Forest Soil Organic Horizon Acidification: Effects of Temperature, Time, and Solution/Soil Ratio1

Abstract

A forest soil organic horizon (Oe) was acidified with HNO3 (0–5 cmol kg−1) in the laboratory at three solution/soil ratios (10, 50, 100), three temperatures (4, 14, 30°C), and for up to 162 h to compare effects of added H+ on pH, soluble Ca, Al, and C with effects of environmental conditions likely to vary seasonally in the forest floor. Soil suspension pH reached a steady state within 48 h, regardless of quantity of H+ added, solution/soil ratio, or temperature. Calcium concentrations also reached a steady state with soil solid phases quickly, and additions of H+ increased quantities of Ca (mmol kg−1) in solution as did increasing solution/soil ratio. Shaking time and temperature had smaller effects on Ca than did H+ additions and solution/soil ratio. In contrast, reactions controlling dissolved organic carbon (DOC) and total soluble aluminum (Alt) were slower than those for pH and Ca. Quantities of DOC and Al going into solution remained nearly constant, so that increasing the solution soil ratio diluted concentrations. Dissolved organic C and soluble Al increased with longer shaking times and higher temperature. The results have implications for designing realistic laboratory experiments on forest floor acidification, for evaluating seasonal and yearly variations in soil solution composition in the field, and in developing accurate and reliable simulation models of H+ reactions in forest soils.