Early indications of soil recovery from acidic deposition in U.S. red spruce forests

Forty to fifty percent decreases in acidic deposition through the 1980s and 1990s led to partial recovery of acidified surface waters in the northeastern United States; however, the limited number of studies that have assessed soil change found increased soil acidification during this period. From existing data, it's not clear whether soils continued to worsen in the 1990s or if recovery had begun. To evaluate possible changes in soils through the 1990s, soils in six red spruce (Picea rubens Sarg.) stands in New York, Vermont, New Hampshire, and Maine, first sampled in 1992 to 1993, were resampled in 2003 to 2004. The Oa-horizon pH increased (P < 0.01) at three sites, was marginally higher (P < 0.1) at one site, and lower (P < 0.05) at the New York site. Total C concentrations in Oa horizons decreased (P < 0.05) at sites where the pH increased, but the cause is uncertain. Exchangeable Al concentrations in Oa horizons decreased (P < 0.05) 20 to 40% at all sites except New York, which showed no change. The Al decrease can be attributed to decreased deposition of SO₄²⁻, which decreased the mobility of Al throughout the upper soil profile. Results indicate a nascent recovery driven largely by vegetation processes.