δ13C, δ15N, N concentration, C/N, and Ca/Al of Pinus densiflora foliage in Korean cities of different precipitation pH and atmospheric NO2 and SO2 levels

Forest decline due to atmospheric pollution is a consequence of long-term chronic exposure, and thus chemical proxies that are sensitive to the pollution are helpful in estimating the impact of atmospheric pollution on forest health. In this study, we explored changes in the isotopic and elemental compositions of needles of red pine (Pinus densiflora) with varying precipitation pH and concentrations of NO2 and SO2 across 18 cities to identify isotopic and chemical signatures that are sensitive to acid deposition. The cities had different intensity of traffic and industrial activities, and thus were expected to have varying levels of atmospheric pollution. The pine needles were analyzed for carbon (δ13C) and nitrogen (δ15N) isotope ratios, N concentrations, C-to-N ratio (C/N), and calcium-to-aluminum ratio (Ca/Al). Analysis of variance, simple linear correlation, and redundancy analysis were used to investigate the variations in foliar chemistry with atmospheric variables. Neither NO2 nor SO2 concentration was correlated with foliar δ13C, δ15N, N concentration, and C/N; whereas precipitation pH was correlated with the foliar parameters, suggesting that foliar chemistry is more sensitive to total acidifying materials than to individual pollutant. The foliar δ13C decreased with lowered precipitation pH, reflecting the increased 13C-depleted CO2 that was co-emitted with acidifying materials from fossil fuel combustion. Foliar δ15N decreased and foliar N concentration increased with lowered precipitation pH, indicating tree uptake of 15N-depleted N from acid deposition. Accordingly, the C/N ratio also decreased with lowered precipitation pH. However, there was no relationship between foliar Ca/Al and precipitation pH; rather. Our result suggests that δ13C, δ15N, N concentration, and C/N of pine needle samples are associated with the level of precipitation pH and thus pine needles could be used as bio-indicators of the impacts of total acidifying pollutants on forest.