δ13C, δ15N, N concentration, and Ca-to-Al ratios of forest samples from Pinus densiflora stands in rural and industrial areas

In an effort to evaluate the potential adequacy of carbon isotope ratio (C-13/C-12, expressed as delta C-13), nitrogen isotope ratio (N-15/N-14, expressed as delta N-15). N concentration, and Ca-to-Al ratio of forest samples as proxies for atmospheric pollution, samples of needle, bark, litter, organic soil, and inorganic soil were collected from red pine (Pinus densiflora) stands located in industrial (polluted) and rural (background and regarded as relatively unpolluted) areas, and their chemical and isotopic compositions were compared. The delta C-13 of needles and bark samples from the industrial area were significantly (P < 0.05) lower than those from the rural area by more than 1.1 parts per thousand, with a greater difference in the needle samples (more than 2 parts per thousand.); which is reflective of C-13-depleted CO2 emissions from fossil fuel combustion in industrial areas. The delta N-15 of all the samples from the industrial area were significantly (P < 0.05) lower than those from the rural area, except for mineral soil, and the differences were between 0.9 parts per thousand. (litter) and 2.9 parts per thousand (organic soil). Such differences could be primarily attributed to a higher deposition of N-15-depleted atmospheric N in the industrial areas. However, no significant differences were noted with regard to the total N concentrations of the samples from different areas; this was probably due to different indigenous soil N availability. With regard to Ca-to-Al ratios, the needle samples had a significantly lower Ca-to-Al ratio in the industrial samples by approximately 17 to 43% probably as the result of enhanced soil acidification by acid deposition as compared with rural areas. Combining all the samples and isotopic and chemical variables, the delta C-13, delta N-15, and Ca-to-Al ratios of the needle samples were identified as the most reliable proxies for atmospheric pollution in the study areas. Further study will be necessary to develop quantitative criteria for the practical use of each indicator in identifying forest ecosystems that are susceptible to atmospheric pollution. (C) 2009 Elsevier B.V. All rights reserved.