Local Alterations to the Nitrogen Cycle as Indicated by Tree Ring and Foliage Stable Isotopes, Xi’an China

Abstract:

China is currently experiencing some of the worst pollution problems on Earth. The increase in nitrogen deposition from industrial pollution sources over the past 30 years, has been substantial enough to increase foliar N uptake in plants growing in unfertilized fields and forests throughout China. The δ¹⁵N signature of foliage and soil have been used to infer changes in the biogeochemical cycling of N in the surrounding ecosystem. The current understanding of global trends in foliar δ¹⁵N however, is limited for the East Asia and Pacific region. Most of the research to date has been conducted in temperate and boreal forests of North America and Europe. In this study, two forested sites were sampled on the Loess Plateau, approximately 80km east of the city of Xi’an, Shaanxi Province, China. The study sites are 1 and 4km down wind of an industrial center including a large nitrogen fertilizer plant. Ecosystem components sampled include soil, forest floor, bole wood, and foliage. We use a combination of δ¹⁵N and δ¹³C of the tree rings, foliage, and soil as indicators of a changing nitrogen cycle and the physiological response of Chinese parasol trees (Firmiana simplex) over time. CN ratios at the study sites suggest that both forested stands are saturated with respect to N. A positive correlation between soil N and foliar δ¹⁵N is attributed to the leaching of N depleted in ¹⁵N with increasing N availability. Despite this positive correlation within the study area, overall foliar δ¹⁵N with a mean of -8.2‰ is low relative to foliage sampled in regions with lower atmospheric N inputs. Foliage and bole wood samples closest to the industrial center have higher δ¹³C, which is consistent with greater exposure to NOx emissions. While difference in δ¹⁵N of ecosystem components between sites is consistent with global trends, the absolute values for the whole study area are relatively low and attributed to N inputs from industrial sources depleted in ¹⁵N.