Soil organic matter is a complex mixture of organic substances inherited from the decay of plants and microorganisms. Its main part, humic substances, is made of a high molecular network which is able to trap small components such as pesticides and phthalates. However, the binding pathways of molecules with humic substances are poorly known. Several molecular studies using selective chemical degradation with labeled reagents have revealed the occurrence of strong covalent bonds between xenobiotics and humic substances. Nonetheless, the occurrence of weak interactions at the molecular level is so far poorly known due to the lack of suitable analytical tools. Recent work involving ~(13)C-NMR characterisation of ~(13)C-labeled organic substances bound with humic substances strongly suggests the existence of weak bonds. Here, we bring indirect molecular and isotopic evidence for the occurrence of noncovalent bonds within the soil matrix. We isotopically distinguish two pools of the same molecular substances, plant wax n-alkanes, within the soil matrix: a first pool amenable by organic extraction of the soil, and a second, bound pool amenable by extraction of the soil humin. This sequestration of apolar substances within humin by noncovalent bonds represents a novel way of preservation of soil organic matter. Furthermore, weak binding of apolar molecules has implications in agricultural and environmental fields.
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