AMMONIUM ADSORPTION IN ALPINE TUNDRA SOIL: IMPACTS ON NUTRIENT TRANSPORT

The nitrogen cycle in alpine tundra ecosystems is complex due to simultaneous multi-component processes resulting in nitrogen conversion between organic and inorganic forms. Reactive N additions to soil have been shown to result in a reduction in base saturation, due to soil acidification.

To examine the impact of NH₄⁺ adsorption on cation transport in an alpine tundra soil, a series of batch equilibration and column leaching studies were conducted using NH₄⁺ concentrations in the 5 – 40 mg L^-1 range, with pH adjusted to 4.5. Soil horizons studied were the O/A1, A2 and Bw collected from a Humic Dystrocryepts. Ammonium additions to individual soil horizons resulted in significant increases in Ca²⁺, Mg²⁺, and K⁺ concentration (base cations) in the batch soil solution, for all ammonium concentrations. Base cation concentration in solution, increased linearly with ammonium concentration for the O/A1, A2, and Bw horizons. Ammonium adsorption was modelled using the Langmuir equation, with the NH₄⁺ energy of adsorption ranging from 19.1 – 23.9 KJ/mole, suggesting physical adsorption within each soil horizon. Leaching experiments, using both composite O/A1, A2, and Bw soil columns and intact soil columns, eluted with 20 mg L^-1 NH₄⁺, showed a rapid NH₄⁺ exchange with base cations, This resulted in elevated base cation concentration within the eluent wetting front. Leachate concentrations, corresponding to the wetting front, exceeded 30 mg L^-1 for Ca²⁺ and 10 mg L^-1 for K⁺, for the composite soil horizon columns. Similar elevated leachate concentrations for Ca²⁺ and K⁺ were observed for the intact soil columns, with Ca²⁺ and K⁺ concentrations exceeding 20 mg L^-1 and 10 mg L^-1, respectively. Nonsymmetrical NH₄⁺ breakthrough curves and cation elution curves suggest competitive adsorption between NH₄⁺, Ca²⁺, Mg²⁺ and K⁺ for similar electrostatic adsorption sites within the O/A1, A2 and Bw horizons.

Cation displacement, initiated by NH₄⁺ adsorption, is postulated to result in a “cascade effect” in which initially displaced cations compete for similar organic matter adsorption sites, which may result in increased nutrient losses in alpine tundra soil, impacting plant biodiversity.