The natural abundance of N-15 in plant and soil-available N indicates a shift of main plant N resources to NO3- from NH4+ along the N leaching gradient

To investigate which of ammonium (NH4+) or nitrate (NO3-) is used by plants at gradient sites with different nitrogen (N) availability, we measured the natural abundance of N-15 in foliage and soil extractable N. Hinoki cypress (Chamaecyparis obtuse Endlicher) planted broadly in Japan was selected for use in this study. We estimated the source proportion of foliar N (NH4+, vs. NO3-) quantitatively using mass balance equations. The results showed that C. obtuse used mainly NH4+ in N-limited forests, although the dependence of C. obtuse on NO3- was greater in other NO3--rich forests. We regarded dissolved organic N (DON) as a potential N source because a previous study demonstrated that C. obtuse can take up glycine. Thus we added DON to our mass balance equations and calculated the source proportion using an isotope-mixing model (IsoSource model). The results still showed a positive correlation between the calculated plant N proportion of NO3- and the NO3- pool size in the soil, indicating that high NO3- availability increases the reliance of C. obtuse on NO3-. Our data suggest the shift of the N source for C. obtuse from NH4+ to NO3- according to the relative availability of NO3-. They also show the potential of the foliar delta N-15 of C. obtuse as an indicator of the N status in forest ecosystems with the help of the delta N-15 values of soil inorganic and organic N. Copyright (C) 2010 John Wiley & Sons, Ltd.