AMMONIUM ADSORPTION IN ALPINE TUNDRA SOIL: IMPACTS ON NUTRIENT TRANSPORT
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.