Dissolved organic matter composition and mineral surface chemistry modulate organic retention and formation of mineral associated organic matter

Abstract

The interaction between dissolved organic matter (DOM) and minerals governs OM stabilisation in soils. Heterogeneous composition of DOM and diverse mineral surface characteristics in soils make it challenging to determine reaction mechanisms at mineral-organic interfaces. To determine the role of DOM and mineral characteristics in the formation of mineral associated organic matter (MAOM), we conducted adsorption experiments using four DOM extracts from pine, Eucalyptus, pasture and wheat residues and six minerals (i.e., kaolinite, montmorillonite, goethite, birnessite, ferrihydrite and allophane) with contrasting charge characteristics, structural order and specific surface area. Poorly crystalline and amorphous minerals generally adsorbed larger amounts of DOM than crystalline oxides and phyllosilicates. The adsorption sequence of functional groups differed with mineral surface chemistry, but C=C/C=O, C-O-C and C-O groups were consistently the most abundant adsorbed moieties. Adsorption involved both mineral-organic and organic-organic interactions, with the latter appearing with increased OM loading, yielding different mineral-OM assemblages depending on OM loading and mineral characteristics. Carboxylic, aromatic and aliphatic organic components predominantly adsorbed on the surfaces of ferrihydrite, allophane, goethite, and kaolinite, whereas quinone and phenolic organic components were preferentially adsorbed on birnessite and montmorillonite. Variations in the molecular composition within DOM also modulated adsorption behaviour at mineral-organic interfaces. The adsorption capacity of wheat DOM on mineral surfaces was lower than that of the other DOM due to its less abundance of reactive functional groups. Our results provide a mechanistic basis for understanding how DOM composition and mineral surface properties regulate the formation of MAOM under acidic conditions.