Temporal pools of individual organic substances in soil

Two pools of plant-derived C31 n-alkane occurring respectively as free and humin-bound forms in the same soil have been distinguished by C/C analyses. The results show that the humin-bound molecule is on the average 7 years older than the free molecule. These findings indicate that apolar organic substances can be trapped temporarily into the soil matrix by weak forces. Dans un même sol, la teneur en carbone 13 de l’alcane en C31 ‘libre’ est différente de celle de l’alcane en C31 ‘lié’ à l’humine. Les résultats montrent l’alcane lié à l’humine est en moyenne 7 ans plus vieux que l’alcane libre. Ainsi, des substances organiques apolaires peuvent être séquestrées temporairement au sein de la matrice du sol par des forces faibles INTRODUCTION Historically, the organic matter occurring in geological media has been classified into two main fractions based on solvent extraction procedures : an extractable fraction including the so-called ‘free’ molecules, and a non-extractable fraction which contain a poorly known, partly polycondensed organic network such as humin and kerogen [1-4]. Nonetheless, small molecules could also be trapped by weak interactions into internal voids of the macromolecular network, as suggested by studies on humic substances and sedimentary organic matter [5-9]. For instance, chemical and physical treatments of fulvic and humic acids have a notable influence on extractable amounts of alkanes and fatty acids [7]. Furthermore, it was found that concentrations of lipids are usually higher in humic-bound fractions than in ‘free’ soil extracts [8]. Such findings suggest that during the decay of living organic matter plant-derived lipids could be split up into various pools of different turnover. If that is the case, then bound molecules should be older than free molecules. This hypothesis is confirmed in this report using C labelling of soil organic matter with maize.