Title: Acidification Sources in Red Alder and Douglas Fir Soils‐Importance of Nitrification
Abstract: 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 N 2 ‐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 H 2 CO 3 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.