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Valentine, D.W., 1990

Influence of topography on soil acidity and hydrogen ion budgets in an arctic landscape

Bibliographic Reference

Valentine, D.W., 1990, Influence of topography on soil acidity and hydrogen ion budgets in an arctic landscape: Durham, North Carolina, Duke University, Ph.D. dissertation, 136 p.


I studied the influence of topography on H+ fluxes and soil acidity along a toposequence north of the Brooks Range in Alaska. Six discrete ecosystems extended from upland tussock tundra down a slope to the willow-dominated floodplain of the Sagavanirktok River. The hillslope consisted of a dry, 'heath' shoulder, a moist 'shrub-lupine' backslope, and a wet 'Equisetum' footslope, and a 'wet sedge' toeslope. I focused on three aspects of soil acidity: (1) the constraints imposed by redox potential and substrate availability on N-mediated H+ fluxes; (2) factors responsible for variation in soil pH; and (3) H+ budgets. I had expected H+ production via nitrification to dominate well-aerated sites and H+ consumption via NO3- reduction to dominate poorly aerated sites. Incubation patterns only partially supported this hypothesis. The heath site consumed up to 80 mmol H+ m-2 60d-1 during the growing season via NO3- consumption and especially NH4+ production. The wet sedge and shrub-lupine sites switched from net NH4+ generation in 1987 to consumption in 1988. In both sites, NO3- production was stimulated by added NH4+ and reversed by added NO3-. Nitrification near N-fixing Alnus crispa (Ait.) Pursh generated far more H+ than elsewhere. Soil pH ranged from 4.9 in the heath site to 7.0 in the willow site. I used an empirical model to attribute pH variation to three factors. Of the 30 possible comparisons among sites, the degree of neutralization of the acid complex was important in 28, acid quantity in 7, and acid strength in 1. I constructed H+ budgets using a simple hydrologic model and soil solution ion concentrations. Acid neutralizing capacity (ANC) was the most important component everywhere except in the heath site. At the two H+-flux extremes, ANC losses dominated H+ generation (260 mmolrm c rm m-2 y-1) in the shrub-lupine site, while ANC inputs dominated H+ consumption (over 50 mmolrm c rm m-2 y-1) in the wet sedge site. The acidic upland sites lost H+ via leached aluminum. Nitrate dynamics were unimportant. The large ANC fluxes highlights the importance of the generation, accumulation, transfer, and oxidation of organic acids to the biogeochemistry of this site.

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