Impact of different nitrogen emission sources on tree physiology as assessed by a triple stable isotope approach

The importance that nitrogen (N) deposition has in driving the carbon (C) sequestration of forests has recently been investigated using both experimental and modeling approaches. Whether increased N deposition has positive or negative effects on such ecosystems depends on the status of the N and the duration of the deposition. By combining δ¹³C, δ¹⁸O, δ¹⁵N and dendrochronological approaches, we analyzed the impact of two different sources of NO_x emissions on two tree species, namely: a broadleaved species (Quercus cerris) that was located close to an oil refinery in Southern Italy, and a coniferous species (Picea abies) located close to a freeway in Switzerland. Variations in the c_i/c_a ratio and the distinction between stomatal and photosynthetic responses to NO_x emissions in trees were assessed using a conceptual model, which combines δ¹³C and δ¹⁸O. δ¹⁵N in leaves, needles and tree rings was found to be a bioindicator of N input from anthropogenic emissions, especially at the oil refinery site. We observed that N fertilization had a stimulatory effect on tree growth near the oil refinery, while the opposite effect was found for trees at the freeway site. Changes in the c_i/c_a ratio were mostly related to variations in δ¹³C at the freeway site and, thus, were driven by photosynthesis. At the oil refinery site they were mainly related to stomatal conductance, as assessed using δ¹⁸O. This study demonstrates that a single method approach does not always provide a complete picture of which physiological traits are more affected by N emissions. The triple isotope approach combined with dendrochronological analyses proved to be a very promising tool for monitoring the ecophysiological responses of trees to long-term N deposition.