SULFATE RETENTION BY SPDOSOLS IN THE PRESENCE OF ORGANIC-LIGANDS

The aim of this work was to study the influence of low molecular weight organic acid concentration (0 to 1600 mumol C L-1) and type (oxalic, citric (CI), salicylic) and of solution pH (3.2 to 4.7) on the kinetics of SO4 sorption by the B horizon of two Spodosols (HER and CON) from the Lower Laurentians, Quebec. Using a miscible displacement technique, the soil materials were leached at a flow rate of 0.2 ml min-1 for 6 h at 25-degrees-C. The slope parameter (1/beta) of the Elovich equation was used as an apparent SO4 sorption rate. The presence of organic ligands in solution reduced SO4 sorption by the HER B and the CON B horizons from 543 to 365 mumol SO4 kg-1 and from 1053 to 642 mumol SO4 kg-1, respectively. For both soils, 1/beta values were always higher (97 to 51 mumol kg-1 ln min-1 (HER) and 256 to 135 mumol kg-1 ln min-1 (CON)) in systems that were free of organic acids; however, the relationships between the properties of the acids and their relative SO4 sorption inhibition capacity was not clear. The extent of SO4 sorption inhibition increased considerably as CI concentration was augmented from 100 to 1600 mumol C L-1. The efficiency of CI in reducing SO4 sorption by the HER B horizon was pH dependent, with SO4 sorption during the experiment decreasing by 23% at pH 3.2 and by 355% (desorption) at pH 4.7. The presence of CI led to the desorption of native exchangeable SO4 when the pH was above 4.2 or when the total C concentration was high (1600 mumol C L-1). The data thus reveal that temporal changes in C cycling can affect SO4 mobility in acidic forest soils with potential implications on the transport of nutrient cations if the total anionic equivalent charge of the soil solution is increased.