Nitrate and Anion Behavior in Alpine Tundra Soil in the Colorado Rocky Mountains

Abstract:

Anthropogenic nitrogen deposition can potentially alter soil biogeochemistry in alpine tundra ecosystems by soil acidification, resulting in accelerated nutrient leaching as well as reduced microbial and plant diversity. Several field studies have simulated various atmospheric nitrogen loading rates and observed changes in above ground biomass, species diversity, and soil buffering capacity. Few studies to date have examined the biogeochemical behavior and transport of nitrogen in alpine tundra soil. The objective of this study is to evaluate nitrate transport in soil and the chemical behavior of associated leached ionic species. To accomplish this, a soil leaching study was conducted using both composite soil columns and intact soil cores collected in Rocky Mountain National Park, CO, USA (3,658 m). Soil columns were leached in a temperature controlled environmental chamber with DI water adjusted for pH and ionic strength. Leachates were collected using a fraction collector and analyzed using IC and ICP-MS. Analysis of collected leachates for intact soil cores indicated a complex mixture of inorganic and organic anions moving in the soil wetting front, with elevated NO₃^- concentration > 15 mg/L. Nitrate concentration decreased rapidly after initial column breakthrough. Leaching of individual soil horizons indicated high NO₃^- concentrations > 15 mg/L in collected pore volumes for both the organic and subsurface horizons. Elevated concentrations of both inorganic (SO₄^2-, F^-) and organic anions (acetate, oxalate) were found in these horizons. Fluctuation of approximately 1-1.5 pH units for the intact soil column leachates and the anion elution order suggests possible complex anion exchange processes in the soil wetting front between various soil solid phases.