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

Author
item WOOD, YVONNE - UC RIVERSIDE, CA
item FENN, MARK - USDA FOREST SERVICE
item MEIXNER, THOMAS - UNIVERSITY OF ARIZONA
item Shouse, Peter
item BREINER, JOAN - USDA FOREST SERVICE
item ALLEN, EDITH - UC RIVERSIDE, CA
item WU, LAOSHENG - UC RIVERSIDE, CA

Submitted to: The Scientific World
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2007
Publication Date: 3/21/2007
Citation: Wood, Y.A., Fenn, M., Meixner, T., Shouse, P.J., Breiner, J., Allen, E., Wu, L. 2007. Smog Nitrogen and the Rapid Acidification of Forest Soil, San Bernardino Mountains, Southern California. The Scientific World. Vol 7:175-180

Interpretive Summary: Soil acidification is a natural process that operates over many thousands of years across forest wild land soils. However, forest soils near urban regions where emissions of anthropogenic nitrogen (N) are high have shown accelerated acidification that is detectable within decades. In this work, we consider the rate of change of soil reactivity in the San Bernardino Mountains located in southern California. During soil sampling at our sites, we observed the consistent presence of subsurface stones between the 40- to 60-cm and the 130- to 170-cm depths. These stones formed a three-dimensional subsurface layer and are indicative of the presence of an ancient land surface which has been uplifted by tectonic activity along the San Andreas Fault. We hypothesized that the stone layers and associated soil textural discontinuity are important to subsurface hydrologic processes through control of the spatial and temporal movement of soil water. In turn, this should affect the rate of soil acidification through control of the movement of NO3 carried as a dissolved solute. Thus, our data suggest that when subsurface stones are present, the depth of early season infiltration is limited, translocating the large surface additions of anthropogenic N as NO3 no deeper than the 80- to 120-cm soil depths. Under these conditions, the upper ~100 cm of soil under high N deposition is rapidly acidifying. In contrast, the soil pHCaCl2 has changed little during the last 3 decades, where low additions of atmospheric N do not provide sufficient NO3 to affect soil acidity.

Technical Abstract: 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.