Nitrogen and sulfur deposition and forest nutrient status in the Valley ofMexico

Mexico City experiences some of the most severe air pollution in the world. Ozone injury has been documented in sensitive tree species in urban and fo rested areas in the Valley of Mexico. However, little is known of the level s of other atmospheric pollutants and their ecological effects on forests i n the Valley of Mexico. In this study bulk throughfall deposition of inorga nic nitrogen (N) and sulfur (S) was measured for one year at a forested sit e upwind (east) and downwind (southwest) of Mexico City. Edaphic and plant (Pinus hartwegii Lindl.) indicators of N and S nutrient status were also me asured. Streamwater NO3- and SO42- concentrations were also determined as a n indicator of watershed-level N and S loss. Annual bulk throughfall deposi tion of inorganic N and S at the high-pollution forested site 23 km southwe st of Mexico City (Desierto de los Leones National Park; DL) was 18.5 and 2 0.4 kg ha(-1). Values for N and S deposition at Zoquiapan (ZOQ), a relative ly low-pollution site 53 km east of Mexico City, were 5.5 and 8.8 kg ha(-1) yr(-1). Foliar concentrations of N, foliar N:P and C:N ratios, extractable soil NO3-, and streamwater NO3- concentrations indicate that the forest at DL is N enriched, possibly as a result of chronic N deposition. Sulfur con centrations in current-year foliage were also slightly greater at DL than a t ZOQ, but S concentrations in one-year-old foliage were not statistically different between the two sites. Streamwater concentrations of NO3- ranged from 0.8 to 44.6 mu Eq L-1 at DL compared to 0.0 to 11.3 mu Eq L-1 at ZOQ. In summary, these findings support the hypothesis that elevated N depositio n at DL has increased the level of available N, increased the N status of P . hartwegii, and resulted in export of excess N as NO3- in streamwater.