Continuing Acidification of Organic Soils across the Northeastern USA: 1984–2001

We conducted a resurvey of the O horizon in 2001 in watersheds previously sampled in 1984 under the Direct/Delayed Response Program (DDRP) to evaluate the effects of reductions in acidic deposition in the northeastern United States. In this 17‐yr interval, median base saturation in the Oa horizon decreased from 56.2% in 1984 to 33.0% in 2001. Effective cation exchange capacity (CEC e ), normalized to soil C concentration, showed no significant change between 1984 and 2001. The change in base saturation was the result of almost equivalent changes in C‐normalized exchangeable Ca (Ca N ) and exchangeable Al (Al N ). The median Ca N declined by more than 50%, from 23.5 to 10.6 cmol c kg −1 C, while median Al N more than doubled, from 8.8 to 21.3 cmol c kg −1 C. We observed the greatest change in soil acid–base properties in the montane regions of Central New England (CNE) and Maine, where base saturation decreased by more than 50% and median soil pH in 0.01 M CaCl 2 (pH s ) decreased from 3.19 to 2.97. Changes in median concentrations of other exchangeable cations were either statistically insignificant (Mg N , K N ) or very small (Na N ). We observed no significant change in the median values of either total soil C content (%C) or total soil N content (%N) over the 17‐yr interval. The acidification of the Oa horizon between 1984 and 2001 occurred despite substantial reductions in atmospheric acidic deposition. Our results may help to explain the surprisingly slow rate of recovery of surface waters.