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Nitrogen deposition effects on western montane ecosystems under a Mediterranean climate.
Fenn, Mark1, 1
ABSTRACT- Chronic atmospheric nitrogen (N) deposition and ozone exposure in montane ecosystems in the Los Angeles Air Basin have caused major ecological perturbations and alterations in carbon and N cycling. The combined effects of ozone and N deposition result in greater C pools in aboveground plant tissues, rapid turnover of ponderosa pine foliage, and increased litter accumulation. The more exposed chaparral and forest watersheds in the transverse ranges are N saturated based on temporal nitrate export patterns, characterized by high baseline concentrations and the highest peak nitrate levels from undisturbed wildland watersheds in North America (>350 eq/L). Nitric oxide emissions (>2.5 kg/ha/yr) from soil are also the highest in North America, rivaling levels found in agricultural soils. High N losses are attributed to the open nature of N cycling in these systems. Net nitrification rates are high (96 kg/ha/yr) and nitrification of mineralized N is rapid and complete. Temporal asynchrony between periods of greatest biotic N demand (spring and early summer) and peak hydrologic fluxes (fall and winter) also favors nitrate leaching. However, recent data on mature tree bole growth responses to long-term fertilization suggest that even under conditions of severe ecosystem N loss, tree growth still responds positively to added N. These findings suggest that these systems are "N saturated" based on N export fluxes, but may be at least intermittently N-limited in terms of plant responses.
KEY WORDS: nitrogen saturation, atmospheric deposition, nitrification, nitrate