Legacy Effects of Sorption Determine the Formation Efficiency of Mineral-Associated Soil Organic Matter

Sorption of dissolved organic matter (DOM) is one major pathway in the formation of mineral-associated organic matter (MOM), but there is little information on how previous sorption events feedback to later ones by leaving their imprint on mineral surfaces and solutions ("legacy effect"). In order to conceptualize the role of legacy effects in MOM formation, we conducted sequential sorption experiments with kaolinite and gibbsite as minerals and DOM derived from forest floor materials. The MOM formation efficiency leveled off upon repeated addition of identical DOM solutions to minerals due to the retention of highly sorptive organic molecules (primarily aromatic, nitrogen-poor, hydrogen-poor, and oxygen-rich molecules), which decreased the sorption site availability and simultaneously modified the mineral surface charge. Organic-organic interactions as postulated in multilayer models played a negligible role in MOM formation. Continued exchange between DOM and MOM molecules upon repeated sorption altered the DOM composition but not the MOM formation efficiencies. Sorption-induced depletion of high-affinity compounds from solutions further decreased the MOM formation efficiencies to pristine minerals. Overall, the interplay between the differential sorptivities of DOM components and the mineral surface chemistry explains the legacy effects that contribute to the regulation of fluxes and the distribution of organic matter in the soil.