The natural abundance of 15 N in plant and soil‐available N indicates a shift of main plant N resources to NO 3 − from NH 4 + along the N leaching gradient

To investigate which of ammonium (NH   4 + ) or nitrate (NO   3 − ) is used by plants at gradient sites with different nitrogen (N) availability, we measured the natural abundance of 15 N 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   4 +vs. NO   3 − ) quantitatively using mass balance equations. The results showed that C . obtusa used mainly NH   4 +in N‐limited forests, although the dependence of C . obtusa on NO   3 −was greater in other NO   3 − ‐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   3 −and the NO   3 −pool size in the soil, indicating that high NO   3 −availability increases the reliance of C . obtusa on NO   3 − . Our data suggest the shift of the N source for C . obtusa from NH   4 +to NO   3 −according to the relative availability of NO   3 − . They also show the potential of the foliar δ 15 N of C . obtusa as an indicator of the N status in forest ecosystems with the help of the δ 15 N values of soil inorganic and organic N. Copyright © 2010 John Wiley & Sons, Ltd.