Response of soil fertility to 25 years of experimental acidification in a temperate hardwood forest

The effects of enhanced acid deposition from the atmosphere, and associated elevated inputs of N, are widely evident, especially for forests where excess N has led to a variety of deleterious effects. These include declines in biodiversity, a response that will likely require considerable time for recovery. The purpose of this study was to determine responses of plant nutrient availability in surface mineral soil to 25 yr of experimental acidification and N addition in a central Appalachian hardwood forest ecosystem. We hypothesized that chronic additions of (NH4)2SO4 will increase mineral N, decrease soil pH, P, and base cations, increase micronutrients (Mn2+ and Fe2+), and increase levels of Al3+. Results supported these predictions, although Mn2+ did not vary significantly. Earlier work on these plots found no response of any of the extractable nutrients to 3 yr of treatment, yet after 25 yr, our results suggest that impacts are apparent in the top 5 cm of the A horizon. We surmise that impacts in these soils may have lagged behind the onset of acidification treatments or that several years of treatment were required to overcome preexisting differences in soil ions. Generally, current findings confirm that (NH4)2SO4 treatments have lowered the pH, enhanced levels of exchangeable Al3+, and increased stream‐water exports of NO3− and base cations—a process that further acidifies soil. The combination of these changes in surface soils, with their high proportion of fine roots, may contribute to the reduced growth and competitiveness of some hardwood species at the acidified site.