PARENT SESSION
Poster Session #27: Nutrient Cycling I.
Tuesday, August 6. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

The effects of atmospheric deposition on forested watersheds in the Los Angeles and Mexico City Air Basins.

FENN, MARK^*,1, DE BAUER, MA. DE LOURDES DE LA I.², PEREA-ESTRADA, VICTOR MANUEL², HERNANDEZ-TEJEDA, TOMAS³, ¹ USDA Forest Service, Riverside, California, mfenn@deltanet.com² Colegio de Postgraduados, Montecillo, Mexico, Mexico³ National Forestry and Agricultural Research Institute (INIFAP), Mexico City, D.F., Mexico

ABSTRACT- The Los Angeles and Mexico City Air Basins are largely surrounded by montane forests and shrublands. In both of these highly populated airsheds, elevated levels of photochemical smog are generated, resulting in significant ozone injury to sensitive forest species. Elevated N deposition in both regions also causes N saturation of the more exposed watersheds, as evidenced by elevated nitrate export in streamwater. Sulphur deposition has declined, but remains high in the Basin of Mexico. Net nitrification rates in soil are high in both regions. However, climatic and edaphic differences between the two regions result in varying ecosystem responses to N deposition. In Mexico the wet season co-occurs with the summer growing season, while in California forests, precipitation primarily occurs as winter snow. The volcanic soils in the Basin of Mexico are naturally N rich compared to the N-poor soils in California, which may explain why foliar growth in the Mexican forests did not respond to N fertilization. Peak nitrate concentrations in streamwater were 2-3 times greater in the California forests, possibly as a result of high runoff fluxes during the winter when biotic N demand is lower. Other ecosystem responses to atmospheric deposition in these two regions will be compared.

KEY WORDS: nitrogen, deposition, streamwater, nitrate