Organic matter coatings of soil minerals affect adsorptive interactions with phenolic and amino acids

Phenolic (PAs) and amino acids (AAs) contribute potentially to the stabilization of organic matter (OM) in soil by forming mineral–organic associations (MOAs). However, little is known about how natural OM coating affects the adsorption of these compounds and their mutual interaction upon adsorption. Therefore, batch adsorption experiments were carried out to study how coatings of minerals with dissolved organic matter (DOM) obtained from forest leaf litter (L‐DOM) and from the O horizon (O‐DOM) affect the adsorption of phenolic acids (salicylic, syringic, ferulic and vanillic acid) and amino acids (lysine, glutamic acid, leucine and phenylalanine) at pH 6. Moreover, the adsorption of PAs on OM‐coated minerals was tested after preconditioning by AAs and vice versa. Adsorption of DOM depended on the mineral (goethite ≫ kaolinite > montmorillonite) and sources of DOM (O‐DOM > L‐DOM for goethite). Coatings of all minerals with both DOM sources reduced the adsorption of PAs and the acidic glutamic acid but enhanced that of the basic lysine. The adsorption of PAs decreased with increasing OM coating of the minerals. Strong bonds between AAs and OM‐coated minerals generally enhanced adsorption of PAs with preconditioning by AAs. This indirectly indicates a multilayer model of MOA. Adsorption of AAs, however, was rarely influenced by preconditioning with PAs. Competition between AAs and PAs generally suppressed the adsorption of AAs on coated minerals, whereas the PAs were hardly affected. These results emphasize that the amount and composition of OM coating must be considered when studying the adsorption of PAs and AAs in soil