A stable isotopic study to determine carbon and nitrogen cycling in a disturbed southern Californian forest ecosystem

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 N-15 and C-13 abundances along with nutritional and edaphic properties were determined in soil, litter, and vegetation samples. Mean bulk nitrogen delta(15)N values of soil and vegetation at Camp Paivika (CP), the most polluted site, were at least 1.7 parts per thousand more enriched than the other, less polluted sites. Mean soil delta(15)NH(4)(+) was also significantly enriched in N-15 at CP compared to Barton Flats (BF), the least polluted site, by 3.8 parts per thousand. Soil delta(15)NO(3)(-) signatures were not statistically different among sites. The litter delta(15)NH(4)(+) values followed a trend similar to that of the soil. Furthermore, the litter delta(15)NO(3)(-) at CP was significantly depleted in N-15 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 delta(13)C 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.