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Magnesium and Potassium Release by Weathering in Spodosols: Grain Surface Coating Effects

Département de Géographie, Université de Montréal, Montréal, Canada H3C 3J7

Marie-Claude Turmel and Paul Beauchemin

Les Laboratoires Savoie-Dufresne, C.P. 927, Succ. Outremont, Montréal, H2V 4R8, Canada

*Corresponding author (courchef{at}ere.umontreal.ca).

ABSTRACT

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 flow rate of 0.05 mL 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 ( < 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 (3.04–4.87 µmol g^–1 h^–1/2) and Al (10.07–36.33 µmol g^–1 h^–1/2) at a faster rate than treated (Fe: 0.28–1.68 µmol g^–1 h^–1/2; Al: 1.21–27.34 µmol 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 µmol g^–1 h^–1/2; K: 0.052–0.073 µmol g^–1 h^–1/2) than in intact (Mg: 0.19–0.61 µmol g^–1 h^–1/2; K: 0.032–0.039 µmol g^–1 h^–1/2) samples for two of the three Spodic B horizons. In the Croche B horizon, the absence of rate differences ( < 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.

Received for publication May 23, 1995.

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