Controls on the Sorption, Desorption and Mineralization of Low-Molecular-Weight Organic Acids in Variable-Charge Soils

Understanding the controls on interactions between soluble organic compounds and the solid soil matrix is important in understanding soil organic matter dynamics in general, including specific impacts on pedogenic processes, nutrient bioavailability, and C sequestration. This study investigated the pH-dependent relationship between low-molecular-weight organic acids (LMWOAs) and variable-charge soils as a control on the retention of an otherwise highly mobile and bioavailable class of organic compounds. Sorption isotherms of three LMWOAs (one amino acid [glycine], one monocarboxylic acid [propionic acid], and one dicarboxylic acid [malonic acid]) were generated for three forest soils using batch equilibration techniques. Before equilibration, soils were saturated with KCl to control for competing ions, and both soils and solutions were adjusted to one of three pH levels (4.0, 6.0, or 8.0 ± 0.1). Bioavailability was assessed by generating desorption isotherms for each LMWOA–pH combination as well as through laboratory incubations for mineralization rates. Results indicate that sorption of the three LMWOAs followed the general trend: malonic acid ≫ glycine > propionic acid. Sorption tended to increase with acidity and short-range-ordered aluminosilicate content, and was correlated (r = 0.69, P < 0.001) with the magnitude of the difference between the positive charge on the mineral surface and the negative charge of the LMWOA. This trend remained true for the amino acid, which demonstrated sorptive maximums that were more strongly correlated with anion exchange (r = 0.62, P = 0.1) than cation exchange (r = 0.05, P = 0.9). These observations, coupled with decreased desorption and lower mineralization rates at lower pH levels, suggest the potential for an electrostatic mechanism to contribute to the abiotic retention of organic matter in variable-charge soils under acidic conditions.