274 SSSAJ: Volume 73: Number 1 • January–February 2009 Soil Sci. Soc. Am. J. 73:274-284 doi:10.2136/sssaj2007.0016 Received 9 Jan. 2007. *Corresponding author (cejohns@syr.edu). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. T he effects of acidic deposition on soils include the deple- tion of base cations, decreasing base saturation and pos- sibly CEC e , increased mobilization of Al, Mn, and H + , and the accumulation of N and S (Blake et al., 1999; Driscoll et al., 2001). These effects have been studied in laboratories, in whole watershed manipulations, and at intensive study sites. Laboratory experiments have shown that strong acid addi- tions to soils result in increased base cation leaching, which returns to preacidification levels once acid additions are ended (Dahlgren et al., 1990). Studies at the Bear Brook Watershed in Maine (BBWM) (Fernandez et al., 2003; Norton et al., 2004), using paired catchments, have concluded that a water- shed treated with bimonthly additions of ammonium sulfate (1800 mol c ha −1 yr −1 ) had lower exchangeable Ca and Mg in all horizons, and that there was increased export of base cat- ions from the watershed over the 9-yr study period. A similar study by Edwards et al. (2002) in West Virginia came to similar conclusions. Rustad et al. (1996) treated Typic Haplorthods at BBWM with H 2 SO 4 and a combined H 2 SO 4 –HNO 3 mix (2000–4000 mol c ha -1 yr -1 ) for 4 yr (1988–1991), then allowed a 2-yr recovery period. They found that after the re- covery period, the soil and soil solution chemistry remained mostly unchanged. They concluded that this particular hard- wood forest soil was not permanently altered by the acid addi- tions and could recover quickly. The mandates of the 1970 and the 1990 Clean Air Act Amendments (CAAA) in the United States, and similar legisla- tion in Canada and Europe, have resulted in ubiquitous de- creases in the wet deposition of sulfate (SO 4 2− ) and hydrogen ion (H + ) across these regions (Stoddard et al., 1999; Driscoll et al., 2003; Kahl et al., 2004). Consequently, since the late 1980s, research at both intensive study sites and on regional scales has focused to the recovery of these aquatic ecosystems following reduced acidic deposition (e.g., Driscoll et al., 1989; Likens et al., 1996; Stoddard et al., 1999; Evans et al., 2001; Skjelkvåle et al., 2001; Clair et al., 2002; Jefferies et al., 2003; Warby et al., 2005), and to a lesser extent, the response of watershed soils. Most of the studies on soil acidification and subsequent recovery from acidic deposition in North America and Europe have been conducted at intensive study sites (Mulder et al., Richard A. F. Warby Chris E. Johnson* Charles T. Driscoll Dep. of Civil and Environmental Engineering 151 Link Hall, Syracuse Univ. Syracuse, NY 13244 FOREST, RANGE & WILDLAND SOILS 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. Abbreviations: Al N , exchangeable aluminum normalized to soil carbon concentration; ANC, acid neutralizing capacity; BBWM, Bear Brook Watershed in Maine; Ca N , exchangeable calcium normalized to soil carbon concentration; CAAA, Clean Air Act Amendments; CEC, cation exchange capacity; CEC e , effective cation exchange capacity; CEC eN , effective cation exchange capacity normalized to soil carbon concentration; CNE, Central New England; DDRP, Direct Delayed Response Project; K N , exchangeable potassium normalized to soil carbon concentration; Mg N , exchangeable magnesium normalized to soil carbon concentration; Na N , exchangeable sodium normalized to soil carbon concentration; pH s , soil pH in 0.01 M calcium chloride (CaCl 2 ); pH w , soil pH in deoinzed water; %C, total soil carbon content; %N, total soil nitrogen content; %BS, soil percent base saturation. Published January, 2009