Declining Acidic Deposition Begins Reversal of Forest-Soil Acidification in the Northeastern U.S. and Eastern Canada.

A consistent decreasing trend in acidic deposition levels over the past several decades has led to partial chemical recovery of surface waters.  However, depletion of soil Ca from acidic deposition has slowed surface water recovery and led to the impairment of both aquatic and terrestrial ecosystems.  Despite the importance of soils in acid-neutralization processes, documentation of acidic deposition effects on soils has been limited, and little information is available on the status of soils with regard to the decline in acidic deposition.  Neither the capacity of soils to recover or the time frame needed to achieve recovery is known.  Through resampling of soils in eastern Canada and the northeastern U.S. possible effects of decreasing acidic deposition on soil exchange chemistry were evaluated.  Results of resampling 27 sites exposed to reductions in wet SO₄^2- deposition of 5.7% to 76% over intervals of 8 to 24 years indicated numerous changes between initial and final samplings.  Decreases of exchangeable Al in the O horizon, and increases in pH in the O and B horizons at a majority of sites provided strong evidence that reversal of soil acidification was underway.   Changes in the O horizon were linked to decreases in SO₄^2- deposition.  Among all study sites, reductions in SO₄^2- deposition were positively correlated with base saturation (P < 0.01), and negatively correlated with exchangeable Al (P < 0.05).  Responses in the B horizon were less clear, and not consistent with the conceptual model of recovery that assumes replenishment of bases from weathering as cation leaching fluxes are reduced by decreases in deposition.