Nitrogen and Sulfur Deposition and Forest Nutrient Status in the Valley of Mexico

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 forested areas in the Valley of Mexico. However, little is known of the levels of other atmospheric pollutants and their ecological effects on forests in the Valley of Mexico. In this study bulk throughfall deposition of inorganic nitrogen (N) and sulfur (S) was measured for one year at a forested site upwind (east) and downwind (southwest) of Mexico City. Edaphic and plant (Pinus hartwegii Lindl.) indicators of N and S nutrient status were also measured. Streamwater NO₃ ^- and SO₄ ^2- concentrations were also determined as an indicator of watershed-level N and S loss. Annual bulk throughfall deposition of inorganic N and S at the high-pollution forested site 23 km southwest of Mexico City (Desierto de los Leones National Park; DL) was 18.5 and 20.4 kg ha^-1. Values for N and S deposition at Zoquiapan (ZOQ), a relatively 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 concentrations in current-year foliage were also slightly greater at DL than at 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 μEq L^-1 at DL compared to 0.0 to 11.3 μEq L^-1 at ZOQ. In summary, these findings support the hypothesis that elevated N deposition 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.