matter decomposition: bridging the gap between Rock\textendashEval pyrolysis and chemical characterization (CPMAS 13C NMR)

International audienceOrganic matter (OM) is a key component of soils but information on its chemistry and behavior in soils is still incomplete. Numerous methods are commonly used to characterize and monitor OM dynamics, but only a few include the qualities required to become routine techniques i.e. simple, rapid, accurate and at low cost. Rock\textendashEval pyrolysis (RE pyrolysis) is a good candidate, as it provides an overview of OM properties by monitoring four components related to the main major classes of organic constituents (from A1 for the labile biological constituents to A4 for the mature refractory fraction). However, a question is still pending: do these four major classes used in the literature reflect a pertinent compositional chemical counterpart? 13C Nuclear Magnetic Resonance Spectroscopy in the solid state (13C CPMAS NMR) has been used to answer this question by collecting information on structural and conformational characteristics of OM. Moreover, in order to avoid the blurring effect of pedogenesis on OM dynamics, a \textquotedblleftless complex OM\textquotedblright source, i.e. compost samples, has been used. Results showed significant and high determination coefficients between classes, indices (of transformation of plant biopolymers, humification\textellipsis) from RE pyrolysis, and the main classes of OM characterized by 13C NMR, e.g. A1 & A2 with labile/easily degradable components (alkyl C et O-alkyl C), A3 & A4 with humified OM (with aromatic C and phenolic C). The R index (contribution of bio-macromolecules) is correlated with phenolic and aromatic C, whereas the I index (related to immature OM) refers to labile\textendash\textendasheasily degradable components (alkyl C et O-alkyl C). The results confirm the pertinence of RE pyrolysis to monitor OM dynamics