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The Impact of Nitrification on Soil Acidification and Cation Leaching in a Red Alder Ecosystem¹

ABSTRACT

The objectives of this study were to investigate the impacts of internal nitrification on soil and soil solution acidity and on the rate of nutrient export through NO^3– mediated leaching. This was achieved by comparing soil chemical properties and soil solution composition within a naturally N-rich red alder (Alnus rubra Bong.) ecosystem to those of an adjacent Douglas-fir [Pseudotsuga menziesil (Mirbel) Franco] forest where soil N levels were significantly lower and no measurable HNO₃ production could be observed. In the red alder system, where > 100 kg ha^–1 yr^–1 of N were added through symbiotic N₂ fixation, the net annual NO^3– leaching past the 40-cm soil depth amounted to 3460 mol charges ha^–1, and NO^3– concentrations in the solutions collected below 40 cm periodically exceeded drinking water standards of 10 mg L^–1. The H⁺ and NO^3– release was most pronounced in the forest floor and top 10 cm of the soil under alder occupancy and caused significant acidification of percolating solutions. Less than 1% of the total H⁺ input from internal (nitrification) and external (atmospheric) sources leached below the 40-cm depth, which was indicative for the strong buffering capacity of this particular soil. The cation displacement reactions involved in this pH buffering caused a 15% decline in base saturation and a significant acidification of the upper part of the soil profile. The presence of large amounts of mobile NO^3– in solution triggered accelerated cation leaching, causing a selective redistribution of primarily exchangeable Ca²⁺ from the A to the B horizon. These field studies lead us to conclude that the rate and the selectivity of NO^3– mediated leaching in a red alder system could significantly lower the exchangeable cation pool in the rooting zone or cause nutrient imbalance, if a site is managed for repeated rotations of red alder.

Key Words: forest soil fertility • internal acidification • nitrogen fixation • nitrogen status • pH buffering • soil base capital

¹ Research sponsored jointly by the National Science Foundation's Ecosystem Studies Program (DEB 78-24395) and the Electric Power Research Inst. under contract RP-1813-1 with Martin Marietta Energy Systems Environ. Sci. Div., Oak Ridge National Laboratory, Oak Ridge, TN.

² Research associate and professor of forest soils, respectively, College of Forest Resources, Univ. of Washington, Seattle, WA 98195.

Received for publication November 14, 1983.

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