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Acidification Sources in Red Alder and Douglas Fir Soils-Importance of Nitrification¹

ABSTRACT

Precipitation, throughfall, forest floor, and soil leachate samples were monitored continuously in 1981 and 1982 in a N-poor Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forest and a red alder (Alnus rubra Bong.) forest growing adjacently on a glacial soil in western Washington. The purpose of the study was to quantify the relative importance of atmospheric vs. natural sources of H⁺ input to forest soil acidification, and to determine the role of N transformation processes in the overall H⁺ balance of soils with different N status. Rainwater samples had an avg pH of 4.7 and annual H⁺ deposition via precipitation averaged 320 mol H⁺ ha^–1 yr^–1. This was modest compared to internal H⁺ production associated with HCO^-₃ and NO^-₃ formation. In the soil under alder cover, which was naturally enriched in N through symbiotic N₂-fixation, nitrification released up to 4500 mol H⁺ ha^–1 annually to the solution percolating through the upper part of the soil profile. In the N-poor soil no nitrification could be observed and N transformation processes had a minor influence on the soil H⁺ balance. The main internal acidification source in this case was H₂CO₃ dissociation releasing 420 mol H⁺ ha^–1 yr^–1. In both instances, soil solutions appeared well buffered against these external and internal acidification sources, and few H⁺ leached below the 40-cm soil depth.

¹ Research sponsored jointly by the National Science Foundation's Ecosystem Studies Program (DEB 78-24395) and the Electric Power Research Institute under contract RP-1813-1 with Martin Marietta Energy Systems Environmental Sciences Division, Oak Ridge National Lab., TN.

² Research Associate and Professor of Forest Soils, College of Forest Resources, Univ. of Washington, Seattle, WA 98195.

Received for publication February 21, 1984. Accepted for publication March 25, 1985.