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

Precipitation, throughfall, forest floor, and soil leachate samples were monitored continuously in 1981 and 1982 in aN-poor Douglasfir (Pseudotsuga men:.iesii (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+ haI yrI. This was modest compared to internal H + production associated with HCOi and NOi formation. In the soil under alder cover, which was naturally enriched in N through symbiotic N1-fixation, nitrification released up to 4500 mol H + haI 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 H1CO) dissociation releasing 420 mol H + haI yrI. 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. Additional Index Words: leaching, acid deposition, soil acidification, pH buffering, Pseudotsuga men:.iesii (Mirb.) Franco, Alnus