Influence of Soil Organic Matter Removal and pH on Oxalate Sorption onto a Spodic Horizon

Oxalate is one of the dominant low molecular weight organic anions in many soils under forest ecosystems. Through its complexation reactions with Fe and Al, both in solution and at mineral surfaces, oxalate increases the availability of nutrients such as P in these soils. This investigation was designed to study (i) the influence of pH on the mechanism of oxalate sorption by the soil clay fraction and whole soil materials from a spodic horizon, and (ii) the effect of soil organic matter removal on oxalate sorption using Fourier‐transform infrared (FTIR) spectroscopy. The removal of soil organic matter and decrease in pH increased oxalate sorption significantly. There was a two‐ to threefold increase in oxalate sorption with removal of organic C at all pH values. Depending on pH, the molar ratio of OH ions released to oxalate sorbed was close to 1 or 2. These results suggest that oxalate formed mondentate and/or binuclear surface complexes at pH 3.5, and bidentate surface complexes at pH 4.5 and 5.5. With oxalate sorption, large amounts of organic C and Al were released from the soil clay fractions and whole‐soil samples. Seven to 10 mol oxalate kg −1 soil clay than by water.