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 inc reases the availability of nutrients such as P in these soils. This in vestigation was designed to study (i) the influence of pH on the mecha nism of oxalate sorption by the soil clay fraction and whole soil mate rials from a spodic horizon, and (ii) the effect of soil organic matte r removal on oxalate sorption using Fourier-transform infrared (FTIR) spectroscopy. The removal of soil organic matter and decrease in pH in creased oxalate sorption significantly, There was a two- to threefold increase in oxalate sorption with removal of organic C at all pH value s, Depending on pH, the molar ratio of OH ions released to oxalate sor bed,vas close to 1 or 2, These results suggest that oxalate formed mon dentate and/or binuclear surface complexes at pH 3.5, and bidentate su rface complexes at pH 4.5 and 5.5. With oxalate sorption, large amount s of organic C and Al were released from the soil clay fractions and w hole-soil samples. Seven to 10 times more organic C was desorbed by lo ading of 10 mol oxalate kg(-1) soil clay than by water.