This study utilized isotope analyses to contrast nitrogen and carbon dynamics at four sites located along an air pollution gradient in the San Bernardino National Forest in southern California. Natural 15N and 13C abundances along with nutritional and edaphic properties were determined in soil, litter, and vegetation samples. Mean bulk nitrogen &dgr;15N values of soil and vegetation at Camp Paivika (CP), the most polluted site, were at least 1.7? more enriched than the other, less polluted sites. Mean soil &dgr;15NH4+ was also significantly enriched in 15N at CP compared to Barton Flats (BF), the least polluted site, by 3.8?. Soil &dgr;15NO3- signatures were not statistically different among sites. The litter &dgr;15NH4+ values followed a trend similar to that of the soil. Furthermore, the litter &dgr;15NO3- at CP was significantly depleted in 15N compared to the other sites. The isotopic discrimination for the eventual production of nitrate from organic nitrogen in soil and litter was maximized at CP and minimized at BF. A stable carbon isotopic gradient of decreasing soil, litter, and foliar &dgr;13C was also observed with increasing site pollution level. These results support the hypothesis that chronic atmospheric deposition has enhanced nitrogen cycling processes and has affected carbon metabolism at CP. ¿ 2000 American Geophysical Union