Document Type


Journal/Book Title/Conference

Soil Science Society of America Journal






American Society of Agronomy

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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.