Role of Soil Organic Acids in Mineral Weathering Processes

Abstract

The soluble organic acids in soils consist largely of complex mixtures of polymeric compounds referred to collectively as fulvic and humic acids. These compounds are relatively refractory, and are broken down only slowly by microorganisms. Low-molecular-mass acids (e.g., acetic, oxalic, formic) exist in dynamic balance: they are rapidly produced and consumed by microorganisms. Their concentrations may be quite high where rapid decomposition of plant material is taking place, and in microenvironments adjacent to roots and fungal hyphae, but are generally low (typically less than 1 mM) in bulk soil solution. Concentrations of organic acids are generally highest in the organic layer at the top of the soil profile and decrease with depth. Particularly high concentrations of dissolved organic acids occur in peatlands and waterlogged soils. Organic acids affect the mineralogy of soils mostly through their ability to complex and transport iron and aluminum, resulting in a characteristic profile development of humid regions. There is considerable controversy as to whether organic acids at natural concentrations significantly accelerate the rate of dissolution of primary silicate minerals. Proposed mechanisms for increasing rates involve formation of “surface complexes” with ions on the surface of the mineral, and lowering of the pH of the soil solution. Neither mechanism is likely to have a significant effect on the dissolution rates of primary minerals of granitic rocks, but may have an effect on more mafic rock types. There will also be significant effects in the microenvironments around rootlets or fungal hyphae, where organic acid concentrations may be much higher than in bulk soil solution.