LIGAND EXCHANGE PROCESSES IN ALPINE TUNDRA SOIL - IMPLICATIONS FOR SOIL ACIDIFICATION AND ALUMINUM TRANSPORT

The transport of naturally occurring F^- in alpine tundra soil is complex due to simultaneous multi-component processes involving ligand exchange between various soil components and aqueous Al³⁺ complexes. Ligand exchange processes, involving the release of OH^-, F^-, and low molecular weight organic acids (LMWOA) into soil solution, are of particular interest, since they can modify both the rate of soil acidification and Al³⁺ toxicity.

To examine ligand exchange processes in alpine tundra soil, a series of batch equilibration and column leaching studies were conducted using either 25 mg L^-1 F^- or 25 and 100 mg L^-1 oxalate solutions. The solution pH of each treatment was adjusted to the corresponding soil pH for the O/A1, A2, and Bw soil horizons. Soil suspension pH was observed to significantly increase for all oxalate and F^- treatments, resulting in pH increases ranging between 0.4 and 1.1 pH units. Both oxalate concentration levels resulted in an increase in F^- concentration in suspension, with the largest F^- release being observed for the 100 mg L^-1 oxalate treatment in the O/A1 horizon. Soil columns eluted with 100 mg L^-1 oxalate, showed continual F^- leaching, with the highest F^- leachate concentration being observed within the first 10 P.V., and the largest quantity of F^- being released by the O/A1 horizon. Accelerated Al leaching was observed for soil columns eluted with 25 mg L^-1 F^-. Aluminum transport corresponded to F^- column breakthrough, with Al leaching increasing in the order A2> Bw> O/A1. Sequential soil horizon leaching with D.I. H₂O→ 25 mg L^-1 F^-→ D.I. H₂O→ 100 mg L^-1 oxalate demonstrated that adsorbed F^- could be displaced by oxalate from soil surfaces, with F^- displacement decreasing in the order O/A1>A2>Bw.

Soil pore water ionic composition within the initial wetting front, with respect to oxalate concentration, appears to be critical in determining the quantity of OH^-, F^- and total aluminum in solution. Results from this study suggest that ligand exchange between F^-, oxalate and OH^- on soil surfaces, and the subsequent release of Al³⁺ into the soil pore water, may be responsible for the formation of aluminum hydroxyl-sulfate solid phases and possible phase transitions between jurbanite (AlOHSO₄), basaluminite (Al₄(SO₄)(OH)₁₀), and alunite (KAl₃(SO₄)₂(OH)₆) in this soil.