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Volume 7 (2007), Pages 175-180
Short Communication

Smog Nitrogen and the Rapid Acidification of Forest Soil, San Bernardino Mountains, Southern California

1Department of Environmental Sciences, University of California, Riverside, CA, USA
2USDA Forest Service, Pacific Southwest Research Station, Riverside, CA, USA
3Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ, USA
4USDA-ARS George E. Brown, Jr. Salinity Laboratory, Riverside, CA, USA
5Department of Botany and Plant Sciences, University of California, Riverside, CA, USA

Received 3 October 2006; Revised 5 February 2007; Accepted 6 February 2007

Academic Editor: Andrzej Bytnerowicz

Copyright © 2007 Yvonne A. Wood et al.


We report the rapid acidification of forest soils in the San Bernardino Mountains of southern California. After 30 years, soil to a depth of 25 cm has decreased from a pH (measured in 0.01 M CaCl2) of 4.8 to 3.1. At the 50-cm depth, it has changed from a pH of 4.8 to 4.2. We attribute this rapid change in soil reactivity to very high rates of anthropogenic atmospheric nitrogen (N) added to the soil surface (72 kg ha–1 year–1) from wet, dry, and fog deposition under a Mediterranean climate. Our research suggests that a soil textural discontinuity, related to a buried ancient landsurface, contributes to this rapid acidification by controlling the spatial and temporal movement of precipitation into the landsurface. As a result, the depth to which dissolved anthropogenic N as nitrate (NO3) is leached early in the winter wet season is limited to within the top ~130 cm of soil where it accumulates and increases soil acidity.