Magnesium and potassium release by weathering in spodosols: Grain surface coating effects

The effect of coatings on the rate of Mg and K release by weathering were determined on four soil horizons (Croche, Sutton, and LaTuque B and Croche C), collected from three Spodosols in southern Quebec, Canada. Rates of Mg, K, Fe, and Al release from the < 2-mm fraction of intact and treated (coatings removed with dithionite-citrate-bicarbonate [DCB] and H2O2) samples were measured in the laboratory using a continuous flow technique. The materials were leached with a 0.0005 M H2SO4 solution adjusted to pH 3.0 at a how rate of 0.05 mt min(-1) for 1080 h. Surface coatings were composed of Al, Fe, Si, and organic C with traces of Ca. Coating removal had no effect on cation release rates (alpha < 0.01) from the Croche C horizon, indicating that mineral surfaces were not altered by treatments with DCB and H2O2. Intact B horizons released Fe (3.01-4.87 mu mol g(-1) h-(1/2)) and Al (10.07-36.33 mu mol g(-1) h(-1/2)) at a faster rate than treated (Fe: 0.28-1.68 mu mol g(-1) h(-1/2); Al: 1.21-27.34 mu mol g(-1) h(-1/2)) samples although effluent pH (pHf) was more acidic in the latter. Magnesium and K release rates were higher in treated (Mg: 0.35-2.03 mu mol g(-1) h(-1/2); K: 0.052-0.073 mu mol g(-1) h(-1/2)) than in intact (Mg: 0.19-0.61 mu mol g(-1) h(-1/2); K: 0.032-0.039 mu mol g(-1) h(-1/2)) samples for two of the three Spodic B horizons. In the Croche B horizon, the absence of rate differences (alpha < 0.01) was attributed to the incomplete removal of Al coatings from treated materials. Because differences in pHf levels could not be invoked to explain all of the observed rate variability, we suggest that the accumulation of coatings was an effective process isolating primary mineral surfaces, therefore reducing the extent of reactive surfaces available for Mg and K release.