Rapid characterisation of organic matter in Brazilian clayey Ferralsols by Rock-Eval pyrolysis
Résumé
Rock-Eval pyrolysis has primarily been designed to obtain quantitative and qualitative information on the organic content of sedimentary rocks for petroleum exploration purposes. However, because it is fast and provides useful basic information, it has latter been applied to soils and immature sediments. A recent study on soil organic matter (SOM) characterisation by Rock-Eval pyrolysis, was realised on soils from the temperate climate zone. Among the few tropical soils that have been studied, Ferralsols present a more complex SOM composition than the other soils. The aim of this study was to complete these previous data and to determine the nature and possible origin of SOM in Ferralsols. With this aim, Brazilian Ferralsols were sampled in the state of Goías, at depth intervals of 0.1m, from the surface down to 1.6 m depth, in two sites under native vegetation (Cerrado) and three others under pasture. In all soils, the total organic carbon (TOC) decreases with depth in an exponential-like manner. The multilobed pyrolysis S2 peaks recorded in the course of programmed pyrolysis under inert atmosphere (N2) provides basic and straightforward information on the heterogeneity of SOM and its degree of evolution. Then, the deconvolution of S2 peaks enables the separation and quantification of three main organic compartments: minor labile compounds, humic-like compounds abundant in the topsoil, and a refractory fraction that may be considered as black carbon after optical examination under the microscope. The refractory fraction that becomes dominant at depth differentiates the Ferralsols from soils from temperate areas. There is no marked distinction in the relative proportions of the different SOM compartments and in their evolution with depth between the profiles under pasture and under native vegetation.