Accounting for the effect of temperature in clarifying the response of foliar nitrogen isotope ratios to atmospheric nitrogen deposition.

Atmospheric nitrogen deposition affects nitrogen isotope composition (δN) in plants. However, both negative effect and positive effect have been reported. The effects of climate on plant δN have not been corrected for in previous studies, this has impeded discovery of a true effect of atmospheric N deposition on plant δN. To obtain a more reliable result, it is necessary to correct for the effects of climatic factors. Here, we measured δN and N contents of plants and soils in Baiwangshan and Mount Dongling, north China. Atmospheric N deposition in Baiwangshan was much higher than Mount Dongling. Generally, however, foliar N contents showed no difference between the two regions and foliar δN was significantly lower in Baiwangshan than Mount Dongling. The corrected foliar δN after accounting for a predicted value assumed to vary with temperature was obviously more negative in Baiwangshan than Mount Dongling. Thus, this suggested the necessity of temperature correction in revealing the effect of N deposition on foliar δN. Temperature, soil N sources and mycorrhizal fungi could not explain the difference in foliar δN between the two regions, this indicated that atmospheric N deposition had a negative effect on plant δN. Additionally, this study also showed that the corrected foliar δN of bulk data set increased with altitude above 1300m in Mount Dongling, this provided an another evidence for the conclusion that atmospheric N deposition could cause N-depletion in plants.