Low-Molecular-Weight Organic Acids in Soils: Sources, Composition, Concentrations, and Functions: A Review

The main sources of the low-molecular-weight aliphatic and aromatic organic acids (LMWOAs) in soils are plant residues, root exudates, and microbial metabolites. In the solutions of upper soil layers of forest ecosystems, citric, oxalic, and malic acids predominate among aliphatic LMWOAs, while substituted and unsubstituted benzoic and cinnamic acids predominate among aromatic LMWOAs. The LMWOA concentrations in plant tissues, root exudates, and microbial metabolites vary from several to some tens of millimoles per liter. In soil solutions, these values decrease by one–two orders of magnitude because of LMWOA sorption and biodegradation, their migration downward the profile, and dilution effect. The sorption of LMWOA anions increases with a decrease in pH and an increase in the concentrations of iron oxides/hydroxides and amorphous aluminosilicate clays. Polybasic acids are more tightly bound to solid phase as compared with monobasic acids. The sorption mechanisms vary depending on pH and LMWOA molecular structure and include formation of innersphere and outersphere surface complexes, ligand exchange, and cation bridge bond. Aromatic LMWOAs can be involved in a hydrophobic interaction. LMWOAs are very rapidly decomposed in the upper horizons, their lifespan amounting to several hours. Some important functions of LMWOAs in soils and landscapes are discussed, including their involvement in the global carbon cycle, mineral weathering, Al and Fe mobilization and translocation, mobilization of Fe and P compounds in the soils deficient in these elements, formation of acid–base buffering of natural waters, and detoxication of soils with increased contents of aluminum, heavy metals, and organic pollutants in soil solutions.