The natural abundance of 15N in plant and soil‐available N indicates a shift of main plant N resources to NO from NH along the N leaching gradient

To investigate which of ammonium (NH) or nitrate (NO) is used by plants at gradient sites with different nitrogen (N) availability, we measured the natural abundance of ¹⁵N in foliage and soil extractable N. Hinoki cypress (Chamaecyparis obtusa Endlicher) planted broadly in Japan was selected for use in this study. We estimated the source proportion of foliar N (NH vs. NO) quantitatively using mass balance equations. The results showed that C. obtusa used mainly NH in N‐limited forests, although the dependence of C. obtusa on NO was greater in other NO‐rich forests. We regarded dissolved organic N (DON) as a potential N source because a previous study demonstrated that C. obtusa 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 NO and the NO pool size in the soil, indicating that high NO availability increases the reliance of C. obtusa on NO. Our data suggest the shift of the N source for C. obtusa from NH to NO according to the relative availability of NO. They also show the potential of the foliar δ¹⁵N of C. obtusa as an indicator of the N status in forest ecosystems with the help of the δ¹⁵N values of soil inorganic and organic N. Copyright © 2010 John Wiley & Sons, Ltd.