Measuring Soil Change to Assess Recovery Potential

Lawrence, G.B.1, Shortle, W.C.2, K.T. Smith2, Roy, K.M.3 and David, M.B.4

Levels of acidic deposition have generally decreased over the past several decades in the northeastern U.S., but so too has the effectiveness of acid-neutralization processes as soils have become depleted of available calcium. As a result, chemical recovery from acidification has been marginal in many of the affected lakes and streams in the Northeast. Furthermore, the recovery in surface waters that has occurred may have resulted from neutralization processes that occur in deep flow paths through till that underlie watersheds with calcium-depleted soils. In this situation, high-flows can still lead to episodic acidification of streams and lakes with short residence times, and terrestrial vegetation is subjected to aluminum mobilization and calcium depletion. Recent work in western Adirondack watersheds has shown that stream-water values of the chemical index BCS in were found to be negative (indicating mobilization of toxic inorganic Al) if base saturation of the upper B horizon was less than 12%. Base saturation values estimated from high-flow stream chemistry data from the Western Adirondack Stream Survey conducted in 2003-2005, ranged from 6% to 12% in 111 of 188 watersheds. Base saturation of these soils is therefore too low to prevent mobilization of toxic Al in most of these watersheds at the current deposition levels. Assessments of recovery potential from acidification need to extend beyond surface water chemistry, but little information is available on the capacity of Ca depleted soils for recovery, or the rate at which this could occur. Retrospective soil studies, although rare, have shown that repeated sampling of soil over time can provide valuable information on soil change. Resampling of soils in 2003 at 6 sites in New York and New England that had been previously sampled and archived in 1992-1993 provided information on how these soils changed during a decade of declining deposition. Only one of 5 sites showed indications of recovery from acidification in the upper B horizon. However, in the Oa horizon, the Ca to Al ratio and base saturation increased at 5 of 6 sites and the CaCl2 extractable pH increased at 4 of 6 sites. These results, in addition to a previously identified link between Oa horizon chemistry and stream water neutralization, suggest that processes in the forest floor may play an important role in the recovery of these catchments.

1 U.S. Geological Survey, 425 Jordan Road, Troy, NY, 12180, Phone: (518) 285-5664; e-mail:
2 USDA Forest Service
4 University of Illinois