GSA Connects 2021 in Portland, Oregon

Paper No. 58-13
Presentation Time: 2:30 PM-6:30 PM


EVANS, Andrew, Department of Earth and Atmospheric Sciences, Metropolitan State University of Denver, Science 2016, Denver, CO 80217 and MCGRATH, Joshua A., Department of Earth and Atmospheric Sciences, Metropolitan State University of Denver, Science 2028, Denver, CO 80217

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.