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

Abstract

Decreasing trends in acidic deposition levels over the past several decades have 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. Nevertheless, documentation of acidic deposition effects on soils has been limited, and little is known regarding soil responses to ongoing acidic deposition decreases. In this study, resampling of soils in eastern Canada and the northeastern U.S. was done at 27 sites exposed to reductions in wet SO4(2-) deposition of 5.7-76%, over intervals of 8-24 y. Decreases of exchangeable Al in the O horizon and increases in pH in the O and B horizons were seen at most sites. Among all sites, reductions in SO4(2-) deposition were positively correlated with ratios (final sampling/initial sampling) of base saturation (P < 0.01) and negatively correlated with exchangeable Al ratios (P < 0.05) in the O horizon. However, base saturation in the B horizon decreased at one-third of the sites, with no increases. These results are unique in showing that the effects of acidic deposition on North American soils have begun to reverse.

Water availability drives gas exchange and growth of trees in northeastern US, not elevated CO2 and reduced acid deposition

“…Over the last decades, this region has experienced simultaneous and significant shifts in moisture availability7, increases in atm. CO 2 concentration, and reductions in acid and nitrogen deposition48 (Fig. 1).…”

High elevation watersheds in the southern Appalachians: Indicators of sensitivity to acidic deposition and the potential for restoration through liming

“…For example, stream ANC was negatively correlated with total soil Ca, and stream Ca:Al was negatively correlated with litterfall Ca:Al. These unexpected patterns may be partially due to insufficient sample size (either too few catchments or too few plots within a catchment to capture the full range of variation), seasonally varying patterns of stream/ soil connectivity, or as yet undescribed patterns of stream and soil recovery from acidic deposition (Lawrence et al, 2015). Long-term forest soil sampling efforts in the 1980s and 1990s showed declining exchangeable Ca and increasing soil acidity linked to forest growth and leaching losses (Johnson et al, 1991;Knoepp and Swank, 1995;Lawrence et al, 1995;Likens et al, 1998).…”

“…Likens and Buso (2012) suggested that this response was due to changes in the stream ion balance from being historically dominated by Ca(HCO 3 ) 2 , then shifting to CaSO 4 , and currently toward NaHCO 3 . In another study, Lawrence et al (2015) examined soil recovery in 27 sites across the NE USA and eastern Canada by resampling soils after 8-24 years. They found that site specific decline in SO 4 deposition was correlated with increased base saturation and decreased exchangeable Al in the O-horizon.…”

Methods of Soil Resampling to Monitor Changes in the Chemical Concentrations of Forest Soils

“…Additional study units can be selected, but each is statistically analyzed separately to evaluate if soil changes have occurred. Statistical results of multiple study units can then be grouped for the purpose of regional analysis, as demonstrated in Lawrence et al 5 . The type and size of the study unit will depend on the monitoring questions being asked and the following study design considerations.…”

“…By linking multiple resampling studies, responses of forest soils to changes in a major environmental driver were identified over an extensive region (Figure 11). The study of Lawrence et al 5 demonstrated that results of soil resampling studies with differing designs can be aggregated to address broad regional problems. Figure 11).…”