Formation of organo-mineral complexes as affected by particle size, pH, and dry - wet cycles

The formation of organo-mineral complexes is an important process controlling carbon cycling and the properties of soil particles. However, differences between the complexes formed in aqueous systems and those found in terrestrial systems have not been clearly recognised. This study simulated the formation of organo-mineral complexes between dissolved humic acid (DHA) and mineral particles using a solid/aqueous adsorption experiment (aqueous system) and a wet–dry cycling methodology (terrestrial system). The clay fraction of mineral particles contributed greatly (>90%) to overall DHA adsorption on soil particles in adsorption experiments where DHA fractionation was clearly observed. Aromatic content was found to be less favourable for adsorption under all conditions. For particles of different sizes, the significance of fractionation increased with adsorption. High adsorption was observed under acid conditions, although there was less fractionation as a result of the acid-facilitated ligand exchange process. When using a wet–dry cycling methodology, a much higher fOC was observed for the complexes than when using the adsorption experiment, and DHA fractionation was not significant. Further, the distinctly decreased E465/E665 ratio suggests that the DHA washed off from acid complexes was different from the original DHA and most likely the result of DHA precipitation during wet–dry cycles under acid conditions.